Mary Jones, Richard Harwood,
Ian Lodge and David Sang
Cambridge IGCSE®
Combined and
Co-ordinated
Sciences
Coursebook
Copyright Material - Review Only - Not for Redistribution
Copyright Material - Review Only - Not for Redistribution
Copyright Material - Review Only - Not for Redistribution
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Combined and
Co-ordinated
Sciences
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Mary Jones, Richard Harwood,
Ian Lodge and David Sang
Cambridge IGCSE®
Coursebook
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University Printing House, Cambridge CB2 8BS, United Kingdom
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It furthers the University’s mission by disseminating knowledge in the pursuit of
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© Cambridge University Press 2017
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ISBN 978-1-316-64590-1 Paperback + Cambridge Elevate enhanced edition (2 years)
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All end-of-chapter questions taken from past papers are reproduced by permission of
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® IGCSE is the registered trademark of Cambridge International Examinations.
Copyright Material - Review Only - Not for Redistribution
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vii
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Introduction
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B1.01 Characteristics of living things
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B8.01 Respiration
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B8.02 Gas exchange in humans
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B8.03 Tobacco smoking
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B8 Respiration and gas exchange
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B9 Coordination and homeostasis
105
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B9.02 The human nervous system
106
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B9.03 The eye
109
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B9.04 Hormones
112
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B9.05 Coordination and response in plants
27
B9.06 Homeostasis
36
B10 Reproduction in plants
B4.01 Types of nutrition
36
B10.01 Asexual and sexual reproduction
B4.02 Photosynthesis
37
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B11.02 Fertilisation and development
138
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B11.03 The menstrual cycle
140
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B11.04 HIV/AIDS
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B6 Transport in plants
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B12 Inheritance
141
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B12.01 Chromosomes
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B12.02 Cell division
147
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B12.03 Inheritance
149
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B11.01 Human reproductive organs
B5.04 The alimentary canal
B6.01 Plant transport systems
B10.03 Comparing sexual and asexual reproduction
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B5.03 Teeth
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B5.02 Digestion
B10.02 Flowers
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B5.01 Diet
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B4.05 Testing leaves for starch
124
B11 Reproduction in humans
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B4.04 Uses of glucose
B5 Animal nutrition
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B4 Plant nutrition
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B3.02 Carbohydrates
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B9.01 Coordination in animals
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B3.04 Proteins
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B2.02 Osmosis
B3.05 Enzymes
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B7.01 The circulatory system
B7.04 Blood
B2.01 Difusion
B3.03 Fats
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B3.01 What are you made of?
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B7 Transport in mammals
82
B2 Movement in and out of cells
B3 Biological molecules
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B7.03 Blood vessels
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B1.03 Cells and organisms
B6.04 Transport of manufactured food
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B1.02 Cells
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B6.03 Transpiration
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Biology
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How to use this book
B6.02 Water uptake
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Contents
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B14 Organisms and their environment
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C5.06 Characteristic reactions of acids
276
C5.07 Acids and alkalis in chemical analysis
279
C5.08 Salts
280
C5.09 Preparing soluble salts
281
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C2.01 The states of matter
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C2.05 Electron arrangements in atoms
244
244
C4.02 Equations for chemical reactions
246
C4.03 Types of chemical reaction
C8.03 The transition elements
332
C8.04 The reactivity of metals
334
C9 Industrial inorganic chemistry
343
C9.01 The extraction of metals by
carbon reduction
344
C9.02 The extraction of metals by electrolysis
348
350
C9.03 Ammonia and fertilisers
353
252
C9.06 Limestone
354
C4.05 Electrolysis
254
C9.07 Recycling metals
355
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C9.05 The chlor-alkali industry
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C4.04 A closer look at reactions,
particularly redox reactions
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C9.04 Sulfur and sulfuric acid
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C4.01 Chemical reactions and equations
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C8.02 Aluminium
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C3.06 Metals, alloys and crystals
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C3.05 The chemical formulae of elements and
compounds
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C8.01 The alkali metals
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C7.02 Rates of reaction
C8 Patterns and properties of metals
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C3.03 Trends across a period
306
C7.04 Reversible reactions
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C7.01 Energy changes in chemical reactions
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C3.01 The Periodic Table – classifying the elements
300
C7.03 Catalysts
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C3 Elements and compounds
C6.05 Moles and solution chemistry
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C7 How far? How fast?
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C6.04 Calculations involving gases
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C6.03 The mole and chemical equations
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C2 The nature of matter
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C6.01 Chemical analysis and formulae
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C2.04 The structure of the atom
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C5.05 Alkalis and bases
C6 Quantitative chemistry
177
C2.03 Atoms and molecules
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C2.02 Separating and purifying substances
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C1.03 The Earth’s crust
C3.02 Trends in groups
C5.04 Acid reactions in everyday life
C5.10 Choosing a method of salt preparation
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Chemistry
C1.01 The atmosphere
C5.03 Metal oxides and non-metal oxides
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C1 Planet Earth
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B14.04 Human influences on ecosystems
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C5.02 Acid and alkali solutions
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B14.03 The carbon cycle
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B14.02 Energy flow
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B14.01 Ecology
C5.01 What is an acid?
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B13.02 Selection
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B13.01 Variation
C5 Acids, bases and salts
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B13 Variation and selection
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C10.03 Alkenes
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C10.04 Hydrocarbon structure
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C10.05 Chemical reactions of the alkanes
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C12.02 Inorganic analysis
395
C12.05 Practical skills
398
P5.03 Hooke’s law
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P6.01 Energy for life
462
P6.02 Forms of energy
463
P6.03 Energy conversions
465
P6.04 Conservation of energy
466
P6.05 Energy calculations
469
P7 Energy resources
476
P7.01 The energy we use
476
P7.02 Energy from the Sun
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P8 Work and power
484
P1.02 Measuring length
404
P8.01 Doing work
484
P1.03 Density
406
P8.02 Calculating work done
485
P1.04 Measuring time
409
P8.03 Power
488
P8.04 Calculating power
489
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P2 Describing motion
P9 The kinetic model of matter
419
P9.01 States of matter
P2.03 Understanding acceleration
419
P9.02 The kinetic model of matter
496
P2.04 Calculating speed and acceleration
422
P9.03 Forces and the kinetic theory
499
P9.04 Gases and the kinetic theory
501
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P2.02 Distance–time graphs
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P2.01 Understanding speed
P3.01 Roller-coaster forces
430
P10 Thermal properties of matter
P3.02 We have lit-of
430
P10.01 Thermal expansion
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P3 Forces and motion
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P1.01 How measurement improves
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P5.02 Stretching springs
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Physics
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C12.04 Experimental design and investigation
P1 Making measurements
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C12.03 Organic analysis
P5 Forces and matter
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C12.01 Chemical analysis
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P6 Energy transformations and
energy transfers
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C12 Chemical analysis and investigation
P4.04 Stability and centre of mass
P5.04 Pressure
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C11.04 Condensation polymerisation
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C11.02 Alternative fuels and energy sources
P4.03 Calculating moments
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C11.01 Petroleum
441
P5.01 Forces acting on solids
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P4.02 The moment of a force
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367
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P4.01 Keeping upright
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C10.06 Chemical reactions of the alkenes
C10.07 Alcohols
P4 Turning efects of forces
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C10.02 Alkanes
P3.04 Force, mass and acceleration
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C10.01 The unique properties of carbon
P3.03 Mass, weight and gravity
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C10 Organic chemistry
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P18 Electrical quantities
P10.03 Designing a thermometer
511
P18.01 Current in electric circuits
596
P18.02 Electrical resistance
600
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P13.02 Reflecting light
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P22.01 Atomic structure
641
P22.02 Radioactivity all around
645
P22.03 The microscopic picture
647
P22.04 Radioactive decay
651
P22.05 Using radioisotopes
653
Glossary
660
Index
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CD-Rom
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P17.03 Explaining static electricity
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Revision checklists
Multiple-choice tests
Glossary (matches the coursebook)
Notes on activities for teachers/technicians
Self-assessment checklists
Activities
Answers to end-of-chapter questions
Answers to questions
Study and revision skills
Helps notes and terms and conditions
581
P17.02 Charging and discharging
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P21.02 Power lines and transformers
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P17.01 A bright spark
630
P21.01 Generating electricity
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P15.02 Electromagnetic waves
P17 Electric charge
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P15.01 Infrared, ultraviolet
P16.02 Magnetic fields
P20.03 Force on a current-carrying conductor
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P16.01 Permanent magnets
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P14.03 Speed, frequency and wavelength
P16 Magnetism
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P20.02 The magnetic efect of a current
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P15 Spectra
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P14.02 Describing waves
616
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P14.01 All at sea!
613
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P14 Properties of waves
P19.03 Combinations of resistors
P21 Electromagnetic induction
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P13.05 Lenses
609
P20.01 Electricity meets magnetism
546
P13.04 Total internal reflection
P19.02 Circuit components
P20 Electromagnetic forces
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P13.01 How far to the Moon?
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P19.01 An international language
P19.04 Electrical safety
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P13.03 Refraction of light
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P12.04 How sounds travel
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P12.03 Seeing sounds
604
P19 Electric circuits
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P12.02 At the speed of sound
P18.04 Electricity and energy
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P12.01 Making sounds
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P11.04 Some consequences of thermal (heat)
energy transfer
P12 Sound
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P18.03 More about electrical resistance
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P11.03 Radiation
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P11.02 Convection
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P11.01 Conduction
P13 Light
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P10.02 Temperature and temperature scales
P11 Thermal (heat) energy transfers
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Acknowledgements
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Cover image: Pery Burge/Science Photo Library
Biology
B1 unit opener, B1.05 JOHN DURHAM/SPL; B1.01, B1.02, 4.01, B5 unit opener, B5.07 Eleanor Jones; B1.03,
B8.03 BIOPHOTO ASSOCIATES/SPL; B1.04, B2 unit opener, B2.01, B5.01-5.04, B9.01, B10.03, B13 unit opener,
B13.01, B13.05a, B14 unit opener, B14.02, B14.03 Geof Jones; B3 unit opener, B3.03 Top-Pics TBK/Alamy
Stock Photo; B3.01 MARTYN F. CHILLMAID/SPL; B3.02. B3.04, B3.05 ANDREW LAMBERT PHOTOGRAPHY/SPL;
B4 unit opener, B4.04 Nigel Cattlin/Alamy Stock Photo; B4.02, B6 unit opener, 6.03 DR KEITH WHEELER/SPL;
B4.03, B6.01, B12.01 POWER AND SYRED/SPL; B5.05 Alex Segre/Alamy Stock Photo; B5.06 Images of Africa
Photobank/Alamy Stock Photo; B6.02, B11 unit opener, B11.01 STEVE GSCHMEISSNER/SPL; B7 unit opener,
B7.02 PHOTOTAKE Inc./Alamy Stock Photo; B7.01 PROF. P. MOTTA/DEPT. OF ANATOMY/UNIVERSITY “LA
SAPIENZA”, ROME/SPL; B8 unit opener, B8.02 Tom Merton/Caiaimage/Getty Images; B8.01 PETER MENZEL/
SPL; B8.04 CORBIN O’GRADY STUDIO/SPL; B9 unit opener Science Photo Library - KTSDESIGN/Getty Images;
B10 unit opener, B10.05 Pictox/Alamy Stock Photo; B10.01 SCIENCE PICTURES LIMITED/SPL; B10.02 IRENE
WINDRIDGE/SPL; B10.04 Mediscan/Alamy Stock Photo; B10.06 DAVID M. PHILLIPS/SPL; B12 unit opener,
B12.05, B13.02a imageBROKER/Alamy Stock Photo; B12.02 CNRI/SPL; B12.03 LEONARD LESSIN/FBPA/SPL;
B12.04, B14.04 blickwinkel/Alamy Stock Photo; B13.02b Sam Sangster/Alamy Stock Photo; B13.03 Mary
Evans Picture Library/Alamy Stock Photo; B13.04 PAT & TOM LEESON/SPL; B13.05b Terry Mathews/Alamy
Stock Photo; B14.01 Richard Wareham Fotografie/Alamy Stock Photo; B14.05 Robert Brook/Alamy Stock
Photo
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Chemistry
C1 unit opener, C1.01 ESA/KEVIN A HORGAN/SPL; C1.02, fig. C5.02b Leslie Garland Picture Library; C1.03
joebelanger/iStock/Getty Images Plus/Getty Images; C2 unit opener, C2.08 PEKKA PARVIAINEN/SPL;
C2.02, C2.03, C2.04, C2.06, C2.07, C3.03, C3.04, C3.07b, C4.01, C4.05-7, C4.08a(i),a(ii),b, C5 unit opener,
Fig C5.02, Fig C5.6b, Fig C5.09b, C7.06, C7.10, C10.03, C12 unit opener, C12.02a,b, C12.03 ANDREW LAMBERT
PHOTOGRAPHY/SPL; C2.05 Courtesy of IBM Archives; C2.01, C4 unit opener, C4.03 CHARLES D. WINTERS/
SPL; C3 unit opener, C3.08 KENNETH LIBBRECHT/SPL; C3.01, C3.05, C8.07, C8.09, C10.01, C12.01 Richard
Harwood; C3.02 Kerstin Waurick/iStock/Getty Images Plus/Getty Images; C3.06, C3.07a, C4.02 MARTYN F.
CHILLMAID/SPL; C4.04a,b, C4.09, C5.03 TREVOR CLIFFORD PHOTOGRAPHY/SPL C5.01 DAVID MUNNS/SPL
C5.04 EUROPEAN SPACE AGENCY/AEOS MEDIALAB/SPL C5.05 Jeremy Pardoe/Alamy Stock Photo; C5.06,
C9.09a MARTIN BOND/SPL; C5.07a, C5.08, C7.04, C7.07, C7.09, C9.04, C10.02 MARTYN F. CHILLMAID/SPL;
C5.07b, C7 unit opener, C7.05, C8.01, C8.10a,b, C11.06b CHARLES D. WINTERS/SPL; C5 (tip) ARNOLD FISHER/
SPL; C6 unit opener zlikovec/Getty Images; C6.01 CHRISTIAN DARKIN/SPL; C7.01 SCOTT CAMAZINE/K.
VISSCHER/SPL; C7.02 TEK IMAGE/SPL; C7.03 Classic Image/Alamy Stock Photo; C7.08 ASTRID & HANNSFRIEDER MICHLER/SPL; C8 unit opener, C8.04 Art Directors & TRIP/Alamy Stock Photo; C8.02 JAMES KINGHOLMES/SPL; C8.03 Chris Mellor/Lonely Planet Images/Getty Images; C8.05 J.C.HURNI, PUBLIPHOTO
DIFFUSION/SPL; C8.06 Print Collector/Hulton Archive/Getty Images; C8.08 (all) VvoeVale/iStock/Getty
Images Plus/Getty Images; C9 unit opener, C9.01 ROSENFELD IMAGES LTD/SPL; C9.02 NOAA/SPL; C9.03 BEN
JOHNSON/SPL; C9.05, fig. C9.13a DIRK WIERSMA/SPL; C9.06 MAXIMILIAN STOCK LTD/SPL; C10 PASIEKA/
Getty Images; C10.04 DAVID R. FRAZIER/SPL; C11 unit opener shotbydave/Getty Images; C11.01 SPUTNIK/
SPL; C11.02, C11.03 PAUL RAPSON/SPL; C11.04 ROGER HARRIS/SPL; C11.05 LEONARD LESSIN/SPL;
C11.06 David Talbot
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Thanks to the following for permission to reproduce images:
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Physics
P1 unit opener, P1.01 GoGo Images Corporation/Alamy Stock Photo; P2 unit opener, P2.03 Gavin Quirke/
Lonely Planet Images/Getty Images; P2.01 TRL LTD./SPL; P2.02, P13.09 Cambridge University Press/Nigel
Luckhurst; P3 unit opener, P3.04 ANDREW WHEELER/SPL; P3.01 Chad Slattery/The Image Bank/Getty
Images; P3.02 Getty Images; P3.03 ERICH SCHREMPP/SPL; P4 unit opener Peter Cade/Getty Images; P4.01
Will Steeley/Alamy Stock Photo; P5 unit opener, P5.02 ALEXIS ROSENFELD/SPL; P5.01 GUSTOIMAGES/SPL;
P6 unit opener, P6.04, P15.02 NASA/SPL; P6.01 Jef Rotman/Nature Picture Library; P6.02 Visions of America,
LLC/Alamy Stock Photo; P6.03, P10 unit opener, P10.01a,b, P10.03, P10.04, P13.02, fig. P13.02a, P13.05,
P13.06, P13.07, P13.08, P14.03b, fig.P14.07a, P14.04a, b, P18 unit opener, fig.P18.01, P18.01a, P18.02, P19.02a,
fig.P19.03a, fig. P19.04a, P19.03, P19.04, P22.05 ANDREW LAMBERT PHOTOGRAPHY/SPL; P6.05 Bernhard
Lang/Photographer’s Choice/Getty Images; P7 unit opener P7.03 Kelly Cheng Travel Photography/ Moment/
Getty Images; P7.01 Jim Wileman/Alamy Stock Photo; P7.02 SEYLLOU/ AFP/Getty Images; P7.04 Steve Allen/
Stockbyte/Getty Images; P7.05 Mint Images-Frans Lanting/Mint Images/Getty Images; P8 unit opener, P8.01
ACE STOCK LIMITED/Alamy Stock Photo; P9 unit opener Charity Burggraaf/Getty Images; P9.01 Agencja
Fotograficzna Caro/Alamy Stock Photo; P10.02 MATT MEADOWS/SPL; P11 unit opener, P11.03 EDWARD
KINSMAN/SPL; P11.01 ShaniMiller/Getty Images; P11.02, P12.04, P19.04 sciencephotos/Alamy Stock Photo;
P12 unit opener PASIEKA/Getty Images; P12.01 ©Bernard Richardson, Cardif University; P12.02a Mode/
Richard Gleed/Alamy Stock Photo; P12.02b Doug Taylor/Alamy Stock Photo; P12.03 David Redfern/Redferns/
Getty Images; P13 unit opener, P13.06, P22 unit opener, P22.08 TEK IMAGE/SPL; P13.01 ROYAL GREENWICH
OBSERVATORY/SPL; P13.03 HANK MORGAN/SPL; P13.04 James Balog/Aurora/Getty Images; Fig. P13.07a,
P14.03a, P20.01, P22.03 SPL; P14 unit opener, Fig. P14.08a BERENICE ABBOTT/SPL; P14.01 Thomas Kitchin
& Victoria Hurst/First Light/Getty Images; P14.02 Rick Strange/Alamy Stock Photo; P14.05 JOHN FOSTER/
SPL; P15 unit opener, P15.03 TONY MCCONNELL/SPL; P15.01 DAVID PARKER/SPL; P15.04 DAVID R. FRAZIER/
SPL; P16 unit opener Sylvie Saivin/ EyeEm/Getty Images; P16.01 CORDELIA MOLLOY/SPL; P16.02 JEREMY
WALKER/SPL; P17 unit opener JKboy Jatenipat.Getty Images; P19 unit opener, P19.01 ROSENFELD IMAGES
LTD/SPL; P19.02 David J. Green - electrical/Alamy Stock Photo; P20 Monty Rakusen/Getty Images; P21 unit
opener, P21.01 ED MICHAELS/SPL; P21.02 standby/Getty Images; P22.01 IBM/SPL; P22.02 PUBLIC HEALTH
ENGLAND/SPL; P22.04 PASCAL GOETGHELUCK/SPL; Fig. P22.12a Leslie Garland Picture Library/Alamy Stock
Photo; P22.06 Crown Copyright - Public Health England; P22.07 Mark Kostich/VETTA/Getty Images
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All biology artworks are by Geof Jones.
Copyright Material - Review Only - Not for Redistribution
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This book has been written to help you obtain the knowledge and skills required for your Cambridge IGCSE®
Combined Science 0653 or Cambridge IGCSE® Co-ordinated Sciences (Double Award) 0654 course. We hope
that you enjoy using it.
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Introduction
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All the Biology topics come first, then Chemistry and then Physics. However, you almost certainly won’t
follow this sequence in your lessons. You will probably find that you study Biology, Chemistry and Physics
alongside each other, so you will use diferent parts of the book in diferent lessons.
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Core and Supplement
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Your teacher will tell you whether you are studying:
or Cambridge IGCSE Co-ordinated Sciences Double Award
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• Cambridge IGCSE Combined Science
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• only the Core part of the syllabus, or the Supplement as well.
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If you study the Core only, you will be entered for Papers 1 (Multiple Choice (Core)) and 3 (Theory (Core)) and
either Paper 5 (Practical Test) or 6 (Alternative to Practical). If you also study the Supplement, you may be
entered for Papers 2 (Multiple Choice (Extended)) and 4 (Theory (Extended)), and either Paper 5 (Practical
Test) or 6 (Alternative to Practical).
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Cambridge IGCSE Combined Science 0653 is a single award syllabus. This means that your final papers
are the equivalent of one IGCSE subject. Cambridge IGCSE Co-ordinated Sciences 0654 is a double award
syllabus. In this case, your final papers are the equivalent of two IGCSE subjects.
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There are sidebars in the margins of the coursebook to show which material relates to each syllabus and
paper. If there is no sidebar, it means that everyone will study this material.
Core
Core
Supplement
You will study the
material:
You will study
everything.
This includes the
material:
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Without a
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material:
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material:
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Supplement
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With a double
black sidebar
With a double
blue sidebar
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With a single
blue sidebar
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Without a
sidebar
With a double
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Use this table to ensure that you study the right material for your syllabus and paper:
Copyright Material - Review Only - Not for Redistribution
With a double
blue sidebar
With a single
black sidebar
With a double
black sidebar
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Questions
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Each chapter has several sets of questions within it. Most of these require quite short answers and simply
test if you have understood what you have just read or what you have just been taught.
Activities
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At the end of each chapter, there are some longer questions testing a range of material from the chapter.
Some of these are past questions from Cambridge exam papers, or similar in style to Cambridge questions.
We would like to thank Cambridge International Examinations for permission to reproduce exam questions.
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Each chapter contains activities. These will help you to develop the practical skills you will
need in your course. There are further activities on the CD-ROM. These are marked with this symbol:
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There are two possible papers aimed at testing your practical skills, called Paper 5 and Paper 6
(Practical Test and Alternative to Practical, respectively). Your teacher will tell you which of these you
will be entered for. You should try to do the activities in this coursebook no matter which of these
papers you are entered for.
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Summary
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CD-ROM
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At the end of each chapter, there is a short list of the main points covered in the chapter. Remember,
though, that these are only very short summaries and you will need to know more detail than this for
your course.
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There is a CD-ROM in the back of the book. You can use the revision checklists on the CD-ROM to check of
how far you have got with learning and understanding each idea.
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The CD-ROM also contains a set of interactive multiple-choice questions which test whether you know and
understand the material from each chapter.
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You will find some self-assessment checklists on the CD-ROM too, which you can print of and use to
assess yourself each time you observe and draw a specimen, construct a results chart, draw a graph from
a set of results or plan an experiment. These are all very important skills, and by using these checklists you
should be able to improve your performance until you can do them almost perfectly every time.
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Workbooks
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There are three workbooks to go with this coursebook – one for each science. If you have the workbooks,
you will find them really helpful in developing your skills, such as handling information and solving
problems, as well as some of the practical skills.
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There are some suggestions on the CD-ROM about how you can do well in your course by studying and
revising carefully.
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How to use this book
This chapter covers
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B9
Coordination and homeostasis
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sections set out the key topics within each unit, and help with navigation through the chapter.
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Worked examples
boxes contain clear definitions of important scientific terms in
each chapter.
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are featured throughout to provide step-by-step guidance for
answering questions.
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Key terms
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the hormones insulin and glucagon
how humans maintain a constant internal
body temperature
how plants respond to stimuli
the role of auxin in shoot growth.
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the human nervous system
neurones and how they work
the diference between voluntary and involuntary actions
reflex actions
the structure and function of the eye
the hormone adrenaline
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This chapter covers:
WORKED EXAMPLE C4.01
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A solution is made up of two parts:
■ the solute: the solid that dissolves
■ the solvent: the liquid in which it dissolves.
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What is the balanced equation for the reaction
between magnesium and oxygen?
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Step 1: Make sure you know what the reactants and
products are. For example, magnesium burns
in air (oxygen) to form magnesium oxide.
Tip boxes contain advice for students to avoid common
Step 2: From this you can write out the word equation:
magnesium + oxygen
Step 3: Write out the equation using the formulae of
the elements and compounds:
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Remember that oxygen exists as diatomic
molecules. This equation is not balanced:
there are two oxygen atoms on the let, but
only one on the right.
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Remember that ice is not always at 0 °C – it may be colder
than that. When you take ice from a freezer, it may be as
cold as −20 °C.
2Mg + O2
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Step 4: Balance the equation:
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MgO
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Mg + O2
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misconceptions and provide support for answering questions.
TIP
magnesium oxide
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2MgO
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Questions
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Activity
are featured throughout each chapter to assess students’
knowledge and understanding of science.
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sections throughout each chapter provide guidance for
conducting practical investigations.
aCtIVITY C7.05
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questiOns
B2.01
Define difusion.
B2.02
List three examples of difusion in living organisms.
B2.03
You will need to think about your knowledge of
particle theory to answer this question.
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The factors afecting reaction rate
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest
possible improvements
! Wear eye protection. Sulfuric acid is corrosive.
You must plan an investigation to discover how one chosen
factor afects the rate of a chemical reaction.
MgSO4 + H2
Mg + H2SO4
3
1 Measure 10 cm of 2 mol/dm3 sulfuric acid into a
boiling tube.
2 Add a 5 cm strip of magnesium ribbon and start
a stopclock.
3 When the reaction stops, record the time taken.
4 List the factors that could speed up or slow down
this reaction.
5 Choose one of these factors and plan an investigation
to discover how it afects the rate.
6 Your investigation should produce suficient results to
enable you to draw a graph.
A worksheet is included on the CD-ROM. The Notes on
activities for teachers/technicians contain details of
how this experiment can be used as an assessment of
skills AO3.2 and AO3.5.
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a What efect does an increase in temperature
have on the kinetic energy of molecules of a
gas or a solute?
b Predict and explain how an increase in
temperature will afect the rate of difusion of
a solute.
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At the end of each chapter, a Summary is included to recap
the key topics.
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the importance of water as a solvent
about osmosis, which is a special kind of difusion,
involving water molecules
how osmosis afects animal cells and
plant cells.
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how difusion results from the random movement
of particles
the factors that afect the rate of difusion
why difusion is important to cells and
living organisms
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You should know:
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Summary
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Check the introduction and the
cover flap for information on how
to use the sidebars in the margins.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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How to use this book
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questions on the IGCSE Combined or IGCSE Co-ordinated Sciences exams.
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End-of-chapter questions
When a force moves, it does work. Copy and complete the following sentences,
writing more or less in the spaces.
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Write this equation in words.
Copy and complete the table to show the units of each quantity in the equation.
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d
Omar and Ahmed are liting weights in the gym. Each lits a weight of 200 N. Omar lits the
weight to a height of 2.0 m, whereas Ahmed lits it to a height of 2.1 m. Who does more work in
liting the weight? Explain how you know.
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Millie and Lily are identical twins who enjoy swimming. Their arms and legs provide
the force needed to move them through the water. Millie can swim 25 m in 50 s.
Lily can swim 100 m in 250 s.
[2]
[2]
Write a word equation showing how work done and energy transferred are related.
[2]
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Calculate the swimming speed of each twin.
Which twin has the greater power when swimming? Explain how you can tell.
a
b
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Unit
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We can calculate work done using this equation: W = F × d.
Quantity
[1]
[1]
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is transferred.
is done.
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Power is the rate at which
Power is the rate at which
a
b
4
[1]
[1]
Power tells us about how quickly work is done. Copy and complete the following sentences,
writing work or energy in the spaces.
a
b
3
When it moves, a bigger force does
work than a smaller force.
The greater the distance moved by the force, the
work it does.
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Following
the summary, there will be selection of exam-style End of chapter
questions to help students to prepare for the type of
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the characteristics of living things
the structure of animal cells and plant cells
the functions of the diferent parts of cells
how to calculate magnification.
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B1
Cells
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B1.01 Characteristics of
living things
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growth: a permanent increase in size
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movement: an action by an organism causing a change of
position or place
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reproduction: the processes that make more of the same kind
of organism
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respiration: the chemical reactions in cells that break down
nutrient molecules and release energy
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excretion: removal from organisms of toxic materials and
substances in excess of requirements
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sensitivity: the ability to detect and respond to changes in
the environment
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nutrition: taking in of materials for energy, growth and development
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of these characteristics are shown in the key terms
box. You should learn these definitions now, but you
will find out much more about each of them later in
this book.
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KEY TERMS
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Biology is the study of living things, which are oten
called organisms. Living organisms have seven features
or characteristics which make them diferent from
objects that are not alive (Figure B1.01). The definitions
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Movement All organisms are able to move
to some extent. Most animals can move their
whole body from place to place, and plants can
slowly move parts of themselves.
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Nutrition Organisms
take substances from their
environment and use them to
provide energy or materials
to make new cells.
Reproduction
Organisms are able to
make new organisms
of the same species as
themselves.
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Figure B1.01 Characteristics of living organisms.
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KEY TERMS
respiration: the chemical reactions in cells that break down
nutrient molecules and release energy for metabolism
reproduction: the processes that make more of the same kind
of organism
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excretion: removal from organisms of the waste products of
metabolism (chemical reactions in cells including respiration),
toxic materials and substances in excess of requirements
sensitivity: the ability to detect or sense stimuli in
the internal or external environment and to make
appropriate responses
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nutrition: taking in of materials for energy, growth and
development; plants require light, carbon dioxide, water and
ions; animals need organic compounds and ions and usually
need water
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growth: a permanent increase in size and dry mass by an
increase in cell number or cell size or both
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organisms under a microscope, we can see that they are
all made of cells.
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In addition to these seven characteristics, living organisms
have another feature in common: when we study living
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movement: an action by an organism or part of an organism
causing a change of position or place
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Respiration All organisms
break down glucose and
other substances inside their
cells, to release energy that
they can use.
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Excretion All organisms
produce unwanted or toxic waste
products as a result of their
metabolic reactions, and these
must be removed from the body.
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Sensitivity All organisms pick up
information about changes in their
environment, and react to
the changes.
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Growth All organisms begin
small and get larger, by the growth
of their cells and by adding new
cells to their bodies.
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cell membrane
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B1.02 Cells
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B1: Cells
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Microscopes
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To see cells clearly, you need to use a microscope
(Figure B1.02). The kind of microscope used in a
school laboratory is called a light microscope because
it shines light through the piece of animal or plant
you are looking at. It uses glass lenses to magnify and
focus the image. A very good light microscope can
magnify about 1500 times, so that all the structures in
Figures B1.03 and B1.04 can be seen.
nucleus
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nuclear envelope
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instead of light and can magnify up to 500 000 times.
This means that a lot more detail can be seen inside
a cell. We can see many structures more clearly, and
also some structures that could not be seen at all with
a light microscope. Pictures made using an electron
microscope are called electron micrographs.
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An electron microscope magnifies up to
× 10 million. With an electron microscope
much more detail can be seen.
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The light microscope magnifies up to
× 1500. With a light microscope you
can see some structures inside a cell,
such as a nucleus.
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A hand lens magnifies
about × 10. Cells can
oten be seen as dots.
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The human eye cannot see
most cells.
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Figure B1.02 Equipment used for looking at biological material.
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cytoplasm
Figure B1.03 A typical animal cell – a liver cell – as seen
using a light microscope.
Photomicrographs of plant and animal cells are shown in
Images B1.01 and B1.02. A micrograph is a picture made
using a microscope. A photomicrograph is a picture made
using a light microscope.
To see even smaller things inside a cell, an electron
microscope is used. This uses a beam of electrons
small vacuole
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All organisms are made of cells. Cells are very
small, so large organisms contain millions of cells.
Some organisms are unicellular, which means that
they are made of just a single cell. Bacteria and yeast are
examples of single-celled organisms.
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cell membrane
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cell wall
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cytoplasm
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nuclear
envelope
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nucleus
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chloroplast
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large vacuole
containing cell
sap
Image B1.02 Cells from the trachea (windpipe) of a
mammal, seen through a light microscope (× 300).
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starch grain inside
chloroplast
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membrane
around vacuole
ACtivity B1.01
Making drawings of biological specimens
Skill:
AO3.3 Observing, measuring and recording
Scientists need to be able to look closely at specimens –
either with the naked eye or using a microscope – and note
significant features in them. It is also important to be able to
make scientific drawings. These need to be simple but clear.
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Figure B1.04 A typical plant cell – a palisade cell – as seen
using a light microscope.
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In this Activity, you will be provided with a specimen of an
animal to draw.
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Here are some points to bear in mind when you draw:
■ Make good use of the space on your sheet of paper. Your
drawing should be large, but do leave space around it so
that you have room for labels.
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Image B1.01 Many plant cells contain green structures,
called chloroplasts. Even if it does not have chloroplasts,
you can still identify a plant cell because it has a cell wall
around it (× 2000).
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B1: Cells
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■ You must use the same units for all the measurements.
Usually, millimetres are the best units to use.
You
should not include any units with the final answer.
■
Magnification does not have a unit. However, you must
include the ‘times’ sign. If you read it out loud, you would
say ‘times five’.
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Here are some points to bear in mind when you
label a diagram:
■ Use a ruler to draw each label line.
■ Make sure the end of the label line actually touches
the structure being labelled.
■ Write the labels horizontally.
■ Keep the labels well away from the edges of
your drawing.
Questions
A1 Measure the length of the lowest ‘tail’ (it is really called
an appendage) on the centipede below. Write your
answer in millimetres.
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QuEStiONS
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B1.01
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Cell membrane
Whatever sort of animal or plant they come from, all cells
have a cell membrane (sometimes called the cell surface
membrane) around the outside. Inside the cell membrane
is a jelly-like substance called cytoplasm, in which are
found many small structures called organelles. The most
obvious of these organelles is usually the nucleus. In a
plant cell, the nucleus is very dificult to see, because it is
right against the cell wall.
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length of real spider
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40 mm
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The cell membrane is a very thin layer of protein and fat.
It is very important to the cell because it controls what goes
in and out of it. It is partially permeable, which means
that it will let some substances through but not others.
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=×5
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Cell structure
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length in drawing
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magnification =
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If an object was 1 mm across, how big
would it look if it were magnified ten times?
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The real spider was 8 mm long. So we can calculate the
magnification like this:
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How many times can a good light
microscope magnify?
B1.02
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size of the real object
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magnification =
For example, measure the length of the spider’s body in
the diagram. You should find that it is 40 mm long.
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A2 The real length of the appendage was 10 mm.
Use this, and your answer to question A1, to
calculate the magnification of the drawing of
the centipede.
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Calculating magnification
Skill:
AO3.3 Observing, measuring and recording
Drawings and photographs of biological specimens are
usually made at a diferent size from the actual object.
The magnification of a diagram or photograph is how
much larger it is than the real thing:
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ACtivity B1.02
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The following are two very important things to notice:
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■ Always use a sharp HB (medium hard) pencil and have a
good eraser with you.
Keep
all lines single and clear.
■
■ Don’t use shading unless it is absolutely necessary.
■ Don’t use colours.
■ Take time to get the outline of your drawing correct first,
showing the right proportions.
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Chloroplasts
Chloroplasts are never found in animal cells, but most
of the cells in the green parts of plants have them. They
contain a green colouring or pigment called chlorophyll.
Chlorophyll absorbs energy from sunlight, and this
energy is then used for making food for the plant by
photosynthesis (Chapter B4).
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Because of the spaces between fibres, even very large
molecules are able to go through the cellulose cell wall.
It is therefore said to be fully permeable.
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Chloroplasts oten contain starch grains, which have been
made by photosynthesis. Animal cells never contain starch
grains. Some animal cells, however, do have granules
(tiny grains) of another substance similar to starch, called
glycogen. These granules are found in the cytoplasm, not
inside chloroplasts.
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Cell wall
All plant cells are surrounded by a cell wall made
mainly of cellulose. Paper, which is made from
cell walls, is also made of cellulose. Animal cells
never have cell walls made of cellulose. Cellulose
belongs to a group of substances called
polysaccharides, which are described in Chapter B2.
Cellulose forms fibres that criss-cross over one another
to form a very strong covering to the cell (Image B1.03).
This helps to protect and support the cell. If the cell
absorbs a lot of water and swells, the cell wall stops it
from bursting.
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Chromosomes are very long, but so thin that they cannot
easily be seen even using the electron microscope.
However, when the cell is dividing, they become short
and thick and can be seen with a good light microscope.
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Table B1.01 compares some features of plant cells and
animal cells.
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vacuoles
A vacuole is a space in a cell, surrounded by a
membrane and containing a solution. Plant cells have very
large vacuoles, which contain a solution of sugars and other
substances, called cell sap. A full vacuole presses outwards
on the rest of the cell, and helps to keep it in shape. Animal
cells have much smaller membrane-bound spaces, called
vesicles, which may contain nutrients or water.
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Cytoplasm
Cytoplasm is a clear jelly. It is nearly all water; about 70%
is water in many cells. Many substances are dissolved in it,
especially proteins. Many diferent metabolic reactions
(the chemical reactions of life) take place in the cytoplasm.
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Nucleus
The nucleus is where the genetic information is stored.
This helps the cell to make the right sorts of proteins.
The information is kept on the chromosomes, which are
inherited from the organism’s parents. The chromosomes
are made of DNA.
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have no cell wall
have a cell membrane
have a cell membrane
have cytoplasm
have cytoplasm
have a nucleus
have a nucleus
oten have chloroplasts
containing chlorophyll
have no chloroplasts
oten have large vacuoles
containing cell sap
have only small vacuoles
oten have starch grains
never have starch
grains; sometimes have
glycogen granules
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have a cellulose cell wall
outside the cell membrane
are oten irregular in shape
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Table B1.01 A comparison of plant and animal cells.
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are oten regular in shape
Image B1.03 Cellulose fibres from a plant cell wall. This
picture was taken using an electron microscope (× 50 000).
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Animal cells
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Plant cells
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B1: Cells
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ACtivity B1.05
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ACtivity B1.03
using a microscope
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Practise using a microscope to look at very
small things.
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Looking at animal cells
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
! Wash your hands thoroughly ater handling the
trachea and cells.
Some simple animal cells line the mouth and trachea
(or windpipe). If you colour or stain the cells,
they are quite easy to see using a light microscope
(see Image B1.02 and the drawing below).
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ACtivity B1.04
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nucleus
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cell wall
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A drawing of onion epidermis cells seen through a
light microscope ater staining with iodine.
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3 Put on a few drops of methylene blue.
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7 Look at the slide under the low power of the microscope.
Note any diferences between what you can see now
and what it looked like before adding the iodine solution.
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5 Use filter paper or blotting paper to clean up
the slide, and then look at it under the low power
of a microscope.
Questions
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6 Make a labelled drawing of a few cells.
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A1 Name two structures which you can see in these cells, but
which you could not see in the tracheal cells (Activity B1.04).
A2 Most plant cells have chloroplasts, but these onion
cells do not. Suggest a reason for this.
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A1 Which part of the cell stained the darkest blue?
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A2 Is the cell membrane permeable or impermeable
to methylene blue? Explain how you worked out
your answer.
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A3 Iodine solution turns blue-black in the presence of
starch. Did any of the onion cells contain starch?
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4 Gently lower a coverslip over the stained cells, trying
not to trap any air bubbles.
Questions
6 Using a pipette, take up a small amount of iodine solution.
Very carefully place some iodine solution next to the edge of
the coverslip. The iodine solution will seep under the edge of
the coverslip. To help it do this, you can place a small piece
of filter paper next to the opposite side of the coverslip,
which will soak up some of the liquid and draw it through.
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2 Put your cells onto the middle of a clean microscope
slide, and gently spread them out. You will probably
not be able to see anything at all at this stage.
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nucleus
cytoplasm
A drawing of tracheal cells seen through a
light microscope ater staining with methylene blue.
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vacuole
small vacuole
1 Using a section liter, gently rub of a little of the lining
from the inside of the trachea provided.
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cell membrane
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Looking at plant cells
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
! Take care with the sharp blade when cutting the onion.
To be able to see cells clearly under a microscope, you
need a very thin layer. It is best if it is only one cell thick.
An easy place to find such a layer is inside an onion bulb.
1 Cut a small piece from an onion bulb, and use forceps
to peel a small piece of thin skin, called epidermis,
from the inside of it. Do not let it get dry.
2 Put a drop or two of water onto the centre of a clean
microscope slide. Put the piece of epidermis into it,
and spread it flat.
3 Gently lower a coverslip onto it.
4 Use filter paper or blotting paper to clean up the slide,
and then look at it under the low power of a microscope.
5 Make a labelled drawing of a few cells. The drawing
below may help you, but do not just copy it.
Do remember not to colour your drawing.
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What sort of cells are surrounded by a
cell membrane?
B1.04
What are plant cell walls made of?
What is the main constituent of cytoplasm?
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What is a vacuole?
What is cell sap?
B1.10
Chloroplasts contain chlorophyll. What does
chlorophyll do?
B1.11
What is stored in the nucleus?
B1.12
Why can chromosomes be seen only when a cell
is dividing?
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B1.08
B1.09
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Table B1.02 lists examples of specialised cells and
the parts of the book where you will find information
about how their structures help them to carry out
their functions.
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Function
near the ends of plant roots
absorb water and mineral salts
Chapter B6, Section B6.02
photosynthesis
Chapter B4, Section B4.03
transport oxygen
Chapter B7, Section B7.04
sperm and egg cells
in testes and ovaries
ciliated cells
lining the trachea and bronchi
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move mucus upward
Chapter B8, Section B8.02
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Without looking back at the beginning of this chapter, decide which five of the following
characteristics are found in all living things:
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photosynthesis
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growth
excretion
speech
List the other two characteristics of all living organisms.
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blood system sight
sensitivity
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nutrition
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the functions of the diferent parts of animal cells and
plant cells
how to calculate magnification using millimetres (mm)
some examples of specialised cells.
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End-of-chapter questions
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the seven characteristics of living organisms
the structure of an animal cell and a plant cell
as seen using a microscope, and be able to
compare them
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You should know:
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fuse together to produce a zygote Chapter B11, Section B11.01
Table B1.02 Some examples of specialised cells.
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in the blood of mammals
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red blood cells
Summary
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palisade mesophyll cells beneath the epidermis of a leaf
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Where you can find
out more
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Where it is found
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root hair cells
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Type of cell
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What does ‘partially permeable’ mean?
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B1.07
What does ‘fully permeable’ mean?
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B1.06
A large organism such as yourself may contain many
millions of cells, but not all the cells are alike. Almost all of
them can carry out the activities which are characteristic
of living things, but many of them specialise in doing
some of these better than other cells do. Muscle cells, for
example, are specially adapted for movement. Most cells
in the leaf of a plant are specially adapted for making food
by photosynthesis.
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B1.03
B1.05
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B1.03 Cells and organisms
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QuEStiONS
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The photograph shows a section through a fruit.
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Make a large diagram of the fruit. You do not need to label your diagram.
The photograph shows the fruit at a magnification of x 0.6. Calculate the diameter of the actual fruit
at the point indicated by the dotted line. Show your working, and remember to include the unit.
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B
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Name the parts of the cell labelled A and B.
State two ways in which you can tell that the cells in the micrograph are plant cells and
not animal cells.
i Measure the maximum diameter of the cell labelled X.
Record your measurement in millimetres.
ii The micrograph has been magnified 250 times.
Calculate the real maximum diameter of the cell labelled X. Show your working clearly.
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[3]
The micrograph shows a group of cells from a plant.
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[5]
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chloroplast, chlorophyll
cell wall, cell membrane
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Distinguish between each of the following pairs of terms:
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makes food by photosynthesis
controls what goes in and out of the cell
stores information about making proteins
contains cell sap
protects the outside of the cell
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a
b
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State which part of a plant cell:
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B1: Cells
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[2]
[2]
[1]
[2]
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movement of materials by difusion
how substances move into and out of cells by difusion through the cell membrane
the efects of osmosis in moving water in and out of cells through the cell membrane
how to investigate the efects of surface area, temperature, concentration gradients and difusion distance on the
rate of difusion
movement of water by osmosis (a special kind of difusion)
how osmosis afects plant tissues.
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B2.01 Difusion
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When they can move freely, particles tend to spread
themselves out as evenly as they can (Figure B2.01).
This happens with gases, solutions, and mixtures of
liquids. Imagine, for example, a rotten egg in one corner
of a room, giving of hydrogen sulfide gas. To begin with,
there will be a very high concentration of the gas near
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Atoms, molecules and ions are always moving. The higher
the temperature, the faster they move. In a solid substance
the particles cannot move very far, because they are
held together by attractive forces between them. In a
liquid they can move more freely, knocking into one
another and rebounding. In a gas they are freer still, with
no attractive forces between the molecules or atoms.
Molecules and ions can also move freely when they are
in solution.
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This chapter covers:
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B2
Movement in and out of cells
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B2: Movement in and out of cells
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low concentration
of oxygen
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Figure B2.02 Difusion of oxygen into a cell. The red dots
represent oxygen molecules.
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the egg, but none in the rest of the room. However,
before long the hydrogen sulfide molecules have spread
throughout the air in the room. Soon, you will not be able
to tell where the smell first came from – the whole room
will smell of hydrogen sulfide.
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The hydrogen sulfide molecules have spread out, or
difused, through the air.
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TIP
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KEy tERM
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Remember that difusion is simply the result of particles
moving about randomly. Cells don’t have to do anything
to make it happen.
Difusion and living organisms
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Living organisms obtain many of their requirements
by difusion. They also get rid of many of their waste
products in this way. For example, plants need
carbon dioxide for photosynthesis. This difuses from
the air into the leaves, through the stomata. It does this
because there is a lower concentration of carbon dioxide
inside the leaf, as the cells are using it up. Outside the
leaf in the air, there is a higher concentration. Carbon
dioxide molecules therefore difuse into the leaf, down
this concentration gradient.
QuEStiONS
B2.01
Define difusion.
B2.02
List three examples of difusion in living organisms.
B2.03
You will need to think about your knowledge of
particle theory to answer this question.
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b Predict and explain how an increase in
temperature will afect the rate of difusion of
a solute.
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a What efect does an increase in temperature
have on the kinetic energy of molecules of a
gas or a solute?
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difusion: the net movement of molecules and ions from a
region of their higher concentration to a region of their lower
concentration down a concentration gradient, as a result of
their random movement
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Oxygen, which is a waste product of photosynthesis,
difuses out in the same way. There is a higher
concentration of oxygen inside the leaf, because it is being
made there. Oxygen therefore difuses out through the
stomata into the air.
Difusion is also important in gas exchange for respiration
in animals and plants (Figure B2.02). Cell membranes are
freely permeable to oxygen and carbon dioxide, so these
easily difuse into and out of cells.
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Figure B2.01 Difusion is the result of the random
movement of particles.
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oxygen difuses down a
concentration gradient
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high concentration
of oxygen
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Cambridge IGCSE Combined and Co-ordinated Sciences
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1 Collect a piece of Visking tubing. Moisten it and rub it
until it opens.
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A1 What colour were the liquids inside and outside the
tubing at the start of the experiment?
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A2 What colour were the liquids inside and outside the
tubing at the end of the investigation?
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Difusion of substances through a membrane
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
You are going to investigate difusion of two diferent
substances dissolved in water (solutes). When a substance
is dissolved, its particles are free to move around.
In this investigation, you will use starch solution and iodine
solution. The solutions will be separated by a membrane
made out of Visking tubing. Visking tubing has microscopic
holes in it. The holes are big enough to let water molecules
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At the start of the experiment, there were starch
molecules inside the tubing but none outside the
tubing. Starch particles are too
to go
through Visking tubing.
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At the start of the experiment, there were iodine
molecules
the tubing but none
the tubing. The iodine molecules difused into the
tubing, down their
gradient.
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When the starch and iodine molecules mixed,
a
colour was produced.
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A4 Did either the starch particles or the iodine
particles difuse through the Visking tubing?
How can you tell?
A5 Copy and complete these sentences.
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A3 When starch and iodine mix, a blue-black colour is
produced. Where did the starch and iodine mix in
your experiment?
ACtivity B2.03
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7 Gently put the Visking tubing into the iodine
solution, so that it is completely covered, as shown
in the diagram.
Questions
investigating factors that afect the rate of
difusion
ev
6 Put some iodine solution into a beaker.
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ACtivity B2.02
5 Rinse the tubing in water, just in case you got any
starch on the outside of it.
8 Leave the apparatus for about 10 minutes.
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A3 Suggest three things that you could have done to
make the colour spread more quickly.
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4 Tie the top of the tubing very tightly, using thread.
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A2 Why had the colour spread through the water at the
end of your experiment?
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2 Tie a knot in one end of the tubing.
3 Using a dropper pipette, carefully fill the tubing with
some starch solution.
A1 Why was it important to leave the water to become
completely still before the crystal was put in?
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Visking tubing
starch solution
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Questions
12
iodine solution
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Demonstrating difusion in a solution
Skill:
AO3.3 Observing, measuring and recording
1 Fill a gas jar with water. Leave it for several hours to let
the water become very still.
2 Carefully place a small crystal of potassium
permanganate into the water.
3 Make a labelled drawing of the gas jar to show how the
colour is distributed at the start of your experiment.
4 Leave the gas jar completely undisturbed for
several days.
5 Make a second drawing to show how the colour
is distributed.
You can try this with other coloured salts as well,
such as copper sulfate or potassium dichromate.
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and iodine molecules through, but not starch molecules,
which are bigger than the holes.
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ACtivity B2.01
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dilute sugar solution
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water molecules
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You can think of a concentration gradient as an imaginary
‘slope’ from high concentration to low concentration. The
net movement of particles is down the slope.
13
It is easiest to think about this if we consider a simple
situation involving just one solute.
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Figure B2.03 illustrates a concentrated sugar
solution, separated from a dilute sugar solution by a
membrane. The membrane has holes or pores in it which
are very small. An example of a membrane like this is
Visking tubing.
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You can see that osmosis is really just a kind of difusion.
It is the difusion of water molecules, in a situation where
the water molecules but not the solute molecules can pass
through a membrane.
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On the right-hand side, the concentration of water
molecules is lower because a lot of space is taken up by
sugar molecules.
Because there are more water molecules on the let-hand
side, at any one moment more of them will ‘hit’ a hole in
the membrane and move through to the other side than
will go the other way (right to let). Over time, there will be
an overall, or net, movement of water from let to right.
This is called osmosis.
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Figure B2.03 Osmosis.
There is a higher concentration of sugar molecules on the
right-hand side of the membrane in Figure B2.03, and a
lower concentration on the let-hand side. If the membrane
was not there, the sugar molecules would difuse from the
concentrated solution into the dilute one until they were
evenly spread out. However, they cannot do this because
the pores in the membrane are too small for them to
get through.
There is also a concentration gradient for the water
molecules. On the let-hand side of the membrane,
there is a high concentration of water molecules.
concentration gradient
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partially permeable
membrane
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diffusion of
water molecules
Water molecules are also very small. Each one is made
of two hydrogen atoms and one oxygen atom. Sugar
molecules are many times larger than this. In Visking
tubing, the holes are big enough to let the water molecules
through, but not the sugar molecules. Visking tubing is
called a partially permeable membrane because it will let
some molecules through but not others.
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sugar
molecule
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Every cell in an organism’s body has water inside it and
outside it. Various substances are dissolved in this water,
and their concentrations may be diferent inside and
outside the cell. This creates concentration gradients,
down which water and solutes will difuse, if they are able
to pass through the membrane.
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Water is one of the most important compounds in living
organisms. It can make up around 80% of some organisms’
bodies. It has many functions, including acting as a solvent
for many diferent substances. For example, substances
are transported around the body dissolved in the water in
blood plasma.
TIP
concentrated sugar solution
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B2.02 Osmosis
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B2: Movement in and out of cells
Copyright Material - Review Only - Not for Redistribution
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Osmosis and animal cells
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cell membrane
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QuEStiONS
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C
In Figure B2.03, there is a high water potential on the lethand side and a low water potential on the right-hand side.
There is a water potential gradient between the two sides.
The water molecules difuse down this gradient, from a
high water potential to a low water potential.
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Figure B2.04 illustrates an animal cell in pure water.
The cytoplasm inside the cell is a fairly concentrated
solution. The proteins and many other substances
dissolved in it are too large to get through the cell
membrane. Water molecules, though, can get through.
Pr
es
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It is actually rather confusing to talk about the
‘concentration’ of water molecules, because the
term ‘concentration’ is normally used to mean
the concentration of the solute dissolved in the water.
It is much better to use a diferent term instead.
We say that a dilute solution (where there is a lot of water)
has a high water potential. A concentrated solution
(where there is less water) has a low water potential.
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Water potential
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Cambridge IGCSE Combined and Co-ordinated Sciences
What is meant by a partially permeable membrane?
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Figure B2.04 Animal cells burst in pure water.
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Osmosis takes place. Water diffuses into the cell through the
partially permeable cell membrane.
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Figure B2.05 illustrates an animal cell in a concentrated
solution. If this solution is more concentrated than the
cytoplasm, then water molecules will difuse out of the
cell. Look at Figure B2.03 to see why.
ev
As the water molecules go out through the cell membrane,
the cytoplasm shrinks. The cell shrivels up.
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So, cell membranes oten separate two diferent solutions –
the cytoplasm and the solution around the cell. If the solutions
are of diferent concentrations, then osmosis will occur.
Water molecules will difuse (by osmosis) from the dilute
solution into the concentrated solution. What happens
to the cell? As more and more water enters the cell, it swells.
The cell membrane has to stretch as the cell gets bigger,
until eventually the strain is too much, and the cell bursts.
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There is always cytoplasm on one side of any cell
membrane. Cytoplasm is a solution of proteins and other
substances in water. There is usually a solution on the
other side of the membrane, too. Inside large animals,
cells are surrounded by tissue fluid. In the soil, the roots of
plants are oten surrounded by a film of water.
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Cell membranes behave very much like Visking tubing.
They let some substances pass through them, but not
others. They are partially permeable membranes.
ev
If you compare this situation with Figure B2.03, you will
see that they are similar. The dilute solution in Figure B2.03
and the pure water in Figure B2.04 are each separated
from a concentrated solution by a partially permeable
membrane. In Figure B2.04, the concentrated solution is
the cytoplasm and the partially permeable membrane is
the cell membrane. Therefore, osmosis will occur.
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osmosis: the net movement of water molecules from a region
of higher water potential (dilute solution) to a region of lower
water potential (concentrated solution), through a partially
permeable membrane
Cell membranes
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more concentrated
solution inside the cell
How would you describe a solution that has a
high concentration of water molecules?
KEy tERM
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Give two examples of partially permeable
membranes.
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B2.07
14
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B2.05
B2.06
pure
water
outside
the cell
br
Which is larger – a water molecule or a
sugar molecule?
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B2.04
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B2: Movement in and out of cells
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Although it is not easy to see, a plant cell also has a
cell surface membrane just like an animal cell. The cell
membrane is partially permeable. A plant cell in pure water
will take in water by osmosis through its partially permeable
cell membrane in the same way as an animal cell. As the
water goes in, the cytoplasm and vacuole will swell.
Pr
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cell membrane
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A plant cell in this state is rather like a blown-up tyre – tight
and firm. It is said to be turgid. The outward pressure of
the cytoplasm on the cell wall is called turgor pressure.
The turgidity of its cells helps a plant that has no wood in
it to stay upright and keeps the leaves firm. Plant cells are
usually turgid.
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more dilute solution
inside the cell
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Osmosis takes place. Water diffuses out of the cell
through the partially permeable cell membrane.
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op
If the solution is very concentrated, then a lot of water will
difuse out of the cell. The cytoplasm and vacuole go on
shrinking. The cell wall, though, is too stif to be able to
shrink much. As the cytoplasm shrinks further and further
into the centre of the cell, the cell wall gets let behind.
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cell wall
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Osmosis and plant cells
Plant cells do not burst in pure water. Figure B2.06
illustrates a plant cell in pure water. Plant cells are
surrounded by a cell wall. This is fully permeable,
which means that it will let any molecules go through it.
Image B2.01 and Figure B2.07 illustrate plant cells in a
concentrated solution. Like the animal cell in Figure B2.05,
the plant cells will lose water by osmosis. The cytoplasm
shrinks and stops pushing outwards on the cell wall. Like a
tyre when some of the air has leaked out, the cell becomes
floppy. It is said to be flaccid. If the cells in a plant become
flaccid, the plant loses its firmness and begins to wilt.
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Figure B2.05 Animal cells shrink in a concentrated solution.
cell membrane
However, the plant cell has a very strong cell wall around it.
The cell wall is much stronger than the cell membrane and
it stops the plant cell from bursting. The cytoplasm presses
out against the cell wall, but the wall resists and presses
back on the contents.
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concentrated
solution outside
the cell
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more
concentrated
solution inside
the cell
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Osmosis takes place. Water diffuses into the cytoplasm
and vacuole through the partially permeable cell surface
membrane. The cell swells and becomes firm.
ev
Image B2.01 These onion cells have been placed in a
concentrated solution. The cytoplasm has shrunk inwards,
leaving big gaps between itself and the cell walls (× 300).
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Figure B2.06 Plant cells become swollen and firm in pure water.
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pure water
outside the cell
Copyright Material - Review Only - Not for Redistribution
15
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surrounded by very concentrated solutions. However,
you can make cells become plasmolysed if you do
Activity B2.04. Plasmolysis usually kills a plant cell because
the cell membrane is damaged as it tears away from the
cell wall.
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cell wall
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QuEStiONS
What happens to an animal cell in pure water?
Explain why this does not happen to a plant cell in
pure water.
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b partially permeable?
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a fully permeable?
B2.11
What is meant by a turgid cell?
B2.12
What is plasmolysis?
B2.13
How can plasmolysis be brought about?
B2.14
In Figure B2.07, what fills space X?
Explain your answer.
B2.15
Describe the events shown in Figures B2.04 and
B2.05 in terms of water potential.
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Which part of a plant cell is:
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ruler
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1 Collect a piece of Visking tubing. Moisten it and rub it
between your fingers to open it. Tie one end tightly.
water
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3 Place a long, narrow glass tube into the tubing,
as shown in the diagram. Tie it very, very tightly,
using thread.
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4 Place the tubing inside a beaker of water, as shown
in the diagram.
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2 Use a dropper pipette to put some concentrated sugar
solution into the tubing.
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narrow glass tube
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Measuring rate of osmosis
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
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A cell like this is said to be plasmolysed. This does not
normally happen because plant cells are not usually
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The cell membrane, surrounding the cytoplasm, tears
away from the cell wall.
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B2.08
B2.09
B2.10
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Figure B2.07 Plant cells become flaccid and may
plasmolyse in a concentrated solution.
ACtivity B2.05
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investigate and describe the efects on plant
tissues of immersing them in diferent solutions
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less concentrated
solution inside
the cell
Osmosis takes place. Water diffuses out of the cytoplasm
and vacuole through the partially permeable cell membrane.
First, the cell shrinks slightly and becomes flaccid. The cell
membrane pulls away from the cell wall, and the
cell is plasmolysed.
16
ACtivity B2.04
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concentrated
solution outside
the cell
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cell membrane
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space X
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
tube held tightly
by thread
Visking tubing
concentrated
sugar solution
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B2: Movement in and out of cells
5 Mark the level of liquid inside the glass tube.
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A3 Use your graph to work out the mean (average) rate at
which the liquid moved up the tube, in mm per second.
(Ask your teacher for help if you are not sure how to do this.)
4
6
8
10
12
14
16
7 Collect a sheet of graph paper. Draw a line graph of
your results. Put time in minutes on the x-axis, and
height in mm on the y-axis.
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■
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Questions
A5 When temperature rises, particles move more quickly.
Describe how you could use this apparatus to carry out
an experiment to investigate the efect of temperature on
the rate of osmosis. Think about the following things.
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A2 Explain why this happened.
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ACtivity B2.06
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Summary
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Water moves from a dilute solution in the soil into the cells in a plant’s roots.
Saliva flows out of the salivary glands into your mouth.
A spot of blue ink dropped into a glass of still water quickly colours all the water blue.
Carbon dioxide goes into a plant’s leaves when it is photosynthesising.
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If Visking tubing containing a sugar solution is put into a beaker of water, the sugar solution moves out
of the tubing by osmosis.
(continued)
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Each of these statements was made by a candidate in an examination. Each one contains at least one error.
Decide what is wrong with each statement, and rewrite it correctly.
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Which of a–d below is an example of i difusion, ii osmosis, or iii neither? Explain your answer in each case.
a
b
c
d
2
Pr
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End-of-chapter questions
1
the importance of water as a solvent
about osmosis, which is a special kind of difusion,
involving water molecules
how osmosis afects animal cells and
plant cells.
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■
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how difusion results from the random movement
of particles
the factors that afect the rate of difusion
why difusion is important to cells and
living organisms
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You should know:
■
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Osmosis and potato strips
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A1 Describe what happened to the liquid level inside the
glass tube.
What will you vary in your experiment?
What will you keep the same?
What will you measure, when will you measure it and
how will you measure it?
How will you record and display your results?
Predict the results that you would expect.
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Every 2 minutes, record the level of the liquid in the
glass tube.
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Pr
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Height of
liquid / mm
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A4 Predict what would have happened to the rate of
osmosis in this experiment if you had used a kind of
Visking tubing with ridges and grooves in it, giving it a
larger surface area. Explain your answer.
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2
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0
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Time /
minutes
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6 Make a copy of this results chart.
Copyright Material - Review Only - Not for Redistribution
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b
c
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Plant cells do not burst in pure water because the cell wall stops water getting into the cell.
When a plant cell is placed in a concentrated sugar solution, water moves out of the cell by osmosis,
through the partially permeable cell wall.
Animal cells plasmolyse in a concentrated sugar solution.
Pr
es
s
Difusion happens faster when the temperature rises.
Oxygen difuses out of a plant leaf during daylight hours.
Water molecules can pass through Visking tubing, but starch molecules cannot.
An animal cell bursts if placed in pure water.
If a plant is short of water, its leaves lose their firmness and the plant wilts.
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b
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The graph below shows the results for two samples of ammonium hydroxide that were investigated.
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60
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70
rs
18
cork
pieces of damp red litmus
paper at 2 cm intervals
cotton wool soaked in
ammonium hydroxide
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cork
[2]
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Define difusion.
The diagram below shows an apparatus that was set up to investigate difusion.
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4
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a
b
c
d
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Explain each of the following.
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d
3
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Cambridge IGCSE Combined and Co-ordinated Sciences
40
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sample B
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30
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sample A
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Time for
litmus to go
blue / s
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50
4
6
8
10
12
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2
s
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Distance along tube / cm
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0
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16
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B2: Movement in and out of cells
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2
6
4
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[Cambridge IGCSE Biology 0610 Paper 2 Q8 November 2004]
es
A student investigates the movement of acid into diferent sized blocks of agar. The agar contains the indicator
phenolphthalein. This indicator is purple in a pH of greater than 8 and colourless in a pH of less than 8.
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Two agar blocks of dimensions 2 cm × 2 cm × 2 cm and 1 cm × 1 cm × 1 cm are labelled A and B,
respectively, and placed in separate beakers as shown in the diagram.
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beaker B
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beaker A
At the beginning of the experiment the colour of the indicator in the agar was purple.
Explain what this colour indicates.
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[1]
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br
a
agar cube
1 cm × 1 cm × 1 cm
id
R
agar cube
2 cm × 2 cm × 2 cm
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5
[3]
[1]
[1]
[2]
Plot the data in the table on a copy of the graph.
Suggest what has caused the litmus paper to go blue.
State which sample of ammonium hydroxide took longest to travel 10 cm along the tube.
What can you suggest about the concentration of sample C? Explain your answer.
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i
ii
iii
iv
25
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8
10
C
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6
12
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Time for red litmus paper
to go blue / s
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s
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Distance of red litmus paper
along tube / cm
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br
id
The table below gives data for a third sample, C, of ammonium hydroxide that was investigated.
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C
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beaker B
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beaker A
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The student adds 10 cm3 hydrochloric acid to each of the beakers to cover the blocks and
then starts the stopclock. She records the time taken for the blocks to become colourless.
Copyright Material - Review Only - Not for Redistribution
(continued)
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Use a ruler to construct a table for the results.
Your table should show the dimensions of each block and the time taken, in seconds,
for each block to go colourless.
Read the stopclocks shown in the diagram. Record the time taken, in seconds,
to complete your table.
i Name the process by which the acid moves into the agar from the solution.
ii Explain the colour change of the agar.
Explain the diference between the times taken for the colour changes in blocks A and B.
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[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 62 Q1 a, b, c & d(i) June 2014]
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Water
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In most organisms, almost 80% of the body is made up of
water. We have seen that cytoplasm is a solution of many
diferent substances in water. The spaces between our
cells are also filled with a watery liquid.
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In this chapter, we will look at each of these kinds of
substances in turn. As you work through your biology
course, you will keep meeting them over and over again.
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The bodies of all living things are made of many diferent
kinds of chemicals. Most of our bodies are made up of
water. We also contain carbohydrates, proteins and fats.
These substances are what our cells are made of. Each of
them is vital for life.
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It will help if you have a basic understanding of the
meanings of the terms atom, element and molecule.
If you are not sure about these, ask your biology or
chemistry teacher to explain them to you.
B3.01 What are you made of?
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why water is important to living organisms
what carbohydrates, fats (lipids) and proteins are made of
how to test for the presence of carbohydrates, lipids and proteins
enzymes and how they act as catalysts
how enzymes are afected by temperature and pH
how to carry out experiments to investigate the efects of changes in temperature and pH on enzyme activity
why enzymes are afected by temperature and pH
planing, carrying out and evaluating your own experiments on enzyme activity.
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This chapter covers:
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B3
Biological molecules
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its molecular formula can be written C6H12O6. This formula
stands for one molecule of this simple sugar, and tells you
which atoms it contains, and how many of each kind.
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Water is also needed for other reasons. For example,
plasma, the liquid part of blood, contains a lot of water,
so that substances like glucose can dissolve in it. These
dissolved substances are transported around the body.
Water is also need to dissolve enzymes and nutrients in
the alimentary canal, so that digestion can take place.
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Although they contain many atoms, simple sugar
molecules are very small (Figure B3.02). They are soluble in
water, and they taste sweet.
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Inside every living organism, chemical reactions are going
on all the time. These reactions are called metabolism.
Metabolic reactions can only take place if the chemicals
which are reacting are dissolved in water. Water is an
important solvent. This is one reason why water is so
important to living organisms. If their cells dry out, the
reactions stop and the organism dies.
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Figure B3.02 Simple sugars, or monosaccharides, have
small molecules and are soluble in water.
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B3.02 Carbohydrates
Carbohydrates include starches and sugars. Their molecules
contain three kinds of atom – carbon (C), hydrogen (H) and
oxygen (O). A carbohydrate molecule has about twice as
many hydrogen atoms as carbon or oxygen atoms.
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The simplest kinds of carbohydrates are the simple sugars
or monosaccharides. Glucose is a simple sugar. A glucose
molecule is made of six carbon atoms joined in a ring,
with the hydrogen and oxygen atoms pointing out from
and into the ring (Figure B3.01). (You don’t need to know
this structure in detail, but you may be interested to see
how the atoms are organised.)
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Sugars
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If two simple sugar molecules join together, a larger
molecule called a complex sugar or disaccharide is
made (Figure B3.03). Two examples of complex sugars are
sucrose (the sugar we use in hot drinks, or on breakfast
cereal, for example) and maltose (malt sugar). Like simple
sugars, they are soluble in water and taste sweet.
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When asked why water is important to organisms,
many students answer ‘so that they do not dry out’.
This is not a good answer – make sure you explain why
the water is needed.
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Figure B3.01 The structure of a glucose molecule.
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If many simple sugars join together, a very large molecule
called a polysaccharide is made. Some polysaccharide
molecules contain thousands of sugar molecules joined
together in a long chain. The cellulose of plant cell walls
is a polysaccharide and so is starch, which is oten found
inside plant cells (Figure B3.04). Animal cells oten contain
a polysaccharide called glycogen. Most polysaccharides
are insoluble, and they do not taste sweet.
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Polysaccharides
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CH2OH
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A glucose molecule contains six carbon atoms, twelve
hydrogen atoms and six oxygen atoms. To show this,
Figure B3.03 Complex sugars (disaccharides), such as
maltose, are made from two simple sugars that have been
joined together.
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B3: Biological molecules
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Testing for carbohydrates
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We can test for the presence of sugars by adding
Benedict’s solution to a food, and heating it. If the food
contains reducing sugar (such as glucose or maltose),
then a brick-red colour will be produced. The mixture
changes gradually from blue, through green, yellow and
orange, and finally to brick red (Image B3.01). If there is no
reducing sugar, then the Benedict’s solution remains blue.
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Functions of carbohydrates
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Carbohydrates are needed for energy. One gram of
carbohydrate releases 17 kJ (kilojoules) of energy.
The energy is released by respiration (Chapter B8).
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The carbohydrate that is normally used in respiration
is glucose. This is also the form in which carbohydrate
is transported around an animal’s body. Human blood
plasma contains dissolved glucose, being transported to
all the cells. The cells then use the glucose to release the
energy that they need to carry out the processes of life.
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Figure B3.04 This is just a small part of a molecule of a
polysaccharide, such as starch.
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Plants store carbohydrates as starch. It is quick and easy
to change glucose into starch, or starch into glucose.
Some plants store large quantities of starch in their seeds
or tubers, and we use these as food.
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The polysaccharide cellulose is used to make the crisscrossing fibres from which plant cell walls are constructed.
Cellulose fibres are very strong, so the cell wall helps to
maintain the shape of the plant cell.
testing foods for sugars
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
! Wear eye protection.
If possible, heat the tubes using a water bath. If you have
to heat directly over a Bunsen flame, use a test-tube
holder and point the opening of the tube away from
people. Take care if using a sharp blade to cut the food.
All simple sugars, and some complex sugars such as
maltose, are reducing sugars. This means that they will
react with a blue liquid called Benedict’s solution. We can
use this reaction to find out if a food or other substance
contains a reducing sugar.
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Animals do not store starch. Instead, they store
carbohydrates in the form of the polysaccharide glycogen.
However, only small quantities of glycogen can be stored.
It is mostly stored in the cells in the liver and the muscles.
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ACtivity B3.01
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Plants also use glucose in respiration, to provide them with
energy. However, they do not transport glucose around
their bodies. Instead, they transport sucrose. The cells
change the sucrose to glucose when they need to use it.
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Image B3.01 Benedict’s test for carbohydrates.
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A1 How could you test a solution to see if it contained iodine?
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5 Record your result in your results chart. If the
Benedict’s solution does not change colour, do not
write ‘no change’. Write down the actual colour that
you see – for example, blue. Then write down your
conclusion from the result of the test.
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What is metabolism?
B3.02
Why do organisms die if they do not have
enough water?
B3.03
Which three elements are contained in
all carbohydrates?
B3.04
The molecular formula for glucose is C6H12O6.
What does this tell you about a glucose molecule?
B3.05
To which group of carbohydrates does each of
these substances belong: a glucose, b starch and
c glycogen?
B3.06
In what form:
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The test for starch is easier, as it does not involve heating.
You simply add iodine solution to a sample of the food.
If there is starch present, a blue-black colour is obtained
(Image B3.02). If there is no starch, the iodine solution
remains orange-brown.
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B3.01
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This test works because the reducing sugar reduces the
blue copper salts to a red compound.
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QuEStiONS
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3 Add some Benedict’s solution. Benedict’s solution is
blue, because it contains copper salts.
4 Heat the tube to about 80 °C, in a water bath.
If there is reducing sugar in the food, a brick-red
precipitate will form.
24
testing foods for starch
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
There is no need to dissolve the food for this test.
1 Draw a results chart.
2 Put a small piece of the food onto a white tile.
3 Add a drop or two of iodine solution.
Iodine solution is brown, but it turns blue-black
if there is starch in the food. Record each of your
results and conclusions.
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2 Cut or grind a little of the food into very small pieces.
Put these into a test tube. Add some water, and shake
it up to try to dissolve it.
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Simple sugar
present
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Colour with
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Food
ACtivity B3.02
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1 Draw a results chart.
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c do animals store carbohydrates in their cells?
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B3.03 Fats
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Fats are also known as lipids. Like carbohydrates, fats contain
only three kinds of atom – carbon, hydrogen and oxygen. A fat
molecule is made of four smaller molecules joined together.
One of these is glycerol. Attached to the glycerol are three
long molecules called fatty acids (Figure B3.05).
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b do animals transport carbohydrates in
their blood?
d do plants transport carbohydrates round
their bodies?
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a do most organisms use carbohydrates
in respiration?
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Fats are insoluble in water. Fats that are liquid at room
temperature are called oils.
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Image B3.02 The black colour shows that the potato
contains starch.
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fat molecule
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Figure B3.05 The structure of a fat molecule.
Image B3.03 A walrus on the Arctic island Spitzbergen.
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The extra energy that fats contain makes them
very useful for storing energy. In mammals, some cells,
particularly ones underneath the skin, become filled
with large drops of fats or oils. These stores can be
used to release energy when needed. This layer of
fat also helps to keep heat inside the body – that is,
it insulates the body. Animals such as walruses,
which live in very cold places, oten have especially
thick layers of fat, called blubber (Image B3.03).
Many plants store oils in their seeds – for example,
peanut, coconut and castor oil. The oils provide a
good store of energy for germination.
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Firstly, you chop the food and shake it up with ethanol.
Although fats will not dissolve in water, they do
dissolve in ethanol. Next, you pour the ethanol
into water. If there is any fat in the food,
then the fat–ethanol mixture breaks up into millions
of tiny droplets when it is mixed with the water.
This mixture is called an emulsion. It looks white
and opaque, like milk (Image B3.04). If there is
no fat in the food, the mixture of water and ethanol
remains transparent.
testing foods for fats
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
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1 Draw a results chart.
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2 Chop or grind a small amount of food, and put some
into a very clean, dry test tube. Add some absolute
(pure) ethanol. Shake it thoroughly.
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3 Put some distilled water in another tube.
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4 Pour some of the liquid part, but not any solid, from
the first tube into the water. A milky appearance shows
that there is fat in the food.
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There are several diferent tests for fats. One of the
best is the ethanol emulsion test.
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Image B3.04 A positive result for the emulsion test.
ACtivity B3.03
Testing for fats and oils
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emulsion of fat
droplets in the
ethanol/water
mixture
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Like carbohydrates, fats and oils can be used in a cell
to release energy. A gram of fat gives about 39 kJ of
energy. This is more than twice as much energy as that
released by a gram of carbohydrate. However, most
cells use carbohydrates first when they need energy and
only use fats when all the available carbohydrates have
been used.
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Functions of fats
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fatty acid molecule
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glycerol molecule
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B3: Biological molecules
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State two uses of fats to living organisms.
B3.09
We get cooking oil mostly from the seeds of
plants. Why do plant seeds contain oil?
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For most protein molecules, their shape directly afects
their function. For example, as you will see in Section B3.05,
the shape of an enzyme molecule determines which
reactions it can catalyse.
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Protein molecules contain some kinds of atoms which
carbohydrates and fats do not (Figure B3.06). As well as
carbon, hydrogen and oxygen, they also contain nitrogen
(N) and small amounts of sulfur (S).
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B3.08
The long chains of amino acids from which protein
molecules are formed can curl up into diferent shapes.
The way in which the chain curls up, and therefore the
three-dimensional shape of the protein molecule, is
determined by the sequence of amino acids in the chain.
Diferent sequences of amino acids result in diferent
shapes of protein molecules.
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Which three elements are found in all fats
and oils?
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B3.07
B3.04 Proteins
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protein. Enzymes are also proteins. You will find out a lot
more about enzymes in Section B3.05.
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B3.13
Give two examples of proteins.
B3.14
State three functions of proteins in living
organisms.
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The test for proteins is called the biuret test (Image B3.05).
This involves mixing the food in water, and then adding
dilute copper sulfate solution. Then dilute potassium
hydroxide solution is gently added. A purple colour
indicates that protein is present. If there is no protein, the
mixture stays blue.
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Image B3.05 The tube on the let shows a negative result for
the biuret test. The tube on the right shows a positive result.
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Unlike carbohydrates, proteins are not normally used to
provide energy. Many of the proteins in the food you eat
are used for making new cells. New cells are needed for
growing, and for repairing damaged parts of the body. In
particular, cell membranes and cytoplasm contain a lot of
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Some proteins are soluble in water; an example is
haemoglobin, the red pigment in blood. Others are
insoluble in water; for example, keratin. Hair and
fingernails are made of keratin.
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In what way are protein molecules similar to
polysaccharides?
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B3.12
Testing for proteins
Figure B3.06 Structure of a protein molecule.
Like polysaccharides, protein molecules are made of
long chains of smaller molecules joined end to end. These
smaller molecules are called amino acids. There are about
20 diferent kinds of amino acid. Any of these 20 can be
joined together in any order to make a protein molecule.
Each protein is made of molecules with amino acids in a
precise order. Even a small diference in the order of amino
acids makes a diferent protein, so there are millions of
diferent proteins which could be made.
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How many diferent amino acids are there?
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a small protein molecule
Functions of proteins
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Name two elements found in proteins that are not
found in carbohydrates.
B3.11
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one kind of amino
acid molecule
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QuEStiONS
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C, H, O
C, H, O
Smaller molecules of
which they are made
simple sugars
(monosaccharides)
fatty acids and glycerol
Solubility in water
sugars are soluble;
polysaccharides are insoluble
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Table B3.01 A comparison of carbohydrates, fats and proteins.
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KEy tERMS
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catalyst: a substance that increases the rate of a chemical
reaction and is not changed by the reaction
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enzymes: proteins that function as biological catalysts
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Not all enzymes help to break things down. Many enzymes
help to make large molecules from small ones. One
example of this kind of enzyme is starch phosphorylase,
which builds starch molecules from glucose molecules
inside plant cells.
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Within any living organism, chemical reactions take
place all the time. They are sometimes called metabolic
reactions. Almost every metabolic reaction is controlled by
catalysts called enzymes. Enzymes are proteins. Without
enzymes, the reactions would take place very slowly, or
not at all. Enzymes ensure that the rates of metabolic
reactions are great enough to sustain life.
Another enzyme which speeds up the breakdown of a
substance is catalase. Catalase works inside the cells
of most living organisms – including both animals and
plants – for example, in liver cells or potato cells. It breaks
down hydrogen peroxide to water and oxygen. This is
necessary because hydrogen peroxide is produced by
many of the chemical reactions that take place inside cells.
Hydrogen peroxide is a very dangerous substance, and it
must be broken down immediately.
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Many chemical reactions can be speeded up by
substances called catalysts. A catalyst alters the rate of a
chemical reaction, without being changed itself.
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B3.05 Enzymes
For example, inside the alimentary canal, large molecules
are broken down to smaller ones in the process of digestion.
These reactions are speeded up by enzymes. A diferent
enzyme is needed for each kind of food. For example,
starch is digested to the sugar maltose by an enzyme called
amylase. Protein is digested to amino acids by protease.
These enzymes are also found in plants – for example, in
germinating seeds, where they digest the food stores for
the growing seedling. Many seeds contain stores of starch.
As the seed soaks up water, the amylase is activated
and breaks down the starch to maltose. The maltose is
soluble, and it is transported to the embryo in the seed.
The embryo uses it to provide energy for growth, and also
to provide glucose molecules that can be strung together
to make cellulose molecules, for the cell walls of the new
cells produced as it grows.
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testing foods for protein
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
! Wear eye protection if available. Potassium hydroxide
is a strong alkali. If you get it on your skin, wash with
plenty of cold water. Take care if using a sharp blade
to cut the food.
The biuret test uses potassium hydroxide solution and
copper sulfate solution. You can also use a ready-mixed
reagent called biuret reagent, which contains these two
substances already mixed together.
1 Draw a results chart.
2 Put the food into a test tube, and add a little water.
3 Add some potassium hydroxide solution.
4 Add two drops of copper sulfate solution.
5 Shake the tube gently. If a purple colour appears, then
protein is present.
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ACtivity B3.04
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making cells, enzymes,
haemoglobin; also used
for energy
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amino acids
some are soluble and some
are insoluble
storage of energy (39 kJ/g);
insulation; making
cell membranes
Table B3.01 compares some properties of carbohydrates,
fats and proteins.
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C, H, O, N
insoluble
easily available energy (17 kJ/g)
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Why organisms
need them
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Proteins
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Carbohydrates
Elements they contain
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B3: Biological molecules
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This is oten known as the lock and key mechanism. You
can think of the enzyme as a lock, and the substrate as a
key that has to perfectly fit into the lock before anything
can happen.
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Sometimes, they are given more specific names than
this. For example, we have seen that the carbohydrase
that breaks down starch is called amylase. The
carbohydrase that breaks down maltose is called
maltase. The carbohydrase that breaks down sucrose
is called sucrase.
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How an enzyme works
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All enzymes have active sites. Each enzyme has an active
site that exactly fits its substrate. This means that each
enzyme can only act on a particular kind of substrate.
Amylase, for example, can break down starch molecules
but cannot break down protein molecules, because they
do not fit into its active site.
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Enzymes are named according to the reaction that they
catalyse. For example, enzymes which catalyse the
breakdown of carbohydrates are called carbohydrases.
If they break down proteins, they are proteases. If they
break down fats (lipids), they are lipases.
R
that is complementary to the shape of its substrate.
The substrate fits into the active site of the enzyme,
forming an enzyme–substrate complex. When the
substrate molecule is in the active site, the enzyme
makes it react – for example, by breaking apart.
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Cambridge IGCSE Combined and Co-ordinated Sciences
starch
maltose
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B3.15
What is a catalyst?
B3.16
What are the catalysts inside a living organism
called?
B3.17
Which kinds of reaction inside a living organism
are controlled by enzymes?
B3.18
Name the substrate and product of a reaction
involving a carbohydrase.
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Figure B3.07 shows how an enzyme causes the
substrate to react. Every enzyme molecule has a
dent in it called its active site. This has a shape
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ie
Pr
amylase
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op
28
QuEStiONS
es
-C
am
br
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id
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w
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A chemical reaction always involves one substance
changing into another. In an enzyme-controlled reaction,
the substance which is present at the beginning of the
reaction is called the substrate. The substance which is
made by the reaction is called the product. For example,
the substrate for the enzyme amylase is starch, and the
product is maltose.
C
An enzyme molecule is like a lock.
ie
The enzyme changes
the substrate into new
molecules called products.
s
-C
-R
am
br
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id
enzyme
w
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The substrate molecule has a
complementary shape to the
enzyme, and can fit into it like a key.
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op
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Pr
op
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es
The substrate must
be a perfect fit.
-R
s
es
-C
am
br
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id
g
Figure B3.07 How an enzyme works. This is oten known as the lock and key mechanism.
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
5 Enzymes are catalysts They are not changed in the
chemical reactions which they control. They can be
used over and over again, so a small amount of enzyme
can change a lot of substrate into product.
w
ge
Properties of enzymes
C
U
ni
op
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B3: Biological molecules
-R
Pr
es
s
-C
2 Enzymes are made inactive by high temperature
This is because they are protein molecules, which are
damaged by heat.
ev
ie
am
br
id
1 All enzymes are proteins This may seem rather odd,
because some enzymes actually digest proteins.
the efect of catalase on hydrogen peroxide
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
Wear eye protection if available. Hydrogen peroxide is
!
a powerful bleach. Wash it of with plenty of water if
you get it on your skin.
Catalase is found in almost every kind of living cell.
It catalyses this reaction:
catalase
water + oxygen
hydrogen peroxide
C
op
es
s
-C
-R
am
br
ev
id
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w
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U
R
ACtivity B3.05
y
4 Enzymes work best at a particular pH pH is a
measure of how acid or alkaline a solution is. Some
enzymes work best in acid conditions (low pH). Others
work best in neutral or alkaline conditions (high pH)
(Figure B3.09).
ni
ev
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w
C
ve
rs
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op
y
3 Enzymes work best at a particular temperature
Enzymes which are found in the human body usually
work best at about 37 °C (Figure B3.08).
6 Enzymes are specific This means that each
kind of enzyme will only catalyse one kind of
chemical reaction.
1 Read through the instructions. Decide what you will
observe and measure, and draw a results table.
2 Measure 10 cm3 of hydrogen peroxide into each of five
test tubes or boiling tubes.
op
y
3 To each tube, add one of the following substances:
10
20
30
a some chopped raw potato
40
50
60
C
0
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Pr
y
Rate of
reaction
b some chopped boiled potato
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Temperature / °C
w
c some fruit juice
id
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Figure B3.08 How temperature afects enzyme activity.
br
ev
d a small piece of liver
4 Light a wooden splint, and then blow it out so that it is
glowing. Gently push the glowing splint down through
the bubbles in your tubes.
5 Record your observations, and explain them as fully as
you can.
ity
Pr
es
s
most
enzymes
ni
ve
rs
Most chemical reactions happen faster at higher
temperatures. This is because the molecules have more
kinetic energy – they are moving around faster, so they
bump into each other more frequently. This means that at
higher temperatures an enzyme is likely to bump into its
substrate more oten than at lower temperatures. They will
also hit each other with more energy, so the reaction is
more likely to take place.
y
ev
w
10
s
-C
Figure B3.09 How pH afects enzyme activity.
ie
8
ev
6
pH
-R
4
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am
br
2
id
g
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C
U
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Temperature and enzyme activity
op
C
Rate of
reaction
ie
w
-R
pepsin (a protease
in the stomach)
op
y
-C
am
e some yeast suspension.
Copyright Material - Review Only - Not for Redistribution
29
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Cambridge IGCSE Combined and Co-ordinated Sciences
However, enzymes are damaged by high temperatures.
For most human enzymes, this begins to happen from
about 40 °C upwards. As the temperature increases
beyond this, the enzyme molecules start to lose their
shape. The active site no longer fits perfectly with the
substrate. The enzyme is said to be denatured. It can
no longer catalyse the reaction.
-R
TIP
U
ni
y
Do not say that enzymes are ‘killed’ by high temperatures.
Enzymes are chemicals, not living organisms.
id
pH and enzyme activity
br
-R
In this investigation, you will test this hypothesis:
Catalase works best at a pH of 7 (neutral).
rs
investigating the efect of pH on the
activity of catalase
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
! Wear eye protection if available. Hydrogen peroxide is a
powerful bleach. Wash it of with plenty of water if you
get it on your skin.
Catalase is a common enzyme which is the catalyst in the
breakdown of hydrogen peroxide, H2O2. Catalase is found in
almost every kind of living cell. Hydrogen peroxide is a
toxic substance formed in cells. The breakdown reaction is
as follows:
2H2O + O2
2H2O2
The rate of the reaction can be determined from the rate of
oxygen production.
One indirect but simple way to measure rate of oxygen
production is to soak up a catalase solution onto a little
square of filter paper and then drop it into a beaker
containing a solution of H2O2. The paper sinks at first, but
as the reaction proceeds, bubbles of oxygen collect on its
surface and it floats up.
The time between placing the paper in the beaker and it
floating to the surface is a measure of the rate of the reaction.
y
ve
1 Label five 50 cm3 beakers pH 5.6, 6.2, 6.8, 7.4, 8.0.
op
2 Measure 5 cm3 of 3% hydrogen peroxide solution into
each beaker.
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ev
C
U
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id
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w
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3 Add 10 cm3 of the correct bufer solution to each beaker.
(A bufer solution keeps the pH constant at a particular value.)
ev
4 Cut out 20 squares of filter paper exactly 5 mm × 5 mm.
Alternatively, use a hole punch to cut out circles of filter
paper all exactly the same size. Avoid handling the
paper with your fingers, as you may get grease onto it.
Use forceps (tweezers) instead.
es
s
-R
br
am
-C
Pr
op
y
ity
7 Prepare a results table like the one below.
op
y
8 Pick up a filter paper square with forceps and dip it into
the leaf extract.
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id
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w
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C
U
9 Make sure you are ready to start timing. Then place
the filter paper square at the bottom of the beaker
containing H2O2 and pH 5.6 bufer solution. (Do not let
it fall near the side of the beaker.) As you put the square
into the beaker, start a stopwatch. Stop the watch when
the paper floats horizontally at the surface.
es
s
-R
br
am
-C
5 Prepare a leaf extract by grinding the leaves in a pestle
and mortar. Add 25 cm3 of water and stir well.
6 Allow the remains of the leaves to settle and then pour
the fluid into a beaker. This fluid contains catalase.
ni
ve
rs
C
w
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ev
R
Why are enzymes damaged by high temperatures?
s
ity
C
w
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B3.21
What is the optimum temperature for the enzyme
in Figure B3.08?
Pr
y
op
ACtivity B3.06
30
B3.20
es
-C
am
The pH of a solution afects the shape of an enzyme. Most
enzymes are their correct shape at a pH of about 7 – that
is, neutral. If the pH becomes very acidic or very alkaline,
then they are denatured. This means that the active site
What is meant by an optimum temperature?
ev
B3.19
ie
w
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QuEStiONS
C
op
C
ve
rs
ity
op
y
The temperature at which an enzyme works fastest is
called its optimum temperature. Diferent enzymes have
diferent optimum temperatures. For example, enzymes
from the human digestive system generally have an
optimum of around 37 °C. Enzymes from plants oten have
optimums around 28 °C to 30 °C. Enzymes from bacteria
that live in hot springs may have optimums as high
as 75 °C.
w
ev
ie
R
Some enzymes have an optimum pH that is not neutral.
For example, there is a protease enzyme in the human
stomach that has an optimum pH of about 2. This is
because we have hydrochloric acid in our stomachs.
This protease must be able to work well in these very
acidic conditions.
Pr
es
s
-C
am
br
id
ev
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no longer fits the substrate, so the enzyme can no longer
catalyse its reaction.
Copyright Material - Review Only - Not for Redistribution
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B3: Biological molecules
10 Record the time in your table and repeat steps 8 and 9
twice more.
w
Questions
12 Pour some of the remaining leaf extract into a test tube
and boil for 2 minutes. Cool under a tap.
A2 Do your results support the hypothesis you were testing,
or do they disprove it? Explain your answer.
Pr
es
s
A3 What is the efect of boiling the extract?
A4 Why do the filter paper squares have to be exactly the
same size?
15 Draw a graph to show time taken for flotation plotted
against pH and compare it with Figure B3.09.
A5 In most experiments in biology, we can never be quite
sure that we would get exactly the same results if we
did it again. There are always some limitations on the
reliability of the data that we collect. Can you think of any
reasons why there may be uncertainty in your results?
For example:
■ Might there have been any variables that
were not controlled and that might have afected
the results?
■ Were you able to measure the volumes and times as
accurately as you would have liked?
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rs
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op
y
14 Calculate the mean (average) time taken at each pH and
enter it into your table.
w
C
-R
-C
13 Repeat steps 8–10, using the boiled extract.
ev
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A1 Does the enzyme have an optimum pH? If it does,
what do your results suggest it to be?
am
br
id
11 Follow steps 8–10 for each of the other pHs.
7.4
8.0
U
Tests 1
br
ev
id
3
w
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2
Mean
6.8
y
6.2
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op
5.6
ie
R
pH
ni
ev
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Time taken for paper to
float in seconds
Pr
y
es
s
-C
-R
am
Boiled extract
ACtivity B3.08
ity
C
op
ACtivity B3.07
investigate the efect of temperature on the
activity of catalase
y
op
br
You should know:
es
s
-R
why enzymes are specific for their particular
substrates
how temperature and pH afect enzyme activity
why temperature and pH afect enzyme activity
how to investigate the efect of temperature and pH
on enzyme activity
how to plan and carry out an investigation into
enzyme activity.
Pr
y
op
-R
s
-C
am
br
ev
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id
g
w
e
C
U
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■
■
es
■
■
■
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■
■
ni
ve
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■
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■
am
■
the functions of water in living organisms
the structure and uses of carbohydrates, and the
Benedict’s test and iodine test to identify them
the structure and uses of fats, and the ethanol
emulsion test
the structure and uses of proteins, and the biuret test
how enzymes work as biological catalysts
about active sites, substrates and products
-C
■
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Summary
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rs
investigate the efect of temperature on the
activity of amylase
31
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Cambridge IGCSE Combined and Co-ordinated Sciences
am
br
id
Name:
Imagine that you have been given two colourless solutions.
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br
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U
ni
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an element found in proteins but not in carbohydrates or lipids
the small molecules that are linked together to form a protein molecule
the reagent used for testing for reducing sugars
the substance which the emulsion test detects
the form in which carbohydrate is transported in a plant
the term that describes all the chemical reactions taking place in an organism.
am
R
a
b
c
d
e
f
3
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2
y
Pr
es
s
glucose
starch
cellulose
glycogen
y
a
b
c
d
-R
For each of these carbohydrates, state: i whether it is a monosaccharide, disaccharide or polysaccharide;
ii whether it is found in plants only, animals only or in both plants and animals; iii one function.
-C
1
ev
ie
End-of-chapter questions
Elements it
contains
y
es
s
-C
-R
am
enzyme
denatured
substrate
product
active site
Pr
op
y
ni
ve
rs
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op
C
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id
g
es
s
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br
c
d
Suggest the optimum temperature for the activity of this protease enzyme.
The stomach contains hydrochloric acid. Suggest the optimum pH for the activity of this
protease enzyme.
Explain why the rate of an enzyme-controlled reaction is relatively slow at low temperatures.
Explain why the rate of the reaction slows down above the enzyme’s optimum temperature.
am
a
b
ity
A protease enzyme is found in the stomachs of humans. It catalyses the breakdown of long chains
of amino acids (proteins) into individual amino acid molecules.
-C
C
w
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op
br
ev
id
Explain the meaning of each of these terms:
a
b
c
d
e
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enzyme
One function
C
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starch
6
Pr
ni
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glucose
5
How to test
for it
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haemoglobin
Carbohydrate,
fat or protein
ity
Substance
rs
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32
es
y
Copy and complete the table below.
op
4
s
-C
Describe how you could find out which of them contains the greater concentration of reducing sugar.
You will need to think carefully about all the diferent variables that you would need to keep constant.
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7
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B3: Biological molecules
ev
ie
am
br
id
Milk is liquid produced by cows and other mammals, on which they feed their young.
Cow’s milk
3.2
Fat / g
3.9
Carbohydrate / g
4.8
120
y
C
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[1]
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[1]
[2]
[1]
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Pr
33
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A circle of plain paper was placed into a Petri dish as shown in the diagram below. Iodine solution was
used to stain the starch in the plain paper.
-R
3
s
2
7
5
4
ten small discs of filter
paper soaked in different
samples of goat amylase
Pr
6
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y
[1]
ev
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id
g
w
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C
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Ten amylase soaked filter paper discs were placed into one of the Petri dishes as shown in
the diagram above.
Ten Petri dishes were set up as in the diagram.
The students lited the filter paper discs at one-minute intervals and recorded the number of areas
where there had been a reaction.
es
s
-R
br
•
•
1
When iodine solution reacts with the starch in the plain paper, what colour would you see?
am
•
circle of plain paper
in the bottom of the
Petri dish
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ie
a
10
ni
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y
-C
9
8
es
am
br
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C
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ni
op
lid of Petri dish
R
ev
[1]
Students investigated samples of amylase from 100 goats. 100 small filter paper discs were each soaked
in a diferent sample of goat amylase. The students tested the activity of these amylase samples using
plain paper. Plain paper contains starch.
op
C
195
[Cambridge IGCSE Combined Science 0653 Paper 21 Q5 a, b, c, d & e June 2013]
y
8
4.9
ge
br
d
e
8.0
Which substance shown in the table is present in the samples of milk in the smallest quantity?
Suggest which substance, not shown in the table, is present in the samples of milk in the
largest quantity.
Explain why both cow’s milk and water-bufalo’s milk produce a violet colour when tested
with biuret solution.
Predict the colour you would see if you added iodine solution to cow’s milk. Explain your answer.
List the components of milk, shown in the table, that provide energy.
id
w
ev
ie
c
4.5
ve
rs
ity
Calcium / mg
C
op
y
-C
Protein / g
a
b
R
Water-bufalo’s milk
Pr
es
s
Substance
-R
The table shows the mass of some of the substances in 100 g samples of milk from two mammals.
Copyright Material - Review Only - Not for Redistribution
(continued)
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b
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Cambridge IGCSE Combined and Co-ordinated Sciences
[1]
am
br
id
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How would the students know that a reaction had taken place?
C
60
12
C
op
c
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6
ni
5
i
Copy and complete the table by calculating the total number of areas where there had been
a reaction ater 4 and 5 minutes. Show your working.
ii Plot a graph using the data from the first two columns, to show the diferences in the
activity of amylase.
iii Suggest two reasons for the diferences in amylase activity of the samples.
Suggest three ways in which you could improve this investigation.
[5]
[2]
[3]
es
[Cambridge IGCSE Biology 0610 Paper 61 Q1 June 2011]
Pr
y
ity
y
op
ev
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H2O
-R
es
Describe how sucrase catalyses the breakdown of sucrose. You should refer to the diagram above
in your answer.
y
op
-R
s
es
-C
am
br
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id
g
w
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C
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C
ity
Pr
op
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a
s
-C
fructose
am
br
id
glucose
w
ge
C
U
sucrase
R
sucrose
ni
ev
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w
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C
34
[2]
Enzymes are biological catalysts. The diagram below shows how the enzyme sucrase breaks down
a molecule of sucrose.
op
9
s
-C
d
-R
am
br
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42
18
4
R
14
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rs
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3
Total number of areas where
there had been a reaction
Pr
es
s
28
op
2
14
y
1
Number of new areas where
there had been a reaction
-C
Time / minutes
-R
If a reaction had not taken place, the students replaced the disc of filter paper for another minute.
This procedure was repeated for five minutes. Their results are recorded in the table below.
Copyright Material - Review Only - Not for Redistribution
[3]
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rs
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R
U
R
ni
25
C
op
50
y
ve
rs
ity
Rate of enzyme
activity /arbitrary
units
Q
Pr
es
s
-C
75
y
op
C
P
100
ev
ie
w
-R
ev
ie
Three enzymes, P, Q and R, were extracted from diferent regions of the alimentary canal of a
mammal. The efect of pH on the activity of the enzymes was investigated at 40 °C. The results
are shown in the diagram below.
am
br
id
b
C
U
ni
op
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B3: Biological molecules
2
3
4
5
6
pH
8
9
10
11
ev
br
[3]
s
-R
am
[2]
es
[Cambridge IGCSE Biology 0610 Paper 33 Q3 a, b(i) (ii) November 2010]
Pr
35
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rs
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ni
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C
ity
Pr
op
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s
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-R
am
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C
U
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R
-C
12
Explain why the investigation was carried out at 40 °C.
Using information in the diagram above, describe the efects of increasing pH on the rate
of activity of enzyme Q.
y
-C
i
ii
7
w
1
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0
id
ge
0
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Pr
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C
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substances passing from one animal to another were
first made by plants. Animal nutrition is described in
Chapter B5.
ni
ve
rs
All living organisms need to take many diferent
substances into their bodies. Some of these may be used
to make new parts, or repair old parts. Others may be used
to release energy. Taking in useful substances is called
feeding, or nutrition.
op
y
Green plants make their own food. They use
simple inorganic substances – carbon dioxide, water
and minerals – from the air and soil. Plants build
these substances into complex materials, making all
the carbohydrates, lipids, proteins and vitamins that
they need.
C
U
w
-R
s
es
am
br
ev
ie
id
g
w
e
Animals and fungi cannot make their own food. They feed
on organic substances that have originally been made
by plants. Some animals eat other animals, but all the
-C
ie
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y
C
w
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s
es
Pr
op
y
B4.01 types of nutrition
R
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ni
U
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■
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■
id
■
br
■
how plants make carbohydrates by photosynthesis
the structure of a leaf
the role of chlorophyll in transferring light energy to chemical energy
how a leaf is adapted for photosynthesis
how to test a leaf for starch
investigating the need for chlorophyll, light and carbon dioxide for photosynthesis
investigating the efect of light intensity on the rate of photosynthesis
why plants need nitrate ions and magnesium ions.
am
R
ev
ie
w
rs
This chapter covers:
■
Pr
36
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op
y
es
s
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B4
Plant nutrition
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B4: Plant nutrition
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B4.02 Photosynthesis
ev
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w
QuEStiONS
y
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op
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id
br
-R
s
es
-C
Pr
y
op
ity
Chlorophyll is the pigment which makes plants look
green. It is kept inside the chloroplasts of plant cells.
When sunlight falls on a chlorophyll molecule, some of the
energy in the light is absorbed. The chlorophyll molecule
then releases the energy. The released energy makes
carbon dioxide combine with water, with the help of
enzymes inside the chloroplast. The glucose that is made
contains energy that was originally in the sunlight. So, in
this processs, light energy is transferred to
chemical energy.
Leaf structure
A leaf consists of a broad, flat part called the lamina
(Figure B4.01), which is joined to the rest of the plant by
a leaf stalk. Running through the leaf stalk are vascular
bundles, which then form the veins in the leaf. These
contain tubes that carry substances to and from the leaf.
y
op
midrib
C
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B4.03 Leaves
Leaves are therefore specially adapted to allow
photosynthesis to take place as quickly and eficiently
as possible.
am
However, sunlight shining onto water and carbon dioxide
still will not make them react together to make glucose.
The sunlight energy has to be trapped, and then used in
the reaction. Green plants have a substance which does
this. It is called chlorophyll.
w
ge
C
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s
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ni
ve
rs
w
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C
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op
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Although a leaf looks thin, it is in fact made up of several
layers of cells. You can see these if you look at a transverse
section (TS) of a leaf under a microscope (Figure B4.02, and
Images B4.01 and B4.02).
id
g
The top and bottom of the leaf are covered with a layer
of closely fitting cells called the epidermis (Figures B4.02
and B4.03, and Image B4.03). These cells do not contain
chloroplasts. Their function is to protect the inner layers
ev
s
C6H12O6 + 6O2
es
am
chlorophyll
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br
sunlight
ie
The balanced equation for photosynthesis is this:
6CO2 + 6H2O
cross-section of vein
Figure B4.01 The structure of a leaf.
To show the number of molecules involved in the reaction,
a balanced equation needs to be written. Carbon dioxide
contains two atoms of oxygen and one of carbon, so its
molecular formula is CO2. Water has the formula H2O.
Glucose has the formula C6H12O6. Oxygen molecules
contain two atoms of oxygen, and so they are written O2.
-C
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transverse section of leaf
glucose + oxygen
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chlorophyll
Pr
sunlight
vein
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The full equation for photosynthesis is written like this:
carbon dioxide + water
margin
lamina
br
am
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The photosynthesis equation
R
What is chlorophyll, and how does it help
the plant?
C
op
ni
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U
R
Which inorganic substances does a plant use to
make carbohydrates?
Photosynthesis happens inside chloroplasts. This is
where the enzymes and chlorophyll are that catalyse and
supply energy for the reaction. In a typical plant, most
chloroplasts are in the cells in the leaves. A leaf is a factory
for making carbohydrates.
photosynthesis: the process by which plants manufacture
carbohydrates from raw materials using energy from light
Chlorophyll
B4.02
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KEy tERM
Give one example of an organic substance.
B4.03
Pr
es
s
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If you just mix carbon dioxide and water together, they will
not make glucose. They have to be given energy before
they will combine. Green plants use the energy of sunlight
for this. The reaction is therefore called photosynthesis
(‘photo’ means light, and ‘synthesis’ means manufacture).
B4.01
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am
br
id
Green plants make the carbohydrate glucose from carbon
dioxide and water. At the same time, oxygen is produced.
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vein
phloem
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spongy
mesophyll
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chloroplast
xylem
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lower
epidermis
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guard cell
stoma
air space
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Figure B4.02 Transverse section through a small part of a leaf.
38
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of cells in the leaf. The cells of the upper epidermis oten
secrete a waxy substance that lies on top of them. It is
called the cuticle, and it helps to stop water evaporating
from the leaf. There is sometimes a cuticle on the
underside of the leaf as well.
C
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ni
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Image B4.01 A photograph taken with a scanning electron
microscope, showing the cells inside a leaf. Scanning
electron microscopes provide 3D images (× 400).
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id
g
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Image B4.02 This photograph was taken using a light
microscope. It shows a transverse section of a leaf from a
tea plant. Can you identify all the diferent layers of cells
labelled in Figure B4.02?
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nucleus
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palisade
mesophyll
mesophyll
layer
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cytoplasm
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vacuole
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cell wall
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upper
epidermis
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cuticle
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B4: Plant nutrition
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nucleus
cytoplasm
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Running through the mesophyll are veins or vascular
bundles. Each vein contains large, thick-walled xylem
vessels (Figure B6.01) for carrying water. There are also
smaller, thin-walled phloem tubes (Figure B6.03) for
carrying away sucrose and other substances that the leaf
has made.
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cell wall
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epidermal cell
The middle layers of the leaf are called the mesophyll
(‘meso’ means middle, and ‘phyll’ means leaf). These cells
all contain chloroplasts. The cells nearer to the top of the
leaf are arranged like a fence or palisade, and they form the
palisade mesophyll. The cells beneath them are rounder,
and arranged quite loosely, with large air spaces between
them. They form the spongy mesophyll (Figure B4.02).
w
ge
In the lower epidermis, there are small openings called
stomata (singular: stoma). Each stoma is surrounded by
a pair of sausage-shaped guard cells which can open
or close the hole. Guard cells, unlike other cells in the
epidermis, do contain chloroplasts.
What is the function of the cuticle?
B4.06
What are stomata?
B4.07
What are guard cells?
B4.08
List three kinds of cell in a leaf which contain
chloroplasts, and one kind which does not.
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guard cell
Leaf adaptations
rs
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Figure B4.03 Surface view of the lower epidermis of a leaf.
Leaves are adapted to obtain carbon dioxide, water
and sunlight.
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Water
Water is obtained from the soil. It is absorbed by the root
hairs, and carried up to the leaf in the xylem vessels.
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br
am
The cells which need the carbon dioxide are the mesophyll
cells, inside the leaf. The carbon dioxide can get into the
leaf through the stomata. It does this by difusion, which is
described in Chapter B2. Behind each stoma is an air space
(Figure B4.02) which connects up with other air spaces
between the spongy mesophyll cells. The carbon dioxide
can therefore difuse to all the cells in the leaf. It can then
difuse through the cell wall and cell membrane of each
cell, and into the chloroplasts.
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Image B4.03 The lower surface of a leaf, showing the
closely fitting cells of the epidermis. The oval openings are
stomata, and the two curved cells around each stoma are
guard cells (× 450).
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Carbon dioxide
Carbon dioxide is obtained from the air. There is not very
much available, because only about 0.04% of the air is
carbon dioxide. Therefore, the leaf must be very eficient
at absorbing it. The leaf is held out into the air by the
stem and the leaf stalk, and its large surface area helps to
expose it to as much air as possible (Figure B4.04).
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Which kind of cell makes the cuticle on a leaf?
B4.05
thick
cytoplasm cell wall nucleus chloroplast
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stoma
B4.04
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QuEStiONS
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Carbon dioxide
diffuses through
air spaces.
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Water in the soil is
absorbed through
root tips.
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It then travels from the xylem vessels to the mesophyll
cells by osmosis, which was described in Chapter B2.
The path it takes is shown in Figures B4.04 and B4.05.
br
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Note that chlorophyll does not ‘attract’ light. It absorbs
energy from light.
-R
ACtivity B4.01
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s
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use a microscope to observe the cells that
cover a leaf
Pr
op
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TIP
The cells that need the sunlight are the mesophyll cells.
The thinness of the leaf allows the sunlight to penetrate
right through it, and reach all the cells. To help this, the
epidermal cells are transparent, with no chloroplasts.
br
What percentage of the air is carbon dioxide?
B4.11
How does carbon dioxide get into a leaf?
B4.12
How does a leaf obtain its water?
B4.13
Give two reasons why the large surface area of
leaves is advantageous to the plant.
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B4.10
Leaves are thin. What purpose does this serve?
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B4.14
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Adaptations of leaves for photosynthesis are shown in
Table B4.01.
What are the raw materials needed for
photosynthesis?
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id
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U
In the mesophyll cells, the chloroplasts are arranged to
get as much sunlight as possible, particularly those in the
palisade cells. The chloroplasts can lie broadside on to
do this, but in strong sunlight, they oten arrange themselves
end on. This reduces the amount of light absorbed.
B4.09
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Figure B4.05 How the raw materials for photosynthesis get
into a palisade cell.
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Figure B4.04 How the materials for photosynthesis get into
a leaf.
ev
Water is
brought from
the roots in
xylem vessels.
ie
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U
Carbon dioxide
diffuses through
stomata from
the atmosphere.
Sunlight
The position of a leaf and its broad, flat surface help it to
obtain as much sunlight as possible. If you look up through
the branches of a tree, you will see that the leaves are
arranged so that they do not cut of light from one another
more than necessary. Plants that live in shady places oten
have particularly big leaves.
R
Water travels
to chloroplasts
by osmosis.
C
op
y
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Sunlight is absorbed
by chlorophyll.
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op
Carbon dioxide
diffuses through
stomata.
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40
sunlight
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to expose as much of the leaf as possible to the
sunlight and air
large surface area
to expose as large an area as possible to the sunlight
and air
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supported by stem and leaf stalk
Pr
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s
to allow sunlight to penetrate to all cells; to allow
CO2 to difuse in and O2 to difuse out as quickly
as possible
no chloroplasts in epidermal cells
to allow CO2 and O2 to difuse to and from all cells
to allow sunlight to penetrate to the mesophyll layer
y
air spaces in spongy mesophyll
to allow CO2 to difuse in and O2 to difuse out
chloroplasts containing chlorophyll present in the
mesophyll layer
to absorb energy from sunlight, so that CO2 will
combine with H2O
palisade cells arranged end on
to keep as few cell walls as possible between sunlight and
the chloroplasts
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stomata in lower epidermis
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thin
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Function
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Adaptation
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B4: Plant nutrition
to expose as much chlorophyll as possible to sunlight
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am
br
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chloroplasts inside palisade cells oten arranged
broadside on
s
to expose as much chlorophyll as possible to sunlight
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-C
chlorophyll arranged on flat membranes inside the
chloroplasts
op
Pr
y
xylem vessels within short distance of every mesophyll cell
to supply water to the cells in the leaf, some of which will
be used in photosynthesis
rs
Table B4.01 Adaptations of leaves for photosynthesis.
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phloem tubes within short distance of every mesophyll cell to take away sucrose and other organic products
of photosynthesis
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B4.04 uses of glucose
br
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sucrose
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starch
Sucrose is used for root
growth or stored in
roots as starch.
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Figure B4.06 The products of photosynthesis.
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Glucose may be turned into starch and stored in the leaf.
Glucose is a simple sugar (Section B3.02). It is soluble in
water, and quite a reactive substance. It is not, therefore, a
very good storage molecule. First, being reactive, it might
get involved in chemical reactions where it is not wanted.
Secondly, it would dissolve in the water in and around
Sucrose is transported
in phloem tubes.
C
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Stored as starch
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glucose
Pr
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Energy may be released from glucose in the leaf.
All cells need energy, which they obtain by the process
of respiration (Chapter B8). Some of the glucose which a
leaf makes will be broken down by respiration, to
release energy.
ie
ev
R
Sucrose is used
for fruit growth or
stored in fruits.
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am
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Used for energy
Sucrose is used for
shoot growth.
ev
id
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One of the first carbohydrates to be made in
photosynthesis is glucose. There are several things that
may then happen to it (Figure B4.06).
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Used to make proteins and other organic
substances
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Image B4.04 This stunted, yellow maize seedling is
sufering from nitrogen deficiency.
C
op
Glucose may be used to make other organic substances.
The plant can use glucose as a starting point for making all
the other organic substances it needs. These include the
carbohydrates sucrose and cellulose. Plants also make
fats and oils.
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The glucose is therefore converted into starch to be
stored. Starch is a polysaccharide, made of many glucose
molecules joined together. Being such a large molecule,
it is not very reactive, and not very soluble. It can be
made into granules which can be easily stored inside
the chloroplasts.
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the plant cells, and might be lost from the cell. Thirdly,
when dissolved, it would increase the concentration of the
solution in the cell, which could afect osmosis.
br
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their crops are growing, to make sure that they do not run
short of these essential substances.
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rs
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The sucrose may later be turned back into glucose again,
to be broken down to release energy, or turned into starch
and stored, or used to make other substances which are
needed for growth.
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A molecule has to be small and soluble to be transported
easily. Glucose has both of these properties, but it is also
rather reactive. It is therefore converted to the complex
sugar sucrose to be transported to other parts of the plant.
Sucrose molecules are also quite small and soluble, but
less reactive than glucose. They dissolve in the sap in the
phloem vessels and can be distributed to whichever parts
of the plant need them (Figure B4.06).
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Changed to sucrose for transport
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am
Plants can also use the sugars they have made in
photosynthesis to make amino acids, which can be
built up into proteins. To do this, they need nitrogen.
Unfortunately, even though the air around us is
78% nitrogen, this is completely useless to plants because
it is very unreactive. Plants have to be supplied with
nitrogen in a more reactive form, usually as nitrate ions.
They absorb nitrate ions from the soil, through their root
hairs, by difusion and active transport. The nitrate ions
combine with glucose to make amino acids. The amino
acids are then strung together to form protein molecules.
nitrate ions
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magnesium ions
What substances does a plant need to be able to
convert glucose into proteins?
B4.17
Explain why a plant that does not get enough
nitrate ions has weak growth.
B4.18
How do parts of the plant such as the roots, which
cannot photosynthesise, obtain food?
to make chlorophyll
C
U
Why needed to make amino
acids, which can
then be used for
making proteins
Why is glucose not very good for storage in a leaf?
B4.16
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Mineral salt
magnesium
B4.15
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nitrogen
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Element
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B4.05 testing leaves for starch
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Iodine solution is used to test for starch. A blue-black
colour shows that starch is present. However, if you put
iodine solution onto a leaf which contains starch,
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Table B4.02 Mineral ions required by plants.
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yellowing between
the veins of leaves
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weak growth,
yellow leaves
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Deficiency
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QuEStiONS
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Table B4.02 shows what happens to a plant if it does
not have enough of these ions. Image B4.04 shows what
happens when a plant does not have enough nitrogen.
Farmers oten add extra mineral ions to the soil in which
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am
br
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id
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Another substance that plants make is chlorophyll.
Once again, they need nitrogen to do this, and also
another element – magnesium. The magnesium,
like the nitrate ions, is obtained from the soil.
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A1 Why was the leaf put into boiling water?
A2 Why did the alcohol become green?
Pr
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43
it will not immediately turn black. This is because the
starch is inside the chloroplasts in the cells. The iodine
solution cannot get through the cell membranes to reach
the starch and react with it.
ni
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rs
Both plants (or leaves) are then treated in exactly the
same way. Any diferences between them at the end of the
investigation, therefore, must be because of the substance
being tested.
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At the end of the investigation, test a leaf from your
experimental plant and one from your control to see if they
have made starch. By comparing them, you can find out
which substances are necessary for photosynthesis.
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es
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Controls
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So, before doing any of these investigations, you must
destarch the plants. The easiest way to do this is to leave
them in a dark cupboard for at least 24 hours. The plants
cannot photosynthesise while they are in the cupboard
because there is no light. So they use up their stores of
starch. To be certain that they are thoroughly destarched,
test a leaf for starch before you begin.
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am
It is very important that the leaves you are testing
should not have any starch in them at the beginning
of the investigation. If they did, and you found
that the leaves contained starch at the end of the
investigation, you could not be sure that they had been
photosynthesising. The starch might have been made
before the investigation began.
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rs
If you do Activities B4.03, B4.04 and B4.05, you can find
out for yourself which substances a plant needs for
photosynthesis. In each investigation, the plant is given
everything it needs, except for one substance. Another
plant is used at the same time. This is a control. The control
is given everything it needs, including the substance being
tested for. Sometimes the control is a leaf, or even a part of
a leaf, from the experimental plant. The important thing is
that the control has all the substances it needs, while the
experimental plant – or leaf – is lacking one substance.
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Destarching plants
s
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br
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Therefore, before testing a leaf for starch, you must break
down the cell membranes, and get rid of the green colour
(chlorophyll). The way this is done is described in
Activity B4.02. The cell membranes are first broken down
by boiling water, and then the chlorophyll is removed by
dissolving it out with alcohol.
ie
A3 Why was the leaf put into alcohol ater being put into
boiling water?
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beaker
Questions
Another dificulty is that the green colour of the leaf and
the brown iodine solution can look black together.
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alcohol
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3 The leaf will now be brittle. Remove it from the alcohol,
and dip it into hot water again to soten it.
boiling tube
leaf
1 Take a leaf from a healthy plant, and drop it into boiling
water in a water bath. Leave for about 30 s. Turn out the
Bunsen flame.
2 Remove the leaf, which will be very sot, and drop it into
a tube of alcohol in the water bath. Leave it until all the
chlorophyll has come out of the leaf.
R
boiling
water
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4 Spread out the leaf on a white tile, and cover it with
iodine solution. A blue-black colour shows that the leaf
contains starch.
Pr
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s
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testing a leaf for starch
Skill:
AO3.1 Using techniques, apparatus and materials
Leaves turn some of the glucose that they make in
photosynthesis into starch. If we find starch in a leaf, that
tells us if it has been photosynthesising.
! Wear eye protection if available.
Take care with the boiling water.
Alcohol is very flammable. Turn out your Bunsen flame
before putting the tube of alcohol into the hot water.
Use forceps to handle the leaf.
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am
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ACtivity B4.02
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B4: Plant nutrition
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4 Leave the plant near a warm, sunny window for a few days.
U
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5 Remove the cover from your leaf, and test the
leaf for starch.
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6 Make a labelled drawing of the appearance of your
leaf ater testing for starch.
br
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Questions
A1 Why was the plant destarched before the beginning of
the experiment?
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Pr
y
A2 Why was part of the leaf let uncovered?
A3 What do your results tell you about light and photosynthesis?
ity
op
ACtivity B4.04
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Questions
U
A1 What was the control in this investigation?
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A2 What do your results tell you about chlorophyll
and photosynthesis?
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to see if chlorophyll is needed for photosynthesis
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
! Wear eye protection if available.
Take care with the boiling water.
Alcohol is very flammable. Turn out your Bunsen
flame before putting the tube of alcohol into the
hot water.
Use forceps to handle the leaf.
1 Destarch a plant with variegated (green and white)
leaves. Then leave your plant in a warm, sunny spot for a
few days.
2 Test one of the leaves for starch (Activity B4.02).
3 Make a drawing of your leaf before and ater testing.
C
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44
leaf
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black paper stencil
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Pr
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to see if light is needed for photosynthesis
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
! Wear eye protection if available.
Take care with the boiling water.
Alcohol is very flammable. Turn out your Bunsen flame
before putting the tube of alcohol into the
hot water.
Use forceps to handle the leaf.
1 Take a healthy bean or Pelargonium plant, growing
in a pot. Leave it in a cupboard for a few days, to
destarch it.
2 Test one of its leaves for starch, to check that it does not
contain any.
3 Using a folded piece of black paper or aluminium foil,
a little larger than a leaf, cut out a shape (see diagram).
Fasten the paper or foil over both sides of a leaf on
your plant, making sure that the edges are held firmly
together. Don’t take the leaf of the plant!
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am
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id
ACtivity B4.03
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Questions
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A1 Why was this investigation done under water?
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A2 This investigation has no control. Try to design one.
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2 Once you have an idea about how you will do your
experiment, write it down as a list of points. Then think
through it again, and make improvements to your plan.
Once you are fairly happy with it, show your teacher. You
must not try to do your experiment until your teacher
says that you may begin.
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■
What will you keep the same in all the tubes or
beakers in your experiment? How will you do this?
■
What will you measure in your experiment? How
will you measure it? When will you measure it?
Will you do repeat measurements and calculate
a mean?
■
How will you record your results? (You can sketch out
a results chart, ready to fill in.)
■
How will you display your results? (You can sketch the
axes of the graph you plan to draw.)
■
What will your results be if your hypothesis is correct?
(You can sketch the shape of the graph you think you
will get.)
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What will you vary in your experiment? How will you
vary it?
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What apparatus and other materials will you need for
your experiment?
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investigating the efect of light intensity on
photosynthesis
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
AO3.5 Evaluating methods
! If you use an electric lamp, keep water well away
from it.
If you did Activity B4.06, you may have noticed that the plant
seemed to produce more bubbles in bright sunlight than
when it was in the shade. This could mean that the rate of
photosynthesis is afected by light intensity.
1 Write down a hypothesis that you will investigate. The
hypothesis should be one sentence, and it should
describe the relationship that you think exists between
light intensity and the rate of photosynthesis. You can
vary light intensity by moving a light source closer to the
plant. The shorter the distance between the light and the
plant, the greater the light intensity.
You can use a water plant in your investigation.
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Photosynthesis in pond weed
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water plant
photosynthesising
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ACtivity B4.08
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inverted funnel
ACtivity B4.07
to see if carbon dioxide is needed for
photosynthesis
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beaker containing
water
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ACtivity B4.06
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oxygen collecting
in the tube
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to show that oxygen is produced in photosynthesis
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
1 Set up the apparatus shown in the diagram. Make sure
that the test tube is completely full of water.
2 Leave the apparatus near a warm, sunny window for a
few days.
3 Carefully remove the test tube from the top of the funnel,
allowing the water to run out, but not allowing the
gas to escape.
4 Light a wooden splint, and then blow it out so that it is
just glowing. Carefully put it into the gas in the test tube.
If it bursts into flame, then the gas is oxygen.
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ACtivity B4.05
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B4: Plant nutrition
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a neat and carefully headed line graph of your results
■
a conclusion, in which you say whether or not your
results support your hypothesis
■
a discussion, in which you use what you know
about photosynthesis to try to explain the pattern in
your results
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4 Finally, write up your experiment in the usual way,
including:
a heading, and the hypothesis that you tested
■
an evaluation of the reliability of your data
■
a diagram of the apparatus that you used, and a full
description of your method
■
an evaluation of your method.
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why plants need nitrate ions and magnesium ions
how to test a leaf for starch
how to do experiments to investigate the
need for chlorophyll, light and carbon dioxide
for photosynthesis
about the importance of a control in an experiment
how to investigate the efect of light intensity on the
rate of photosynthesis.
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■
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the equation for photosynthesis
the role of chlorophyll in photosynthesis
the structure of a leaf
how a leaf is adapted to carry out photosynthesis
eficiently
how a plant uses and stores the carbohydrates made
in photosynthesis
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You should know:
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Summary
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a neat and carefully headed table of results, including
means if you decided to do repeats
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This is a good thing to do. Make careful notes about all
the changes that you make.
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3 Once you have approval from your teacher, you should
do your experiment. Most scientific researchers find that
they want to make changes to their experiment once they
actually begin doing it.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Copy and complete this table to show how, and for what purpose, plants obtain these substances.
Obtained from
Used for
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End-of-chapter questions
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Nitrates
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Water
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Magnesium
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chloroplast and chlorophyll
palisade mesophyll and spongy mesophyll
organic substances and inorganic substances
guard cell and stoma
a
b
c
Write the word equation for photosynthesis.
Describe how a leaf obtains the two substances on the let hand side of your equation.
Describe what happens to the two substances on the right hand side of your equation.
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3
Explain the diference between each of these pairs of terms.
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Carbon dioxide
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6
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a
b
transport
storage
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A student did an experiment with two potted plants, each of which had been enclosed in a transparent
polythene bag for a period of two days. During this time, the plants were exposed to bright light.
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Which carbohydrate does a plant use for each of these purposes? Explain why.
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There is an air space behind each stoma.
The epidermal cells of a leaf do not have chloroplasts.
Leaves have a large surface area.
The veins in a leaf branch repeatedly.
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b
c
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Explain how each of the following helps a leaf to photosynthesise.
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B4: Plant nutrition
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In one of the polythene bags there was a chemical which absorbs carbon dioxide.
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plant B
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leaf from plant B without chemical
that absorbs carbon dioxide
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The black paper was removed from each leaf.
The leaves were placed in hot water for two minutes.
The leaves were removed from the hot water, and placed in a test-tube of hot alcohol for five minutes.
The leaves were dipped briefly back into the hot water.
The leaves were spread out on a white tile, and covered with iodine solution.
a
Make a copy of the diagrams of the two leaves. Label the diferent areas of each leaf to show the
colours that you would expect to see ater each leaf had been treated with iodine solution.
In the starch test, explain the reasons for
i placing the leaf in the hot water at the beginning,
ii placing the leaf in hot alcohol
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•
•
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Both leaves were then tested for starch.
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part covered
with black paper
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part covered
with black paper
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uncovered part
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uncovered part
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One leaf was removed from each plant, and labelled drawings of the two leaves were made as shown below.
leaf from plant A with chemical
that absorbs carbon dioxide
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chemical to
absorb carbon
dioxide
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black paper
soil
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transparent
polythene
bag
plant A
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Both plants had part of their leaves covered with black paper, as shown in the diagram below.
Copyright Material - Review Only - Not for Redistribution
[3]
[1]
[1]
(continued)
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The purpose of the experiment with the black paper was to see if light is needed for
photosynthesis. However, another student criticised this experiment, saying that the black
paper might have prevented photosynthesis by preventing gas exchange.
Suggest a modification of the experiment that would overcome this criticism.
ii Another modification of the experiment would be to use just one plant, and enclose diferent
part-covered leaves on this plant with and without the chemical that absorbs carbon dioxide.
Explain why this might be considered to be a better experiment.
Describe an experiment that you could do to show that chlorophyll in a leaf is needed for photosynthesis.
y
The diagram shows a section through a leaf.
A
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Give the letters that indicate i a stoma, ii the cuticle and iii a vascular bundle.
i The upper layers of a leaf are transparent. Suggest an advantage to a plant of this feature.
ii The cuticle is made of a waxy material. Suggest an advantage to a plant of this feature.
iii State two functions of vascular bundles in leaves.
Most photosynthesis in plants happens in leaves.
i Name the two raw materials needed for photosynthesis.
ii Photosynthesis produces glucose.
Describe how plants make use of this glucose.
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[2]
[4]
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Adapted from [Cambridge IGCSE Biology 0610 Paper 21 Q4 November 2010]
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[3]
[1]
[1]
[2]
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B
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48
[1]
[3]
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[1]
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 62 Q1 June 2013]
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Cambridge IGCSE Combined and Co-ordinated Sciences
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• minerals
• water
• fibre roughage
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Animals get their food from other organisms – from plants
or other animals. They cannot make their own food as
plants do.
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A diet which contains all of these seven types of nutrient,
in the correct amounts and proportions, is called a
balanced diet.
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Energy needs
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Every day, a person uses up energy. The amount you use
partly depends on how old you are, which sex you are and
what job you do. A few examples are shown in Figure B5.01.
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carbohydrates
proteins
fats
vitamins
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The food an animal eats every day is called its diet. Most
animals need seven types of nutrient in their diet. These are:
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B5.01 Diet
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the functions of amylase, protease and lipase and
where they are secreted
the functions of hydrochloric acid in gastric juice
the role of bile
the significance of villi and the structure of a villus
the roles of capillaries and lacteals in villi.
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a balanced diet
nutrients and their sources
the needs of diferent people for diferent amounts of
energy in their diet
why we need to digest the food that we eat
teeth
the structure of the alimentary canal, and the
functions of each of its parts
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This chapter covers:
■
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B5
Animal nutrition
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339
948
66
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2214
chocolate biscuits
2197
cornflakes
1567
402
custard
496
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cottage cheese
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1016
fish (fresh)
340
french fries
1065
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Pr
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fried liver
1016
fruit yoghurt
405
ice cream
698
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1293
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lettuce
36
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marmalade
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melon
150
pawpaw
160
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oranges
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peas
161
1925
rice
1536
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plain biscuits
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roast chicken
599
roast peanuts
2364
sardines
906
spaghetti
1612
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sugar
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tomatoes
932
1682
60
unsweetened fruit juice
143
white bread
991
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1698
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oatmeal
1035
272
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milk
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Table B5.01 Energy content of some diferent kinds of food.
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chocolate
stewed steak
As well as providing you with energy, food is needed for
many other reasons. To make sure that you eat a balanced
diet you must eat foods containing carbohydrate, fat and
protein. You also need each kind of vitamin and mineral,
fibre and water. These substances are called nutrients.
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1682
lentils
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cheddar cheese
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A person’s diet may need to change at diferent times of
their life. For example, a woman will need to eat a little
more each day when she is pregnant, and make sure that
she has extra calcium and iron in her diet, to help to build
her baby’s bones, teeth and blood. She will also need to
eat more while she is breast feeding. Most people find
that they need to eat less as they reach their 50s and 60s,
because their metabolism slows down.
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fish (dried, salt)
All food contains some energy. Scientists have worked out
how much energy there is in particular kinds of food. You
can look up this information. A few examples are given in
Table B5.01. You may remember that one gram of fat contains
about twice as much energy as one gram of protein or
carbohydrate (Section B3.03). This is why fried foods should
be avoided if you are worried about putting on weight.
Nutrients
373
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adult pregnant
female female
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boiled white (Irish) potatoes
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child
teenage teenage adult
(aged 8) male
female male
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boiled egg
carrots
The energy you use each day comes from the food you eat.
If you eat too much food, some of the extra will probably be
stored as fat. If you eat too little, you may not be able to obtain
as much energy as you need. This will make you feel tired.
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canned peaches
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bananas
cabbage
Figure B5.01 Daily energy requirements.
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270
brown bread
Daily energy
requirements
6
/ MJ
50
baked beans
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0
Energy content / kJ/100 g
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Food
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If your diet doesn’t contain all of these nutrients, your body
will not be able to work properly.
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Vitamins are organic substances which are only needed
in tiny amounts. If you do not have enough of a vitamin,
you may get a deficiency disease. Table B5.02 provides
information about vitamins C and D.
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The structures of molecules of carbohydrates, fats and
proteins, and their uses in the body, are described in
sections B3.02 to B3.04. Images B5.01, B5.02, B5.03 and
B5.04 show foods that are good sources of these nutrients.
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Vitamins
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Image B5.03 Some good sources of fats.
51
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Image B5.01 Some good sources of carbohydrates.
s
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Why it is needed
Deficiency disease
C
citrus fruits (such as
oranges, limes), raw
vegetables
to make the stretchy protein
collagen, found in skin and
other tissues; keeps tissues in
good repair
scurvy, which causes pain in joints and muscles,
and bleeding from gums and other places; this
used to be a common disease of sailors, who
had no fresh vegetables during long voyages
D
butter, egg yolk (and
helps calcium to be absorbed,
can be made by the skin for making bones and teeth
when sunlight falls on it)
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C
rickets, in which the bones become sot and
deformed; this disease was common in young
children in industrial areas, who rarely got out
into the sunshine
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Table B5.02 Vitamins.
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Foods that contain it
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Vitamin
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Image B5.04 Some good sources of fibre.
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Image B5.02 Some good sources of proteins.
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B5: Animal nutrition
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calcium,
Ca
milk and other dairy
products, bread
for bones and teeth;
for blood clotting
iron, Fe
liver, red meat,
egg yolk,
dark green vegetables
for making haemoglobin, the
red pigment in blood which
carries oxygen
Deficiency disease
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Why it is needed
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Foods that contain it
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Mineral
element
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Table B5.03 Minerals.
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Minerals
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Fish and white meat such as chicken do not contain much
saturated fat, so eating more of these and less red meat
may help to cut down the risk of heart disease.
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Most people can control their weight by eating normal,
well-balanced meals and taking regular exercise. Crash
diets are not a good idea, except for someone who is very
overweight. Although a person may manage to lose a lot
of weight quickly, he or she will almost certainly put it on
again once he or she stops dieting.
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Image B5.05 Being very overweight increases the
risk of many diferent, and serious, health problems.
Weight around your middle has been shown to be
linked to heart disease.
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Dairy products such as milk, cream, butter and cheese
contain a lot of saturated fat. So do red meat and eggs.
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The kind of fat found in animal foods is called saturated
fat. These foods also contain cholesterol. Some research
suggests that people who eat a lot of saturated fat and
cholesterol are more likely to get heart disease than
people who do not. This is because fat deposits build up
on the inside of arteries, making them stifer and narrower.
If this happens in the coronary arteries supplying the
heart muscle with blood, then not enough blood can get
through. The heart muscles run short of oxygen and cannot
work properly. This is called coronary heart disease.
The deposits can also cause a blood clot, which results in a
heart attack (Section B7.02).
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C
One common form of fibre is the outer husk of cereal
grains, such as oats, wheat and barley. This is called bran.
Some of this husk is found in wholemeal bread. Brown or
unpolished rice is also a good source of fibre.
Fat and heart disease
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People who take in more energy than they use up get
fat. Being very fat is called obesity (Image B5.05). Obesity
is dangerous to health. Obese people are more likely to
get heart disease, strokes and diabetes. The extra weight
placed on the legs can cause problems with the joints,
especially knees.
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All plant foods, such as fruits and vegetables, contain fibre
(Image B5.04). This is because the plant cells have cellulose
cell walls. Humans cannot digest cellulose.
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Obesity
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Fibre helps to keep the alimentary canal working properly.
Food moves through the alimentary canal because the
muscles contract and relax to squeeze it along. This is called
peristalsis. The muscles are stimulated to do this when
there is food in the alimentary canal. Sot foods do not
stimulate the muscles very much. The muscles work more
strongly when there is harder, less digestible food, like fibre,
in the alimentary canal. Fibre keeps the digestive system in
good working order and helps to prevent constipation.
anaemia, in which there are not enough red
blood cells so the tissues do not get enough
oxygen delivered to them
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Fibre
brittle bones and teeth;
poor blood clotting
But vegetable oils are usually unsaturated fats. These, and
also oils from fish, do not increase the risk of heart disease,
so it is sensible to use these instead of animal fats when
possible. Vegetable oil can be used for frying instead
of butter or lard. Polyunsaturated spreads can be used
instead of butter.
Minerals are inorganic substances. Once again, only small
amounts of them are needed in the diet. Table B5.03
shows two of the most important ones.
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B5: Animal nutrition
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Image B5.06 The older
boy is thin, but has
a swollen abdomen,
suggesting that he
is sufering from
kwashiorkor. This photo
was taken at a refugee
camp in Ethiopia.
rs
Malnutrition is caused by not eating a balanced diet. One
common form of malnutrition is kwashiorkor (Image B5.06).
This is caused by a lack of protein in the diet. It is most
common in children between the ages of nine months and
two years, ater they have stopped feeding on breast milk.
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B5.01
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A balanced diet contains these nutrients:
carbohydrates
vitamins
Pr
op
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fats
minerals
proteins
water
a Which of these nutrients are organic, and
which are inorganic?
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The most severe forms of malnutrition result from a lack
of both protein and energy in the diet. Severe shortage of
energy in the diet causes marasmus, in which a child has
body weight much lower than normal, and looks emaciated.
b Which of these nutrients can provide energy?
c What is the role of fibre in the diet?
B5.03
What is coronary heart disease?
B5.04
What is the diference between starvation and
malnutrition?
Malnutrition can also be the result of having too much of
something in your diet, e.g. too much fat, leading to obesity.
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What is meant by a deficiency disease?
Give two examples of deficiency diseases.
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B5.05
B5.06
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List three health problems associated with obesity.
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B5.02
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QuEStiONS
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Children sufering from kwashiorkor are always
underweight for their age. But they may oten look quite
fat, because their diet may contain a lot of carbohydrate.
If they are put onto a high-protein diet, they usually begin
to grow normally again.
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Kwashiorkor is oten caused by poverty, because the child’s
carers do not have any high-protein food to give to the
child. But sometimes it is caused by a lack of knowledge
about the right kinds of food that should be eaten.
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In some parts of Africa, for example, several years of
drought can mean that the harvests do not provide
enough food to feed all the people. Despite help from
other countries, many people have died from starvation.
Even if there is enough food to keep people alive, they may
sufer from malnutrition.
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Starvation and malnutrition
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testing foods for vitamin C
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
AO3.5 Evaluating methods
The DCPIP test is used to find out if a food contains
vitamin C. DCPIP is a blue liquid. Vitamin C causes DCPIP to
lose this colour.
First, try out the test:
1 Measure 2 cm3 of DCPIP into a clean test tube.
2 Use a dropper pipette to add lemon juice to the DCPIP.
Count how many drops you need to add before the
DCPIP loses its colour.
You can use this test to compare the concentration of
vitamin C in diferent liquids. The less liquid you have
to add to the DCPIP to make it lose its colour, the more
vitamin C there is in the liquid.
3 Plan and carry out an experiment to test one of the
following hypotheses.
a Fresh lemon juice contains more vitamin C than
other types of lemon juice.
b Raw potato contains more vitamin C per g than
boiled or baked potato.
c Freezing vegetables or fruit juices reduces their
vitamin C content.
d Storing vegetables in a refrigerator retains more
vitamin C than storing them at room temperature.
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ACtivity B5.01
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53
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starch
amylase
simple sugars
protein
protease
amino acids
fat
lipase
fatty acids and glycerol
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Table B5.04 Functions of digestive enzymes.
Once pieces of food have been ground up, the large
molecules present are then broken down into small ones.
This is called chemical digestion. It involves a chemical
change from one sort of molecule to another. Enzymes
are involved in this process (Chapter B3). Figure B5.03
summarises how mechanical and chemical digestion work
together to produce small molecules the body can use.
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The food that is eaten by mammals usually contains some
large molecules of protein, carbohydrate and fat. Before
these molecules can be absorbed, they must be broken
down into small ones. This is called digestion.
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Figure B5.03 shows what happens to the three kinds of
nutrients that need to be digested – fats, proteins and
carbohydrates. Look at one column at a time, and work
down it, to follow what happens to that type of food as it
passes through the alimentary canal.
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Small molecules
produced
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The alimentary canal of a mammal is a long tube running
from one end of its body to the other (Figure B5.02).
Before food can be of any use to the animal, it has to get
out of the alimentary canal and into the bloodstream.
This is called absorption. To be absorbed, molecules of
food have to get through the walls of the alimentary canal.
They need to be quite small to be able to do this.
Enzyme that
breaks it down
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chemical digestion: the breakdown of large insoluble
molecules into small soluble molecules
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absorption: the movement of digested food molecules
through the wall of the intestine into the blood
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Mechanical and chemical digestion
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assimilation: the movement of digested food molecules into the
cells of the body where they are used, becoming part of the cells
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Oten the food an animal eats is in quite large pieces.
These pieces of food need to be broken up by teeth, and
by churning movements of the alimentary canal. This is
called mechanical digestion.
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egestion: passing out of food that has not been digested, as
faeces, through the anus
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digestion: the breakdown of large, insoluble food molecules
into small, water-soluble molecules using mechanical and
chemical processes
mechanical digestion: the breakdown of food into smaller
pieces without chemical change to the food molecules
Simple sugars, water, vitamins and minerals are already
small molecules, and they can be absorbed just as they are.
They do not need to be digested.
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ingestion: taking substances (e.g. food, drink) into the body
through the mouth
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Large carbohydrate molecules, such as polysaccharides,
have to be broken down into simple sugars
(monosaccharides). Proteins are broken down to amino
acids. Fats are broken down to fatty acids and glycerol
(Table B5.04).
54
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B5.02 Digestion
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2 Digestion Large,
insoluble molecules of
food are broken down to
small molecules.
3 Absorption The small
molecules are absorbed
into the blood.
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Figure B5.02 How an animal deals with food.
4 Egestion Food which
could not be digested
or absorbed is removed
from the body.
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1 ingestion Food
is taken into the
alimentary canal.
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Proteins
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Water in digestive juices
dissolves some food.
Water in digestive juices
dissolves some food.
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Teeth break down large
pieces of food into
smaller ones.
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small pieces of
food and some
food in solution
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Proteases break down
protein molecules to
polypeptide molecules.
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Peptidases break down
polypeptides to amino
acid molecules.
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B5.03 teeth
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QuEStiONS
What is digestion?
B5.08
Name two groups of food that do not need to
be digested.
B5.09
What does digestion change each of these kinds
of food into: a polysaccharides, b proteins and
c fats?
B5.10
What is meant by chemical digestion?
Teeth help with the ingestion and mechanical digestion
of the food we eat.
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B5.07
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Teeth can be used to bite of pieces of food. They
then chop, crush or grind them into smaller pieces.
This gives the food a larger surface area, which makes
it easier for enzymes to work on the food in the
digestive system. It also helps soluble parts of the food
to dissolve.
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Maltase breaks maltose
down to glucose
molecules.
glucose
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amino acid
Figure B5.03 Digestion.
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maltose
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Amylase breaks starch
molecules down to
maltose molecules.
polypeptides
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Lipase breaks down fat
molecules to fatty acid
and glycerol molecules.
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fat molecules
glycerol
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starch molecule
protein molecules
fatty acid
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small pieces
of food and
some food
in solution
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Bile salts break down
large drops of fat into
smaller ones.
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Teeth break down large
pieces of food into
smaller ones.
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Teeth break down large
pieces of food into
smaller ones.
fat droplets
Carbohydrates
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Fats
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ACtivity B5.02
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Checking your teeth
Types of teeth
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The structure of a tooth is shown in Figure B5.04.
The part of the tooth which is embedded in the gum is
called the root. The part which can be seen is the crown.
The crown is covered with enamel. Enamel is the hardest
substance made by animals. It is very dificult to break or
chip it. However, it can be dissolved by acids. Bacteria feed
on sweet foods let on the teeth. This makes acids, which
dissolve the enamel, and decay sets in.
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Cambridge IGCSE Combined and Co-ordinated Sciences
Most mammals have four kinds of teeth (Figures B5.05 and
B5.06). Incisors are the sharp-edged, chisel-shaped teeth
at the front of the mouth. They are used for biting of pieces
of food. Canines are the more pointed teeth at either side
of the incisors, used for gripping food. Premolars and
molars are the large teeth towards the back of the mouth.
They are used for chewing food, crushing it into smaller
pieces to help with mechanical digestion. In humans, the
molars right at the back are sometimes called wisdom
teeth. They do not grow until much later in the person’s
development than the others.
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In the middle of the tooth is the pulp cavity. It contains
nerves and blood vessels. These supply the cytoplasm in
the dentine with food and oxygen.
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The root of the tooth is covered with cement. This
has fibres growing out of it. These attach the tooth to
the jawbone, but allow it to move slightly when biting
or chewing.
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Plaque is sot and easy to remove at first, but if it is let it
hardens to form tartar, which cannot be removed by brushing.
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orbit
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cranium
gum
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pulp cavity
containing
nerves and
blood vessels
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cement
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root
upper
jaw
incisor
molar
premolar
canine
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Figure B5.05 A human skull, showing the diferent types
of teeth.
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lower
jaw
blood supply for
the tooth
Figure B5.04 Longitudinal section of an incisor tooth.
jaw articulation
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jawbone
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fibres attaching
tooth to
jawbone
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dentine
crown
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enamel
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Tooth decay is a common problem. It is caused by
bacteria. You have large numbers of bacteria living in your
mouth, most of which are harmless. However, some of
these bacteria, together with substances from your saliva,
form a sticky film over your teeth, especially next to the
gums and in between the teeth. This is called plaque.
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Dental decay
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Under the enamel is a layer of dentine, which is rather like
bone. Dentine is quite hard, but not as hard as enamel. It
has channels in it which contain living cytoplasm.
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side view
Canines are very similar
to incisors in humans.
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1 Don’t eat too much sugar. If you never eat any sugar,
you will not have tooth decay. But nearly everyone
enjoys sweet foods, and if you are careful you can
still eat them without damaging your teeth. The
rule is to eat sweet things only once or twice a day,
preferably with your meals. The worst thing you can
do is to suck or chew sweet things all day long. And
don’t forget that many drinks also contain a lot of sugar.
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1 Particles of sugary foods
get trapped in cracks in
the teeth.
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3 Make regular visits to a dentist. Regular dental
check-ups will make sure that any gum disease or
tooth decay is stopped before it really gets a hold.
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3 There are nerves in the
pulp cavity, so the tooth
becomes very painful if
the infection gets this far.
What are incisors, and what are they used for?
B5.12
What is plaque?
B5.13
Explain how plaque can cause tooth decay.
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Figure B5.07 Tooth decay.
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B5.11
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2 Use a fluoride toothpaste regularly. Fluoride makes
your teeth more resistant to decay. Drinking water
which contains fluoride, or brushing teeth with a
fluoride toothpaste, makes it much less likely that you
will have to have teeth filled or extracted. Regular and
thorough brushing also helps to remove plaque, which
will prevent gum disease and reduce decay.
2 Bacteria feeding on the
sugar form acids, which
dissolve a hole in the
enamel and dentine.
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If sugar is let on the teeth, bacteria in the plaque will
feed on it. They use it in respiration, changing it into acid.
The acid gradually dissolves the enamel covering the
tooth, and works its way into the dentine (Figure B5.07).
Dentine is dissolved away more rapidly than the enamel.
If nothing is done about it, the tooth will eventually have to
be taken out.
There are several easy things which you can do to keep
your teeth healthy and free from pain.
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Molars, like premolars,
are used for grinding.
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Figure B5.06 Types of human teeth.
Premolars have wide surfaces,
for grinding food.
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Incisors are chisel shaped, for
biting off pieces of food.
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front view
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4 The infection can spread
rapidly through the pulp
cavity, and may form an
abscess at the root of
the tooth.
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Cambridge IGCSE Combined and Co-ordinated Sciences
The mouth
The alimentary canal is a long tube which runs from the
mouth to the anus. It is part of the digestive system. The
digestive system also includes the liver and the pancreas.
Food is ingested using the teeth, lips and tongue. The teeth
then bite or grind the food into smaller pieces, increasing
its surface area. The tongue mixes the food with saliva and
forms it into a bolus. The bolus is then swallowed.
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Saliva is made in the salivary glands. It is a mixture of
water, mucus and the enzyme amylase. The water helps to
dissolve substances in the food, allowing us to taste them.
The mucus helps the chewed food to bind together to form
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salivary duct
epiglottis
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salivary gland
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oesophagus
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trachea
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gall bladder
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pancreas
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sphincter muscle
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pancreatic duct
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duodenum
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ileum
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colon
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rectum
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anus
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Figure B5.08 The human digestive system.
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anal sphincter
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appendix
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small
intestine
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caecum
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stomach
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bile duct
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sphincter muscle
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liver
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diaphragm
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58
bolus of food
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palate
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nasal cavity
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Each section of the alimentary canal has its own part
to play in the digestion, absorption and egestion
of food. Figure B5.08 shows the main organs of the
digestive system.
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B5.04 the alimentary canal
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large
intestine
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A tube called the pancreatic duct leads from the pancreas
into the duodenum. Pancreatic juice, which is a fluid made
by the pancreas, flows along this tube.
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The oesophagus
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Bile
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When you swallow, a piece of cartilage covers the entrance
to the trachea. It is called the epiglottis, and it stops food
from going down into the lungs.
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As well as pancreatic juice, another fluid flows into the
small intestine. It is called bile. Bile is a yellowish green,
alkaline, watery liquid, which helps to neutralise the acidic
mixture from the stomach. It is made in the liver, and then
stored in the gall bladder. It flows to the small intestine
along the bile duct.
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The stomach
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Like all parts of the alimentary canal, the stomach wall
contains goblet cells which secrete mucus. It also
contains other cells which produce protease enzymes and
others which make hydrochloric acid.
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Villi
The stomach can store food for quite a long time. Ater one
or two hours, the sphincter at the bottom of the stomach
opens and lets the chyme into the duodenum.
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The small intestine
The carbohydrase enzyme maltase breaks down
maltose to glucose. Proteases finish breaking down any
polypeptides into amino acids. Lipase completes the
breakdown of fats to fatty acids and glycerol.
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Several enzymes are secreted into the small intestine.
They are made in the pancreas, which is a creamcoloured gland, lying just underneath the stomach.
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The small intestine is the part of the alimentary canal
between the stomach and the colon. It is about 5 m long.
It is called the small intestine because it is quite narrow.
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As well as receiving enzymes made in the pancreas, the
small intestine makes some enzymes itself. They are made
by cells in its walls.
The inner wall of the small intestine is covered with
millions of tiny projections. They are called villi (singular:
villus). Each villus is about 1 mm long (Figures B5.09 and
B5.10, and Image B5.07). Cells covering the villi make
enzymes. The enzymes do not come out into the lumen of
the small intestine; they stay close to the cells which make
them. These enzymes complete the digestion of food.
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The acid also helps to protect you from harmful
microorganisms that may be in the food, by denaturing
enzymes in them.
Bile does not contain any enzymes. It does, however, help
to digest fats. It does this by breaking up the large drops
of fat into very small ones, increasing their surface area
and making it easier for the lipase in the pancreatic juice
to digest them into fatty acids and glycerol. This is called
emulsification, and it is done by salts in the bile called
bile salts. Emulsification is a type of mechanical digestion.
Bile also contains sodium hydrogencarbonate, which
helps to neutralise the acidic chyme from the stomach
and therefore provides a suitable pH for the activity of the
enzymes in pancreatic juice.
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The stomach has strong, muscular walls. The muscles
contract and relax to churn the food and mix it with the
enzymes and mucus. The mixture is called chyme.
The main protease enzyme in the stomach is pepsin.
It begins to digest proteins by breaking them down into
polypeptides. Pepsin works best in acid conditions.
The acid also helps to kill any bacteria in the food.
These enzymes do not work well in acid environments,
but the chyme which has come from the stomach contains
hydrochloric acid. Pancreatic juice contains sodium
hydrogencarbonate which partially neutralises the acid.
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There are two tubes leading down from the back of the
mouth. The one in front is the trachea or windpipe, which
takes air down to the lungs. Behind the trachea is the
oesophagus, which takes food down to the stomach.
The entrance to the stomach from the oesophagus is
guarded by a ring of muscle called a sphincter. This muscle
relaxes to let the food pass into the stomach.
This fluid contains many enzymes, including amylase,
protease and lipase. Amylase breaks down starch to
maltose. Trypsin is a protease, which breaks down proteins
to polypeptides. Lipase breaks down fats (lipids) to fatty
acids and glycerol.
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a bolus, and lubricates it so that it slides easily down the
oesophagus when it is swallowed. Amylase begins to digest
starch in the food to the sugar maltose. Usually, it does
not have time to finish this because the food is not kept in
the mouth for very long. However, if you chew something
starchy (such as a piece of bread) for a long time, you may
be able to taste the sweet maltose that is produced.
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B5: Animal nutrition
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goblet cell, which
makes mucus
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Cambridge IGCSE Combined and Co-ordinated Sciences
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lacteal, which
absorbs digested
fats
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Image B5.07 This micrograph shows thousands of villi
covering the inner wall of the small intestine. It is magnified
about 20 times.
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blood capillaries,
which absorb
small molecules
such as amino
acids and sugars
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0.1 mm
Figure B5.09 Longitudinal section through a villus.
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blood from
aorta
blood to hepatic
portal vein
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diffusion down a concentration gradient
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6 m
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active transport
Figure B5.11 Absorption of digested nutrients into a villus.
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Figure B5.10 Detail of the surface of a villus.
lymph to lymphatic
vessels and then
the heart
mass flow
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mucus
goblet cell
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nucleus
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Some
fats are
absorbed
into lacteals.
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basement
membrane
microvilli
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Small molecules,
such as water,
amino acids,
sugars and some
fats, minerals
and vitamins,
are absorbed
into capillaries
through diffusion
and active
transport.
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vein, returning blood
to the liver
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artery, bringing blood
from the heart
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60
By now, most carbohydrates have been broken down to
simple sugars, proteins to amino acids, and fats to fatty
acids and glycerol. These molecules are small enough to
pass through the wall of the small intestine and into the
blood. This is called absorption (Figure B5.11).
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Absorption of digested food
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How this helps absorption take place
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B5: Animal nutrition
This gives plenty of time for digestion to be completed, and for
digested food to be absorbed as it slowly passes through.
It has villi. Each villus is covered with cells
which have even smaller projections on them,
called microvilli.
This gives the inner surface of the small intestine a very large
surface area.
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The larger the surface area, the faster nutrients can be absorbed.
Monosaccharides, amino acids, water, minerals and vitamins, and
some fats, pass into the blood, to be taken to the liver and then
round the body.
Fats are absorbed into lacteals.
Villi have walls only one cell thick.
The digested nutrients can easily cross the wall to reach the
blood capillaries and lacteals.
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Villi contain lacteals, which are part of the
lymphatic system.
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Villi contain blood capillaries.
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It is very long – about 5 m in an adult human.
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Table B5.05 How the small intestine is adapted for absorbing digested nutrients.
The small intestine is especially adapted to allow
absorption to take place very eficiently. Some of its
features are listed in Table B5.05.
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Table B5.06 gives a summary of digestion in the human
alimentary canal.
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The large intestine
The large intestine is given this name because it is a wider
tube than the small intestine.
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The liver has an especially important role in the
metabolism of glucose. If there is more glucose than
necessary in the blood, the liver will convert some of it to
the polysaccharide glycogen, and store it. You can find out
more about this in Section B9.06.
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By the time the food reaches the last part of the large
intestine, called the rectum, most of the substances which
can be absorbed have gone into the blood. All that remains
is indigestible food (fibre, or roughage), bacteria, and some
dead cells from the inside of the alimentary canal. This
mixture forms the faeces, which are passed out at intervals
through the anus. This process is called egestion.
A model of absorption
Why do the walls of the stomach secrete
hydrochloric acid?
B5.16
Which two digestive juices are secreted into the
small intestine?
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B5.15
How do bile salts help in digestion?
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B5.17
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Name two parts of the alimentary canal where
amylase is secreted. What does it do?
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ACtivity B5.03
B5.14
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QuEStiONS
ni
ve
rs
C
w
ie
ev
R
Ater they have been absorbed into the blood in the villi,
the nutrients are taken to the liver. The liver processes
some of them, before they go any further. Some of
these nutrients can be broken down, some converted
into other substances, some stored and the remainder
let unchanged.
The nutrients, dissolved in the blood plasma, are then
taken to other parts of the body where they may become
assimilated as part of a cell.
U
R
Not all the food that is eaten can be digested, and this
undigested food cannot be absorbed in the small intestine.
It travels on, through the caecum, past the appendix
and into the first part of the large intestine, the colon.
In humans, the caecum and appendix have no function.
In the colon, more water and salt are absorbed. However,
the colon absorbs much less water than the small intestine.
Assimilation
op
ni
ev
ve
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w
C
ity
op
Pr
y
es
s
Water, mineral salts and vitamins are also absorbed in the
small intestine. The small intestine absorbs between 5 and
10 dm3 of water each day.
Copyright Material - Review Only - Not for Redistribution
61
ve
rs
ity
Substrate
salivary
glands
amylase
starch
in pits in
wall of
stomach
pepsin
starch
maltose
trypsin
proteins
lipase
fats
ie
ev
-R
s
es
Pr
bile salts
emulsify fats
bile pigments
excretory
products
sucrase
sucrose
glucose and
fructose
lactase
lactose
glucose and
galactose
peptidase
polypeptides amino acids
lipase
fats
fatty acids
and glycerol
es
s
-R
ev
ie
w
C
glucose
op
y
rs
ity
fatty acids
and glycerol
maltose
-C
am
br
id
ge
U
R
sodium
reduces
hydrogencarbonate acidity of
chyme
polypeptides
maltase
ni
no juice
by cells
secreted;
covering
enzymes
the villi
remain in or
on the cells
covering
the villi
ve
w
ie
ev
ileum
none
acid
environment
for pepsin;
kills bacteria
in food
y
amylase
br
am
-C
liver,
stored
in gall
bladder
hydrochloric acid
C
op
curdled milk
protein
y
op
bile
C
62
polypeptides
rennin
milk protein
(only in
young
mammals)
ni
pancreas
id
pancreatic
juice
ge
U
R
duodenum
Functions
of other
substances
-R
proteins
w
w
ev
ie
Pr
es
s
gastric
juice
C
op
y
stomach
Other substances
in juice
maltose
ve
rs
ity
oesophagus none
Product
w
Enzymes
in juice
ev
ie
Where
made
ge
saliva
-C
mouth
Juices
secreted
am
br
id
Part of the
canal
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
C
ity
Pr
op
y
All of the digestive juices contain water and mucus. The water is used for the digestion of large molecules to small ones.
It is also a solvent for the nutrients and enzymes. Mucus acts as a lubricant. It also forms a covering over the inner
surface of the alimentary canal, preventing enzymes from digesting the cells.
y
op
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
R
ev
ie
w
ni
ve
rs
Table B5.06 Summary of digestion in the human alimentary canal.
Copyright Material - Review Only - Not for Redistribution
ve
rs
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ev
ie
am
br
id
Summary
w
ge
C
U
ni
op
y
B5: Animal nutrition
■
■
■
C
op
y
the functions of amylase, protease and lipase
the structure and functions of the alimentary canal
and other organs of the digestive system
the diferent types of teeth, and how to care for them
the structure of teeth
the causes of dental decay.
ni
w
Match each function described below with one or more of the nutrients in the list. Some of the
nutrients will match up with more than one function, and some of the functions will match up to
more than one nutrient.
ie
a
fibre
calcium
iron
es
water
Pr
make bones strong
provides energy
used to make enzymes, haemoglobin and hair
keeps the muscles in the alimentary canal working well
used to make haemoglobin
dissolves substances in cells, so that metabolic reactions can take place
helps calcium to be absorbed from the digestive system
C
U
ni
op
ve
rs
ity
63
For each of the nutrients listed in part a, state one food that is a good source of this nutrient.
w
digestion, absorption
small intestine, large intestine
enamel, dentine
bile, pancreatic juice
-R
s
es
Pr
ity
y
op
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
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ev
ie
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ni
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C
op
y
-C
a
b
c
d
ev
id
ie
With the aid of examples wherever possible, explain the diferences between each of the following
pairs of terms.
br
2
vitamin C
ge
b
i
ii
iii
iv
v
vi
vii
protein
am
R
ev
ie
w
C
op
y
vitamin D
fat
-R
-C
carbohydrate
s
am
br
ev
id
1
ge
U
R
End-of-chapter questions
y
ev
ie
w
C
■
■
ve
rs
ity
op
y
■
■
Pr
es
s
■
that balanced diets difer for diferent people
how poor diet can afect health, including starvation,
obesity and coronary heart disease
the causes and efects of protein-energy malnutrition
why food needs to be digested before it can
be absorbed
-C
■
-R
You should know:
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
ev
ie
-R
B
op
y
Pr
es
s
-C
A
w
C
ve
rs
ity
C
D
J
ev
ie
w
ge
The diagram below shows the human digestive system.
am
br
id
3
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
ie
w
ge
H
ev
br
Name each of the parts labelled A to J on the diagram.
Give the letters (not the name) of each of the following parts:
i two parts where amylase is secreted
ii two parts where protease is secreted
iii one part where lipase is secreted
iv the part where hydrochloric acid is secreted
v two parts where water is absorbed
vi the part where egestion takes place.
Pr
rs
C
ve
w
y
Copy and complete these sentences about digestion, using words from the list. You may use each
word once, more than once or not at all.
duodenum fatty
glycerol
hydrochloric
ileum
ingestion
large
pancreas
proteins
small
starch
trachea urinary
id
br
ie
oesophagus
C
carbohydrates
fats
w
amylase
ev
gall
U
amino
ge
acids
am
R
ni
op
ie
4
ev
ity
op
y
es
s
-C
-R
am
a
b
64
y
F
G
id
i
C
op
U
R
ni
E
mucus
-R
The teeth, lips and tongue help to take food into the mouth. This is called
es
, which
Pr
op
y
ity
The food travels down the
to the stomach. Here,
acid is secreted,
which provides ideal conditions for the enzyme pepsin to work. Pepsin is a protease, and begins the
digestion of
ni
ve
rs
C
w
ie
,
s
-C
The food is mixed with saliva from the salivary glands. Saliva contains the enzyme
which digests
to the sugar maltose. Saliva also contains
lubricates the chewed food making it easy to swallow.
y
op
id
g
ev
es
s
-R
br
Which two of these nutrients are organic substances?
Explain why none of these nutrients need to be digested before they are absorbed.
am
a
b
ie
Calcium, iron, vitamin C and vitamin D are nutrients required in small amounts in the diet.
-C
5
w
e
C
U
R
ev
Ater leaving the stomach, the food enters the
, which is the first part of the
intestine. Here, juices from the
and
bladder flow in.
They contain amylase, protease and lipase. Lipase digests fats to
and
Copyright Material - Review Only - Not for Redistribution
[1]
[2]
ve
rs
ity
w
ge
ev
ie
-R
Pr
es
s
A
y
C
w
ge
U
C
op
ni
ev
ie
w
C
ve
rs
ity
op
y
B
R
[3]
[4]
[6]
br
ev
id
ie
Name the teeth labelled A, B and C.
Outline the functions of tooth A and tooth C.
Draw and label a diagram to show the internal structure of the tooth labelled C.
-R
am
a
b
c
65
C
ity
op
Pr
glass rod
es
s
-C
Two experiments were set up as in the diagram below. The apparatus is a model of the digestion
and absorption of starch in the alimentary canal.
y
7
[1]
[2]
[3]
The diagram shows the teeth in the upper jaw of a human.
-C
6
Name two foods that contain calcium.
Describe the role of vitamin D in the body.
Describe and explain the deficiency symptoms of a lack of iron in the diet.
am
br
id
c
d
e
C
U
ni
op
y
B5: Animal nutrition
ve
ie
w
rs
visking tubing
y
op
C
starch and amylase
w
ge
U
R
ni
ev
water
br
ev
id
ie
The piece of visking tubing represents the alimentary canal. The visking tubing is a selectively permeable
membrane, which allows smaller molecules to pass through it but not larger molecules.
-R
am
In experiment 1, the liquid inside the visking tubing was starch solution.
s
-C
In experiment 2, this liquid was a mixture of starch solution and amylase solution.
Pr
e
es
y
op
s
-R
id
g
br
am
-C
blue
brown
blue
brown
red
brown
red
Conclusion
C
U
R
2
Colour obtained
w
Test
iodine test
Benedict’s test
water in the beaker iodine test
Benedict’s test
inside visking
iodine test
tubing
Benedict’s test
water in the beaker iodine test
Benedict’s test
ie
ity
Liquid tested
inside visking
tubing
ni
ve
rs
Experiment
1
ev
ie
w
C
The results are shown in the table.
ev
op
y
es
Both experiments were let for one hour. Ater this time, the liquid inside the visking tubing and the water
in the beaker were each tested with iodine solution and with Benedict’s solution.
Copyright Material - Review Only - Not for Redistribution
(continued)
ve
rs
ity
w
ge
ev
ie
i Copy the table, and then fill in the colour that was obtained for the top row.
ii In the top row of the last column of the table, state what can be concluded from this result.
iii Fill in the remainder of the last column of the table, stating in each case whether starch or
sugar is present or absent.
i Explain what the results from experiment 2 tell you about the efect of amylase on
starch molecules.
ii What do the results from experiment 2 tell you about the ability of sugar molecules to pass
through visking tubing? Explain your answer.
For the experiment shown in the diagram:
i state which part of the alimentary canal is represented by the visking tubing
ii state what the beaker of water represents.
Using the results of these experiments, explain why starch needs to be digested in the
alimentary canal.
U
R
w
ge
a
b
c
[2]
[1]
[1]
[1]
[2]
[1]
s
es
y
Pr
ity
rs
op
amount of
starch
remaining
in the
test-tube
op
y
ve
ni
ev
w
2
3
4
5
6
7
8
ev
1
ie
id
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C
U
R
time / minutes
-R
am
br
0
[1]
[1]
ity
Pr
op
y
es
s
-C
i State the time at which the rate of digestion of starch is fastest.
ii Name the product of the digestion of starch in this test-tube.
iii Make a copy of the graph. On the same axes, sketch a graph to show the result that would
be expected if the experiment is repeated at 25 °C.
Explain why starch is digested by the body.
C
[2]
[1]
y
op
-R
s
-C
am
br
ev
ie
id
g
w
e
C
U
R
ev
ie
w
ni
ve
rs
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q4 November 2014]
es
d
[1]
-R
am
br
ev
id
ie
Define the term digestion.
Name the enzyme that digests starch in the alimentary canal.
In an experiment, a starch-digesting enzyme was added to a starch suspension in a test-tube at 35 °C.
The graph shows how the amount of starch remaining in the test-tube changed over the next
eight minutes.
ie
w
C
66
[2]
[Cambridge IGCSE Combined Science 0653 Paper 61 Q1 November 2013]
-C
8
[1]
[1]
y
ni
C
op
ev
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d
ve
rs
ity
c
w
C
op
y
Pr
es
s
-C
-R
am
br
id
a
b
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
op
y
ve
rs
ity
ni
C
U
ev
ie
w
ge
-R
am
br
id
Pr
es
s
-C
y
ni
C
op
y
ve
rs
ity
op
C
w
ev
ie
-R
ity
rs
transpiration
the efects of temperature and humidity on
transpiration rate
how sucrose and amino acids are transported
through a plant.
■
C
op
y
ve
■
br
ev
id
ie
w
ge
R
■
■
ni
ev
■
why plants need transport systems
the position and function of xylem
how plants absorb and transport water
how the surface area of root hairs increases the rate of
absorption of water and ions
U
■
s
Pr
y
op
ie
w
C
This chapter covers:
■
67
es
-C
am
br
ev
id
ie
w
ge
U
R
B6
Transport in plants
es
Pr
op
y
Xylem
ity
A xylem vessel is like a long drainpipe (Image B6.01 and
Figure B6.01). It is made of many hollow, dead cells, joined
end to end. The end walls of the cells have disappeared,
so a long, open tube is formed. Xylem vessels run from the
roots of the plant, right up through the stem. They branch
out into every leaf.
ni
ve
rs
C
Xylem vessels contain no cytoplasm or nuclei. Their walls
are made of cellulose and lignin. Lignin is very strong, so
xylem vessels help to keep plants upright. Wood is made
almost entirely of lignified xylem vessels.
-R
s
es
am
br
ev
ie
id
g
Water, though, comes from further away. Plants absorb
water through their roots, and this water must be
transported up to the leaves. The transport system that
does this is made up of xylem.
w
e
C
U
op
y
Plants have branching shapes. This gives them a
large surface area in relation to their volume. It means
that most cells are close to the surface. Carbon dioxide
can easily reach them by difusion from the air.
-C
w
ie
ev
s
-C
All organisms need to obtain various substances
from their environment. For plants, these substances
are carbon dioxide and water for photosynthesis,
and mineral ions, which they absorb from
the ground.
R
Plants also have a second transport system, made up of
phloem. Phloem transports sucrose and amino acids from
the leaves where they are made, to other parts of the plant
such as its roots and flowers.
-R
am
B6.01 Plant transport
systems
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
y
C
w
C
op
w
ie
id
ev
Vascular bundles
br
transverse section
ge
U
ni
ev
ie
R
Like xylem vessels, phloem tubes are made of many
cells joined end to end. However, their end walls have not
completely broken down. The cells are called sieve tube
elements. Sieve tube elements contain cytoplasm,
but no nucleus. They do not have lignin in their cell walls.
The cells in phloem are still alive, whereas those in
xylem are hollow and dead.
ve
rs
ity
op
y
Phloem
Image B6.01 This is a scanning electron micrograph of
xylem vessels (× 1800).
ity
op
Pr
y
es
s
-C
-R
am
Xylem vessels and phloem tubes are usually found close
together. A group of xylem vessels and phloem tubes is
called a vascular bundle.
op
endodermis
cortex
phloem
w
ni
ve
rs
C
ity
thick cell wall,
containing lignin
Pr
op
y
es
s
-C
-R
am
br
ev
id
ie
w
ge
C
U
R
ni
ev
Longitudinal section
The positions of vascular bundles in roots and shoots are
shown in Figures B6.02 and B6.03. In a root, vascular tissue
is found at the centre, whereas in a shoot vascular bundles
are arranged in a ring near the outside edge. Vascular
bundles are also found in leaves (Figure B4.01). They help
to support the plant.
y
ve
ie
w
rs
C
68
identify the positions of xylem vessels in roots,
stems and leaves
Pr
es
s
-C
-R
am
br
id
ev
ie
w
ACtivity B6.01
y
w
cambium
ie
ev
-R
stele
Figure B6.02 Transverse section of a root.
s
-C
gap where end wall of
cell has been lost
es
am
br
id
g
e
C
U
R
space, containing
no cytoplasm
Figure B6.01 Xylem vessels.
xylem
op
ev
ie
thin area of cell wall,
which is called a pit
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
vascular
bundle
xylem
w
5 Use a razor blade to cut across the stem of the plant
about half-way up. Take great care when using a razor
blade and do not touch its edges.
am
br
id
ev
ie
ge
cortex
phloem
C
U
ni
op
y
B6: Transport in plants
y
C
op
U
R
ge
A2 Why was it important to wash the roots of the plant:
ie
id
b before cutting sections?
ev
br
am
B6.03
What is a vascular bundle?
Pr
What do phloem tubes carry?
ity
B6.02
y
op
ie
id
to see which part of a stem transports water
and solutes
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
AO3.5 Evaluating methods
! Take care with the sharp blade when cutting the
stem sections.
1 Take a plant, such as Impatiens, with a root system
intact. Wash the roots thoroughly.
2 Put the roots of the plant into eosin solution.
Leave overnight.
3 Set up a microscope.
4 Remove the plant from the eosin solution, and
wash the roots thoroughly.
w
ge
C
U
ni
ev
ve
rs
What do xylem vessels carry?
ie
B6.01
A3 Design an experiment to investigate the efect of one
factor (for example, light intensity, temperature,
wind speed) on the rate at which the dye is
transported up the stem. Remember to write down
your hypothesis, and to think about variables.
When you have completed your plan, ask your
teacher to check it for you. Then carry out your
experiment and record and display your results.
Write down your conclusions, and discuss them in
the light of your knowledge about transport in plants.
You should also evaluate the reliability of your results
and suggest how to improve your experiment if you
were able to do it again.
-R
es
s
-C
C
op
y
QuEStiONS
w
w
a before putting it into the eosin solution?
epidermis
Figure B6.03 Transverse section of a stem.
R
8 Observe the section under a microscope. Make a
labelled drawing of your section.
A1 Which part of the stem contained the dye? What does
this tell you about the transport of water and solutes
(substances dissolved in water) up a stem?
cambium
ACtivity B6.02
7 Choose your thinnest section, and mount it in a drop of
water on a microscope slide. Cover with a coverslip.
Questions
ni
ev
ie
w
C
ve
rs
ity
op
y
Pr
es
s
-C
-R
6 Now cut very thin sections across the stem. Try to get
them so thin that you can see through them. It does
not matter if your section is not a complete circle.
-R
s
ni
ve
rs
ity
Pr
op
y
Image B6.02 shows the end of a root, magnified.
At the very tip is a root cap. This is a layer of cells
which protects the root as it grows through
the soil. The rest of the root is covered by a layer of
cells called the epidermis.
w
e
C
U
op
y
The root hairs are a little way up from the root tip. Each
root hair is a long epidermal cell (Images B6.02 and B6.03).
Root hairs do not live for very long. As the root grows,
they are replaced by new ones.
ev
ie
id
g
Root hairs are very tiny, but there is a very large number of
them. This means that they have a very large surface area,
which increases the rate at which they can absorb water
and ions.
es
s
-R
br
am
-C
C
w
ie
ev
R
Plants roots have root hairs. The function of root hairs is
to absorb water and mineral ions from the soil.
es
-C
am
br
ev
B6.02 Water uptake
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69
ve
rs
ity
ev
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op
y
Pr
es
s
-C
-R
am
br
id
root hairs
w
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
y
C
op
Image B6.03 Part of a transverse section across a root,
showing root hairs (× 100).
ni
ev
ie
w
C
ve
rs
ity
epidermis
br
am
rs
C
ve
w
y
op
C
w
ge
U
ni
ie
ev
R
The root hairs are on the edge of the root. The xylem
vessels are in the centre. Before the water can be taken
to the rest of the plant, it must travel to these xylem vessels.
-R
am
br
ev
id
ie
Water passes across the root, from
cell to cell, by osmosis. It also seeps
between the cells.
epidermis
of root
Water is drawn up the xylem
vessels, because transpiration is
constantly removing water from
the top of them.
y
op
cortex of root
es
s
-R
ev
film of water
Figure B6.04 How water is absorbed by a plant.
-C
ie
id
g
br
am
soil particle
w
e
root
hair
C
U
ev
ie
w
ni
ve
rs
C
ity
Pr
op
y
es
s
-C
Water enters the root
hairs by osmosis.
R
Once water reaches the xylem, it moves up the xylem
vessels in the same way that a drink moves up a straw
when you suck it. When you suck a straw, you are reducing
the pressure at the top of the straw. The liquid at the
bottom of the straw is at a higher pressure, so it flows up
the straw into your mouth.
ity
op
Pr
y
es
Water moves into a root hair by osmosis. The cytoplasm
and cell sap inside it are quite concentrated solutions. The
water in the soil is normally a more dilute solution. Water
therefore difuses into the root hair, down its concentration
gradient, through the partially permeable cell surface
membrane (Section B2.02).
s
-C
-R
Image B6.02 A root tip (× 70).
70
The path it takes is shown in Figure B6.04. It travels by
osmosis through the cortex, from cell to cell. Some of it
may also just seep through the spaces between the cells,
or through the cell walls, never actually entering a cell at
all. Eventually it reaches the xylem vessels in the middle
of the root. These transport it all the way up through the
stem and into the leaves.
ev
id
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w
ge
U
R
root cap
Copyright Material - Review Only - Not for Redistribution
xylem vessel
ve
rs
ity
The same thing happens with the water in xylem vessels.
The pressure at the top of the vessels is lowered, while the
pressure at the bottom stays high. Water therefore flows
up the xylem vessels.
op
y
B6.03 transpiration
w
ve
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Transpiration is the loss of water vapour from a plant. Most
of this loss takes place from the leaves.
C
You can think of the way that water moves into a root hair,
across to the xylem vessels, up to the leaves and then out
into the air in terms of water potential.
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Water containing
dissolved minerals
moves up the root
and stem in the
xylem vessels.
Water evaporates
from the leaves.
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• The stomata, when open, allow water vapour to difuse
easily out of the leaf. This reduces the water potential
inside the leaf, which encourages more water to
evaporate from the surfaces of the mesophyll cells.
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Water enters
root hairs by
osmosis.
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• The hollow, narrow xylem vessels provide an easy
pathway for water to flow all the way up from the roots
to the very top of the plant.
• The many air spaces inside the leaf mean that there
is a large surface area of wet cells from which water
can evaporate into the air. This increases the rate of
evaporation, drawing more water out of the xylem and
speeding up the flow of water up the plant.
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Figure B6.05 The transpiration stream.
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• The root hair cells provide a huge surface area through
which water can be absorbed. This increases the quantity
of water that can move into the plant at any one moment.
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We can now see how well the structure of a plant is adapted
to help it to take up water and move it up through the plant.
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The low water potential in the leaves is caused by the loss
of water vapour from the leaves by transpiration. This
produces a ‘pull’ from above, drawing water up the plant.
Water molecules have a strong tendency to stick together.
This is called cohesion. When the water is ‘pulled’ up the
xylem vessels, the whole column of water stays together.
Without cohesion, the water column would break apart
and the whole system would not work.
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The movement of water from the roots, up through the
xylem vessels, to the mesophyll cells and then out through
the stomata, is known as the transpiration stream
(Figure B6.05).
R
You may remember that water moves down a water
potential gradient, from a high water potential to a low water
potential (Section B2.02). All along this pathway, the water is
moving down a water potential gradient from one place to
another. The highest water potential is in the solution in the
soil, and the lowest water potential is in the air.
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Some of this film of moisture evaporates from the
cells, and this water vapour difuses out of the leaf
through the stomata. Water from the xylem vessels in
the leaf will travel to the mesophyll cells by osmosis to
replace it.
The constant loss of water from the leaves reduces the
efective pressure at the top of the xylem vessels, so
that water flows up them. So it is transpiration that is
ultimately responsible for causing water to move up
through a plant.
y
If you look back at Figure B4.03, you will see that there are
openings on the surface of the leaf called stomata. There
are usually more stomata on the underside of the leaf, in
the lower epidermis. The mesophyll cells inside the leaf are
each covered with a thin film of moisture.
Water potential gradient
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How is the pressure at the top of the xylem vessels
reduced? It happens because of transpiration.
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transpiration: loss of water vapour from plant leaves
by evaporation of water at the surfaces of the mesophyll
cells followed by difusion of water vapour through the stomata
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B6: Transport in plants
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Measuring transpiration rates
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Figure B6.06 illustrates apparatus which can be used to
compare the rate of transpiration in diferent conditions. It
is called a potometer. By recording how fast the air/water
meniscus moves along the capillary tube you can compare
how fast the plant takes up water in diferent conditions.
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op
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There are many diferent kinds of potometer, so yours may
not look like this. The simplest kind is just a long glass tube
which you can fill with water. A piece of rubber tubing slid
over one end allows you to fix the cut end of a shoot into it,
making an airtight connection. This works just as well as the
one in Figure B6.06, but is much harder to refill with water.
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Questions
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screw clip
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capillary
tube
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Figure B6.06 A potometer.
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airtight seal
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reservoir
containing
water
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transpiring
branch of the
plant, drawing up
water from the
potometer
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Transpiration is increased by high temperatures and
low humidity.
temperature
On a hot day, water will evaporate quickly from the leaves of
a plant. Transpiration increases as temperature increases.
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A3 Why is it important to use forceps, not fingers, for
handling cobalt chloride paper?
Conditions that afect transpiration rate
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A2 Why does this surface lose water faster than the other?
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A1 Which piece of cobalt chloride paper turned pink first?
What does this tell you about the loss of water
from a leaf?
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It is not easy to measure how much water is lost from the
leaves of a plant. It is much easier to measure how fast the
plant takes up water. The rate at which a plant takes up
water depends on the rate of transpiration – the faster a
plant transpires, the faster it takes up water.
Pr
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to see which surface of a leaf loses most water
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.4 Interpreting and evaluating observations
and data
Cobalt chloride paper is blue when dry and pink when wet.
Use forceps to handle it.
1 Use a healthy, well-watered potted plant, with leaves
which are not too hairy. Fix a small square of blue
cobalt chloride paper onto each surface of one leaf,
using clear sticky tape. Make sure there are no air
spaces around the paper.
2 Leave the paper on the leaf for a few minutes.
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ACtivity B6.03
Copyright Material - Review Only - Not for Redistribution
air/water
meniscus
ruler
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What is a potometer used for?
B6.08
Explain how a temperature, and b humidity afect
the rate of transpiration.
y
using a potometer to compare rates of transpiration
under diferent conditions
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
1 Set up the potometer as in Figure B6.06. The stem of
the plant must fit exactly into the rubber tubing, with
no air gaps. Petroleum jelly will help to make an
airtight seal.
2 Fill the apparatus with water, by opening the clip.
3 Close the clip again, and leave the apparatus in a light,
airy place. As the plant transpires, the water it loses is
replaced by water taken up the stem. Air will be drawn
in at the end of the capillary tube.
4 When the air/water meniscus reaches the scale,
begin to record the position of the meniscus every
two minutes.
5 When the meniscus reaches the end of the
scale, refill the apparatus with water from the
reservoir as before.
6 Now repeat the investigation, but with the apparatus in
a diferent situation. You could try each of these:
■ blowing it with a fan
■ putting it in a cupboard
■ putting it in a refrigerator
■ putting it into very dry or very moist air
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Questions
4 Record the mass of each plant every day, at the same
time, for at least a week.
op
y
A1 Under which conditions did the plant transpire a most
quickly, and b most slowly?
A2 You have been using the potometer to compare the
rate of uptake of water under diferent conditions.
Does this really give you a good measurement of the
rate of transpiration? Explain your answer.
U
Questions
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A1 Which plant lost mass? Why?
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A2 Do you think this is a good method of measuring
transpiration rate? How could it be improved?
am
What are stomata?
7 Draw graphs of your results.
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3 Place both plants on balances, and record
their masses.
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B6.06
B6.07
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What is transpiration?
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B6.05
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Explain how water goes into root hairs.
ACtivity B6.05
to measure the rate of transpiration of a
potted plant
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
1 Use two similar well-watered potted plants. Enclose
one plant entirely in a polythene bag, including its pot.
This is the control.
2 Enclose only the pot of the second plant in a polythene
bag. Fix the bag firmly around the stem of the plant, as
in the diagram, and seal with petroleum jelly.
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B6.04
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ACtivity B6.04
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QuEStiONS
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Humidity
Humidity means the moisture content of the air. The
higher the humidity, the less water will evaporate from the
leaves. This is because there is not much of a difusion
gradient for the water between the air spaces inside the
leaf, and the wet air outside it. Transpiration decreases as
humidity increases.
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B6: Transport in plants
Copyright Material - Review Only - Not for Redistribution
73
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B6.04 transport of manufactured
food
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Leaves make carbohydrates by photosynthesis. They also
use some of these carbohydrates to make amino acids,
proteins, oils and other organic substances.
translocation: the movement of sucrose and amino acids
in phloem, from regions of production (source) to regions
of storage, or to regions of utilisation in respiration or
growth (sink)
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Sources and sinks
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The part of a plant from which sucrose and amino acids
are being translocated is called a source. The part of the
plant to which they are being translocated is called a sink.
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Summary
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potometer
stoma
transpiration
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a long tube made of empty cells joined end to end
hard, strong tubes that help to support a plant
an extension from a cell near the tip of a root, which absorbs water from the soil
the loss of water vapour from the leaves of a plant
a small gap between the cells of the epidermis of a plant
a piece of apparatus used for measuring the rate at which a plant shoot takes up water
am
a
b
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f
xylem vessel
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root hair
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Match each of the following terms with its description. For some of the terms, there may be more than
one description that matches them.
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End-of-chapter questions
1
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about transpiration and the conditions that
afect its rate
how transpiration moves water up xylem vessels
the role of phloem tubes in translocation of sucrose
and amino acids
about sources and sinks.
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rs
■
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■
why plants need transport systems
where xylem and phloem are found in roots,
stems and leaves
how xylem vessels transport water and mineral ions
the adaptations of root hairs for rapid uptake of
water and ions
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You should know:
■
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When a plant is actively photosynthesising and growing,
the leaves are generally the major sources of translocated
material. They are constantly producing sucrose, which is
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Phloem can transfer sucrose in either direction – up or
down the plant. This is not true for the transport of water
in the xylem vessels. That can only go upwards, because
transpiration always happens at the leaf surface, and it is
this that provides the ‘pull’ to draw water up the plant.
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carried in the phloem to all other parts of the plant. These
parts – the sinks – include the roots and flowers. The roots
may change some of the sucrose to starch and store it.
The flowers use the sucrose to make fructose (an especially
sweet-tasting sugar found in nectar). Later, when the fruits
are developing, quite large amounts of sucrose may be used
to produce sweet, juicy fruits to attract animals.
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Some of the organic food material, especially sugar, that
the plant makes is transported in phloem tubes. It is carried
from the leaves to whichever part of the plant needs it. This
is called translocation. The sap inside the phloem tubes
therefore contains a lot of sugar, particularly sucrose.
74
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
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2
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B6: Transport in plants
root cortex cells
air spaces in leaf
root hairs
leaf mesophyll cells
Pr
es
s
Write these parts in the correct order, to describe the pathway of water through a plant.
For each part in your list, state whether the water is in the form of a liquid or a gas as it passes through it.
y
The diagrams show a transverse section of a stem, and a transverse section of a root.
y
C
op
Explain what is meant by the term transverse section.
Make a copy of the diagram that shows a transverse section of a stem. Label the xylem tissue.
Make a copy of the diagram that shows a transverse section of a root. Label the xylem tissue.
On your two diagrams, label the position of the phloem tissue.
s
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Pr
Most of the water taken up by plants replaces water lost in transpiration.
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A student used a potometer to investigate the efect of wind speed on the rate of water uptake by a
leafy shoot. As the shoot absorbs water the air bubble moves upwards.
Pr
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The student’s apparatus is shown in the diagram below.
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beaker of water
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air bubble
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coloured water
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capillary tube
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75
A potometer is a piece of apparatus that is used to measure water uptake by plants.
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4
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d
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stomata
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xylem
a
b
3
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The list below includes some of the parts of a plant through which water moves as it passes from the soil
into the air.
Copyright Material - Review Only - Not for Redistribution
(continued)
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Cambridge IGCSE Combined and Co-ordinated Sciences
35
8
40
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0.4
5
2.4
5
4.0
5
7.0
2
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Calculate the rate of water uptake at the highest wind speed.
Describe the efect of increasing wind speed on the rate of water uptake. You may use figures from
the table to support your answer.
State two environmental factors, other than wind speed, that the student should keep constant
during the investigation.
br
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The roots of most plants have root hairs near their tips.
Researchers grew two types of plants, A and B, in soil with diferent concentrations of phosphate ions.
They measured the mean number of root hairs in a small area of the roots, and also the mean length
of the root hairs.
The table shows their results.
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1.41
high
1.85
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[2]
[2]
[3]
[2]
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[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q3 b & c November 2013]
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b
225
Describe two ways in which the addition of phosphate ions to the soil afects the root hairs
in type A plants.
ii Compare the efect of adding phosphate ions to the soil for type A plants and for type B plants.
iii Explain why a reduction in the length of its root hairs could reduce the rate of growth of a plant.
Farmers oten add fertilisers containing phosphate ions, potassium ions and nitrate ions to the soil in
which they grow crops.
Explain why adding nitrate ions to the soil helps the crop plants to grow faster and larger.
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1.70
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175
low
id
B
1.26
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high
Mean length of root
hairs / micrometres
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low
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A
Mean number of root
hairs per unit area
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Type of plant Phosphate
concentration
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a
[2]
[Cambridge IGCSE Biology 0610 Paper 31 Q4 b, c & d June 2009]
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5
[2]
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[1]
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10
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20
6
Rate of water
uptake / mm
per minute
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4
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12
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2
a
b
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0
Distance travelled by Time /
the air bubble / mm
minutes
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Wind speed /
metres per
second
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The student used a fan with five diferent settings and measured the wind speed.
The results are shown in the table below.
Copyright Material - Review Only - Not for Redistribution
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The blood in the let-hand side of the heart has come
from the lungs. It contains oxygen, which was picked
up by the capillaries surrounding the alveoli. It is called
oxygenated blood.
This oxygenated blood is then sent around
the body. Some of the oxygen in it is taken up by
the body cells, which need oxygen for respiration
(Chapter B8). When this happens, the blood becomes
deoxygenated. The deoxygenated blood is
brought back to the right-hand side of the heart.
It then goes to the lungs, where it becomes
oxygenated again.
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Oxygenating the blood
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Figure B7.01 illustrates the general layout of the human
circulatory system. The arrows show the direction of blood
flow. If you follow the arrows, beginning at the lungs, you can
see that blood flows into the let-hand side of the heart, and
then out to the rest of the body. It is brought back to the righthand side of the heart, before going back to the lungs again.
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The main transport system of all mammals, including
humans, is the blood system, also known as the circulatory
system. It is a network of tubes, called blood vessels.
A pump, the heart, keeps blood flowing through the
vessels. Valves in the heart and blood vessels make sure
the blood flows in the right direction.
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B7.01 the circulatory system
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the double circulatory system and its advantages
the structure and function of the heart
how the atria, ventricles and valves function
how exercise afects the heart
coronary heart disease and possible risk factors
the structure and functions of arteries, veins and capillaries
what blood contains, and its functions in the body.
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This chapter covers:
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77
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B7
Transport in mammals
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Oxygenated blood
is carried to all the
cells in the body
from the left side
of the heart.
heart
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Figure B7.02 The circulatory system of a fish.
right side
of heart
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op
In a fish, though, the low-pressure blood just carries
on around the fish’s body. This means that blood
travels much more slowly to a fish’s body organs than it
does in a mammal.
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Deoxygenated
blood is returned to the
right side of the heart.
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Oxygen diffuses from the
blood to the body cells.
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The circulatory system shown in Figure B7.01 is a double
circulatory system. This means that the blood passes
through the heart twice on one complete circuit of the
body. We can think of the circulatory system being made
up of two parts – the blood vessels that take the blood
to the lungs and back, called the pulmonary system,
and the blood vessels that take the blood to the rest of
the body and back, called the systemic system. Double
circulatory systems are found in all mammals, and also in
birds and reptiles. However, fish have a circulatory system
in which the blood passes through the heart only once on
a complete circuit. This is called a single circulatory system
and is shown in Figure B7.02.
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B7.02 the heart
The function of the heart is to pump blood around the
body. It is made of a special type of muscle called cardiac
muscle. This muscle contracts and relaxes regularly,
throughout life.
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Figure B7.03 is a section through a heart. It is divided into
four chambers. The two upper chambers are called atria.
The two lower chambers are ventricles. The chambers on
the let-hand side are completely separated from the ones
on the right-hand side by a septum.
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This is particularly important when you think about the
delivery of oxygen for respiration. Any tissues that are
metabolically very active need a lot of oxygen delivered to
them as quickly as possible, and this delivery is much more
efective in a mammal than in a fish.
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The double circulatory system
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Double circulatory systems have some advantages
over single circulatory systems. When blood flows
through the tiny blood vessels in a fish’s gills, or a
mammal’s lungs, it loses a lot of the pressure that
was given to it by the pumping of the heart. In a mammal,
this low-pressure blood is delivered back to the heart,
which raises its pressure again before sending it of to
the rest of the body.
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left side
of heart
Figure B7.01 The general layout of the circulatory system
of a human, as seen from the front.
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Oxygen diffuses from the
blood to the body cells.
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Deoxygenated
blood is carried
to the lungs.
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Oxygen diffuses
into the blood.
78
Oxygen diffuses into
the blood from the gills.
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alveolus in the lung
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let ventricle
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tendon
supporting valve
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vena cava
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from body
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one-way valve
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right atrium
let atrium
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to body
pulmonary
vein
from lungs
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vena cava
one-way valve
aorta
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from head
one-way valve
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w
to head
pulmonary
to lungs
artery
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B7: Transport in mammals
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septum
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right ventricle
79
If you look at Figures B7.01 and B7.03, you will see that
blood flows into the heart at the top, into the atria. Both of
the atria receive blood. The let atrium receives blood from
the pulmonary veins, which come from the lungs. The right
atrium receives blood from the rest of the body, arriving
through the venae cavae (singular: vena cava).
rs
The let ventricle, however, pumps blood all around the
body. The let ventricle has an especially thick wall of
muscle to enable it to do this. The blood flowing to the
lungs in the pulmonary artery has a much lower pressure
than the blood in the aorta.
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op
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ACtivity B7.01
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Describe the human circulatory system, using the
words blood vessels, pump and valves.
B7.02
What is oxygenated blood?
B7.03
Where does blood become oxygenated?
B7.04
Which side of the heart contains
oxygenated blood?
B7.05
Explain the diference between a double
circulatory system and a single circulatory system.
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B7.01
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What are the advantages of a double
circulatory system?
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B7.06
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There is also a diference in the thickness of the walls of
the right and let ventricles. The right ventricle pumps
blood to the lungs, which are very close to the heart.
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The function of the ventricles is quite diferent from the
function of the atria. The atria simply receive blood, from
either the lungs or the body, and supply it to the ventricles.
The ventricles pump blood out of the heart to other parts
of the body. To help them do this, the ventricles have much
thicker, more muscular walls than the atria.
QuEStiONS
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Dissecting a heart
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From the atria, the blood flows into the ventricles.
The ventricles then pump it out of the heart. They do this
by contracting the muscle in their walls. The strong cardiac
muscle contracts with considerable force, squeezing
inwards on the blood inside the heart and pushing it out.
The blood in the let ventricle is pumped into the aorta,
which takes the blood around the body. The right ventricle
pumps blood into the pulmonary artery, which takes it to
the lungs.
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Figure B7.03 Vertical section through a human heart.
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Cambridge IGCSE Combined and Co-ordinated Sciences
B7.08
Where are the one-way valves found in the heart?
B7.09
Which parts of the heart pump blood into
a the pulmonary artery, and b the aorta?
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Coronary heart disease
• Smoking cigarettes Several components of cigarette
smoke, including nicotine, cause damage to the
circulatory system. Stopping smoking is the single most
important thing a smoker can do in order to reduce
their chances of getting coronary heart disease.
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C
op
Blockage of the coronary arteries is called coronary heart
disease (CHD). It is a very common cause of illness and
death, especially in developed countries. We know several
factors that increase a person’s risk of getting coronary
heart disease.
Why does the let ventricle have a thicker wall than
the right ventricle?
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In Figure B7.04, you can see that there are blood
vessels on the outside of the heart. They are called the
coronary arteries. These vessels supply blood to the
heart muscles.
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Why do the ventricles have thicker walls
than the atria?
B7.11
R
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Which parts of the heart receive blood from
a the lungs, and b the body?
B7.10
If a coronary artery gets blocked – for example, by a
blood clot – the cardiac muscles run short of oxygen.
They cannot respire, so they cannot obtain energy to allow
them to contract. The heart therefore stops beating.
This is called a heart attack or cardiac arrest.
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B7.07
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aorta
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pulmonary
artery
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vena cava
from head
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• Diet There is evidence that a diet high in salt,
saturated fats (fats from animals) or cholesterol
increases the chances of getting coronary heart disease.
To reduce the risk, it is good to eat a diet containing a
very wide variety of foods, with not too many fats in it
(though we do need some fat in the diet to stay healthy).
Oils from plants and fish, on the other hand, can help to
prevent heart disease.
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It may seem odd that this is necessary, when the heart
is full of blood. However, the muscles of the heart are so
thick that the nutrients and oxygen in the blood inside
the heart would not be able to difuse to all the muscles
quickly enough. The heart muscle needs a constant supply
of nutrients and oxygen, so that it can keep contracting
and relaxing. The coronary arteries supply this.
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pulmonary
veins
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vena cava
from body
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muscle of
let ventricle
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Figure B7.04 External appearance of a human heart.
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coronary
artery
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B7: Transport in mammals
• Obesity Being very overweight increases the risk of
coronary heart disease. Keeping your body weight at
a suitable level, and taking plenty of exercise, helps to
maintain the coronary arteries in a healthy condition.
y
• Genes Some people have genes that make it more
likely they will get coronary heart disease. We say that
they have a genetic predisposition towards it. There is
not really anything you can do about this. However, if
several people in your family have had problems with
their hearts, then this could mean that you have these
genes. In that case, it is important to try hard to reduce
the other risk factors by having a healthy life-style.
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Valves in the heart
We have seen that the heart beats as the cardiac muscles
in its walls contract and relax. When they contract, the
heart becomes smaller, squeezing blood out. When they
relax, the heart becomes larger, allowing blood to flow into
the atria and ventricles.
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A good way to measure the rate of your heart beat is to
take your pulse rate. A pulse is caused by the expansion
and relaxation of an artery, caused by the heart pushing
blood through it. Your pulse rate is therefore the same as
your heart rate. You can find a pulse wherever there is an
artery fairly near to the surface of the skin. Two suitable
places are inside your wrist, and just to the side of the big
tendons in your neck.
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Pr
Explain why your pulse rate is the same as
your heart rate.
B7.13
Why does your heart need to beat faster
when you do exercise?
B7.14
Where and what is the pacemaker?
B7.15
Explain what makes your heart beat faster
when you exercise.
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B7.12
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The rate at which the heart beats is controlled by a patch
of muscle in the right atrium called the pacemaker.
The pacemaker sends electrical signals through the walls
of the heart at regular intervals, which make the muscle
contract. The pacemaker’s rate, and therefore the rate of
heart beat, changes according to the needs of the body.
For example, during exercise, when extra oxygen is needed
QuEStiONS
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Describe and explain the action of
the atrioventricular valves when the
ventricles contract.
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B7.16
op
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When a person exercises, their heart beats faster. This is
because their muscles are using up oxygen more quickly
in respiration, to supply the energy needed for movement.
A faster heart rate means faster delivery of blood to the
muscles, providing oxygen.
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to find the efect of exercise on the rate of
heart beat
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ACtivity B7.02
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There are one-way valves between the let
atrium and ventricle and between the right atrium
and ventricle (Figure B7.05). These valves are called
atrioventricular valves.
The function of these valves is to stop blood flowing from
the ventricles back to the atria. This is important, so that
when the ventricles contract, the blood is pushed up into
the arteries, not back into the atria. When the ventricles
contract, the pressure of the blood pushes the valves
upwards. The tendons attached to them stop them from
going up too far.
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You may be able to feel your heart beating if you put your
hand on your chest. Most people’s hearts beat about
60 to 75 times a minute when they are resting. If you put
your head against a friend’s chest, or use a stethoscope,
you can also hear the sounds of the valves closing with
each heart beat. They sound rather like ‘lub-dup’. Each
complete ‘lub-dup’ represents one heart beat.
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Heart beat
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• Stress We all need some stress in our lives, or they
would be very dull. However, unmanageable or longterm stress appears to increase the risk of developing
heart disease. Avoiding severe or long-term stress
is a good idea, if you can manage it. Otherwise, it is
important to find ways to manage stress.
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The signal for this is an increase in the pH of the blood.
During exercise, muscles respire more quickly than usual,
in order to release the energy needed for movement.
This increase in respiration rate means that more carbon
dioxide is produced, and this dissolves in the blood.
A weak acid is formed, lowering the pH of the blood.
Receptor cells in the brain sense this drop in pH, and this
triggers an increase in the frequency of the nerve impulse
sent to the pacemaker.
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by the muscles, the brain sends impulses along nerves to
the pacemaker, to make the heart beat faster.
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81
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B7.03 Blood vessels
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s
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Do not say that capillaries are one cell thick. It is their
walls that are one cell thick.
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Veins
The capillaries gradually join up again to form veins. By
the time the blood gets to the veins, it is at a much lower
pressure than it was in the arteries. The blood flows more
slowly and smoothly now. There is no need for veins to
have such thick, strong, elastic walls.
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The blood does not flow smoothly through the arteries.
It pulses through, as the ventricles contract and relax.
The arteries have elastic tissue in their walls which can
stretch and recoil with the force of the blood. This helps to
make the flow of blood smoother. You can feel your arteries
stretch and recoil when you feel your pulse in your wrist.
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If the veins were narrow, this would slow down the blood
even more. To help keep the blood moving easily through
them, the space inside the veins, called the lumen, is much
wider than the lumen of the arteries.
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Capillaries
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The arteries gradually divide to form smaller and
smaller vessels (Figure B7.07 and Image B7.01).
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TIP
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These are the capillaries, which are very small and
penetrate to every part of the body. No cell is very far
away from a capillary.
The function of the capillaries is to take nutrients,
oxygen and other materials to all the cells in the body,
and to take away waste materials. To do this, their walls
must be very thin so that substances can get in and
out easily. The walls of the smallest capillaries are only
one cell thick.
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There are three main kinds of blood vessels: arteries,
capillaries and veins (Figure B7.06). Arteries carry
blood away from the heart. They divide again and again,
and eventually form very tiny vessels called capillaries.
The capillaries gradually join up with one another to form
large vessels called veins. Veins carry blood towards the
heart. These vessels are compared in Table B7.01.
When blood flows out of the heart, it enters the arteries.
The blood is then at very high pressure, because it has
been forced out of the heart by the contraction of the
muscular ventricles. Arteries therefore need very strong
walls to withstand the high pressure of the blood flowing
through them.
R
ventricular contraction: the muscles of the
atria relax. The muscles of the ventricles
contract. Blood is forced out of the
ventricles into the arteries.
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Figure B7.05 How the heart pumps blood.
Arteries
The muscles of
the ventricles
contract, forcing
blood out of the
ventricles.
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Atrial contraction: the muscles of the atria
contract. The muscles of the ventricles
remain relaxed. Blood is forced from the
atria into the ventricles.
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The
atrioventricular
valves open.
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op
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op
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The muscles of
the atria contract,
squeezing the blood
into the ventricles.
The muscles of
the atria relax,
allowing blood to
flow into the heart
from the veins.
Relaxation: all muscles are relaxed.
Blood flows into the heart.
82
The semilunar valves
are forced open by the
pressure of the blood.
The atrioventricular
valves are forced shut
by the pressure of the
blood.
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The semilunar valves
remain shut.
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The semilunar valves shut, preventing
blood from flowing into the ventricles.
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The valves in the veins
are forced shut by the
pressure of the blood,
stopping the blood from
flowing back into the
veins.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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thick outer wall
smooth lining
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pressure from
body muscles
open valve
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smooth lining
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thin layer
of muscles
and elastic
fibres
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large
lumen
closed valve
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wall made of a
single layer of cells
A vein
fairly thin
outer wall
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A capillary
very small lumen
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Image B7.01 A capillary, shown in blue, snakes its way
through muscle tissue (× 600).
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thick layer of muscles
and elastic fibres
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small
lumen
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An artery
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B7: Transport in mammals
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Figure B7.06 Sections through three types of
blood vessels.
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Figure B7.08 Valves in a vein: the valves are like pockets set
in the wall of the vein.
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artery
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arteriole (small artery)
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vein
Naming blood vessels
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capillary network
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Figures B7.09 and B7.10 illustrate the positions of the main
arteries and veins in the body.
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Figure B7.07 A capillary network.
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Veins have valves in them to stop the blood flowing
backwards (Figure B7.08). Valves are not needed in the
arteries, because the force of the heart beat keeps blood
moving forwards through them.
Blood is also kept moving in the veins by the contraction
of skeletal muscles around them. The large veins in your
legs are squeezed by your leg muscles when you walk. This
helps to push the blood back up to your heart.
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venule (small vein)
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pressure from
body muscles
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83
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Arteries
carry blood
away from
the heart
Capillaries
supply all
cells with their
requirements,
and take away
waste products
very thin, only one
cell thick
return blood to
the heart
quite thin,
containing far less
muscle and elastic
tissue than arteries
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no need for strong walls, as most of the
blood pressure has been lost; thin walls
and narrow lumen bring blood into
close contact with body tissues
wide; contains valves
no need for strong walls, as most of
the blood pressure has been lost; wide
lumen ofers less resistance to blood
flow; valves prevent backflow
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very narrow, just wide
enough for a red blood
cell to pass through
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Table B7.01 Arteries, veins and capillaries.
pulmonary
artery
Why do arteries have elastic walls?
B7.20
What is the function of capillaries?
B7.21
Why do veins have a large lumen?
B7.22
How is blood kept moving in the large veins of
the legs?
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artery to
legs
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vein from
legs
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Red blood cells
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Red blood cells are made in the bone marrow of some
bones, including the ribs, vertebrae and some limb bones.
They are produced at a very fast rate – about 9000 million
per hour.
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Figure B7.09 Plan of the main blood vessels in the
human body.
Plasma is mostly water. Many substances are dissolved
in it. Soluble nutrients such as glucose, amino acids and
mineral ions are carried in the plasma.
Plasma also transports hormones and carbon dioxide.
More details about the substances carried in blood plasma
are provided in Table B7.02. The functions of components
of blood are summarised in Table B7.03.
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renal artery
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B7.19
The liquid part of blood is called plasma. Floating in the
plasma are cells. Most of these are red blood cells. A much
smaller number are white blood cells. There are also small
fragments formed from special cells in the bone marrow,
called platelets (Figure B7.11 and Image B7.02).
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renal vein
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Why do arteries need strong walls?
B7.04 Blood
artery to
liver
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B7.18
aorta
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vein from
liver
Which type of blood vessels carry blood a away
from, and b towards the heart?
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pulmonary vein
vena cava
from body
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artery to head
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B7.17
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QuEStiONS
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vena cava
from head
How structure fits function
thick and strong,
relatively narrow; it
strength and elasticity needed to
containing muscles varies with heart beat, as withstand the pulsing of the blood as it
and elastic tissues it can stretch and recoil
is pumped through the heart
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Veins
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Width of lumen
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Structure of wall
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Function
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artery to head
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vein from head
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B7: Transport in mammals
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pulmonary
artery
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pulmonary
veins
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artery to small
intestine
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renal vein
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Red cells are red because they contain the pigment
haemoglobin. This carries oxygen. Haemoglobin is a protein,
and contains iron. It is this iron that readily combines with
oxygen where the gas is in good supply. It just as readily gives
it up where the oxygen supply is low, as in active tissues.
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doughnut-shaped
red blood cell, with
no nucleus
white blood cell
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Figure B7.11 Blood cells.
Another unusual feature of red blood cells is their shape. They
are biconcave discs – like a flat disc that has been pinched in
on both sides. This, together with their small size, gives them
a relatively large surface area compared with their volume.
This high surface area to volume ratio speeds up the rate at
which oxygen can difuse in and out of the red blood cell.
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platelets
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The lack of a nucleus in a red blood cell means that
there is more space for packing in millions of molecules
of haemoglobin.
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Red cells have to be made so quickly because they do not
live for very long. Each red cell only lives for about four
months. One reason for this is that they do not have a
nucleus (Figure B7.11).
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artery to leg
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vein from leg
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renal artery
s
hepatic portal vein
Figure B7.10 The main arteries and veins in the human body.
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hepatic artery
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hepatic vein
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aorta
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vena cava
from head
vena cava
from body
artery to arm
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vein from arm
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Destination
water
absorbed in small intestine
and colon
all cells
plasma proteins
(including fibrinogen
and antibodies)
fibrinogen made in the
liver; antibodies made
by lymphocytes
remain in the blood
lipids including
cholesterol and
fatty acids
absorbed in the ileum; also
derived from fat reserves in
the body
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to the liver, for
breakdown; to adipose
tissue, for storage; to
respiring cells, as an
energy source
mineral ions,
e.g. Na+, Cl–
absorbed in the ileum
and colon
to all cells
secreted into the blood by
endocrine glands
to all parts of the body
carbon dioxide is released by
all cells as a waste product of
respiration
to the lungs for
excretion
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Excess glucose is converted to
glycogen and stored in the liver.
Excess ions are excreted by
the kidneys.
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Hormones only afect their target cells.
Hormones are broken down by the
liver, and their remains are excreted by
the kidneys.
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Most carbon dioxide is carried as
hydrogencarbonate ions (HCO3–) in the
blood plasma.
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Breakdown of fats yields energy –
heart muscle depends largely on
fatty acids for its energy supply. High
cholesterol levels in the blood increase
the risk of developing heart disease.
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absorbed in the ileum; also
to all cells, for energy
produced by the breakdown of release by respiration
glycogen in the liver
dissolved gases,
e.g. carbon dioxide
Excess is removed by the kidneys.
Fibrinogen helps in blood clotting.
Antibodies kill invading pathogens.
carbohydrates,
especially glucose
hormones
Notes
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Component
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plasma
water, containing many substances
in solution
Functions
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1 liquid medium in which cells and platelets can float
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2 transports CO2 in solution
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3 transports nutrients in solution
4 transports hormones in solution
5 transports heat
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2 make antibodies
help in blood clotting
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small fragments of cells, with no nucleus
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Table B7.03 Components of blood.
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platelets
1 engulf and destroy pathogens
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variable shapes, with nucleus
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white cells
1 transport oxygen
2 transport small amount of CO2
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biconcave discs with no nucleus,
containing haemoglobin
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6 transports antibodies
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red cells
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Structure
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Component
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Table B7.02 Some of the main components of blood plasma.
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B7: Transport in mammals
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List five substances that are transported
in plasma.
B7.24
What is the function of red blood cells?
B7.25
What is unusual about the structure of red
blood cells?
B7.26
What is haemoglobin?
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the mechanism by which heart rate is changed
during exercise
the structure and functions of arteries, veins
and capillaries
how the structures of arteries, veins and capillaries
help them to carry out their functions
the names of the major blood vessels
how to recognise red blood cells, white blood cells,
platelets and plasma
the functions of these components of blood.
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about the double circulatory system
the advantages of a double circulatory system
the structure of the heart and how it works
reasons for the diference in thickness of the walls of
the heart chambers
factors that increase the risk of developing coronary
heart disease (CHD)
what happens during one heart beat, including the
roles of the valves
how exercise afects heart rate
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What are platelets?
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B7.27
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B7.23
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You should know:
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QuEStiONS
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Blood clotting stops pathogens getting into the body
through breaks in the skin. Normally, your skin provides
a very efective barrier against the entry of bacteria
and viruses. Blood clotting also prevents too much
blood loss.
Summary
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Platelets are small fragments of cells, with no nucleus.
They are made in the red bone marrow, and they are
involved in blood clotting.
Image B7.02 Blood seen through a microscope. The large
cell is a white cell. The others are all red cells. There are
also a few platelets (× 1700).
■
phagocyte, with lobed nucleus;
it can engulf bacteria
Platelets
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lymphocyte, with
a large nucleus
Figure B7.12 Two types of white blood cell.
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White cells are easily recognised, because, unlike red blood
cells, they do have a nucleus, which is oten quite large and
lobed (Figures B7.11 and B7.12 and Image B7.02). They can
move around and can squeeze out through the walls of
blood capillaries into all parts of the body. Their function is
to fight pathogens (disease-causing bacteria and viruses)
and to clear up any dead body cells. Some of them do this
by taking in and digesting bacteria, in a process called
phagocytosis. Others produce chemicals called antibodies.
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White blood cells
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The small size of the red blood cell is also useful in
enabling it to squeeze through even the tiniest capillaries.
This means that oxygen can be taken very close to every
cell in the body.
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Identify the components of blood that have each of the following functions.
a
b
c
d
e
Arteries, veins, capillaries, xylem vessels and phloem tubes are all tubes used for transporting substances
in mammals or flowering plants. Describe how each of these tubes is adapted for its particular function.
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transporting carbon dioxide
destroying bacteria
transporting oxygen
clotting
transporting glucose
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The diagram shows a section through a blood capillary.
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[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q6 June 2013]
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The diagram shows two cells found in human blood.
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The actual diameter of a red blood cell is 0.007 mm (7 µm).
Calculate the magnification of the diagram. Show your working.
Describe three diferences between the structure of a red blood cell and a white blood cell.
i State the function of a red blood cell.
ii Explain how the structure of a red blood cell helps it to carry out this function.
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[2]
[2]
[2]
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Describe how cell A transports oxygen.
Describe the function of cell B.
Outline the functions of a blood capillary.
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a
b
c
cell B
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cell A
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artery, vein
deoxygenated blood, oxygenated blood
atrium, ventricle
red blood cell, white blood cell
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Explain the diference between each of the following pairs.
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3
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a glucose molecule would travel through on its way from your digestive system to a muscle in your leg
a carbon dioxide molecule would travel through on its way from the leg muscle to your lungs.
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Using Figure B7.11 to help you, list in order the blood vessels and parts of the heart which:
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End-of-chapter questions
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[3]
[3]
[1]
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B7: Transport in mammals
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The diagram shows how the volume of the let ventricle changes over a time period of 1.3 seconds.
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A
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0.1
0.2
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0.6
0.7
0.8
0.9
Time in seconds
1.2
1.3
1.4
1.5
1.6
[1]
[1]
[2]
[2]
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Heart surgeons may stop the heart beating during operations. While this happens, blood is pumped
through a heart-lung machine that oxygenates the blood. The diagram below shows a heart-lung
machine in use.
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oxygenated blood returned to body
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pump
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reservoir of
deoxygenated
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blood removed
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from body
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let ventricle
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drainage tubes
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oxygenator
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8
1.0
How many complete heart beats are shown in the diagram?
i Use the graph to calculate how long one heart beat takes. Show your working.
ii Use your answer to b i to calculate the heart rate in beats per minute. Show your working.
Describe what is happening between points A and B on the graph.
Describe how the valves between the atria and ventricles help to ensure a one-way flow of blood
through the heart.
Make a copy of the graph shown above. On your graph, sketch a line to show the volume of the
right ventricle during this time period.
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0.3
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B
Volume of
let ventricle
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(continued)
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Name the structures labelled A to D.
Name the blood vessels E and F.
The heart-lung machine is used so that surgeons can operate on the arteries supplying heart
muscle. These arteries may be diseased. Name these arteries and explain how they may
become diseased.
Suggest why a patient is put on a heart-lung machine during such an operation.
Humans have a double circulatory system. There is a low pressure circulation and a high
pressure circulation.
Explain how the structure of the heart enables it to pump blood into two circulations at
diferent pressures.
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90
[4]
[2]
[3]
[2]
[4]
[Cambridge IGCSE Biology 0610 Paper 32 Q1 November 2011]
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Every living cell needs energy. In humans, our cells need
energy for:
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• making protein molecules by linking together amino
acids into long chains
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The main nutrient used to provide energy in cells is
glucose. Glucose contains a lot of chemical energy. In order
to make use of this energy, cells have to break down the
glucose molecules and release the energy from them.
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• making new cells, so that we can grow
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• cell division, so that we can repair damaged tissues and
can grow
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The food is digested – that is, broken down into smaller
molecules – which are absorbed from the intestine into the
blood. The blood transports the nutrients to all the cells in
the body. The cells take up the nutrients that they need.
• contracting muscles, so that we can move parts of
the body
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• producing heat inside the body, to keep the body
temperature constant even if the environment is cold.
All of this energy comes from the food that we eat.
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B8.01 Respiration
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the features of gas exchange surfaces in animals
the diferences between inspired and expired air
the efects of physical activity on breathing
how the gas exchange system is protected from
pathogens and particles
the efects of smoking tobacco.
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why organisms need energy
how respiration provides organisms with energy
aerobic respiration
anaerobic respiration
gas exchange in humans
the structure and function of the gas exchange system
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This chapter covers:
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B8
Respiration and gas exchange
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They do this in a series of metabolic reactions called
respiration. Like all metabolic reactions, respiration
involves the action of enzymes.
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Using yeast to make bread
Aerobic respiration
Flour contains a lot of starch, and also protein – especially
a protein called gluten. To make bread, the flour is mixed
with water and yeast to make dough (Image B8.01).
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Most of the time, our cells release energy from glucose by
combining it with oxygen. This is called aerobic respiration.
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Bread is made from flour, which is made by grinding the
grains (seeds) of cereal crops. Most bread is made from
wheat flour.
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The balanced equation is:
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carbon dioxide + water
glucose + oxygen
6CO2 + 6H2O
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aerobic respiration: the chemical reactions in cells that use
oxygen to break down nutrient molecules to release energy
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Anaerobic respiration
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Anaerobic respiration also makes alcohol, but this is all
broken down when the bread is baked. Baking also kills
the yeast.
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KEy tERM
alcohol + carbon dioxide
Aerobic respiration
Anaerobic respiration
uses oxygen
does not use oxygen
no alcohol or
lactic acid made
alcohol (in yeast and plants)
or lactic acid (in animals)
is made
large amount of energy
released from each
molecule of glucose
much less energy released
from each molecule
of glucose
carbon dioxide made
carbon dioxide is made by
yeast and plants, but not
by animals
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As in aerobic respiration, carbon dioxide is made. Plants
can also respire anaerobically like this, but only for short
periods of time.
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Table B8.01 A comparison of aerobic and
anaerobic respiration.
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lactic acid
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glucose
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Some of the cells in your body, particularly muscle cells,
can respire anaerobically for a short time. They make lactic
acid instead of alcohol and no carbon dioxide is produced.
This happens when you do vigorous exercise and your
lungs and heart cannot supply oxygen to your muscles as
quickly as they are using it.
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Yeast, a single-celled fungus, can respire anaerobically.
It breaks down glucose to alcohol.
Table B8.01 compares aerobic and anaerobic respiration.
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anaerobic respiration: the chemical reactions in cells
that break down nutrient molecules to release energy without
using oxygen
glucose
Image B8.01 Making bread dough in a bakery in Iran.
Amylase enzymes break down some of the starch in the
dough to make maltose and glucose, which the yeast
can use in anaerobic respiration. It produces bubbles of
carbon dioxide. These get trapped in the dough. Gluten
makes the dough stretchy, so the carbon dioxide bubbles
cause the dough to rise.
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It is possible to release energy from sugar without using
oxygen. It is not such an eficient process as aerobic
respiration and not much energy is released per glucose
molecule, but the process is used by some organisms.
It is called anaerobic respiration (‘an’ means without).
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KEy tERM
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C6H12O6 + 6O2
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This happens in a series of small steps, each one
controlled by enzymes. We can summarise the reactions of
aerobic respiration as a word equation:
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B8: Respiration and gas exchange
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A4 What happened to the limewater in each of your pieces
of apparatus? What does this show?
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investigating heat production by
germinating peas
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What is the purpose of respiration?
B8.02
What is the energy released by respiration
used for?
B8.03
What is anaerobic respiration?
B8.04
Name an organism which can respire
anaerobically.
B8.05
List three ways in which anaerobic respiration in
humans difers from aerobic respiration.
B8.06
List two ways in which anaerobic respiration
in humans difers from anaerobic respiration
in yeast.
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ACtivity B8.03
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Comparing the energy content of two kinds
of food
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B8.02 Gas exchange in humans
limewater
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yeast in
boiled,
cooled sugar
solution
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liquid
paraffin
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6 Leave your apparatus in a warm place.
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A1 Why is it important to boil the water?
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A2 Why must the sugar solution be cooled before adding
the yeast?
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If you look again at the aerobic respiration equation,
you can see that carbon dioxide is made. This is a waste
product and it must be removed from the organism.
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A3 What is the liquid parafin for?
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If you look back at the aerobic respiration equation
in Section B8.01, you will see that two substances are
needed. They are glucose and oxygen. The way in which
cells obtain glucose is described in Chapters B4 and B5.
Animals get sugar from carbohydrates which they eat.
Plants make theirs by photosynthesis.
Oxygen is obtained in a diferent way. Animals and
plants get their oxygen directly from their surroundings.
For humans, and most plants, this is the air.
U
Questions
Gas exchange surfaces
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7 Observe what happens to the limewater ater
half an hour.
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B8.01
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QuEStiONS
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investigating the production of carbon dioxide by
anaerobic respiration in yeast
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
1 Boil some water, to drive of any dissolved air.
2 Dissolve a small amount of sugar in the boiled water,
and allow it to cool.
3 When it is cool, add yeast and stir with a glass rod.
4 Set up the apparatus as in the diagram. Add the liquid
parafin by trickling it gently down the side of the tube,
using a pipette.
5 Set up an identical piece of apparatus, but use boiled
yeast instead of living yeast.
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ACtivity B8.02
A6 Describe a method you could use to compare the rate
of carbon dioxide production by yeast using diferent
kinds of sugar. Remember to describe the variables you
will change, those you will control and how, and how
you will collect, record and analyse your results.
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A5 What new substance would you expect to find in the
sugar solution containing living yeast at the end of
the experiment?
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ACtivity B8.01
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In organisms, there are special areas where the oxygen
enters and carbon dioxide leaves. One gas is entering and
the other leaving, so these are surfaces for gas exchange.
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Gas exchange surfaces have special characteristics that
help the process to be quick and eficient.
the trachea
From the nose or mouth, the air passes into the windpipe
or trachea. At the top of the trachea is a piece of cartilage
called the epiglottis. This closes the trachea and stops
food going down the trachea when you swallow. This is
a reflex action that happens automatically when food
touches the sot palate.
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The pathway to the lungs
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1 They are thin to allow gases to difuse across them quickly.
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2 They are close to an eficient transport system to take
gases to and from the exchange surface.
Just below the epiglottis is the voice box or larynx.
This contains the vocal cords. The vocal cords can be
tightened by muscles so that they make sounds when
air passes over them. The trachea has rings of cartilage
around it which keep it open.
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4 They have a good supply of oxygen (oten brought by
breathing movements).
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3 They have a large surface area, so that a lot of gas can
difuse across at the same time.
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the alveoli
At the end of each bronchiole are many tiny air sacs
or alveoli (Figure B8.02). This is where gas exchange
takes place.
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ACtivity B8.04
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Examining lungs
the bronchi
The trachea goes down through the neck and into
the thorax. The thorax is the upper part of your body from
the neck down to the bottom of the ribs and diaphragm.
In the thorax, the trachea divides into two. The two
branches are called the right and let bronchi (singular:
bronchus). One bronchus goes to each lung and then
branches out into smaller tubes called bronchioles.
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Figure B8.01 shows the structures which are involved in
gas exchange in a human. The most important are the two
lungs. Each lung is filled with many tiny air spaces called
air sacs or alveoli. It is here that oxygen difuses into the
blood. Because they are so full of spaces, lungs feel very
light and spongy to touch. The lungs are supplied with air
through the windpipe or trachea.
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The human gas exchange system
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larynx
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bronchiole
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pleural fluid
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alveoli (air sacs)
Figure B8.01 The human gas exchange system.
heart
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pleural membranes
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rib cross-section
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internal intercostal muscle
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external intercostal muscle
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cartilage supporting trachea
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trachea
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diaphragm
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alveolus
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basement
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Gas exchange in the lungs
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The walls of the alveoli are the gas exchange surface.
Tiny capillaries are closely wrapped around the outside of
the alveoli (Figure B8.04). Oxygen difuses across the walls
of the alveoli into the blood (Figure B8.05). Carbon dioxide
difuses the other way.
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air space in
alveolus
red blood
cell
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blood plasma
elastic
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Figure B8.04 Section through part of the lung, magnified.
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The walls of the alveoli have several features which make
them an eficient gas exchange surface.
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white blood cell which can
destroy bacteria that get
into the alveolus
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This protects the lungs from any harmful microorganisms –
pathogens – that might be in the air, reducing the chance
of getting infections in the lungs. It also stops too many
particles (for example soot, dust) from reaching the lungs,
where they might cause inflammation.
cell in wall
of alveolus
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cell in wall
of capillary
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There are other cells that have microscopic hair-like
extensions called cilia. These beat in a synchronised wave
(like a Mexican wave), sweeping the mucus up towards the
back of the throat. Once there, it is swallowed.
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nucleus
ciliated cell
Figure B8.03 Part of the lining of the respiratory passages.
Goblet cells
Some of the cells that line the passages through which air
moves towards the alveoli are goblet cells (Figure B8.03).
These cells secrete sticky mucus. As the air passes over the
mucus, microorganisms and particles of dust in the air get
trapped in it.
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Blood vessels return
oxygenated blood to
the pulmonary veins.
Figure B8.02 Alveoli.
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goblet cell
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Blood vessels bring
blood without much
oxygen from the
pulmonary arteries.
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mucus released
from goblet cell
cilia beating
air
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bronchiole
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B8: Respiration and gas exchange
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air moves in and out
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carbon dioxide
diffuses out of
blood
oxygen
diffuses into
blood
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Questions
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• They have a good supply of oxygen. Your breathing
movements keep your lungs well supplied with oxygen.
A1 In which tube did bubbles appear when you breathed
out? Explain why.
A2 In which tube did bubbles appear when you breathed
in? Explain why.
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A3 What happened to the liquid in tube A?
A4 What happened to the liquid in tube B?
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A5 What do your results tell you about the relative amounts
of carbon dioxide in inspired air and expired air?
What is the larynx?
B8.08
What is the function of the cilia in the
respiratory passages?
B8.09
Where does gas exchange take place in a human?
B8.10
How many cells does an oxygen molecule
have to pass through, to get from an alveolus
into the blood?
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B8.07
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ACtivity B8.06
ev
investigating the efect of exercise on rate and
depth of breathing
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QuEStiONS
limewater or
hydrogencarbonate
indicator solution
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• They have a large surface area. In fact, the surface area
is enormous. The total surface area of all the alveoli in
your lungs is over 100 m2.
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A
• They have an excellent transport system. Blood is
constantly pumped to the lungs along the pulmonary
artery. This branches into thousands of capillaries which
take blood to all parts of the lungs. Carbon dioxide
in the blood can difuse out into the air spaces in the
alveoli and oxygen can difuse into the blood. The blood
is then taken back to the heart in the pulmonary vein,
ready to be pumped to the rest of the body.
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rubber tubing
Pr
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• They are very thin. They are only one cell thick.
The capillary walls are also only one cell thick.
An oxygen molecule only has to difuse across this
small thickness to get into the blood.
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Figure B8.05 Gas exchange in an alveolus.
96
Comparing the carbon dioxide content of inspired
air and expired air
Skills:
AO3.1 Using techniques, apparatus and materials
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
The rubber tubing must be sterilised before you use it.
Don’t blow or suck hard when doing this experiment,
just breathe gently.
You can use either limewater or hydrogencarbonate
indicator solution for this experiment. Limewater changes
from clear to cloudy when carbon dioxide dissolves in it.
Hydrogencarbonate indicator solution changes from red
to yellow.
1 Set up the apparatus as in the diagram.
2 Breathe in and out gently through the rubber tubing.
Do not breathe too hard. Keep doing this until the
liquid in one of the tubes changes colour.
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moist surface
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wall of capillary
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wall of alveolus
ACtivity B8.05
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Reason for diference
21%
16%
Oxygen is absorbed across the gas exchange surface,
then used by cells in respiration.
0.04%
4%
Carbon dioxide is made inside respiring cells and difuses
out across the gas exchange surface.
variable
always high
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Water content
(humidity)
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Carbon dioxide
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Expired air
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Inspired air
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Oxygen
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B8: Respiration and gas exchange
Gas exchange surfaces are made of living cells, so must be
kept moist; some of this moisture evaporates into the air.
Table B8.02 compares the composition of inspired and
expired air.
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(aerobic respiration) in the liver. So, even though you do
not need the energy any more, you go on breathing faster
and more deeply, and your heart rate continues to be
high. You are taking in and transporting extra oxygen to
break down the lactic acid. The faster heart rate also helps
to transport lactic acid as quickly as possible from the
muscles to the liver.
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Table B8.02 A comparison of inspired and expired air.
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Exercise and breathing rate
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Sometimes, cells may need a lot of oxygen very quickly.
Imagine you are running in a race. The muscles in your legs
are using up a lot of energy. The cells in the muscles will be
respiring – combining oxygen with glucose – as fast as they
can, to release energy for muscle contraction.
ity
A lot of oxygen is needed to work as hard as this. You
breathe deeper and faster to get more oxygen into your
blood. Your heart beats faster to get the oxygen to the leg
muscles as quickly as possible.
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While you were running, you built up an oxygen debt.
You ‘borrowed’ some extra energy, without ‘paying’ for
it with oxygen. Now, as the lactic acid is combined with
oxygen, you are paying of the debt. Not until all the lactic
acid has been used up does your breathing rate and rate of
heart beat return to normal (Image B8.02).
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All the cells in your body need oxygen for respiration and
all of this oxygen is supplied by the lungs. The oxygen is
carried by the blood to every part of the body.
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But however much the breathing rate is increased,
eventually a limit is reached. The heart and lungs simply
cannot supply oxygen to the muscles any faster. But more
energy is still needed for the race. How can that extra
energy be found?
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Extra energy can be produced by anaerobic respiration.
Some glucose is broken down without combining it
with oxygen.
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lactic acid + energy
glucose
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When you stop running, you will have quite a lot of
lactic acid in your muscles and your blood. This lactic
acid must be broken down by combining it with oxygen
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As explained in Section B8.01, this does not release
very much energy, but a little extra might make all
the diference.
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Image B8.02 These sprinters will pay back their oxygen
debts ater the race.
The rate at which your breathing muscles work – and
therefore your breathing rate – is controlled by the brain.
The brain constantly monitors the pH of the blood that
flows through it. If there is a lot of carbon dioxide or lactic
acid in the blood, this causes the pH to fall. When the
brain senses this, it sends nerve impulses to the muscles
that cause breathing movements, the diaphragm and the
intercostal muscles. The nerve impulses stimulate these
muscles to contract harder and more oten. The result is a
faster breathing rate and deeper breaths.
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This happens because the high rate of respiration in
the muscles produces a lot of carbon dioxide. The
concentration of carbon dioxide in the blood therefore
increases. This is sensed by the brain, which responds by
increasing the breathing rate.
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Cambridge IGCSE Combined and Co-ordinated Sciences
leading to hypertension. Smokers have a much greater
chance of developing coronary heart disease than
non-smokers.
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One public health concern is that these dangers exist for
both smokers and non-smokers. The possible damage is just
as real for non-smokers who are in a smokers’ environment.
They breathe in smoke from burning cigarettes, and from
smoke exhaled by smokers. This is termed passive smoking.
In many countries, smoking is now banned in all public
places. It is also very strongly recommended that parents do
not smoke anywhere near their children.
tar
Tar contains many diferent chemicals, some of which are
carcinogens – that is, they can cause cancer. The chemicals
can afect the behaviour of some of the cells in the respiratory
passages and the lungs, causing them to divide uncontrollably.
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am
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Everyone knows that smoking damages your health, but
still people do it. Figure B8.06 shows smoking rates in
some countries.
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B8.03 tobacco smoking
ie
Carbon monoxide reduces
the oxygen-carrying capacity
of the blood.
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op
Figure B8.07 Some of the substances in cigarette smoke.
ni
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Particulates damage
lung surfaces.
Nicotine damages the circulatory system, making blood
vessels get narrower. This can increase blood pressure,
Pr
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2750 and more
2500–2749
2250–2499
2000–2249
1750–1999
1500–1749
1250–1499
1000–1249
750–999
500–749
250–499
0–249
Unknown
es
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Figure B8.06 The map shows the mean number of cigarettes smoked per person, per year.
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Tar causes lung cancer and
many other kinds of cancer.
rs
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Nicotine
Nicotine afects the brain. It is a stimulant, which means
it makes a person feel more alert. Nicotine is an addictive
drug. This is why smokers oten find it extremely dificult to
give up.
s
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id
Figure B8.07 shows the main components of tobacco
smoke. There are, in fact, many more substances in
tobacco smoke, and researchers are still finding out more
about them and the damage that each of them can do to
the smoker’s health.
Nicotine is addictive.
w
ge
Components of tobacco smoke
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There is therefore less surface area across which gas
exchange can take place. The person is said to have
emphysema. They find it dificult to get enough oxygen
into their blood. A person with emphysema may not be
able to do anything at all active, and eventually they may
not even have the energy to walk.
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Smoking and lung cancer
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It was in the 1950s that people first began to realise
that there was a link between smoking cigarettes and
getting lung cancer. The person at the forefront of this
new understanding was a medical researcher called
Richard Doll (Image B8.04).
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Smoking increases the risk of developing high blood
pressure. As the blood passes through the lungs, it absorbs
many substances from cigarette smoke. Some of these
make the walls of the arteries get thicker and harder.
The walls cannot stretch and recoil as easily as the blood
surges through them. Smoking also makes it more likely
that a blood clot will form inside blood vessels, including
the coronary arteries that supply the wall of the heart
with oxygenated blood. People who smoke have a greatly
increased risk of developing coronary heart disease.
w
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Smoking and heart disease
Smoke particles
Smoke particles are little particles of carbon and other
materials that are present in cigarette smoke. They get
trapped inside the lungs. White blood cells try to remove
them and secrete chemicals that are intended to get rid
of these invading particles. Unfortunately, the chemicals
secreted by the white blood cells can do serious damage
to the lungs themselves, resulting in chronic obstructive
pulmonary disease (COPD). The delicate walls of the
alveoli tend to break down (Image B8.03).
b
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a
Several of the chemicals in cigarette smoke harm the cells
lining the respiratory passages. You may remember that
these cells clean the air as it passes through, stopping
bacteria and dust particles from getting down to the lungs
(Section B8.02). Figure B8.08 shows how smoking afects
this cleaning mechanism.
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Carbon monoxide
Carbon monoxide is a poisonous gas which afects the
blood. The carbon monoxide difuses from the lungs into
the blood and combines with haemoglobin inside the red
blood cells. This means that less oxygen can be carried.
The body cells are therefore deprived of oxygen. This is
not good for anyone, but it is especially harmful for a baby
growing in its mother’s uterus. When the mother smokes,
these harmful chemicals go into the baby’s blood. The
carbon monoxide can prevent it from growing properly.
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The cells divide over and over again, forming a lump or
tumour. If this tumour is malignant, this is cancer. Cells
may break away from the first tumour and spread to other
parts of the body, where new tumours will grow. Almost
everyone who gets lung cancer is a smoker, or has lived or
worked in an environment where they have been breathing
in other people’s cigarette smoke. Smoking cigarettes
increases the risk of developing many diferent kinds of
cancer. All forms of cancer are more common in smokers
than in non-smokers.
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B8: Respiration and gas exchange
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Image B8.03 a Healthy lung tissue with many small air spaces, b lung tissue with emphysema – air spaces are fewer, larger
and have thicker walls between them (× 60).
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Goblet cells work faster
than usual, producing
extra mucus.
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2500
15
2000
1500
10
1000
500
5
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4000
females
25
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Pr
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Cigarettes smoked per year
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4500
3500
3000
20
2500
15
2000
1500
10
1000
5
500
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0
0
1911 1921 1931 1941 1951 1961 1971 1981 1991 2001
Year
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Figure B8.09 Lung cancer deaths and smoking rates in the
UK between 1911 and 2001.
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Doll published the results of his research in a journal in
1950, but it was many years before everyone was prepared
to accept the link between smoking and lung cancer.
The dificulty was that you could not really do a controlled
experiment on it. Instead, researchers had to rely on
looking for a correlation between these two factors.
ev
The graphs in Figure B8.09 show that there is a correlation
between the number of cigarettes smoked per year and
the number of deaths from lung cancer.
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3000
0
0
1911 1921 1931 1941 1951 1961 1971 1981 1991 2001
Year
At that time, doctors were becoming concerned
about the rapid rise of lung cancer in the British
population. No one knew why this was happening.
Richard Doll interviewed lung cancer patients in
20 hospitals in London, trying to find out if they
had anything in common. His initial theory was
that this was something to do with the new substance,
tarmac, that was being used to build roads.
However, it rapidly became clear to him that all of
these people were smokers. Very quickly, he himself
stopped smoking.
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25
3500
Figure B8.08 How smoking damages the
respiratory system.
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30
males
Deaths per year / thousands
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4000
The mucus provides a good place for bacteria to live. The
bacteria can cause chronic (long-term) infections in the
lungs and bronchi. Mucus in the lungs makes it difficult for
oxygen and carbon dioxide to diffuse between the alveoli
and the blood.
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100
4500
es
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Mucus trickles
down to the
lungs and stays
there.
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Image B8.04 Richard Doll, who was the first person to
realise that smoking causes lung cancer.
Deaths per year / thousands
There are fewer cilia
and those that remain
work less well.
Cigarettes smoked per year
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Airway of a smoker
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Pr
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Cilia beat
and sweep
mucus up to
the mouth.
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Normal airway
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For example, we know that tar contains chemicals
that afect the genes in cell nuclei. These chemicals
can damage the normal control mechanisms of a cell,
so that it begins to divide over and over again. This is
how cancer begins.
■
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■
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■
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■
the diferences between the composition of
inspired air and expired air
the reasons for these diferences
the diferences in composition between inspired
and expired air
the reasons for these diferences
why breathing rate and depth increases
during exercise, and remains high for some
time aterwards
the efects of smoking on the gas exchange
system and on the risk of developing coronary
heart disease.
■
y
■
why humans and other organisms need energy
about the release of energy from food in respiration
the equation for aerobic respiration
the equations for anaerobic respiration in yeast
and in humans
the structure and functions of the organs of the
human respiratory system
the features of the human gas exchange surface that
adapt it for its function
how goblet cells, mucus and ciliated cells help to
protect the gas exchange surface from pathogens
and particles
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You should know:
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Summary
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Pr
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For many years, tobacco companies tried to play down this
link. They suggested many other possible reasons for the
correlation, because they did not want people to stop smoking.
However, much research has now been done on the efects of
smoking on health, and we now understand how smoking –
both passive and active – can cause lung and other cancers.
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B8: Respiration and gas exchange
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List five diferent uses of energy in the human body.
Name the process that releases energy from food, so that the body can use it.
Write a word equation for the process that you have named in b.
2
a
b
List three diferences between inspired air and expired air.
Explain the reasons for these diferences.
3
a
b
c
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lactic acid or alcohol made
energy released from glucose
carbon dioxide made
heat released
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Describe, in detail, the pathway of an oxygen molecule as it moves from the air outside your body,
into your blood, and to a cell in a muscle in your arm. You could write your answer in words, or use a
flow diagram, or perhaps a mixture of both. You will need to think about what you have learnt about the
human transport system, as well as what you have learnt in this chapter.
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For each description, state whether it applies to aerobic respiration, to anaerobic respiration or to both.
a
b
c
d
5
Explain what is meant by the term gas exchange.
Where is the gas exchange surface in humans?
List three features of the human gas exchange surface that help it work eficiently.
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b
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End-of-chapter questions
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101
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air drawn out
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3
insects
4
limewater
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2
limewater
id
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1
potassium
hydroxide
solution
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air in
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A student is investigating one of the characteristics of living things using insects.
She sets up the apparatus shown in the diagram below.
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Cambridge IGCSE Combined and Co-ordinated Sciences
s
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A girl breathed into a machine that recorded the volume of the air that she breathed in and out.
The results were recorded as a graph of volume against time. The diagrams show results obtained
when she was resting and when she was exercising.
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s
During
exercise
Pr
30
40
50
Time / seconds
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60
0
70
10
20
30
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40
Time / seconds
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10
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0
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Volume
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Volume
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0.5 dm3
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At rest
R
[1]
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 62 Q4 November 2014]
am
7
[1]
[3]
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102
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b
[1]
[1]
[1]
[1]
[1]
i State the purpose of the limewater in flask 2.
ii Predict the appearance of the limewater in flask 2 ater 10 minutes.
i State the purpose of the limewater in flask 4.
ii Predict the appearance of the limewater in flask 4 ater 10 minutes.
Suggest a control for this experiment.
i State the appearance of the liquid in flask 4 at the end of the experiment if it had
contained water and Universal (full range) Indicator rather than limewater.
ii Explain your answer to d i.
Name the process inside living cells that is responsible for the changes that are observed
in this experiment.
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Air is drawn through the apparatus from let to right as shown. The potassium hydroxide in
flask 1 removes any carbon dioxide from the air.
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50
60
70
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[1]
[1]
[1]
[1]
[4]
[4]
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30
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The graph shows how a student’s breathing rate changed during and ater exercise.
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d
Use the first graph to find how many breaths per minute the girl took while she was resting.
Use the second graph to find how many breaths per minute the girl took while she was exercising.
Use the first graph to find the volume of the first breath that she took while she was resting.
(Remember to include the unit in your answer.)
Use the second graph to find the volume of the second breath that she took while
she was exercising.
Explain how these changes in rate and depth of breathing helped the girl to do the exercise.
Describe the mechanism that brought about these changes in rate and depth of breathing in
the girl’s body.
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br
id
a
b
c
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B8: Respiration and gas exchange
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25
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20
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Breaths per minute 15
103
exercise stops
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10
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exercise starts
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4
6
12
14
16
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10
Calculate the increase in the student’s breathing rate from when he started to exercise, to its
maximum rate.
Calculate how long it took, ater he finished exercise, for his breathing rate to return to normal.
Explain why his breathing rate did not return to normal immediately ater exercise stopped.
Describe and explain how you would expect the student’s heart rate to change during the
16-minute period shown on the graph.
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b
c
d
8
Time / minutes
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5
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[2]
[2]
[4]
[4]
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Information was collected about the relative death rates of men in diferent categories. The men were
divided into categories according to whether they smoked or not, and if they did smoke, at what age
they started. The data are shown in the bar chart below.
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Cambridge IGCSE Combined and Co-ordinated Sciences
2
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1
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Relative death
rate in men
40–60 years
Pr
es
s
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3
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20 –24
2
2 –29
29 non-smoker
100 –14 15 –199 20
25
Age in years regular smoking started
1.6
es
1–9
2.0
20–29
Pr
y
10–19
30–39
2.4
2.2
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[4]
y
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Using the data in the table, draw a bar chart similar to the one provided earlier in this question.
Using the information in the graph provided earlier and the graph you drew, state three
diferent conclusions about the connection between cigarette smoking and risk of dying
between ages 40–60 years.
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104
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1.0
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Relative death rates in men
40–60 years
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Number of cigarettes smoked
each day
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The men in the study were also divided into categories according to the number of cigarettes smoked
per day. These data are shown in the table below.
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[3]
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Animals need fast and eficient communication systems
between their receptors and efectors. This is partly
because most animals move in search of food. Many
animals need to be able to respond very quickly to catch
their food or to avoid predators.
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To make sure that the right efectors respond at the
right time, there needs to be some kind of communication
system between receptors and efectors. If you touch
something hot, pain receptors on your fingertips send
an impulse to your arm muscles to tell them to contract,
pulling your hand away from the hot surface.
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One of the characteristics of all living things is the ability
to detect and respond to changes in its environment.
This is known as sensitivity. Changes in an organism’s
environment are called stimuli (singular: stimulus)
and are sensed by specialised cells called receptors.
The organism responds using efectors. Muscles are
efectors, and they may respond to a stimulus by
contracting. Glands can also be efectors. For example,
if you smell good food cooking, your salivary glands may
respond by secreting saliva.
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B9.01 Coordination in animals
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the hormones insulin and glucagon
how humans maintain a constant internal
body temperature
how plants respond to stimuli
the role of auxin in shoot growth.
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the human nervous system
neurones and how they work
the diference between voluntary and involuntary actions
reflex actions
the structure and function of the eye
the hormone adrenaline
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This chapter covers:
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B9
Coordination and homeostasis
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The way in which receptors pick up stimuli, and then pass
information on to efectors, is called coordination.
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The dendrites pick up electrical signals from other
neurones lying nearby. These electrical signals are called
nerve impulses. The signal passes to the cell body, then
along the axon, which might pass it to another neurone.
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Most animals have two methods of sending information
from receptors to efectors. The fastest is by means of
nerves. The receptors and nerves make up the animal’s
nervous system. A slower method, but still a very
important one, is by means of chemicals called hormones.
Hormones are part of the endocrine system.
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To enable them to do this, they have long, thin fibres of
cytoplasm stretching out from the cell body. The longest fibre
in Figure B9.01 is called an axon. Axons can be more than a
metre long. The shorter fibres are called dendrons or dendrites.
The central nervous system
All mammals (and many other animals) have a central
nervous system (CNS) and a peripheral nervous system.
The CNS is made up of the brain and spinal cord (Figure B9.02).
The peripheral nervous
brain
system is made up of nerves
and receptors.
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The human nervous system is made of special cells called
neurones. Figure B9.01 illustrates a particular type of
neurone called a motor neurone.
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B9.02 the human nervous system
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When a receptor detects
a stimulus, it sends an
electrical impulse to the
brain or spinal cord. The
brain or spinal cord receives
the impulse and sends
an impulse on, along the
appropriate nerve fibres, to Figure B9.02 The human
central nervous system.
the appropriate efector.
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body
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Reflex arcs
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In the spinal cord, the neurone passes an impulse on to
several other neurones. Only one is shown in Figure B9.04.
These neurones are called relay neurones, because they relay
the impulse on to other neurones. The relay neurones pass the
impulse on to the brain. They also pass it on to an efector.
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myelin sheath
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In this case, the efectors are the muscles in your arm. The
impulse travels to the muscle along the axon of a motor
neurone. The muscle then contracts, so that your hand is
pulled away.
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Figure B9.01 A human motor neurone.
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nerve ending
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nucleus of cell which
makes myelin sheath
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Figures B9.03 and B9.04 show how these impulses are
sent. If your hand touches a hot plate, this information is
detected by a sensory receptor in your finger. The receptor
generates a nerve impulse, which travels to the spinal cord
along the axon of a sensory neurone (Figure B9.05).
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node of Ranvier
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spinal cord
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dendrite
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Like the rest of the nervous
system, the CNS is made
up of neurones. Its role is to
coordinate the information
travelling through the
nervous system.
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Neurones contain the same basic parts as any animal
cell. Each has a nucleus, cytoplasm and a cell membrane.
However, their structure is specially adapted to be able to
carry signals very quickly.
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B9: Coordination and homeostasis
This sort of reaction is called a reflex action. You do not
need to think about it. Your brain is made aware of it, but
you only consciously realise what is happening ater the
message has been sent on to your muscles.
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reflex action: a means of automatically and rapidly
integrating and coordinating stimuli with the responses of
efectors (muscles and glands)
relay
neurone
Relay neurone
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effector
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Figure B9.05 The structure of sensory, motor and relay
neurones.
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cell body of the relay neurone
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spinal nerve
cell body of
the sensory
neurone
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A nerve impulse from the
motor neurone makes the
muscle contract.
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sensory neurone
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cell body of the motor
neurone
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Figure B9.04 A reflex arc.
axon of the
motor neurone
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The hand
touches a hot
plate.
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cell body
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receptor
direction of travel of
electrical impulse
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sensory neurone
Figure B9.03 Schematic diagram of a reflex arc.
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impulses
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motor neurone
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cell body
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Reflex actions are very useful, because information gets
from the receptor to the efector as quickly as possible.
You do not waste time in thinking about what to do. The
pathway along which the nerve impulse passes – the
sensory neurone, relay neurone and motor neurone – is
called a reflex arc. Figure B9.05 shows the structure of
these three types of neurone.
Motor neurone
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Sensory neurone
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spinal cord
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B9.06
Where are the cell bodies of each of these types
of neurone found: a sensory neurone, b relay
neurone, and c motor neurone?
B9.07
What is the value of reflex actions?
B9.08
Describe two reflex actions, other than the
response to touching something hot, and the
knee jerk reflex.
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What is the function of the central nervous
system?
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List three ways in which neurones are specialised
to carry out their function of transmitting
electrical impulses very quickly.
B9.05
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B9.04
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Measuring reaction time using a ruler
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Give two examples of efectors.
B9.02
What are the two main communication systems in
an animal’s body?
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ACtivity B9.01
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Image B9.01 shows a person’s reflex actions being tested –
you may have had this test yourself. Another reflex action
is described in Section B9.03.
Reflex actions are examples of involuntary actions.
They are not under conscious control. Many of our actions,
however, are voluntary. They happen because we decide
to carry them out. For example, reading this book is a
voluntary action.
to measure mean reaction time
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
The time taken for a nerve impulse to travel from a
receptor, through your CNS and back to an efector is
very short. It can be measured, but only with special
equipment. However, you can get a reasonable idea of the
time it takes if you use a large number of people and work
out an average time.
1 Get as many people as possible to stand in a circle,
holding hands.
2 One person lets go of his or her neighbour with the
let hand, and holds a stopwatch in it. When everyone
is ready, this person simultaneously starts the
stopwatch, and squeezes his or her neighbour’s hand
with the right hand.
3 As soon as each person’s let hand is squeezed,
he or she should squeeze his or her neighbour with the
right hand. The message of squeezes goes all round
the circle.
4 While the message is going round, the person with the
stopwatch puts it into the right hand, and holds his
or her neighbour’s hand with the let hand. When the
squeeze arrives, he or she should stop the watch.
5 Keep repeating this, until the message is going round
as fast as possible. Record the time taken, and also the
number of people in the circle.
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ACtivity B9.02
Image B9.01 The knee jerk reflex is an example of a reflex
action. A sharp tap just below the knee stimulates a
receptor. This sends impulses along a sensory neurone into
the spinal cord. The impulse then travels along a motor
neurone to the thigh muscle, which quickly contracts and
raises the lower leg.
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List three ways in which neurones are similar to
other cells.
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B9: Coordination and homeostasis
conjunctiva
Questions
iris
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6 Now try again, but this time make the message of
squeezes go the other way around the circle.
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A1 Using the fastest time you obtained, work out the
mean time it took for one person to respond to the
stimulus they received.
A3 Did the nerve impulse go as quickly when you changed
direction? Explain your answer.
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A2 Did people respond faster as the experiment went on?
Why might this happen?
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The rest of the eye simply helps to protect the retina,
or to focus light onto it.
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Each eye is set in a bony socket in the skull, called the
orbit. Only the very front of the eye is not surrounded by
bone (Figure B9.07).
The fluid is washed across your eye by your eyelids when
you blink. The eyelids, eyebrows and eyelashes also help
to stop dirt from landing on the surface of your eyes.
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Figure B9.06 shows the internal structure of the eye.
The part of the eye that contains the receptor cells is the
retina. This is the part which is actually sensitive to light.
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The structure of the human eye
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Even the part of the eye inside the orbit is protected.
There is a very tough coat surrounding it called the sclera.
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The front of the eye is covered by a thin, transparent
membrane called the conjunctiva, which helps to protect
the parts behind it. The conjunctiva is always kept moist
by a fluid made in the tear glands. This fluid contains an
enzyme called lysozyme, which can kill bacteria.
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The eye is a receptor organ. Its function it to detect light,
and to transfer the energy in the light to electrical energy
in a nerve impulse.
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tear duct
Figure B9.07 The eye from the front.
A4 If you have access to the internet, find a site that allows
you to measure your reaction time and try it out. Do
you think the website gives you more reliable results
than the ‘circle’ method? Compare the results you
obtain, and discuss the advantages and disadvantages
of each method.
B9.03 the eye
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muscle attaching eye to skull
choroid
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optic nerve
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lens
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aqueous humour with
salts to nourish the lens
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blind spot
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ciliary muscle
semi-solid vitreous humour
supporting the eyeball
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suspensory ligament
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Figure B9.06 Section through a human eye (seen from above). (Note: you do not need to learn the labels for sclera, choroid,
aqueous humour and vitreous humour, but you may find these helpful if you do Activity B9.05.)
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the retina
The retina is at the back of the eye. When light falls on
a receptor cell in the retina, the cell sends an electrical
impulse along the optic nerve to the brain. The brain
sorts out all the impulses from each receptor cell, and
builds up an image. Some of these receptor cells are
sensitive to light of diferent colours, enabling us to see
coloured images.
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These two sets of muscles are said to be antagonistic
muscles. This means that they work together
but have opposite efects. When one contracts, the
other relaxes.
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The responses of the iris to diferent light intensities are
examples of a reflex action. Although the nerve impulses
go into the brain, we do not need to think consciously
about what to do. The response of the iris to light intensity
(the stimulus) is fast and automatic. Like many reflex
actions, this is very advantageous: it prevents damage to
the retina that could be caused by very bright light falling
onto it.
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The closer together the receptor cells are, the clearer the
image the brain will get. The part of the retina where the
receptor cells are packed most closely together is called
the fovea. This is the part of the retina where light is
focused when you look straight at an object.
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There are no receptor cells where the optic nerve leaves
the retina. This part is called the blind spot. If light falls
on this place, no impulses will be sent to the brain.
Try Activity B9.03.
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TIP
Remember that some reflex actions can involve the
brain rather than the spinal cord. However, they do not
involve the parts of the brain that are involved in making
conscious decisions.
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Figure B9.08 The iris reflex.
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In the middle of the iris is a gap called the pupil. The size of
the pupil can be adjusted. The wider the pupil is, the more
light can get through to the retina. In strong light, the iris
closes in and makes the pupil small. This stops too much
light getting in and damaging the retina.
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In dim light, the
radial muscles
in the iris
contract.
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the iris
In front of the lens is a circular piece of tissue called the
iris. This is the coloured part of your eye. The iris contains
pigments, which absorb light and stop it getting through to
the retina.
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In bright light,
the circular
muscles in the
iris contract.
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Behind the retina is a black layer called the choroid.
The choroid absorbs all the light ater it has been through
the retina, so it does not get scattered around the inside
of the eye. The choroid is also rich in blood vessels which
nourish the eye.
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Can you always see the image?
Hold this page about 45 cm from your face. Close the let
eye, and look at the cross with your right eye. Gradually
bring the page closer to you. What happens? Can you
explain it?
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ACtivity B9.03
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To allow it to adjust the size of the pupil, the iris contains
muscles. Circular muscles lie in circles around the pupil.
When they contract, they make the pupil constrict, or get
smaller. Radial muscles run outwards from the edge of
the pupil. When they contract, they make the pupil dilate,
or get larger (Figure B9.08). This is called the iris reflex (or
sometimes the pupil reflex).
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B9: Coordination and homeostasis
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Focusing light
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Adjusting the focus
Not all light rays need bending by the same amount
to focus them onto the retina. Light rays coming from
an object in the distance will be almost parallel to one
another. They will not need much bending (Figure B9.10).
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Light rays coming from a nearby object are going away
from one another, or diverging. They will need to be bent
inwards quite strongly (Figure B9.11).
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The cornea
bends the
light rays.
Dissecting a sheep’s eye
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light focused
on the retina
The thin lens bends
the light rays slightly.
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Figure B9.10 Focusing on a distant object.
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The cornea
bends the
light rays.
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object
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light rays diverging only slightly
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An upside-down image
is formed on the retina.
Figure B9.09 How an image is focused onto the retina.
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The lens bends
the light rays.
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ACtivity B9.05
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light rays
from object
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The cornea is responsible for most of the refraction of
the light. The lens makes fine adjustments.
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For the brain to see a clear image, there must be a
clear image focused on the retina. Light rays must be
bent, or refracted, so that they focus exactly onto the
retina. The humours inside the eye are transparent
and colourless so that light can pass through
them easily.
Figure B9.09 shows how the cornea and lens focus
light onto the retina. The image on the retina is upside
down. The brain interprets this so that you see it the
right way up.
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Looking at human eyes
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
It is best to perform this experiment with a partner,
although it is possible to use a mirror and look at your
own eyes.
1 First identify all the following structures: eyebrows;
eyelashes; eyelids; conjunctiva; pupil; iris;
cornea; sclera; small blood vessels; openings to
tear ducts.
Figure B9.06 will help you to do this.
2 Make a diagram of a front view of the eye and label
each of these structures on it.
3 Use this book to find out the functions of each
structure you have labelled. Write down these
functions, as briefly as you can, next to each label or
beneath your diagram.
4 Ask your partner to close his or her eyes, and cover
them with something dark to cut out as much
light as possible. (Alternatively, you may be able to
darken the whole room.) Ater about 3 or 4 minutes,
quickly remove the cover (or switch on the lights)
and look at your partner’s eyes as they adapt to
the light. What happens? What is the purpose of
this change?
5 Explain how this change is brought about.
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ACtivity B9.04
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The cornea
bends the
light rays.
What is meant by the term stimulus?
B9.10
Which part of the eye contains cells which are
sensitive to light?
List, in order, the parts of the eye through which
light passes to reach the retina.
B9.12
Name two parts of the eye which refract light rays.
B9.13
What is meant by the term accommodation?
B9.14
a What do the ciliary muscles do when you are
focusing on a nearby object?
b What efect does this have on:
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i the suspensory ligaments?
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The thick lens bends
the light rays greatly.
B9.11
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ii the lens?
Figure B9.11 Focusing on a nearby object.
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The shape of the lens can be adjusted to bend light
rays more, or less. The thicker the lens, the more it
will bend the light rays. The thinner it is, the less
it will bend them. This adjustment in the shape of the
lens, to focus light coming from diferent distances,
is called accommodation.
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B9.09
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object
QuEStiONS
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light focused
on the retina
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diverging greatly
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Once the hormone is in the blood, it is carried to all parts
of the body, dissolved in the blood plasma. Although the
blood carries many hormones, each afects only certain
parts of the body. These are called its target organs.
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So far, we have seen how nerves can carry electrical
impulses very quickly from one part of an animal’s body
to another. But animals also use chemicals to transmit
information from one part of the body to another.
The chemicals are called hormones. Hormones are made
in special glands called endocrine glands, which secrete
(release) the hormones directly into blood capillaries.
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Figure B9.12 shows how the shape of the lens is changed.
It is held in position by a ring of suspensory ligaments.
The tension on the suspensory ligaments, and thus
the shape of the lens, is altered by means of the ciliary
muscle. When this muscle contracts, the suspensory
ligaments are loosened. When it relaxes, they are pulled
tight. When the suspensory ligaments are tight, the
lens is pulled thin. When they are loosened, the lens
gets thicker.
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Endocrine glands
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B9.04 Hormones
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Nearby object
The suspensory
ligaments are
slackened.
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The suspensory
ligaments are
pulled tight.
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The ciliary muscle
contracts.
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The ciliary muscle relaxes.
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Distant object
The lens is
allowed to bulge.
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Side view of eye
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Side view of eye
Front view
of eye
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The lens is
pulled thin.
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Figure B9.12 How the shape of the lens is changed.
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Front view
of eye
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Nervous
system
Describe two situations in which adrenaline is
likely to be secreted.
B9.17
How does adrenaline help to prepare the
body for action?
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Like animals, plants are able to respond to their
environment, although usually with much slower
responses than those of animals.
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B9.16
B9.05 Coordination and response
in plants
Two important stimuli for plants are light and gravity.
Shoots normally grow towards light. Roots do not usually
respond to light, but a few grow away from it. Shoots
tend to grow away from the pull of gravity, while roots
normally grow towards it (Figures B9.13 and B9.14).
It is very important to the plant that its roots and shoots
grow in appropriate directions. Shoots must grow
upwards, away from gravity and towards the light, so that
the leaves are held out into the sunlight. The more light
they have, the better they can photosynthesise. Flowers,
too, need to be held up in the air, where insects, birds or
the wind can pollinate them.
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How are hormones transported around
the body?
efect of a hormone may
last longer
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B9.15
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chemicals travel
more slowly
These responses are called tropisms. A tropism is a
growth response by a plant, in which the direction of the
growth is afected by the direction of the stimulus.
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QuEStiONS
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impulses travel
very quickly
In general, plants respond to stimuli by changing their
rate or direction of growth. They may grow either towards
or away from a stimulus. Growth towards a stimulus is
said to be a positive response, and growth away from a
stimulus is a negative response.
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chemicals carried dissolved
in the blood plasma
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The blood vessels in your skin and digestive system
contract so that they carry very little blood. This makes
you go pale, and gives you ‘butterflies in your stomach’.
As much blood as possible is needed for your brain and
muscles in the emergency.
Table B9.01 compares the nervous and endocrine systems.
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impulses transmitted
along nerve fibres (axons
and dendrons)
Table B9.01 A comparison of the nervous and endocrine
systems of a mammal.
Adrenaline also causes the liver to release glucose into the
blood. This provides extra glucose for the muscles, so that
they can release energy from it (by respiration) and use the
energy for contracting.
ie
information transmitted
in the form of chemicals
called hormones
efect of a nerve impulse
usually only lasts for a
very short time
U
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Adrenaline causes the pupils in the eye to widen.
This allows more light into the eye, which might help
you to see the danger more clearly.
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information transmitted
in the form of
electrical impulses
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There are two adrenal glands, one above each kidney.
They make a hormone called adrenaline. When you are
frightened, excited or keyed up, your brain sends impulses
along a nerve to your adrenal glands. This makes them
secrete adrenaline into the blood.
Adrenaline has several efects which are designed to help
you to cope with danger known as the ‘fight or flight’
response. For example, it makes your heart beat faster,
supplying oxygen to your brain and muscles more quickly.
This gives them more energy for fighting or running away.
It also increases breathing rate, so that more oxygen can
enter the blood in the lungs.
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made up of
secretory cells
made up of neurones
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Adrenaline
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hormone: a chemical substance produced by a gland, carried
by the blood, which alters the activity of one or more specific
target organs and is then destroyed by the liver
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Endocrine
system
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B9: Coordination and homeostasis
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Petri dish B
seedling
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moist cotton wool
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Whichever way up a seed is planted, its
radicle always grows downwards.
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Figure B9.13 The response to gravity in a Coleus shoot.
114
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before
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clinostat
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7 Make labelled drawings of one seedling from each dish.
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Roots, though, need to grow downwards, into the soil in
order to anchor the plant in the soil, and to absorb water
and minerals from between the soil particles.
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A2 Why was dish B put onto a clinostat and not simply let
in a light place?
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A3 Explain what happened to the seedlings in dish C.
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A4 What was the control in this experiment?
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Plant hormones
phototropism: a response in which a plant grows towards or
away from the direction from which light is coming
We have seen that for an organism to respond to a
stimulus, there must be a receptor to pick up the
stimulus, an efector to respond to it, and some kind of
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gravitropism: a response in which a plant grows towards or
away from gravity
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KEy tERMS
Questions
A1 How did the seedlings in dish A respond to light from
one side? What is the name for this response?
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Figure B9.14 The response to gravity in a root.
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to find out how shoots respond to light
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
1 Label three Petri dishes A, B and C. Line each with
moist cotton wool or filter paper, and put about six
peas or beans in each.
2 Leave all three dishes in a warm place for a day or two,
until the seeds begin to germinate. Check that they do
not dry out.
3 Now put dish A into a light-proof box with a slit in one
side, so that the seedlings get light from one side only.
4 Put dish B onto a clinostat (see diagram) in a light
place. The clinostat will slowly turn the seedlings
around, so that they get light from all sides equally.
If you do not have a clinostat, arrange to turn the
dish by hand three or four times per day to achieve a
similar efect.
5 Put dish C into a completely light-proof box.
6 Leave all the dishes for a week, checking that they do
not dry out.
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ACtivity B9.06
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If the tip is cut off and separated from the rest of the
coleoptile by a piece of agar jelly, the coleoptile still grows
towards the light.
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Tropisms are controlled by hormone-like chemicals in
the plant. Tropisms are examples of chemical control of
plant growth.
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Figure B9.15 An experiment to investigate the method by
which shoots respond to light.
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Figure B9.15 shows an experiment that can be done to
find out which part of a shoot picks up the stimulus of light
shining onto it. The sensitive region is the tip of the shoot.
This is where the receptor is.
Even light
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Cells on this side
grow quickly.
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Cells on
this side
grow
slowly.
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Auxin
One kind of plant hormone is called auxin. Auxin is
being made all the time by the cells in the tip of a shoot.
The auxin difuses downwards from the tip, into the rest of
the shoot.
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Auxin made in the tip
diffuses evenly down the
shoot. The shoot grows
straight up.
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Auxin concentrates on the shady
side. This causes the shady side
to grow faster than the light side,
so the shoot bends towards
the light.
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Figure B9.16 Auxin and phototropism.
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Auxin makes the cells just behind the tip get longer.
The more auxin there is, the faster the cells grow. As they
grow, they get longer (elongate). Without auxin, they will
not grow (Figure B9.16).
light
Auxin is made here.
These two parts of the shoot must be communicating with
one another somehow. They do it by means of chemicals
called plant hormones.
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unidirectional light
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The part of the shoot which responds to the stimulus is the
part just below the tip. This is the efector.
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light
But if a piece of mica separates the tip from the rest of the
coleoptile, then it does not grow towards the light.
This suggests that the response to light is caused by a
substance which is made in the tip and diffuses down
the coleoptile.
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Mica; substances
cannot diffuse
through this.
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Plants, however, do not have complex sense organs,
muscles or nervous systems. So how do they manage to
respond to stimuli like light and gravity?
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light
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Agar jelly; most
substances can
diffuse through
this.
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communication system in between. In mammals, the
receptor is oten part of a sense organ, and the efector is a
muscle or gland. Information is sent between them along
nerves, or sometimes by means of hormones.
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light
If the tip of the coleoptile is cut off and then replaced, the
coleoptile will still grow towards the light.
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to find out how roots respond to gravity
Skills:
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
AO3.5 Evaluating methods
You are going to design this investigation yourself. You can
use similar techniques to those in Activity B9.06. This is the
hypothesis you are going to test:
Roots grow towards gravity.
When you have written your plan, get it checked by your
teacher before you try to carry it out. Write it up in the
usual way, including a discussion and evaluation.
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ACtivity B9.07
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B9: Coordination and homeostasis
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115
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B9.06 Homeostasis
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The cells inside your body, however, do not have a changing
environment. Your body keeps the environment inside you
almost the same, all the time. In the fluid surrounding your
cells, the temperature and amount of water are kept almost
constant. So is the concentration of glucose. Keeping this
internal environment constant is called homeostasis.
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ity
In this section, you will see two examples of how
homeostasis is carried out in humans. The nervous system
and various endocrine glands are involved, as well as the
skin, pancreas and liver.
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to find out how auxin afects shoots
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homeostasis: the maintenance of a constant internal
environment
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What part of the shoot is sensitive to light?
B9.19
What part of the shoot responds to light?
B9.20
How do these parts communicate with each
other? How is this like or unlike a similar system in
a mammal?
B9.21
How does the normal response of a shoot to light
help the plant?
B9.22
How does a root respond to gravity?
the skin
One of the most important organs involved in temperature
regulation in mammals is the skin. Figure B9.17 shows a
section through human skin.
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B9.18
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Human skin is made up of two layers. The top layer is
called the epidermis, and the lower layer is the dermis.
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All the cells in the epidermis have been made in the layer
of cells at the base of it. These cells are always dividing.
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Some animals - including ourselves - are very good
at controlling their body temperature. They can keep
their temperature almost constant, even though the
temperature of their environment changes. All mammals
can do this, and so can birds.
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Control of body temperature
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to find out which part of a shoot is sensitive
to light
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ACtivity B9.09
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ACtivity B9.08
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With the stem in the horizontal position, auxin tends to
collect on the lower side of the stem, causing faster growth
there. Therefore, the stem curves upward.
In the same way, in the bean seedlings shown in
Figure B9.14, auxin has built up on the lower surface of
the root. The efect here, however, is the opposite to that
in the Coleus shoot. This amount of auxin slows down
the growth on this side, so the radicle bends downwards.
Homeostasis is very important. It helps your cells to work as
eficiently as possible. Keeping a constant temperature of
around 37 °C helps enzymes to work at the optimum rate.
Keeping a constant amount of water means that your cells
are not damaged by absorbing or losing too much water
by osmosis. Keeping a constant concentration of glucose
means that there is always enough fuel for respiration.
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This explains why, if a potted Coleus plant is placed on its
side in a dark room overnight, the shoot will bend upwards
(Figure B9.13). Since there is no light, we can presume the
result to be a response to gravity. (What other precaution
should we take to be sure of this?)
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The environment (surroundings) of a living organism is
always changing. Think about your own environment. The
temperature of the air around you changes. For example, if
you live in a temperate country, it might be –10 °C outside
on a cold day in winter, and 23 °C indoors. If you live in the
tropics, the outside temperature may be over 40 °C.
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When, however, light shines onto a shoot from one side,
the auxin at the tip concentrates on the shady side.
This makes the cells on the shady side elongate faster than
the ones on the bright side, so the shoot bends towards
the light.
Maintaining the internal environment
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When light shines onto a shoot from all around, auxin
is distributed evenly around the tip of the shoot. The
cells all grow at about the same rate, so the shoot grows
straight upwards. This is what normally happens in plants
growing outside.
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hair erector
muscle
hair
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venule
(small vein)
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fat cells
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arteriole
(small artery)
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Figure B9.17 A section through human skin.
The new cells that are made gradually move towards the
surface of the skin. As they go, they die, and fill up with a
protein called keratin. The top layer of the skin is made up
of these dead cells. It is called the cornified layer.
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Beneath the skin is a layer of fat, called adipose tissue.
This fatty tissue is made up of cells which contain large
drops of oil. This layer helps to insulate your body against
heat loss, and it also acts as an energy reserve.
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The cornified layer protects the soter, living cells
underneath, because it is hard and waterproof. It is always
being worn away and replaced by cells from beneath. On
the parts of the body which get most wear – for example,
the soles of the feet – it grows thicker.
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the hypothalamus
A part of the brain called the hypothalamus is at the
centre of the control mechanism that keeps internal
temperature constant. The hypothalamus coordinates
the activities of the parts of the body that can bring
about temperature changes.
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Some of the cells in the epidermis contain a dark brown
pigment, called melanin. Melanin absorbs the harmful
ultraviolet rays in sunlight, which would damage the living
cells in the deeper layers of the skin.
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You can see sweat glands in Figure B9.17. These secrete a
liquid called sweat. Sweat is mostly water, with small amounts
of salts and urea dissolved in it. It travels up the sweat ducts,
and out onto the surface of the skin through the sweat pores.
As we will see, sweat helps in temperature regulation.
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• Muscles in some parts of the body contract and relax
very quickly. This produces heat. It is called shivering.
The heat generated in the muscles warms the blood as
it flows through them. The blood distributes this heat all
over the body.
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The hypothalamus acts like a thermostat. It contains
temperature receptors that sense the temperature of the
blood running through it. If this is above or below 37 °C,
then the hypothalamus sends electrical impulses, along
nerves, to the parts of the body which have the function of
regulating your body temperature.
If your body temperature drops below 37 °C, nerve
impulses from the hypothalamus cause the following
things to happen (Figure B9.18).
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There are also blood vessels and nerve endings. These
nerve endings are sensitive to touch, pain, pressure and
temperature, so they help to keep you aware of changes in
your environment.
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Here and there, the epidermis is folded inwards, forming
a hair follicle. A hair grows from each one. Hairs are
made of keratin.
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dermis
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shunt vessel
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pressure
receptor
blood
capillary
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epidermis
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neurone
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sweat gland
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temperature
receptors
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sweat pore
hair follicle
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cornified layer
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B9: Coordination and homeostasis
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117
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• The arterioles supplying the capillaries near the surface
of the skin get wider – they become dilated. This is
called vasodilation. More blood therefore flows through
them. Because a lot of blood is so near the surface of
the skin, heat is readily lost from the blood into the air.
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Figure B9.19 summarises the way in which the
hypothalamus, skin and muscles work together to keep
your internal body temperature constant. In fact, it is not
possible to keep it perfectly constant – the system only
manages to keep it within narrow set limits.
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Sweat
evaporates from
the skin surface,
cooling it.
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Arterioles in the
skin constrict, so not
much blood flows
through them.
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More blood
is brought to
the surface
capillaries where
it can lose heat.
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Erector muscles
contract, pulling
hairs up on end.
Arterioles supplying
the capillaries dilate,
bringing more blood
to the capillaries.
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Erector muscles
relax, so the
hairs lie flat on
the skin and trap
less air.
Capillaries are
supplied with
less blood from
arterioles, so they
remain narrow.
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When the body is too hot
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The upright hairs trap
a layer of warm air
next to the skin, which
insulates it.
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When the body is too cold
We have seen that, when the temperature of your
blood rises above the norm, the hypothalamus senses
this. It responds by sending nerve impulses to your skin
that bring about actions to help cool the blood.
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• The erector muscles in the skin relax, so that the hairs lie
flat on the skin.
118
The blood vessels do not move up and down through the
skin. They just get wider and narrower.
Negative feedback
If body temperature rises above 37 °C, nerve impulses from
the hypothalamus cause the following things to happen.
• The sweat glands secrete sweat. The sweat lies on the
surface of the hot skin. The water in it then evaporates,
taking heat from the skin with it, thus cooling the body.
TIP
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• The arterioles that supply the blood capillaries near
the surface of the skin become narrower, or constricted.
This is called vasoconstriction. Only a very little blood
can flow in them. The blood flows through shunt
vessels and the deep-lying capillaries instead. Because
these are deep under the skin, beneath the insulating
fatty tissue, the blood does not lose so much heat
to the air.
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• The erector muscles in the skin contract, pulling the
hairs up on end. In humans, this does not do anything
very useful – it just produces ‘goose pimples’. In a hairy
animal such as a cat, though, it traps a thicker layer of
warm air next to the skin. This prevents the skin from
losing more warmth. It acts as an insulator.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Figure B9.18 How skin helps with temperature regulation.
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The arteriole supplying the sweat
gland dilates, bringing more blood so
the gland can make more sweat.
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B9: Coordination and homeostasis
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The body gains heat.
mechanisms to lose
heat
respiration slows
hair lies flat
surface blood vessels dilate
sweating
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respiration increases
muscles work
hair stands up
surface blood vessels contract
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temperature
receptors in
the skin
hypothalamus
(control centre)
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The body is too cold.
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The body is too hot.
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The body loses heat.
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taking place and starts of another set of actions that help
to raise the blood temperature.
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So, all the time, the hypothalamus is monitoring small
changes in the temperature of your blood. As soon as this
rises above normal, actions take place that help to reduce
the temperature. Then, as soon as the hypothalamus
senses the lowered temperature, it stops these actions
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3 Now collect some hot water. Pour water into each of your
containers until they are almost full. Immediately take
the temperature of each one and record your results for
time 0.
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5 Draw a line graph to display your results.
Questions
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A1 a State two variables that are kept constant in
this experiment.
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b Why is it important to keep these variables
constant?
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A2 a Calculate the number of °C by which the large
container cooled during your experiment.
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b Calculate the number of °C by which the small
container cooled during your experiment.
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A3 Do your results support the hypothesis that you were
testing? Explain your answer.
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4 Take readings every 2 minutes for at least 14 minutes.
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Experiment to investigate the efect of size on
rate of cooling
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
Temperature regulation is an important part of homeostasis.
We lose heat from our bodies to the air around us. Cells produce
more heat to prevent the body temperature from dropping.
In this investigation, you will use containers of hot water
to represent a human body. The experiment will test
this hypothesis:
A large body cools more slowly than a small one.
1 Take two test tubes or other containers, identical except
that one is large and one is small. You will also need
two thermometers.
2 Read through what you are going to do. Draw a results
chart in which you can write your results as you go along.
Remember to put the units in your table headings.
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ACtivity B9.10
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This process is called negative feedback. The term
‘feedback’ refers to the fact that, when the hypothalamus
has made your skin take action to increase heat loss,
information about the efects of these actions is ‘fed back’
to it, as it senses the drop in the blood temperature. It is
called ‘negative’ because the information that the blood
has cooled down stops the hypothalamus making your
skin do these things.
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When the cooler blood reaches the hypothalamus, this
responds by sending nerve impulses to your skin that
bring about actions to help reduce the rate at which heat
is lost from the blood. At the same time, the rate of heat
production in the muscles is increased.
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Figure B9.19 Maintaining body temperature in a steady state.
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mechanisms to gain
or save heat
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Cambridge IGCSE Combined and Co-ordinated Sciences
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On the other hand, too much glucose in the blood is
not good either, as it can cause water to move out of
cells and into the blood by osmosis. This leaves the cells
with too little water for them to carry out their normal
metabolic processes.
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ACtivity B9.11
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The control of blood glucose concentration is carried out
by the pancreas and the liver (Figure B9.20).
Give two functions of the fatty tissue beneath
the skin.
B9.24
Explain how sweating helps to cool the body.
B9.25
Name the organ which coordinates
temperature regulation.
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Scattered through the pancreas, however, are groups of
cells called islets of Langerhans. These cells do not make
pancreatic juice. They make two hormones called insulin
and glucagon. These hormones help the liver to control
the amount of glucose in the blood. Insulin has the efect
of lowering blood glucose concentration, and glucagon
does the opposite.
ni
Explain what vasodilation is, and how it helps to
cool the body.
B9.27
Explain what is meant by negative feedback.
U
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If you eat a meal that provides a lot of glucose, the glucose
is absorbed through the walls of the small intestine,
and the concentration of glucose in the blood goes up.
The islets of Langerhans detect this rise in the blood
glucose concentration and secrete insulin into the blood.
When insulin reaches the liver, it causes the liver to absorb
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Pr
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Glucose is released from
the liver into the blood
and the blood glucose
concentration rises.
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normal levels of
blood glucose
The liver breaks down
glycogen into
glucose.
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Figure B9.20 How blood glucose concentration is regulated.
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high blood
glucose
low blood
glucose
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Glucagon is secreted.
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id
The blood glucose concentration falls.
pancreas
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Insulin is secreted.
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Liver cells use some
glucose in respiration
and store some
glucose as
glycogen.
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The control of the concentration of glucose in the blood is
a very important part of homeostasis. Cells need a steady
supply of glucose to allow them to respire; without this,
they cannot release the energy they need. Brain cells are
especially dependent on glucose for respiration, and they
die quite quickly if they are deprived of it.
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Control of blood glucose concentration
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B9.26
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The pancreas is two glands in one. Most of it is an ordinary
gland with a duct. It makes pancreatic juice, which
flows along the pancreatic duct into the duodenum
(Section B5.04).
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QuEStiONS
B9.23
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investigating the efect of evaporation on the
rate of cooling
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B9: Coordination and homeostasis
glucose from the blood. Some of this glucose is used
for respiration, but some is converted into the insoluble
polysaccharide glycogen. This is stored in the liver.
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If the blood glucose concentration falls too low, the
pancreas secretes glucagon. This causes liver cells to break
down glycogen to glucose, and release it into the blood.
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sensory neurone
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relay neurone
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has its cell body in the central nervous system
carries nerve impulses away from a receptor
carries nerve impulses towards its cell body
carries nerve impulses away from its cell body
is entirely inside the central nervous system
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motor neurone
radial muscles
relaxation
relay neurone
retina
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suspensory ligaments
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sensory neurone
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receptor
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lens
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efector
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cornea
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contraction
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conjunctiva
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circular muscles
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Choose the term from the list that matches each of the descriptions. You may use each term once,
more than once or not at all.
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motor neurone
Identify the type of neurone – sensory, relay or motor – that matches each of these descriptions.
For some descriptions, more than one type of neurone may match.
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electrical impulse
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receptor
3
■
What is the correct term for this type of reaction?
Using each of the following words at least once, but not necessarily in this order, explain how this
reaction is brought about.
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a
b
2
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If you step on a sharp object, muscles in your leg will rapidly pull your foot away.
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End-of-chapter questions
efector
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■
about gravitropism and phototropism in plants
how to do experiments to investigate gravitropism and
phototropism in plants
the role of auxin in controlling gravitropism
and phototropism
what homeostasis is, and how the skin and brain help
to keep body temperature constant
about the concept of negative feedback
how the pancreas and liver help to regulate the
concentration of glucose in the blood.
■
ie
■
the structure of the human nervous system
how information is passed as electrical impulses
along neurones
about reflex actions, and the diferent kinds of
neurone involved in a reflex arc
the structure and function of the eye, including the
pupil reflex and accommodation
what hormones are, and how the hormone adrenaline
is involved in the fight or flight response
diferences and similarities between the nervous and
hormonal control systems
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You should know:
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Summary
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(continued)
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Homeostasis means keeping your body temperature constant.
When we are cold, our hairs stand on end, which keeps us warm.
The fatty layer under the skin stops cold air getting into the body.
When we are too hot, our sweat glands secrete a cold liquid that cools us down.
When you are too hot, your blood capillaries move closer to the skin surface.
Insulin is an enzyme that changes glucose to glycogen.
ie
When a person is submerged in cold water, their body temperature can drop very quickly. This is because
heat is transferred quickly, by conduction, from the warm body into the cold water. An experiment was
carried out to see if it is better to stay still if you fall into cold water, or to try to swim.
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es
s
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Two men sat for minutes, in air at a temperature of °C.
They then got into a swimming pool, where the water was also at a temperature of °C.
Person A swam for the next 30 minutes. Person B lay still in the water.
The body temperatures of both men were measured at minute intervals throughout the experiment.
The results are shown in the graph.
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lay still
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37.0
B
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37.5
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•
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5
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36.5
36.0
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35.0
34.0
10
20
30
40
Time / minutes
swam
50
60
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34.5
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[2]
[4]
[2]
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State the body temperature of each man at the start of the experiment.
Explain why their body temperatures remained roughly constant for the first 30 minutes of
the experiment.
Explain why the body temperatures of both men dropped between 30 minutes and 60 minutes.
Suggest why person A’s temperature dropped faster than person B’s temperature during this
time period. (This is quite a dificult question! You may find thinking about exchange surfaces
is helpful.)
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35.5
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a
b
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A
Body temperature /°C
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Each of these sentences contains incorrect information. Identify what is wrong, and then write a sentence
that provides correct information.
op
4
a nerve cell that transmits impulses from the central nervous system to an efector
a cell that is sensitive to a stimulus
the part of the eye that refracts light rays most strongly
the part of the eye that contains receptor cells
the action of the ciliary muscle when the eye is focusing on a nearby object
the muscles in the iris that contract to reduce the amount of light entering the eye
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Cambridge IGCSE Combined and Co-ordinated Sciences
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[3]
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A
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50
1.00 pm
2.00 pm
3.00 pm
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i Explain why the concentration of glucose in the blood rises between A and B.
ii Explain why the concentration of glucose in the blood falls between B and C
The graph shows that the blood glucose concentration remains fairly constant between C and D.
Explain the role of negative feedback in keeping blood glucose level constant.
[3]
[3]
Define the term sensitivity.
Some seedlings are supported on their sides in a light-proof container and let for five days.
The diagram below shows what happens.
[2]
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es
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ater five days
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stand stem
root
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[1]
[2]
[2]
[3]
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 32 Q10 November 2014]
e
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Pr
Name the response of the seedlings shown in the diagram.
With reference to the diagram, describe how the survival chances of a plant are increased
by the type of response shown by:
• the roots
• the stems
iii Explain the role of auxin in the responses of the roots and stems shown by these seedlings.
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[3]
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light-proof
vertical cork mat container
seedlings
pinned to
cork mat
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4.00 pm
Time of day
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7
D
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B
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d
[1]
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150
Blood glucose
100
concentration
/ mg per 100 cm3
[3]
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Explain why body cells need a constant supply of glucose.
In healthy humans, the blood normally contains about 90 mg of glucose per 100 cm3 of blood.
Name the gland that secretes the hormones that help to keep this concentration fairly constant.
The graph below shows the changes in concentration of blood glucose ater a meal containing starch.
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B9: Coordination and homeostasis
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123
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Asexual reproduction
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In reproduction, each new organism obtains a set of
chromosomes from its parent or parents.
Chromosomes are long threads, made of DNA, found
in the nucleus of every cell, and they contain sets of
instructions known as genes. As you will find out in
Chapter B12, these genes vary slightly from one another
in diferent individuals.
op
Sexual reproduction
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In sexual reproduction, the parent organism produces
sex cells called gametes. Eggs and sperm are examples
of gametes. Two of these gametes join and their nuclei fuse
together. This is called fertilisation. The new cell formed
by fertilisation is called a zygote. The zygote divides again
and again, and eventually grows into a new organism.
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s
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es
Reproduction is one of the fundamental characteristics
of all living things. Each kind of organism has its own
particular method of reproducing, but all of these methods
fit into one of two categories – asexual reproduction or
sexual reproduction.
Asexual reproduction involves just one parent. Some of the
parent organism’s cells divide by a kind of cell division that
produces new cells containing exactly the same genes as the
parent cell. The new cells are therefore genetically identical
to the parent cell and to each other. They grow into new
organisms, which are all genetically identical to each other
and to their single parent. Look at Images B10.01 and B10.02.
s
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B10.01 Asexual and sexual
reproduction
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the structure and function of the parts of a flower
the diferences between insect-pollinated and
wind-pollinated flowers
how pollination and fertilisation take place
the conditions that seeds need for germination.
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asexual and sexual reproduction
the nuclei of gametes and zygotes
the advantages and disadvantages of sexual reproduction
the advantages and disadvantages of asexual
reproduction
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This chapter covers:
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B10
Reproduction in plants
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B10: Reproduction in plants
KEy tERMS
asexual reproduction: a process resulting in the production
of genetically identical ofspring from one parent
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Gametes
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Gametes are diferent from ordinary cells, because they
contain only half as many chromosomes as usual. This is
so that when two of them fuse together, the zygote they
form will have the correct number of chromosomes.
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In many organisms, there are two diferent kinds of
gamete. One kind is quite large, and does not move much.
This is called the female gamete. In humans, the female
gamete is the egg. In flowering plants the female gamete is
a nucleus inside the ovule (Figures B10.02 and B10.03).
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The same is true for plants. For example, the cells of a
eucalyptus tree have 22 chromosomes. Their male and
female gametes each have 11 chromosomes. When these
fuse together, they produce a zygote, inside a seed, that
has 22 chromosomes.
Male gametes and female gametes
Image B10.02 This photograph shows a leaf from a plant
called Kalanchoe. It grows tiny plantlets along the edges of
its leaves, which will eventually drop of and grow their own
roots, becoming separate plants.
am
An egg or sperm, though, only has 23 chromosomes – a
single set. It is called a haploid cell. Gametes are always
haploid. When two gametes fuse together, they form a
diploid zygote.
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Humans, for example, have 46 chromosomes in each of
their body cells. But human egg and sperm cells only
have 23 chromosomes each. When an egg and sperm
fuse together at fertilisation, the zygote which is formed
will therefore have 46 chromosomes, the normal number
(Figure B10.01).
The 46 chromosomes in an ordinary human cell are of
23 diferent kinds. There are two of each kind. This is
because there are two sets of chromosomes in the cell.
One set came from the father, and one set from the
mother. A cell which has the full number of chromosomes,
with two complete sets, is called a diploid cell.
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Image B10.01 Hydra is a tiny animal that lives in freshwater
ponds and lakes. This one is reproducing by growing a bud
from itself. The bud will eventually break away to form a
separate Hydra.
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sexual reproduction: a process involving the fusion of the
nuclei of two gametes (sex cells) to form a zygote, and the
production of ofspring that are genetically diferent from each
other
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The zygote contains chromosomes from both its
parents. It can have any combination of their genes.
Sexual reproduction therefore produces ofspring that
are genetically diferent from each other and from
their parents.
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125
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23
In sexual reproduction, cells in
testes and ovaries divide by meiosis,
producing gametes, with half the
number of chromosomes.
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On the outside of the flower are the sepals. The sepals
protect the flower while it is a bud. Sepals are oten green.
rs
Just inside the sepals are the petals. These are oten
brightly coloured. The petals attract insects to the flower.
The petals of some flowers have lines running from
top to bottom. These lines are called guide-lines,
because they guide insects to the base of the petal.
Here, there is a gland called a nectary. The nectary
makes a sugary liquid called nectar, which insects
feed on.
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B10.01 Explain why ofspring produced by asexual
reproduction are genetically identical.
B10.03 What is a zygote?
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B10.04 Why do gametes contain only half the normal
number of chromosomes?
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B10.05 What is meant by a diploid cell?
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B10.06 Name one part of your body where you have
diploid cells.
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B10.08 Give one example of a haploid cell.
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The female parts of diferent kinds of flower vary. One of
the diferences is the arrangement of the ovules in the
ovary. Figure B10.03 shows one arrangement.
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Do not use the word ‘flower’ when you mean ‘plant’. A plant
is a complete organism. A flower is just part of a plant.
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The female part of the flower is in the centre. It consists of
one or more carpels. A carpel contains an ovary. Inside
the ovary are many ovules, which contain the female
gametes. At the top of the ovary is the style, with a
stigma at the tip. The function of the stigma is to catch
pollen grains.
Many flowering plants can reproduce in more than one
way. Oten, they can reproduce asexually and also sexually,
by means of flowers.
TIP
Inside the petals are the stamens. These are the male
parts of the flower. Each stamen is made up of a long
filament, with an anther at the top. The anthers contain
pollen grains, which contain the male gametes.
s
B10.07 What is meant by a haploid cell?
B10.02 Flowers
The function of a flower is to make gametes, and to
ensure that fertilisation will take place. Figure B10.02
illustrates the structure of an insect-pollinated flower.
Image B10.03 shows flowers of Eucryphia.
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Flower structure
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The other sort of gamete is smaller, and usually moves
actively in search of the female gamete. This is called the
male gamete. In humans, the male gamete is the sperm.
In flowering plants, the male gamete is found inside the
pollen grain. It does not move by itself, but is carried to the
female gamete by a pollen tube (Figure B10.06).
B10.02 What is a gamete?
When the male and female gametes
join together, a zygote is formed
which has the full number of
chromosomes.
ie
Figure B10.01 Sexual reproduction in humans.
QuEStiONS
y
egg
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The cells in a human body each
contain 46 chromosomes.
46
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meiosis
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female body cell
46
fertilisation
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male body cell
126
zygote
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sperm
meiosis
46
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Cambridge IGCSE Combined and Co-ordinated Sciences
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ovule
filament
sepal
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stamen
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ovary
anther
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carpel
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stigma
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petal
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B10: Reproduction in plants
nectary
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receptacle
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flower stalk
pollen grains
caught by stigma
stigma
style
127
ovary wall
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ovule
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Figure B10.02 A generalised flower.
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Image B10.03 Eucryphia flowers. Eucryphia is a tree that
grows wild in South America.
receptacle
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Figure B10.03 Section through the female part of a flower.
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Pollen looks like a fine powder. It is oten yellow. Under
the microscope, you can see the shape of individual
grains (Image B10.04). Pollen grains from diferent kinds
of flower have diferent shapes. Each grain is surrounded
by a hard coat, so that it can survive in dificult conditions
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Figure B10.04a illustrates a young anther, as it looks before
the flower bud opens. You can see in Figure B10.04b
that the anther has four spaces or pollen sacs inside it.
Some of the cells around the edge of the pollen sacs divide
to make pollen grains. When the flower bud opens, the
anthers split open (Figure B10.04c). Now the pollen is on
the outside of the anther.
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The male gametes are inside the pollen grains, which are
made in the anthers.
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Pollen grains and ovules
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mature pollen grains
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Figure B10.04 How pollen is made.
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anther
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c Transverse section through
a mature flower anther
pollen sac, containing
developing pollen grains
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Pollination is oten carried out by insects (Image B10.05).
Insects such as bees come to the flowers, attracted by
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For fertilisation to take place, the male gametes must
travel to the female gametes. The first stage of this journey
is for pollen to be taken from the anther where it was made
to a stigma. This is called pollination.
b Transverse section through
a young flower anther
lines along which
the anther will
split
filament
y
Pollination
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a A young flower anther
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if necessary. The coat protects the male gametes that are
inside the grains, as the pollen is carried from one flower
to another.
The female gametes are inside the ovules, in the ovary.
Each ovule contains a nucleus. Fertilisation happens when
a pollen grain nucleus fuses with an ovule nucleus.
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Image B10.04 These pollen grains from a daisy flower are
sticking to the surface of a petal. The electron micrograph
is magnified about × 800.
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investigating the structure of a flower
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
! Take care with the sharp knife blade.
During this investigation, make large, labelled
drawings of the structures that you observe.
1 Take an open, fresh-looking flower. Suggest two ways
in which the flower advertises itself to insects.
2 Gently remove the sepals from the outside of the
flower. Look at the sepals on a flower bud, near the top
of the stem. What is the function of the sepals?
3 Now remove the petals from your flower. Make a
labelled drawing of one of them, to show the markings.
What is the function of these markings?
4 Find the stamens. If you have a young flower, there will
be pollen on the anthers at the top of the stamens.
Dust some onto a microscope slide, and look at it
under a microscope. Draw a few pollen grains.
5 Now remove the stamens. What do you think is the
function of the filaments?
6 Using a hand lens, try to find the nectaries at the
bottom of the flower. What is their function?
7 The carpel is now all that is let of the flower. Find an
ovary, style and stigma. Look at the stigma under a
binocular microscope or a lens. What is its function,
and how is it adapted to perform it?
8 Using a sharp blade, make a clean cut lengthways
through the ovary, style and stigma. You have made a
longitudinal section. Find the ovules inside the ovary.
How big are they? What colour are they? About how
many are there?
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ACtivity B10.01
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B10: Reproduction in plants
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their colour and strong sweet scent. The bee follows the
guide-lines to the nectaries, brushing past the anthers as it
goes. Some of the pollen sticks to its body.
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The bee then goes to another flower, looking for more
nectar. Some of the pollen it picked up at the first flower
sticks onto the stigma of the second flower when the bee
brushes past it. The stigma is sticky, and many pollen
grains get stuck on it. If the second flower is from the same
species of plant as the first, pollination has taken place.
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KEy tERM
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Image B10.05 The bee has come to the flower to collect
nectar. Pollen gets stuck to its body, and the bee will then
carry this to the next flower it visits.
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In some plants, it is the wind which carries the pollen
between flowers. Figure B10.05 shows a grass flower,
which is an example of a wind-pollinated flower.
Image B10.06 shows pollen grains from a grass flower.
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large amounts of
light pollen
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Figure B10.05 An example of a wind-pollinated flower.
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Ater pollination, the male gamete inside the pollen grain
on the stigma still has not reached the female gamete. The
female gamete is inside the ovule, and the ovule is inside
the ovary.
If the pollen grain has landed on the right kind of stigma,
it begins to grow a tube. You can try growing some pollen
tubes, in Activity B10.03. The pollen tube grows down through
the style and the ovary, towards the ovule (Figure B10.06).
It secretes enzymes to digest a pathway through the style.
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Fertilisation
large feathery
stigma hanging
outside the flower
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Table B10.01 compares insect-pollinated and windpollinated flowers.
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Wind pollination
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anthers dangling
outside the flower
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pollination: the transfer of pollen grains from the male part
of the plant (anther of stamen) to the female part of the plant
(stigma)
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Pollination
Skills:
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
You are going to design and carry out an investigation to
test this hypothesis:
Bees visit yellow flowers more oten than
flowers of other colours.
You will need to carry out this investigation outdoors.
It will be much easier to control variables if you make
artificial flowers rather than using real ones. You can make
them using coloured plastic to make ‘petals’, surrounding
a central area where you can put a little pot of sugar
solution. You will need to do your experiment on a sunny
day, when there are plenty of bees flying.
Remember to think about controlling variables. Think
about exactly how you will count the bee visits, how you
will record them and how you will display your results.
Write a simple conclusion from your results, and then
discuss the results in the light of what you know about
pollination. (You might also be interested in finding out
about how bees see colour.) Evaluate your experiment,
and suggest improvements you could make.
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ACtivity B10.02
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ovule
integuments
(outer
covering) of
ovule
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small, inconspicuous petals, or no petals at all
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no scent
no nectaries
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oten have nectaries at the base of petals
Wind-pollinated
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stigma inside flower, where insect has to brush past it to
reach nectar
stigmas large and feathery and dangling outside the flower,
where pollen in the air may land on it
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anthers dangling outside the flower, where they catch
the wind
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anthers inside flower, where insect has to brush past them
to reach nectar
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oten strongly scented
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micropyle
Figure B10.06 Fertilisation in a flower.
large, conspicuous petals, oten with guide-lines
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female nucleus
in ovule
Insect-pollinated
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wall of ovary
male nucleus
travelling down
tube
Image B10.06 Grass pollen (× 35 000).
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style
pollen tube
beginning to
grow
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stigma
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smooth, light pollen, which can be blown in the wind
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sticky or spiky pollen grains, which stick to insects
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quite large quantities of pollen made, because some will be very large quantities of pollen made, because most will be
eaten or will be delivered to the wrong kind of flower
blown away and lost
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Table B10.01 A comparison between insect-pollinated and wind-pollinated flowers.
The pollen nucleus (male gamete) travels along the pollen
tube, and into the ovule. It fuses with the ovule nucleus
(female gamete). Fertilisation has now taken place.
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Once the ovules have been fertilised, many of the parts of
the flower are not needed any more. The sepals, petals and
stamens have all done their job. They wither and fall of.
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One pollen grain can only fertilise one ovule. If there are
many ovules in the ovary, then many pollen grains will be
needed to fertilise them all.
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Inside the ovary, the ovules start to grow. Each ovule now
contains a zygote, which was formed at fertilisation. The
ovule is now called a seed.
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The seed contains a tiny embryo plant and also food
for the embryo. In a bean seed, the food is stored in two
cream-coloured cotyledons containing starch and protein.
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ACtivity B10.03
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Growing pollen tubes
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Seeds
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QuEStiONS
B10.09 What is the function of a flower?
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B10.10 In which part of a flower are male gametes made?
B10.11 In which part of a flower are female
gametes made?
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A seed contains hardly any water. When it was formed on
the plant, the water in it was drawn out, so that it became
dehydrated. Without water, almost no metabolic reactions
can go on inside it. The seed is inactive or dormant.
This is very useful, because it means that the seed can
survive harsh conditions, such as cold or drought, which
would kill a growing plant.
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B10: Reproduction in plants
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3 Put tube B in a refrigerator.
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ACtivity B10.04
to find the conditions necessary for the germination of
tomato seeds
Skills:
AO3.2 Planning
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and data
! Pyrogallol is very caustic. Your teacher will handle it
for you. You should not use it yourself.
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A2 Suggest why each of these conditions is needed for
successful germination.
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A1 What three conditions do tomato seeds need
for germination?
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2 Put tubes A, D and E in a warm place in the laboratory,
in the light.
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6 Leave your seeds for a day or so. Then complete your
results table to show which seeds have germinated.
Questions
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5 Construct a results table and begin to fill it in to show
what conditions the seeds in each tube have.
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4 Put tube C in a warm, dark cupboard.
1 Set up five tubes as shown in the diagram. Pyrogallol
absorbs oxygen.
cotton wool
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In warm,
dark place
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pyrogallol in
sodium hydroxide
solution
In warm,
In warm,
light place
light place
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In cold,
dark place
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water
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In warm,
light place
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water
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water
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perforated
zinc
platform
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wet
cotton
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tomato
seeds
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B10.13 Why do wind-pollinated flowers usually produce
more pollen than insect-pollinated ones?
B10.14 Ater pollination, how does the male gamete reach
the ovule?
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A seed must be in certain conditions before it will begin
to germinate. You can find out what they are if you do
Activity B10.04.
B10.12 What is pollination?
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Cambridge IGCSE Combined and Co-ordinated Sciences
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equally well adapted and are likely to grow well. This is
especially true if there is plenty of space for them in that
area. However, if it is getting crowded, then it may not be
a good thing for the parent to produce new ofspring that
grow all around it.
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ACtivity B10.05
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B10.03 Comparing sexual and
asexual reproduction
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Another advantage of asexual reproduction is that a single
organism can reproduce on its own. It does not need to
wait to be pollinated, or to find a mate. This can be good if
there are not many of those organisms around – perhaps
there is only a single one growing in an isolated place.
In that case, asexual reproduction is definitely the best
option. Do remember, though, that even a single plant may
be able to reproduce sexually, by pollinating its flowers
with its own pollen.
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to find the efect of storage time on the
germination rate of seeds
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In asexual reproduction, some of the parents’ cells divide
in a way that makes new cells that are genetically identical
to the parent cell. They are clones. Asexual reproduction
does not produce genetic variation.
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Many plants can reproduce in two ways – asexually and
sexually. Which is better?
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Sometimes, it is a good thing not to have any variation.
If a plant, for example, is growing well in a particular
place, then it must be well adapted to its environment.
If its ofspring all inherit the same genes, then they will be
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how insect pollination and wind pollination take place
the diferences between insect-pollinated and
wind-pollinated flowers
how fertilisation happens in a flower
how to investigate the environmental conditions that
seeds need to make them germinate.
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the diferences between asexual reproduction and
sexual reproduction
the advantages to a species of asexual reproduction
and sexual reproduction
the names and functions of the parts of a flower
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You should know:
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Summary
In flowering plants, sexual reproduction produces seeds,
which are likely to be dispersed over a wide area. This
spreads the ofspring far away from the parents, so that
they are less likely to compete with them. It also allows
them to colonise new areas.
You will find out more about variation, and its importance
for evolution, in Chapter B13.
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Is it useful or not to have genetic variation among
ofspring? This depends on the circumstances.
132
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But in sexual reproduction, some of the parents’ cells
divide by a method that produces gametes, which have
only half as many chromosomes as the parent cell. When
two sets of chromosomes in the two gametes combine
at fertilisation, a new combination of genes is produced.
So sexual reproduction produces ofspring that are
genetically diferent from their parents.
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However, if the plant is not doing very well in its
environment, or if a new disease or pest has come along
to which it is not resistant, then it could be an advantage
for its ofspring to be genetically diferent from it. There is
a good chance that at least some of the ofspring may be
better adapted to that environment, or be resistant to
that disease.
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B10: Reproduction in plants
1
fertilisation
gamete
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seed
sexual reproduction
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a sex cell; it can be male or female
a cell formed by the fusion of the nuclei of two gametes
a type of reproduction that produces new individuals that are genetically identical to their parent
the transfer of pollen from an anther to a stigma
an ovule ater fertilisation
the fusion of the nuclei of two gametes
a type of reproduction that produces genetically diferent ofspring
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Construct a two-column table, with the headings Asexual reproduction and Sexual reproduction.
Write each of these statements in the correct column.
only one parent involved
one or two parents involved
involves gametes
involves fertilisation
zygote formed
all ofspring genetically identical
genetic variation among ofspring
a
A student investigated the conditions needed for the germination of mustard seeds.
The diagram below shows the apparatus at the start of his experiment.
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B
D
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– 4 °C
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•
•
•
•
•
•
•
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b
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d
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asexual reproduction
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Match each of these words with its definition.
pollination
3
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End-of-chapter questions
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moist
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Tubes A to D were placed in the laboratory at room temperature. Tube E was placed in a freezer at –4 °C.
[1]
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Which one of these factors should the student have kept the same for all of the tubes?
Choose from the list: age of seeds, amount of water, temperature.
Ater three days, the seeds in tubes B and D had germinated. The seeds in all the other
tubes had not germinated.
Use these results to deduce the conditions needed for the germination of mustard seeds.
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seeds
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black
card
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[3]
(continued)
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Cambridge IGCSE Combined and Co-ordinated Sciences
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In a tropical rainforest, the trees oten grow very closely together, which reduces the amount of
light reaching the forest floor.
The seeds of many species of rainforest trees will not germinate unless they get plenty of light.
i Suggest why this is an advantage to the seedlings.
ii In a separate experiment, the student used seeds of rainforest trees. State the tube in
the diagram above in which the result would difer from those he obtained for mustard seeds.
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The ovary of a flower contains one or more ovules. The ovules contain female gametes. Ater fertilisation, an ovule
becomes a seed containing an embryo plant.
The diagram shows a pea seed developing inside a pod.
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B
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A
pea seed
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Explain the meaning of each of the following terms:
gamete
fertilisation
ii Parts A and B in the diagram remain from the flower.
State the name of part A and the function of part B of these parts in the flower.
iii Suggest the part of the flower from which structure C developed.
Four sets of pea seeds were placed in Petri dishes containing either damp soil or
damp filter paper. They were let in diferent conditions, shown in the table below.
[2]
[2]
[1]
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C
warm
light
damp filter paper
warm
dark
damp soil
cold
light
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damp filter paper
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dark
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B
cold
C
damp soil
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Conditions
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Set
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Predict which sets of seeds will germinate. Explain your answer.
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[1]
[Cambridge IGCSE Combined Science 0653 Paper 22 Q4 a & b June 2010]
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[1]
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[3]
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A pea seed was planted in a pot. When the seed had grown into a young plant, the pot was placed
on its side, in a room where light was coming from all sides.
The diagram shows the young pea plant three days ater the pot had been placed on its side.
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The diagram below shows a banana plant producing suckers.
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[2]
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Name the type of reproduction that is shown in the diagram.
Suggest two advantages to the growers of banana plants of using this type of reproduction
to propagate their plants.
Banana plants can be killed by fungal diseases, such as black sigatoka and Panama disease.
Explain why a population of bananas produced by the method shown in the diagram could
all be wiped out by the same disease.
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[2]
[3]
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q8 June 2013]
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Name the response shown by the pea plant in the diagram above.
Suggest how this response will help the plant to reproduce sexually.
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B10: Reproduction in plants
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[2]
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At the base of the uterus is a narrow opening, guarded by
muscles. This is the neck of the uterus, or cervix. It leads to
the vagina, which opens to the outside.
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The opening from the bladder, called the urethra, runs
in front of the vagina, while the rectum is just behind it.
The three tubes open separately to the outside.
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Figure B11.01 shows the reproductive organs of a woman.
The female gametes, called eggs, are made in the two
ovaries. Leading away from the ovaries are the oviducts,
sometimes called Fallopian tubes. They do not connect
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The female reproductive organs
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Humans, like all mammals, reproduce sexually. A new
life begins when a male gamete fuses with a female one,
forming a zygote. This is how you and every other human
being was formed.
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directly to the ovaries, but have a funnel-shaped opening
just a short distance away.
The two oviducts lead to the womb or uterus. This has
very thick walls, made of muscle. It is quite small – only
about the size of a clenched fist – but it can stretch a great
deal when a woman is pregnant.
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B11.01 Human reproductive
organs
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the structure and functions of the male and female human reproductive systems
adaptations of egg and sperm cells
fertilisation and implantation
the diferences between male and female gametes
the functions of the placenta and amnion
the menstrual cycle
HIV/AIDS.
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This chapter covers:
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B11
Reproduction in humans
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oviduct or
Fallopian tube
uterus wall
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vagina
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Figures B11.02 and B11.03 show the reproductive organs of
a man. The male gametes, called spermatozoa or sperm,
are made in two testes (singular: testis). These are outside
the body, in two sacs of skin called the scrotum.
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The sperm are carried away from each testis in a tube
called the sperm duct. The sperm ducts from the testes
join up with the urethra just below the bladder. The urethra
continues downwards and opens at the tip of the penis. The
urethra can carry both urine and sperm at diferent times.
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Gamete production
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Sperm are made continually in the testes from the age of
about 12 to 14 years old. Figure B11.05 shows the structure
of a sperm.
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sperm duct
Sperm production is very sensitive to heat. If they get too
hot, the cells in the tubules will not develop into sperm.
This is why the testes are outside the body, where they are
cooler than they would be inside.
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penis
When she reaches the age of around 10 to 14 years old,
some of these eggs will begin to mature. Usually, only one
develops at a time. When it is mature (Figure B11.04), an
egg cell bursts out of the ovary and into the funnel at the
end of the oviduct. This is called ovulation. In humans,
it happens once a month.
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prostate
gland
epididymis
scrotum
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Figure B11.02 Side view of the male reproductive organs.
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testis
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Figure B11.03 The male reproductive organs.
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bladder
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urethra
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penis
Eggs begin to be formed inside a girl’s ovaries before she
is born. At birth, she will already have thousands of partly
developed eggs inside her ovaries.
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scrotum
Where the sperm ducts join the urethra, there is a gland
called the prostate gland. This makes a fluid which the
sperm swim in.
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epididymis
testis
The male reproductive organs
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duct
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erectile
tissue
cervix
Figure B11.01 The female reproductive organs.
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prostate
gland
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urethra
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ovary
uterus lining
(endometrium)
erectile
tissue
bladder
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B11: Reproduction in humans
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begins in the testes, travels along the sperm ducts,
and into the penis. The sperm are squeezed along,
out of the man’s urethra and into the woman’s vagina.
This is called ejaculation.
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layer of jelly
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cell surface membrane
cytoplasm
containing yolk –
an energy store
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The fluid containing the sperm is called semen.
Ejaculation deposits the semen at the top of the vagina,
near the cervix.
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Sperm can only swim at a rate of about 4 mm per
minute, so it takes quite a while for them to get as far
as the oviducts. Many will never get there at all. But one
ejaculation deposits about a million sperm in the vagina,
so there is a good chance that some of them will reach
the egg.
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Figure B11.04 A human egg cell.
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diameter 0.1 mm
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Very slowly, the egg travels towards the uterus. Cilia lining
the oviduct help to sweep it along.
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If the egg is not fertilised by a sperm within 8–24 hours
ater ovulation, it will die. By this time, it has only travelled
a short way along the oviduct. So a sperm must reach
an egg while it is quite near the top of the oviduct if
fertilisation is to be successful.
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When the man is sexually excited, blood is pumped into
spaces inside the penis, so that it becomes erect. To bring
the sperm as close as possible to the egg, the man’s penis
is placed inside the vagina of the woman. This is called
sexual intercourse.
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Image 11.01 This sperm cell is swimming over the surfaces
of the ciliated cells in the oviduct.
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Sperm are pushed out of the penis into the vagina. This
happens when muscles in the walls of the tubes containing
the sperm contract rhythmically. The wave of contraction
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Ater ovulation, the egg is caught in the funnel of the oviduct.
The funnel is lined with cilia which beat rhythmically, wating
the egg into the entrance of the oviduct.
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B11.02 Fertilisation and
development
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middle piece, containing
mitochondria to release
energy for swimming
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Figure B11.05 A human sperm.
length 0.05 mm
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tail (flagellum), which produces
swimming movements
nucleus, containing
chromosomes
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head
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The sperm are still quite a long way from the egg.
They swim, using their tails, up through the cervix, through
the uterus, and into the oviduct (Image B11.01 and
Figure B11.06).
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nucleus containing
chromosomes
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acrosome – a vesicle
containing enzymes, to
dissolve a way through
the jelly surrounding
the egg cell
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Sperm are let in the
top of the vagina.
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Implantation
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3 The zygote
divides.
5 The cells in the ball
keep dividing as it moves
down the oviduct. It is
now called an embryo.
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placenta forming
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6 Implantation. The
embryo sinks into the
sot lining of the uterus.
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1 Ovulation. A mature
follicle bursts, and
releases an egg into
the oviduct.
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4 Ater several
hours, a ball of
cells is formed.
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2 Fertilisation. A sperm
nucleus fuses with
the egg nucleus,
forming a zygote.
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It takes several hours for the embryo to reach the uterus,
and by this time it is a ball of 16 or 32 cells. The uterus has
a thin, spongy lining, and the embryo sinks into it. This is
called implantation (Figure B11.08).
The embryo grows and develops in the uterus for
approximately nine months. Ater that time, muscles in the
wall of uterus contract and push the baby out through the
cervix and vagina. The baby is still attached to the uterus
by the umbilical cord and the placenta. The placenta falls
away from the uterus wall and passes out through the
vagina. It is called the aterbirth.
As soon as the successful sperm enters the egg,
the egg membrane becomes impenetrable, so that
no other sperm can get in. The unsuccessful sperm
will all die.
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When the sperm nucleus and the egg nucleus have fused
together, they form a zygote. The zygote continues to move
slowly down the oviduct. As it goes, it divides to form a
ball of cells. This is called an embryo. The embryo obtains
food from the yolk of the egg.
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Figure B11.07 Fertilisation.
One sperm enters the egg. Only the head of the sperm
goes in; the tail is let outside. The nucleus of the sperm
fuses with the nucleus of the egg. This is fertilisation
(Figure B11.07).
Figure B11.08 Implantation.
The nucleus of the
successful sperm fuses
with the egg nucleus.
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The head of one sperm
penetrates the egg
membrane.
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Sperm swim through
the uterus and into
the oviduct.
Figure B11.06 How sperm get to the egg (sperm and egg
are drawn to diferent scales).
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The egg
membrane stops
more sperm
getting through.
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If there is an egg in the
oviduct, it will be fertilised.
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B11.02 Where is the prostate gland, and what is
its function?
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B11.01 What is the name for the narrow opening between
the uterus and the vagina?
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B11.08 What is implantation?
B11.07 Study Figures B11.04 and B11.05. Construct a
table to compare the size, structure, numbers and
ability to move of sperm and eggs, and explain
how these features adapt them to carry out
their functions.
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B11.09 What is a fetus?
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B11.11 List two substances which pass from the mother’s
blood into the fetus’s blood.
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B11.10 How is the fetus connected to the placenta?
The fetus is surrounded by a strong membrane, called the
amnion. This makes a liquid called amniotic fluid. This
fluid helps to support the embryo and to protect it.
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QuEStiONS
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thin wall of
placenta
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B11.12 Why does the uterus wall become thick and
spongy before ovulation?
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B11.13 What happens if the egg is not fertilised?
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Figure B11.09 Part of the placenta. (Note that you do not
need to know about the structure of the placenta, but this
may help you to understand what it does.)
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umbilical cord
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Figure B11.10 shows what happens in the ovaries and the
lining of the uterus during the human menstrual cycle.
vein
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artery
fetus’s blood
separated from
mother’s blood
by thin wall of
placenta
If the egg cell is not fertilised, it is dead by the time it
reaches the uterus. It does not sink into the spongy wall,
but continues onwards, down through the vagina. As the
spongy lining is not needed now, it gradually disintegrates.
It, too, is slowly lost through the vagina. This is called
menstruation, or a period. It usually lasts for about
five days. Ater menstruation, the lining of the uterus builds
up again, so that it will be ready to receive the next egg,
if it is fertilised.
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to mother
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B11.03 the menstrual cycle
Usually, one egg is released into the oviduct every month
in an adult woman. Before the egg cell is released, the
lining of the uterus becomes thick and spongy, to prepare
itself for a fertilised egg cell. It is full of tiny blood vessels,
ready to supply the embryo with food and oxygen if it
should arrive.
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space filled
with the
mother’s
blood
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B11.05 How does an egg travel along the oviduct?
Oxygen and dissolved nutrients in the mother’s blood
difuse across the placenta into the fetus’s blood, and are
then carried along the umbilical cord to the fetus. Carbon
dioxide and other waste materials (excretory products)
difuse the other way, and are carried away in the mother’s
blood. As the fetus grows, the placenta grows too. By the
time of birth, the placenta will be a flat disc, about 12 cm in
diameter and 3 cm thick.
lining of uterus
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B11.04 Where are sperm made?
B11.06 Where does fertilisation take place?
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B11.03 Explain how ovulation happens.
Ater eleven weeks, the embryo has developed into a
fetus. The placenta is joined to the fetus by the umbilical
cord. Inside the cord are two arteries and a vein. The
arteries take blood from the fetus into the placenta, and
the vein returns the blood to the fetus.
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QuEStiONS
The cells in the embryo, now buried in the sot wall of
the uterus, continue to divide. As the embryo grows, a
placenta also grows, which connects it to the wall of the
uterus (Figure B11.09). The placenta is sot and dark red,
and has finger-like projections called villi. The villi fit closely
into the uterus wall. The placenta is where substances are
exchanged between the mother’s blood and the embryo’s.
It is the embryo’s life support system.
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The placenta and amnion
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B11: Reproduction in humans
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HIV infects a particular a type of white blood cell. Over
a long period of time, HIV slowly destroys these cells.
Several years ater infection with the virus, the numbers of
certain kinds of white blood cells are so low that they are
unable to fight against other pathogens (disease-causing
organisms) efectively.
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A person who has the virus in their body is said to be
HIV-positive. Several years ater initial infection with HIV,
a person is likely to develop symptoms of AIDS unless
they are given efective treatment. They become very
vulnerable to other infections, such as pneumonia.
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genetic material
(RNA)
10 nm
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Figure B11.11 The human immunodeficiency virus, HIV.
A nanometre (nm) is 1 × 10 –9 m, so this virus is very,
very small.
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The disease AIDS, or acquired immune deficiency
syndrome, is caused by HIV. HIV stands for human
immunodeficiency virus. Figure B11.11 shows this virus.
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protein
Sexually transmitted infections are caused by bacteria or
viruses that can be passed from one person to another
during sexual intercourse. By far the most important of
these infections is HIV/AIDS.
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B11.04 Hiv/AiDS
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The follicle turns into a corpus
luteum. The lining of the uterus
becomes more vascular, ready
to receive the embryo if the egg
is fertilised.
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Menstruation. As the egg has
not been fertilised, the thick
uterus lining is not needed. It
breaks down, and is gradually
lost through the vagina.
Figure B11.10 The menstrual cycle.
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Ovulation. The follicle bursts,
releasing an egg cell from the
ovary. Fertilisation could
take place.
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Inside the ovary, a
follicle containing an egg
cell develops. The uterus
lining is repaired.
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Cambridge IGCSE Combined and Co-ordinated Sciences
They may develop cancer, because one function of
white blood cells is to destroy body cells which may be
beginning to produce cancers. Brain cells are also quite
oten damaged by HIV. A person with AIDS usually dies
from a collection of several illnesses.
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Preventing HIV transmission
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The virus that causes AIDS cannot live outside the human
body. In fact, it is an especially fragile virus – much less
tough than the cold virus, for example. You can only
become infected with HIV through direct contact of your
body fluids with those of someone with the virus.
This can be in one of the following ways.
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From mother to baby
The virus, HIV, can pass from a mother to her baby during
pregnancy, during birth, or during breast-feeding.
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The more sexual partners a person has, the higher the
chance of them becoming infected with HIV. In some parts
of the world, where it is common practice for men to have
many diferent sexual partners, very high percentages
of people have developed AIDS. This is so in some parts
of Africa and Asia, and also amongst some homosexual
communities in parts of Europe and the USA.
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The best way of decreasing the risk of transmission of
the virus to a baby is to treat an HIV-positive woman with
antiretroviral drugs before and during her pregnancy.
Mothers who are HIV-positive may also be advised not to
breast feed their baby.
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Summary
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the functions of the amnion and the placenta
about the changes in the ovaries and the uterus during
the menstrual cycle
about HIV/AIDS and ways in which it can be prevented
from spreading.
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the structure and functions of the male and female
reproductive organs
how and where fertilisation takes place
how the structures of sperm and egg cells are adapted
to their functions
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You should know:
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The best way of avoiding infection with HIV is never to
have more than one sexual partner. If everyone did that,
People who have to deal with accidents, such as police
and paramedics, must always be on their guard against
HIV if there is blood around. They oten wear protective
clothing, just in case a bleeding accident victim is infected
with HIV.
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through sexual activity
HIV can live in the fluid inside the vagina, rectum and
urethra, and also in the blood. During sexual intercourse,
fluids from one partner come into contact with fluids of the
other. It is very easy for the virus to be passed on in this way.
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Blood can also be transferred from one person to another
if they share hypodermic needles. This most commonly
happens in people who inject drugs, such as heroin.
Many drug users have died from AIDS. It is essential that
any hypodermic needle used for injection is sterile.
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through blood contact
Many cases of AIDS have been caused by HIV being
transferred from one person’s blood to another. In the
1970s and 1980s, when AIDS first appeared, and before
anyone knew what was causing it, blood containing
HIV was used in transfusions. People being given the
transfusions were infected with HIV, and later developed
AIDS. Now all blood used in transfusions in most countries
is screened for HIV before it is used.
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There is still no cure for AIDS, though drugs can greatly
increase the life expectancy of a person infected with
HIV. These are called antiretroviral drugs. Researchers are
always trying to develop new drugs and vaccines which
will kill the virus without damaging the person’s own cells.
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then the virus would immediately stop spreading. Using
condoms is a good way of lowering the chances of the
virus passing from one person to another during sexual
intercourse – though it does not rule it out.
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B11: Reproduction in humans
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the place where an egg is fertilised
the organ where eggs are made
the organ in which an embryo develops
a ring of muscle at the base of the uterus
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The diagram shows a fetus developing in the uterus.
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F
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Write the name of the parts of the female reproductive system that match each description.
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End-of-chapter questions
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a
The diagram below shows two gametes: a sperm cell and an egg cell.
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Name each of the parts labelled A to I.
Describe the function of part C.
Outline the function of part F.
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egg cell
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[1]
[1]
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i State one way in which both of these cells difer from other cells of the body.
ii Suggest an advantage of the egg cell being larger than the sperm cell.
iii A fertilised egg divides into a ball of cells and becomes attached to the lining of the uterus.
Explain why it is important that this ball of cells soon becomes attached to the lining
of the uterus.
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sperm cell
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[4]
(continued)
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placenta
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b The diagram below shows a developing fetus inside its mother’s body.
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B
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Identify the parts labelled A, B and C.
State what causes blood to flow along A.
State a function of the fluid inside structure B.
State two substances which pass from the mother to the fetus, and two waste substances
which pass from the fetus to the mother.
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flagellum
digestive enzymes
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nucleus
With reference to your diagram, explain how the structure of a sperm adapts it for its function.
Describe how a human egg cell is adapted for its function.
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cytoplasm
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cell membrane
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Make a copy of the diagram. On your diagram label the following parts:
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The diagram shows a human sperm.
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[Cambridge O Level Human and Social Biology 5096 Paper 21 Q1 a & b November 2011]
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A
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[3]
[4]
[3]
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5
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B11: Reproduction in humans
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The graph shows the number of people in the Caribbean who were known to be infected with HIV,
who had AIDS and who died from AIDS, between 1982 and 2008.
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2000
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AIDS deaths
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500
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1986
1990
1994
1998
Year
2006
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[3]
[2]
[4]
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With reference to the graph, describe the changes in the number of people infected with
HIV between 1982 and 2008.
Suggest why the actual number of people infected with HIV may be greater than the
numbers shown on the graph.
Suggest the reasons for the shape of the graphs between 2004 and 2008.
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c
2002
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1982
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b
AIDS cases
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1000
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HIV cases
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1500
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Number per 100 000 of population
2500
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Most of the time, the chromosomes are too thin to be
seen except with an electron microscope. But when
a cell is dividing, they get shorter and fatter so they
can be seen with a light microscope. Image B12.01
shows human chromosomes seen with a powerful
electron microscope.
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Each species of organism has its own number and variety
of genes. This is what makes their body chemistry, their
appearance and their behaviour diferent from those of
other organisms.
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Humans have a large number of genes. You have
46 chromosomes inside each of your cells, all with
many genes on them. Every cell in your body has an
exact copy of all your genes. But, unless you are an
identical twin, there is no one else in the world with
exactly the same combination of genes that you have.
Your genes make you unique.
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Each chromosome contains one very long molecule of
DNA. The DNA molecule carries a code that instructs
the cell about which kinds of proteins it should make.
Each chromosome carries instructions for making many
diferent proteins. A part of a DNA molecule coding for one
protein is called a gene.
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In the nucleus of every cell there are a number of long
threads called chromosomes.
The genes on your chromosomes determine all sorts
of things about you – what colour your eyes or hair are,
whether you have a snub nose or a straight one, and
whether you have a genetic disease such as cystic fibrosis.
You inherited these genes from your parents.
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B12.01 Chromosomes
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chromosomes and genes
haploid and diploid nuclei
cell division by mitosis and meiosis
how to use genetic diagrams to predict and explain inheritance.
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This chapter covers:
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B12
Inheritance
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B12: Inheritance
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KEy tERMS
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Image B12.01 A scanning electron micrograph of human
chromosomes. You can see that each one is made of two
identical chromatids, linked at a point called the centromere.
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chromosome: a thread-like structure of DNA, carrying genetic
information in the form of genes
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B12.02 Cell division
You began your life as a single cell – a zygote – formed
by the fusion of an egg cell and a sperm cell. The nuclei
of each of these gametes contained a single complete
set of 23 chromosomes. When they fused together, they
produced a zygote with 46 chromosomes.
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Image B12.03 Chromosomes of a woman, arranged
in order.
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Mitosis
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Soon ater the zygote was formed, it began to divide over
and over again, producing a ball of cells that eventually
grew into you. Each time a cell divided, the two new cells
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Images B12.02 and B12.03 show the chromosomes in a
cell of a man and of a woman. They have been arranged
in order, largest first. You can see that there are two
chromosomes of each kind, because they are from diploid
cells. In each pair, one is from the person’s mother and the
other from their father. The two chromosomes of a pair are
called homologous chromosomes.
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diploid nucleus: a nucleus containing two sets of
chromosomes (e.g. in body cells)
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haploid nucleus: a nucleus containing a single set of
unpaired chromosomes (e.g. in gametes)
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A cell with a single set of chromosomes, such as a gamete,
is said to be haploid. The nucleus of the zygote contained
two sets of chromosomes. It was a diploid cell.
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Image B12.02 Chromosomes of a man, arranged in order.
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gene: a length of DNA that codes for a protein
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produced were provided with a perfect copy of the two
sets of chromosomes in the original zygote. The new cells
produced were all genetically identical.
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Meiosis
Gametes have only half the number of chromosomes of
a normal body cell. They have one set of chromosomes
instead of two. This is so that when they fuse together,
the zygote formed has two sets.
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Just before mitosis takes
place, the chromosomes in
the parent cell are copied.
centromere which joins Each copy remains attached
the two chromatids
to the original one, so each
together
chromosome is made up of
two identical threads joined
together (Figure B12.01).
two identical
chromatids
The two threads are
called chromatids, and
the point where they are
Figure B12.01 A chromosome held together is called
the centromere.
just before division.
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chromosome
Human gametes are formed by the division of cells in the
ovaries and testes. The cells divide by a special type of cell
division called meiosis. Meiosis shares out the chromosomes
so that each new cell gets only one of each type.
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Mitosis is the way in which any cell – plant or animal –
divides when an organism is growing, or repairing a
damaged part of its body. It produces new cells to replace
damaged ones. For example, if you cut yourself, new skin
cells will be made by mitosis to help to heal the wound.
Mitosis is also used in
asexual reproduction.
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Figure B12.03 summarises what happens during meiosis.
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Figure B12.02 shows what happens when a cell with
four chromosomes (two sets of two) divides by mitosis.
Two new cells are formed, each with one copy of each of
the four chromosomes. As the new cells grow, they make
new copies of each chromosome, ready to divide again.
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meiosis: reduction division in which the chromosome number
is halved from diploid to haploid, resulting in genetically
diferent cells
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The parent cell contains four
chromosomes.
During growth of the cell, an exact copy is made of each
chromosome. The cells are now ready to divide again.
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During mitosis, each chromosome splits. One chromatid
from each chromosome goes into each daughter cell.
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Figure B12.02 Chromosomes during the life of a cell dividing by mitosis.
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You may remember that one of each pair of homologous
chromosomes came from the person’s mother, and
one from their father. During meiosis, the new cells
get a mixture of these. So a sperm cell could contain a
chromosome 1 from the man’s father and a chromosome 2
from his mother, and so on. There are all sorts of diferent
possible combinations. This is one of the reasons why
gametes are genetically diferent from the parent cell.
Meiosis produces genetic variation.
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This type of cell division, which produces genetically
identical cells, is called mitosis.
mitosis: nuclear division giving rise to genetically identical cells
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B12: Inheritance
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The parent cell contains
four chromosomes.
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First division – meiosis i
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Homologous chromosomes pair together.
Crossing over takes place.
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Homologous chromosomes separate. One
from each pair goes into each daughter cell.
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Second division – meiosis ii
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Each chromosome separates into two
chromatids. One chromatid of each kind goes
into each daughter cell.
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Figure B12.03 Summary of chromosome behaviour during meiosis.
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centromere
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position of
eye colour genes
Figure B12.04 You have two copies of each kind of
chromosome in your cells. Each copy carries genes for
the same characteristic in the same position.
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inheritance: the transmission of genetic information from
generation to generation
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Because you have two complete sets of chromosomes
in each of your cells, you have two complete sets of
genes. Each chromosome in a homologous pair contains
genes for the same characteristic in the same positions
(Figure B12.04). This is true for all animals and most plants.
Let us look at one kind of gene to see how it behaves, and
how it is inherited.
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two chromatids of one
chromosome
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We have seen that chromosomes each contain many
genes. We think there are about 20 000 human genes,
carried on our two sets of 23 chromosomes.
a pair of homologous chromosomes
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B12.03 inheritance
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Image B12.04 Chinchillas can have grey fur or charcoal (black) fur.
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In chinchillas, genes determine the colour of the fur. The
genes are sets of instructions for producing the proteins
that cause diferent fur colours.
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For example, we can call the allele that gives grey fur G,
and the allele that gives charcoal fur g.
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Figure B12.05 Genotypes for the fur colour gene
in chinchillas.
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If the two alleles for this gene in its cells are the same –
that is, GG or gg – the chinchilla is said to be homozygous.
If the two alleles are diferent – that is, Gg – then it
is heterozygous.
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homozygous: having two identical alleles of a
particular gene
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heterozygous: having two diferent alleles of a
particular gene
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allele: a version of a gene
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In each cell in a chinchilla’s body, there are two copies of
the gene giving instructions about which kind of fur colour
protein to make. This means that there are three possible
combinations of alleles. A chinchilla might have two G
alleles, GG. It might have one of each, Gg. Or it might have
two g alleles, gg (Figure B12.05).
G
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There are several diferent forms of the fur colour gene.
The diferent forms are called alleles. We can refer to these
alleles using letters as symbols.
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Genes and alleles
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B12: Inheritance
Genotype and phenotype
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Only chinchillas with the genotype gg – homozygous
recessive – have charcoal fur.
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If its genotype is GG, then it has grey fur. If its genotype
is gg it has charcoal fur. If its genotype is Gg it has grey fur.
Alleles in gametes
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The genes that a chinchilla has are its genotype. Its
genotype for fur colour could be GG, Gg or gg.
Imagine a male chinchilla that has the genotype Gg. It is
a carrier for charcoal fur. In its testes, sperm are made by
meiosis. Each sperm cell gets either a G allele or g allele.
Half of his sperm cells have the genotype G and half have
the genotype g.
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You can see that, in this example, the chinchilla’s
phenotype for colour depends entirely on its genotype.
This is not always true. Some features, such as how big
it grows, can be afected by what it eats, as well as by its
genotype. However, for the moment, we will only consider
the efect that genotype has on phenotype, and not worry
about efects that the environment might have.
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The features the chinchilla has are called its phenotype.
This can include what it looks like – for example, the colour
of its fur – as well as things which we cannot actually see,
such as what kind of protein it has in its cell membranes.
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QuEStiONS
B12.01 What are chromosomes made of?
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We have seen that there are three diferent possible
genotypes for chinchilla fur colour, but only two
phenotypes. We can summarise this as follows:
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B12.05 If a normal human cell has 46 chromosomes,
how many chromosomes are there in a human
sperm cell?
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grey
grey
charcoal
B12.06 Using the symbols N for normal wings, and n
for vestigial (very small) wings, write down
the following:
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The allele g is recessive. A recessive allele only afects the
phenotype when there is no dominant allele present.
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When the chinchillas mate, hundreds of thousands of sperm
will begin a journey towards the egg. Half of them will carry
a G allele, and half will carry a g allele. If there is an egg in
the female’s oviduct, it will probably be fertilised. There is an
equal chance of either kind of sperm getting there first.
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dominant: an allele that is expressed if it is present
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recessive: an allele that is only expressed when there is no
dominant allele of the gene present
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Genes and fertilisation
The eggs that are made in the female’s ovaries are also
made by meiosis. She can only make one kind of egg.
All of the eggs will contain a g allele.
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b the possible genotypes of its gametes.
If this heterozygous chinchilla is crossed with a female
with charcoal fur (genotype gg), will their ofspring have
charcoal fur?
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a the genotype of a fly which is heterozygous for
this characteristic.
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This happens because the allele G is dominant to the allele g.
A dominant allele has just as much efect on phenotype when
there is only one of it as when there are two of it. A chinchilla
that is homozygous for a dominant allele has the same
phenotype as a chinchilla that is heterozygous. A heterozygous
chinchilla is said to be a carrier of the charcoal colour,
because it has the allele for it but does not have charcoal fur.
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GG
Gg
gg
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phenotype
B12.04 a The allele for brown eyes is dominant to
the allele for blue eyes. Write down suitable
symbols for these alleles.
b What is the phenotype of a person who is
heterozygous for this characteristic?
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Dominant and recessive alleles
genotype
B12.03 What are alleles?
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phenotype: the observable features of an organism
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B12.02 What are homologous chromosomes?
genotype: the genetic makeup of an organism in terms of the
alleles present
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KEy tERMS
Each gamete has only one of each kind of chromosome
instead of two, as in the body cells. So, for example,
human egg and sperm cells have 23 chromosomes,
not 46 as in other cells. These cells, therefore, only carry
one of each pair of alleles of all the genes.
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If a sperm carrying a G allele wins the race, then the zygote
will have a G allele from its father and a g allele from its
mother. Its genotype will be Gg. When the baby chinchilla
is born, it will have the genotype Gg.
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What happens if both parents are heterozygous?
g
zygotes
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Parents’ phenotypes
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grey
Parents’ genotypes
Gg
Gg
G or g
G or g
Gametes
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Another cross
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Figure B12.06 Fertilisation between a heterozygous grey
chinchilla and a chinchilla with charcoal fur.
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First, write down the phenotypes and genotypes of the
parents. Next, write down the diferent types of gametes
they can make, like this.
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gg
g
gg
charcoal
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Gg
grey
This example illustrates the inheritance of one pair
only of contrasting characteristics. This is known as
monohybrid inheritance.
The next step is to write down what might happen during
fertilisation. Either kind of sperm might fuse with an egg.
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g
About one quarter of the ofspring would be
expected to have charcoal fur, and three quarters
would have grey fur.
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G or g
Gg
grey
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Gg
GG
grey
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Parents’ genotypes
G
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charcoal
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sperm
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grey
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Parents’ phenotypes
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There is a standard way of writing out all this information.
It is called a genetic diagram.
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eggs
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Ofspring genotypes and phenotypes
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Genetic diagrams
Gametes
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g
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gg
charcoal
So we would expect approximately half of the ofspring
to be heterozygous with grey fur, and half to be
homozygous, with charcoal fur. Another way of putting this
is to say that the expected ratio of grey fur to charcoal fur
would be 1:1.
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sperm
To finish your summary of the genetic cross, write out in
words what you would expect the ofspring from this
cross to be.
g
g
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Gg
grey
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g
G
G
This part of the genetic diagram is called a Punnett square.
fertilisation
152
g
A female of genotype gg
produces eggs of genotype g .
A male of genotype Gg
produces equal numbers of
G and g sperm.
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But if a sperm carrying a g allele manages to fertilise the
egg, then the baby will have the genotype gg, like its
mother (Figure B12.06).
egg
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Ofspring genotypes and phenotypes
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B12: Inheritance
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to be charcoal. But we should not be too surprised if they
have three ofspring with charcoal fur.
However, if the rabbits are heterozygous, then they are
likely to produce ofspring with diferent coat colours. They
are not pure-breeding. Heterozygous individuals are not
pure-breeding.
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Image B12.05 Genetic diagrams do not tell us
how many ofspring there will be – just the probabilities
of any one ofspring having a particular phenotype.
There are five kittens in this family, and it looks as
though the ratio of grey to black-and-white fur is
approximately 1:1.
Sex determination
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Probabilities in genetics
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The last pairs of chromosomes in Images B12.03 and B12.04
are responsible for determining what sex a person will
be. They are called the sex chromosomes (Figure B12.07).
A woman’s chromosomes are both alike and are called
X chromosomes. She has the genotype XX. A man, though,
only has one X chromosome. The other, smaller one is a
Y chromosome. He has the genotype XY.
You can work out sex inheritance in just the same way
as for any other characteristic, but using the letter
symbols to describe whole chromosomes, rather than
individual alleles.
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In the last example, there were four possible
ofspring genotypes at the end of the cross. This does
not mean that the two chinchillas will have four ofspring.
It simply means that each time they have ofspring,
these are the possible genotypes that they might have
(Image B12.05).
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For any one ofspring, there is a 1 in 4 chance that its
genotype will be GG, and a 1 in 4 chance that its genotype
will be gg. There is a 2 in 4, or rather 1 in 2, chance that its
genotype will be Gg.
female
male
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With small numbers like this, probabilities do not always
match reality. If you had the patience to toss your coin up a
few thousand times, though, you will almost certainly find
that you get much more nearly equal numbers of heads
and tails.
X
X
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Figure B12.07 The sex chromosomes.
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So, if the parent chinchillas in the last example had eight
ofspring, we might expect six of them to be grey and two
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The same thing applies in genetics. The ofspring
genotypes which you work out are only probabilities.
With small numbers, they are unlikely to work out exactly.
With very large numbers of ofspring from one cross,
they are more likely to be accurate.
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However, as you know, probabilities do not always work
out. If you toss a coin up four times, you might expect it to
turn up heads twice and tails twice. But does it always do
this? Try it and see.
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Some populations of animals or plants always have
ofspring just like themselves. For example, a rabbit
breeder might have a strain of rabbits which all have
brown coats. If he or she interbreeds them with one
another, all the ofspring always have brown coats as well.
The breeder has a pure-breeding strain of brown rabbits.
Pure-breeding strains are always homozygous for the
pure-breeding characteristics.
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Pure breeding
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XX
X or Y
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XX
female
sperm
XY
male
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ACtivity B12.01
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Ofspring genotypes and phenotypes
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B12.08 a In humans, the allele for red hair, b, is recessive
to the allele for brown hair, B. A man and his
wife both have brown hair. They have five
children, three of whom have red hair, while
two have brown hair. Explain how this may
happen, using a genetic diagram.
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B12.07 Using a complete genetic diagram, work out
what kind of ofspring would be produced if
the heterozygous fly in question B12.06 mated
with a fly homozygous for normal wings.
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brown
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b The pedigree diagram shows hair colour
in three generations of a family. Squares
represent males and circles represent females.
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3
red
red
A2 Is this what you would have expected?
Explain your answer.
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A3 Why must you close your eyes when choosing the
beads?
What are the genotypes of persons 1 and 3?
What is the phenotype of person 2?
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A4 Why must you put the beads back into the beakers
ater they have ‘mated’?
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Questions
A1 In the first cross, what kinds of ofspring were
produced, and in what ratios?
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‘Breeding’ beads
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations and
data
In this investigation, you will use two beakers of beads.
Each beakers represents a parent. The beads represent
the gametes they make. The colour of a bead represents
the genotype of the gamete. For example, a red bead
might represent a gamete with genotype A, for ‘tongue
rolling’. A yellow bead might represent a gamete with the
genotype a, for ‘non-tongue rolling’.
1 Put 100 red beads into the first beaker. These represent
the gametes of a person who is homozygous for
‘tongue rolling’, AA.
2 Put 50 red beads and 50 yellow beads into the second
beaker. These represent the gametes of a heterozygous
person with the genotype Aa.
3 Close your eyes, and pick out one bead from the first
beaker, and one from the second. Write down the
genotype of the ‘ofspring’ they produce. Put the two
beads back.
4 Repeat step 3 100 times.
5 Now try a diferent cross – for example, Aa crossed
with Aa.
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So each time a child is conceived, there is a 1:1 chance of it
being either sex.
QuEStiONS
B12.09 In Dalmatian dogs, the allele for black spots
is dominant to the allele for liver spots. If a
breeder has a black-spotted dog, how can he
or she find out whether it is homozygous or
heterozygous for this characteristic? (Hint: the
breeder will need to cross the dog with another
one, and look at the ofspring.) Use genetic
diagrams to explain your answer.
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Gametes
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Parents’ genotypes
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Parents’ phenotypes
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Summary
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B12: Inheritance
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End-of-chapter questions
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gene, allele
dominant, recessive
homozygous, heterozygous
genotype, phenotype
mitosis, meiosis
haploid, diploid
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The leaves of tomato plants can have leaves with smooth or indented edges. The allele for indented
edges is dominant, and the allele for smooth edges is recessive.
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Write down the genotypes of a homozygous smooth plant and a homozygous indented plant.
A pure-breeding (homozygous) smooth plant was crossed with a homozygous indented plant.
All of the ofspring had indented leaves.
Construct a complete genetic diagram to explain how this happened.
Several of these indented ofspring were crossed together. There were 302 plants with indented
leaves and 99 with smooth leaves.
Construct a complete genetic diagram to explain this result.
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[4]
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155
Explain the diference between each of the following pairs of terms:
a
b
c
d
e
f
4
Choose suitable symbols for the alleles of the flower colour gene.
Which allele is dominant, and which is recessive? Explain how you worked this out.
Write down all the possible genotypes for flower colour in this plant, and the phenotypes they will produce.
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In a species of plant, flower colour can be red or white. Heterozygous plants have red flowers.
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Suggest suitable symbols for these two alleles.
Write down the three possible genotypes for these alleles.
Write down the phenotype that each of these genotypes will produce.
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In guinea pigs, the allele for smooth fur is dominant to the allele for rough fur.
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about genotypes and phenotypes involving dominant
and recessive alleles
how to use genetic diagrams to predict or explain the
results of crosses
how to interpret pedigree diagrams.
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about chromosomes and genes
how and why cells divide by mitosis
how and why cells divide by meiosis
about haploid and diploid nuclei
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You should know:
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[5]
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Rabbits are oten kept as pets. People try to breed rabbits with unusual colours, such as himalayan
colouring. The diagram shows a rabbit with himalayan fur colour. The rabbit’s fur is white with some
black areas.
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[2]
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 32 Q9 November 2013]
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Rabbits, like humans, keep their internal body temperature constant. The body temperature of
a rabbit is 38.5 °C.
i Explain how a rabbit generates heat within its body.
ii Suggest how the fur of a rabbit helps to maintain its body temperature higher than that
of its environment.
iii When himalayan rabbits are first born, they are white all over. The black colour develops
gradually. The black pigment is produced by the action of an enzyme that is only active
at temperatures below 25 °C.
Use this information to suggest a reason for the distribution of black fur on the body of
a himalayan rabbit.
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Completely white fur and himalayan-coloured fur are produced by two alleles of a gene.
When a white rabbit and a himalayan rabbit are bred together, all the ofspring are white.
When two of these white ofspring are bred together, one quarter of their ofspring are
himalayan and three quarters are white.
i Identify which allele is dominant and which is recessive, and suggest suitable symbols
for the two alleles.
ii Two rabbits that are heterozygous for these alleles are crossed.
Construct a genetic diagram, using your symbols from part i, to explain the results of this cross.
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continuous and discontinuous variation
genetic and environmental variation
mutation and what causes it
natural selection and adaptation
selective breeding.
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This chapter covers:
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B13.01 variation
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You have only to look around a group of people to see that
they are diferent from one another. Some of the more
obvious diferences are in height or hair type. We also
vary in intelligence, blood groups, whether we can roll
our tongues or not, and many other ways. Diferences
between the features of diferent individuals are called
phenotypic variation.
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There are two basic kinds of variation. One kind is
discontinuous variation. Blood groups are an example
of discontinuous variation. Everyone fits into one of four
definite categories – each of us has group A, B, AB or O.
There are no in-between categories.
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By describing variation as continuous or discontinuous,
we can begin to explain how organisms vary. But the cause
of the variation is another question altogether.
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The other kind is continuous variation. Height is an
example of continuous variation. There are no definite
heights that a person must be. People vary in height,
between the lowest and highest extremes.
You can try measuring and recording discontinuous
and continuous variation in Activity B13.01. Your results
for continuous variation will probably look similar to
Figure B13.01. This is called a normal distribution.
Most people come in the middle of the range, with fewer
at the lower or upper ends. Human height (Image B13.01)
shows a normal distribution.
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B13
Variation and selection
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Height
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Figure B13.01 A normal distribution curve. This is a graph
that shows the numbers of people of diferent heights.
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Image B13.02 a The presence of horns in cattle is controlled
by a dominant allele of a gene. b Polled (hornless) cattle
have two copies of the recessive allele of this gene.
Image B13.01 Human height shows continuous variation.
What characteristic here shows discontinuous variation?
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Number
of people
at each
height
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(Figure B13.02). The tree’s genotype gives it the potential to
grow tall, but it will not realise this potential unless its roots
are given plenty of space and it is allowed to grow freely.
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One reason for the diferences between individuals is
that their genotypes are diferent. This is called genetic
variation. Blood groups, for example, are controlled by
genes. There are also genes for hair colour, eye colour,
height and many other characteristics (Image B13.02).
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QuEStiONS
B13.01 Decide whether each of these features shows
continuous variation or discontinuous variation
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b foot size in humans
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Environmental variation
a blood group in humans
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c leaf length in a species of tree
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Another important reason for variation is the diference
between the environments of individuals. Pine trees possess
genes that enable them to grow to a height of about 30 m.
But if a pine tree is grown in a very small pot, and has its
roots regularly pruned, it will be permanently stunted
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d presence of horns in cattle
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B13.02 For each of the examples in a to d above, suggest
whether the variation is caused by genes alone, or
by both genes and environment.
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Genetic variation
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In general, discontinuous variation is caused by genes
alone. Continuous variation is oten influenced by both
genes and the environment.
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variation: diferences between individuals of the same species
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B13: Variation and selection
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Causes of genetic variation
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Variation caused by the
environment is not inherited.
A cutting from a bonsai pine tree
would grow into a full size tree, if
given sufficient space.
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Another type of mutation afects whole chromosomes.
For example, when eggs are being made by meiosis in
a woman’s ovaries, the chromosome 21s sometimes
do not separate from one another. One of the daughter
cells therefore gets two chromosome 21s and the other
one gets none. The cell with none dies. The other one
may survive, and eventually be fertilised by a sperm. The
zygote from this fertilisation will have three copies of
chromosome 21. The child that grows from the zygote has
Down’s syndrome.
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A bonsai pine tree is dwarfed by being grown in a very small
pot, and continually pruned.
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Many diferent chemicals are known to increase the risk
of a mutation happening. The heavy metals lead and
mercury and their compounds can interfere with the
process in which DNA is copied. If this process goes wrong,
the daughter cells will get faulty DNA when the cell divides.
Chemicals that cause mutations are called mutagens.
mutation: a change in a gene or chromosome
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Meiosis
During sexual reproduction, gametes are formed by
meiosis. In meiosis, homologous chromosomes exchange
genes, and separate from one another, so the gametes
which are formed are not all exactly the same. Meiosis
produces new cells that are genetically diferent from the
parent cell.
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Figure B13.02 The inheritance of variation.
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A dwarf pony, such as a Shetland pony, is small because of its
genes. The offspring of Shetland ponies are small like their
parents, no matter how well they are fed and cared for.
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Children with Down’s have characteristic facial features
and are usually very happy and friendly people. However,
they oten have heart problems and other physical and
physiological dificulties.
Mutations oten happen for no apparent reason. However,
we do know of many factors which make mutation more
likely. One of the most important of these is ionising
radiation. Radiation can damage the bases in DNA
molecules. If this happens in the ovaries or testes, then the
altered DNA may be passed on to the ofspring.
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Mutation
Sometimes, a gene may suddenly change. This is called
mutation. Mutation is how new alleles are formed.
Mutations are the only source of brand-new characteristics
in the gene pool. So mutations are really the source of all
genetic variation.
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There are several ways in which genetic variation occurs.
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In the 19th century, several ideas were put forward
to suggest how this might have happened. By far the
most important was suggested by Charles Darwin
(Image B13.03). He put forward his theory in a book called
On the Origin of Species, published in 1859.
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Number of
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Length / cm
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Measuring variation
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
1 Make a survey of at least 30 people, to find out whether
or not they can roll their tongue. Record your results.
2 Measure the length of the third finger of the let hand of
30 people. Take the measurement from the knuckle to
the finger tip, not including the nail.
3 Divide the finger lengths into suitable categories, and
record the numbers in each category, like this.
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ACtivity B13.01
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Fertilisation
Any two gametes of opposite types can fuse together at
fertilisation, so there are many possible combinations
of genes which may be produced in the zygote. In an
organism with a large number of genes, the possibility of
two ofspring having identical genotypes is so small that it
can be considered almost zero.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Image B13.03 A portrait of Charles Darwin at the age of 72.
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Questions
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4 Draw a histogram of your results.
A1 Which characteristic shows continuous variation, and
which shows discontinuous variation?
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Struggle for existence
Most populations do not generally increase rapidly in size,
so there must be considerable competition for survival
between the organisms.
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Over the many millions of years that living things have
existed, there have been gradual changes in organisms
and populations. Fossils tell us that many animals and
plants that once lived no longer exist.
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Over-production
Most organisms produce more young than will survive
to adulthood.
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A3 The mean finger length is the total of all the finger
lengths, divided by the number of people in your
sample. What is the mean finger length of the sample?
B13.02 Selection
variation
Most populations of organisms contain individuals which
vary slightly from one to another. Some slight variations
may better adapt some organisms to their environment
than others.
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A2 Your histogram may be a similar shape to the curve in
Figure B13.01. This is called a normal distribution. The
category, or class, which has the largest number of
individuals in it is called the modal class. What is the
modal class for finger length in your results?
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Darwin’s theory of how evolution could have happened
may be summarised like this.
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B13: Variation and selection
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Gradual change
In this way, over a period of time, the population
will lose all the poorly adapted individuals. The
population will gradually become better adapted
to its environment.
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Darwin proposed his theory before anyone understood
how characteristics were inherited. Now that we know
something about genetics, his theory can be stated slightly
diferently. We can say that natural selection results in the
alleles producing advantageous phenotypes being passed
on to the next generation more frequently than the alleles
which produce less advantageous phenotypes.
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2 Over-production. The cacti produce
large numbers of offspring.
In the wet season they flower.
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4 Survival of the fittest. The cacti
with the longest roots are able to
obtain water, while the others die
of dehydration.
5 Advantageous characteristics passed
on to offspring. The long-rooted cacti
reproduce, producing offspring more
likely to be long-rooted themselves.
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3 Struggle for existence. During the
dry season, there is competition
for water.
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1 Genetic variation. In a population
of cacti, some have longer roots
than others.
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The theory is called the theory of natural selection,
because it suggests that the best-adapted organisms
are selected to pass on their characteristics to the
next generation.
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process of adaptation: the process resulting from natural
selection, by which populations become more suited to their
environment over many generations
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Advantageous characteristics passed
on to ofspring
Only these well-adapted organisms will survive and
be able to reproduce successfully, and will pass on the
alleles that produce advantageous characteristics
to their ofspring.
This process continues over time, generation ater
generation. Gradually, the individuals in successive
generations of a species gain more and more
advantageous features – that is, features that adapt them
to their environment. We can describe evolution as the
change in adaptive features over time, as the result of
natural selection.
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Survival of the fittest
Only the organisms which are really well adapted to their
environment will survive (Figures B13.03 and Image B13.04).
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Figure B13.03 An example of how natural selection might occur.
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Image B13.04 When large numbers of organisms, such as these wildebeest of the East African plains, live together,
there is competition for food, and the weaker ones are likely to be killed by predators. Individuals best adapted to their
environment survive and reproduce.
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An example of natural selection
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The population of bacteria in the person’s body may be
several million. The chances of any one of them mutating
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This does, in fact, happen quite frequently. This is one
reason why pharmaceutical companies have developed
many diferent antibiotics – if some bacteria become
resistant to one, they can be treated with another.
The more we use an antibiotic, the more we are exerting
a selection pressure that favours the resistant forms.
If antibiotics are used too oten, we may end up with
resistant strains of bacteria that are very dificult to
control. A form of the bacterium Staphylococcus aureus
has become resistant to several diferent antibiotics, and
is known as MRSA. This can cause infections that are very
dificult to treat.
Many diferent kinds of bacteria are no longer afected by
antibiotics such as penicillin. They are said to be resistant
to the antibiotic.
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Antibiotics are substances that kill bacteria, or stop them
reproducing, but do not harm human cells. For example,
the antibiotic penicillin works by stopping bacteria from
forming cell walls. When a person who is infected with
bacteria is treated with penicillin, the bacteria are unable
to grow new cell walls, and the bacterial cells burst open.
Antibiotics are used globally to cure people of bacterial
infections, many of which might otherwise be fatal.
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Darwin’s theory of natural selection provides a good
explanation of how resistance to antibiotics has arisen and
spread in populations of bacteria.
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to a form which is not afected by penicillin is quite low, but
because there are so many bacteria, it could well happen.
If it does, the mutant bacterium will have a tremendous
advantage. It will be able to go on reproducing while all
the others cannot. Soon, its descendants may form a huge
population of penicillin-resistant bacteria (Figure B13.04).
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B13: Variation and selection
bacterium resistant
to antibiotic
Antibiotic is added, which kills the
bacteria that are not resistant.
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Humans can also bring about changes in living organisms,
by selecting certain individuals for breeding. Figure B13.05
and Images B13.05a and b show examples of the results
of this kind of selection. For example, from the varied
individuals amongst a herd of cattle, the breeder chooses
the ones with the characteristics he or she wants to
appear in the next generation. He or she then allows these
individuals, and not the others, to breed. If this selection
process is repeated over many generations, these
characteristics will become the most common ones in
the population.
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However, what humans think are desirable characteristics
would oten not be at all advantageous to the plant or
animal if it was living in the wild. Modern varieties of cattle,
for example, selected over hundreds of years for high milk
yield or fast meat production, would stand little chance of
surviving for long in the wild.
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Selective breeding
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Figure B13.04 How resistance to antibiotics increases in a population of bacteria.
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This process is called artificial selection. It has been
going on for thousands of years, ever since humans first
began to cultivate plants and to domesticate animals.
It works in just the same way as natural selection.
Individuals with ‘advantageous’ characteristics breed,
while those with ‘disadvantageous’ ones do not.
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Figure B13.05 Wild and cultivated apples.
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Image B13.05 a White Park cattle, like these in England,
are a very old breed. They are thought to be quite similar
to original wild cattle. b Friesian cattle have been bred for
high milk yield.
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The resistant one multiplies and forms
a population of resistant bacteria just
like itself.
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bacteria not resistant
to antibiotic
In a population of bacteria, not every one
is alike. By chance, one may have a gene
that makes it resistant to an antibiotic.
bacteria resistant
to antibiotic
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antibiotic
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bacterium resistant
to antibiotic
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QuEStiONS
B13.03 Why is it unwise to use antibiotics unnecessarily?
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B13.04 Imagine you are a farmer with a herd of dairy
cattle. You want to build up a herd with a very high
production of milk. You have access to sperm samples
from bulls, for each of which there are records of the
milk production of his ofspring. What will you do?
B13.05 Wheat is attacked by many diferent pests,
including a fungus called yellow rust.
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a Describe how you could use artificial selection
to produce a new variety of wheat which is
naturally resistant to yellow rust.
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b When resistant varieties of wheat are produced, it
is found that ater a few years they are infected by
yellow rust again. Explain how this might happen.
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Summary
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Some farmers are now beginning to think diferently about
the characteristics they want in their animals and plants.
Instead of enormous yields as their first priority, they are
now looking for varieties that can grow well with less
fertiliser or pesticides in the case of food plants, and with
less expensive housing and feeding in the case of animals.
Luckily, many of the older breeds with these characteristics
have been conserved, and these can now be used to breed
new varieties with ‘easy-care’ characteristics.
You should know:
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how natural selection results in adaptation
and evolution
how antibiotic-resistant strains of bacteria develop
about selective breeding.
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about continuous and discontinuous variation,
and what causes them
about mutation
how natural selection happens
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environment
matched
mutation
selection
sex
genes
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adapted
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Copy and complete the following sentences, using words from the list. You may use each word once,
more than once or not at all.
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End-of-chapter questions
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Distinguish between each of these pairs of terms.
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continuous variation, discontinuous variation
natural selection, artificial selection
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A population of organisms that can reproduce sexually oten becomes adapted to a
new environment more quickly than a population that can only reproduce asexually.
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Suggest explanations for each of the following.
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Cell division by mitosis does not usually produce variation, unless there is a change in the DNA,
called
Most mutations are harmful, because they make an organism less well
to its environment.
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Variation can be defined as diferences between individuals of the same
Sometimes,
the diferences are clear-cut, and each individual fits into one of a small number of defined categories.
This is called
variation. This kind of variation is caused by the organisms’
In other cases, the diferences have no definite categories. This is called
variation.
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There is variation in the way in which human ear lobes are naturally joined to the head.
The diagram below shows the two versions.
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Changes in the characteristics of a species may continue to happen even ater it has
become well adapted to its environment.
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B13: Variation and selection
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Feel your own ear lobes and record whether you have attached or free ear lobes.
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free
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attached
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i What can you conclude from these results?
ii Calculate the approximate ratio of free to attached ear lobes in this group.
iii Explain how this ratio might help in understanding the way in which the attachment
of ear lobes is inherited.
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[Cambridge O Level Biology 5090 Paper 62 Q2 June 2010]
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Reed warblers are small birds that migrate over long distances between western Africa
and northern Europe.
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5
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10
female
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14
male
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Number of students with
attached ear lobes
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Total
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female
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male
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Number of students with
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Age / year
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The results of a survey of the ear lobes of some students are shown in the table below.
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The photograph below shows a reed warbler, Acrocephalus scirpaceus.
Copyright Material - Review Only - Not for Redistribution
(continued)
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Cambridge IGCSE Combined and Co-ordinated Sciences
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A study was carried out in Sweden into the efects of natural selection on wing length in
reed warblers.
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The wings of young reed warblers reach their maximum length a few days ater leaving the nest.
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At this age the wing length in millimetres of each bird was recorded. Each bird was identified by
putting a small ring around one of its legs.
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When the birds were caught in net traps as adults, the information on the rings was used to identify
specific birds and their ages.
The length of time between ringing and trapping was recorded for each bird that was identified before
it was released.
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66
70 or more
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270
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The researchers concluded that reed warblers with a wing length of 66–67 mm had the best
chance of survival.
i Describe the evidence from the table that supports this conclusion.
ii The researchers also suggested that more evidence was needed to make this conclusion.
Suggest what other evidence would show that birds with wings 66–67 mm in length have
the best chance of survival.
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[3]
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[4]
[Cambridge IGCSE Biology 0610 Paper 32 Q5 b, c & d November 2011]
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Scientists have discovered that genes are responsible for wing length in reed warblers.
The most common length of wing has been 66–67 mm for many generations of these birds.
Explain how natural selection may be responsible for maintaining the mean wing length
of reed warblers at 66–67 mm.
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Explain why wing length is an example of continuous variation.
Suggest a feature of reed warblers, other than wing length, that shows continuous variation.
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total = 771
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63 or less
Mean age at
trapping / days
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Number of birds
trapped
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Wing length at
ringing / mm
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The mean age at trapping was calculated for birds with each wing length. The results are shown in
the table.
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B14.01 Ecology
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There are many words used in ecology with which it is
useful to be familiar.
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The area where an organism lives is called its habitat.
The habitat of a tadpole might be a pond. There will probably
be many tadpoles in the pond, forming a population of
tadpoles. A population is a group of organisms of the same
species, living in the same area at the same time.
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ecosystem: a unit containing all of the organisms and their
environment, interacting together, in a given area, e.g. a lake
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The living organisms in the pond, the water in it, the
stones and the mud at the bottom make up an
ecosystem. An ecosystem consists of a community and
its environment (Figure B14.01).
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One very important way of studying living things is to
study them where they live. Animals and plants do not
live in complete isolation. They are afected by their
surroundings, or environment. Their environment is also
afected by them. The study of the interaction between
living organisms and their environment is called ecology.
But tadpoles will not be the only organisms living in the
pond. There will be many other kinds of animals and
plants making up the pond community. A community is
all the organisms, of all the diferent species, living in the
same habitat.
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food chains, webs and ecosystems
eficiency of energy transfer in food chains
the carbon cycle and how human activities afect it
water pollution.
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This chapter covers:
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B14
Organisms and their environment
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167
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The pond is a habitat.
All the inhabitants of the pond
make up a community.
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Figure B14.01 A pond and its inhabitants – an example of an ecosystem.
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KEy tERMS
producer: an organism that makes its own organic nutrients,
usually using energy from sunlight, through photosynthesis
Animals get their food, and therefore their energy, by
ingesting (eating) plants, or by eating animals which have
eaten plants.
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All living organisms need energy. They get energy from food,
by respiration. All the energy in an ecosystem originates
from the Sun. Some of the energy in sunlight is captured by
plants, and used to make food – glucose, starch and other
organic substances such as fats and proteins. These contain
some of the energy from the sunlight. When the plant needs
energy, it breaks down some of this food by respiration.
Animals are consumers. An animal which eats plants is
a primary consumer, because it is the first consumer
in a food chain. An animal which eats that animal is a
secondary consumer, and so on along the chain. Primary
consumers are also called herbivores, and higher level
consumers are carnivores.
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B14.02 Energy flow
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consumer: an organism that gets its energy by feeding on
other organisms
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herbivore: an animal that gets its energy by eating plants
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• When an organism uses food for respiration, some of
the energy released from the food is lost as heat energy
to the environment.
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food chain: a diagram showing the flow of energy from one
organism to the next, beginning with a producer
• When one organism eats another, it rarely eats
absolutely all of it. For example, the grasshopper in the
food chain in Figure B14.02 may eat almost all of the
parts of the plant above ground, but it will not eat the
roots. So not all of the energy in the plant is transferred
to the grasshopper.
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food web: a network of interconnected food chains
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Every food chain begins with green plants because only
they can capture the energy from sunlight. They are called
producers, because they produce food.
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Producers and consumers
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KEy tERMS
Energy losses
As energy is passed along a food chain, some of it is lost to
the environment. This happens in many ways.
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Many diferent food chains link to form a food web.
Figure B14.03 shows an example of a food web.
carnivore: an animal that gets its energy by eating other
animals
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The sequence by which energy, in the form of chemical
energy in food, passes from a plant to an animal and then
to other animals is called a food chain. Figure B14.02
shows one example of a food chain.
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All the organisms of one
species make up a population.
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The pond and its inhabitants
make up an ecosystem.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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grasshopper – a primary consumer
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flycatcher
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broad-winged
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ocelot
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Figure B14.02 A food chain.
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grasshopper
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squirrel
plant material
including leaves,
fruits and seeds
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Figure B14.03 A food web.
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flycatcher – a secondary consumer
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plants – primary producers
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B14: Organisms and their environment
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energy from Sun
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available
energy
available
energy
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heat lost in
respiration
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heat lost in
respiration
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This means that, the further you go along a food chain,
the less energy is available for each successive group of
organisms (Figure B14.04). The plants get a lot of energy
from the Sun, but only a fraction of this energy is absorbed
by the grasshoppers, and only a fraction of that is
absorbed by the flycatchers. This explains why predators
are usually much rarer than herbivores, and why there are
usually many more plants than animals in an ecosystem.
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• When an animal eats another organism as food,
enzymes in its digestive system break down most
of the large food molecules, so that they can be
absorbed. But not all of the food molecules are
digested and absorbed, and the ones that are not
are eventually lost from the body in the faeces.
These faeces contain energy that is lost from this
food chain.
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Figure B14.04 Energy losses in a food chain.
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QuEStiONS
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Each stage in a food chain is called a trophic level
(‘trophic’ means feeding).
B14.01 Where does all the energy in living organisms
originate from?
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One very important group of organisms, which is easy
to overlook when you are studying an ecosystem, is the
decomposers. They feed on waste material from animals
and plants, and on their dead bodies. Many fungi and
bacteria are decomposers.
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Studying an ecosystem
Skills:
AO3.3 Observing, measuring and recording
AO3.4 Interpreting and evaluating observations
and data
In this activity, you will try to work out some food chains
in an ecosystem. Remember that you must disturb the
ecosystem as little as possible. Do not take plants or
animals away from the ecosystem unless your teacher tells
you that you can do this. If you have a digital camera, take
photographs of the organisms rather than collecting them.
1 Search the area thoroughly and try to identify all the
types of plants in the area. If you cannot identify a
plant, and there appears to be a lot of it, then collect
samples of leaves and flowers to take back to your
laboratory, where you can spend longer trying to find
out what it is. Better still, take photographs of the plant
so that you do not need to take samples from it.
2 Try to identify any small animals you see. Where
possible, take photographs of each kind of animal.
3 Make notes about the large animals in the area, such as
the types of bird present and what they are feeding on.
4 In the laboratory, with your teacher’s assistance, try to
identify all the organisms you found.
5 Use books or the internet to find out what some of the
animals feed on.
6 Construct a food web for this ecosystem.
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decomposer: an organism that gets its energy from dead or
waste organic matter
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Decomposers are extremely important, because they help
to release substances from dead organisms, so that they
can be used again by living ones. One of these substances
is carbon.
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ACtivity B14.01
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The carbon cycle
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Carbon is a very important component of living things,
because it is an essential part of carbohydrates, fats
and proteins.
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Some of the glucose is then broken down by the plant in
respiration. The carbon in the glucose becomes part of a
carbon dioxide molecule again, and is released back into
the air.
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Some of the carbon in the plant will be eaten by animals.
The animals respire, releasing some of it back into the air
as carbon dioxide.
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When the plant or animal dies, decomposers will feed on them.
The carbon becomes part of the decomposers’ bodies. When
they respire, they release carbon dioxide into the air again.
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Figure B14.05 shows how carbon circulates through an
ecosystem. The air contains about 0.04% carbon dioxide. When
plants photosynthesise, carbon atoms from carbon dioxide
become part of glucose or starch molecules in the plant.
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B14.04 Why are there rarely more than five links in a
food chain?
Decomposers
trophic level: the position of an organism in a food chain,
food web or pyramid of biomass or numbers
170
B14.03 Why are green plants called producers?
B14.03 the carbon cycle
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Many organisms feed at more than one trophic level.
You, for example, are a primary consumer when you eat
vegetables, a secondary consumer when you eat meat
or drink milk, and a tertiary consumer when you eat a
predatory fish such as a salmon.
KEy tERM
B14.02 Write down a food chain a which ends with
humans, b is in the sea, and c that has five
links in it.
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Because there is less energy available as you go up the
trophic levels, there are fewer organisms at each level.
This loss of energy limits the length of food chains.
They rarely have more than five trophic levels, as there
is not enough energy let to support a sixth.
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Trophic levels
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Cambridge IGCSE Combined and Co-ordinated Sciences
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carbon dioxide
in the air
combustion
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carbon
compounds
in fossil fuels
photosynthesis
respiration
combustion
respiration
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171
B14.05 Name the only process shown in Figure B14.05
that removes carbon dioxide from the air.
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QuEStiONS
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B14.06 Name two carbon compounds that are found in
the body tissues of animals.
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B14.07 Explain what will happen to the quantity of
carbon dioxide in the air if fossil fuels are burnt
faster than they are formed.
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Deforestation
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Humans have always cut down trees. Wood is an excellent
fuel and building material. The land on which trees
grow can be used for growing crops for food, or to sell.
One thousand years ago, most of Europe was covered
by forests. Now, most of them have been cut down.
The cutting down of large numbers of trees is called
deforestation (Image B14.01).
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Deforestation destroys the habitats of many organisms.
This can lead to extinction of species. Deforestation
also causes loss of soil, flooding, and an increase in the
concentration of carbon dioxide in the atmosphere.
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A rainforest is a very special place, full of many diferent
species of plants and animals. More diferent species live in
a small area of rainforest than in an equivalent area of any
other habitat in the world. We say that rainforest has a high
species diversity.
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Image B14.01 When rainforest is cut down and burnt,
as here in Brazil, habitats are destroyed, large amounts of
carbon dioxide are released and soil nutrients are lost.
Rainforests occur in temperate and tropical regions
of the world (Image B14.02). Recently, most concern
about deforestation has been about the loss of tropical
rainforests. In the tropics, the relatively high and constant
temperatures, and high rainfall, provide perfect conditions
for the growth of plants (Image B14.03).
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B14.04 Human influences on
ecosystems
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urine, faeces,
death
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carbon compounds in
microorganisms and other
decomposers in soil
Figure B14.05 The carbon cycle.
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carbon compounds
in plants
feeding
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gasolene
carbon compounds
in animals
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coal
death and longterm subjection to
high pressures and
temperatures
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B14: Organisms and their environment
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Cambridge IGCSE Combined and Co-ordinated Sciences
The loss of part of a rainforest means a loss of a habitat for
many diferent species of animals. Even if small ‘islands’
of forest are let as reserves, these may not be large
enough to support a breeding population of the animals.
Deforestation threatens many species of animals and
plants with extinction.
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Oxygen concentrations in the atmosphere are also
afected. With fewer trees photosynthesising, less
oxygen is released by them, so atmospheric oxygen
concentrations may fall.
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When people in industrialised countries get concerned
about the rate at which some countries are cutting down
their forests, it is very important they should remember
that they have already cut down most of theirs. Most
tropical rainforests grow in developing countries, and in
some countries many of the people are very poor.
The people may cut down the forests to clear land
on which they can grow food. It is dificult to expect
someone who is desperately trying to produce food,
to keep their family alive, not to do this, unless you
can ofer some alternative. International conservation
groups such as the World Wide Fund for Nature, and
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Image B14.03 Unspoilt tropical rainforest in
Sarawak, Malaysia.
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The loss of so many trees can also afect the
water cycle. While trees are present and rain falls,
a lot of it is taken up by the trees, and transported into
their leaves. It then evaporates, and goes back into the
atmosphere in the process of transpiration. If the trees
have gone, then the rain simply runs of the soil and into
rivers. Much less goes back into the air as water vapour.
The air becomes drier, and less rain falls. This can make
it much more dificult for people to grow crops and
keep livestock.
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Image B14.02 This rainforest is growing in a part of Chile
where the climate is temperate (with cold winters and
warm summers) and there is very high rainfall. It has an
enormous species diversity.
Deforestation also afects the concentration of carbon
dioxide in the atmosphere. The forest trees take carbon
dioxide from the air to use in photosynthesis, and they
return oxygen to the air. When the trees are cut down,
there are fewer plants to remove carbon dioxide. Moreover,
as the cut-down trees decay or are burnt, the carbon in
their bodies is converted to carbon dioxide and is released
into the air.
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When an area of rainforest is cut down, the soil
under the trees is exposed to the rain. The soil of a
rainforest is very thin. It is quickly washed away once
it loses its cover of plants. This soil erosion may make
it very dificult for the rainforest to grow back again,
even if the land is let alone. The soil can also be
washed into rivers, silting them and causing flooding
(Figure B14.06).
Copyright Material - Review Only - Not for Redistribution
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The land is overgrazed.
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The removal of grass cover
allows soil to be blown or
washed away.
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The soil structure is
impoverished and is
blown or washed away.
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The thin soil is
washed away by
the rain.
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Wheat crops are grown
every year.
Rivers carry the
topsoil away.
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Trees are cut down.
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B14: Organisms and their environment
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Figure B14.06 How human activities can increase soil erosion.
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Most organisms that live in water respire aerobically, and
so need oxygen. They obtain their oxygen from oxygen gas
which has dissolved in the water. Anything which reduces
the amount of oxygen available in the water can make it
impossible for fish or other aquatic organisms to live there.
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B14.08 Explain how extensive deforestation can afect
the amount of carbon dioxide in the air.
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Algae and green plants in the river grow faster when they
are supplied with these extra nitrates. They may grow so
much that they completely cover the water. They block
out the light for plants growing beneath them, which die.
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B14.09 Explain how deforestation can cause soil erosion
and flooding.
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Farmers and horticulturists use fertilisers to increase the
yield of their crops. The fertilisers usually contain nitrates
and phosphates. Nitrates are very soluble in water. If
nitrate fertiliser is put onto soil, it may be washed out in
solution when it rains. This is called leaching. The leached
nitrates may run into streams and rivers. This causes a
reduction in the oxygen levels in the water, which kills fish
and other aquatic organisms. Untreated sewage has a
similar efect.
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There are two main sources of pollution which can reduce
oxygen levels in fresh water. They are fertilisers and
untreated sewage.
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The greatest pressure on the rainforest may come
from the country’s government in the big cities,
rather than the people living in or near the rainforest.
The government may be able to obtain large amounts of
money by allowing logging companies to cut down forests
and extract the timber. A way of getting round this could
be to allow countries to sell ‘carbon credits’ to other,
richer countries. In 2009, Indonesia did this. The idea is
that other countries give money to Indonesia to use in
conserving their forests, and that these countries are
then allowed to produce more carbon dioxide from their
industrial activities.
QuEStiONS
Water pollution
Many organisms live in water. They are called aquatic
organisms. Aquatic habitats include fresh water, such
as streams, rivers, ponds and lakes, and also marine
environments – the sea and oceans.
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governments of the richer, developed countries such
as the USA, can help by providing funds to the people
or governments of developing countries to try to help
them to provide alternative sources of income for
people. Many of the most successful projects involve
helping local people to make use of the rainforest in a
sustainable way.
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Even the plants on the top of the water eventually die.
When they do, their remains are a good source of food for
bacteria, which are decomposers.
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Sunlight can penetrate deep into the
water, allowing water plants to grow.
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This whole process is called eutrophication (Image B14.04
and Figure B14.07). It can happen whenever nutrients for
plants or bacteria are added to water. As well as fertilisers,
other pollutants from farms, such as slurry from buildings
where cattle or pigs are kept, or from pits where grass is
rotted down to make silage, can cause eutrophication.
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Water with high concentrations of nutrients is low in
oxygen, so few animals can live in it.
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The bacteria breed rapidly. The large population of
bacteria respires aerobically, using up oxygen from the
water. Soon, there is very little oxygen let for other living
things. Those which need a lot of oxygen, such as fish, have
to move to other areas, or die.
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Water with few nutrients is rich in oxygen, and
supports a variety of animal life.
clear water
run-off from fertilisers,
animal waste and silage
containing nitrates and
other nutrients
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Large populations of
algae and bacteria grow.
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Figure B14.07 Eutrophication. Nutrients flowing into the
water increase algal and bacterial growth. This reduces
oxygen concentration, killing fish.
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No light gets through the water,
so no water plants grow.
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No fish
can live in
this water.
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dissolved
oxygen
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Untreated sewage can also cause eutrophication
(Figure B14.08). Sewage does not usually increase the
growth of algae, but it does provide a good food source
for many kinds of bacteria. Once again, their population
grows, depleting the oxygen levels.
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Image B14.04 The huge growth of algae in this polluted
pond has provided food for aerobic bacteria. These have
used up most of the oxygen in the water, so the fish died.
fish
Amount
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point at which untreated
sewage is discharged
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Figure B14.08 The efect of raw sewage on a stream.
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Distance downstream
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Efluent from properly treated sewage does not cause
eutrophication. It is raw (untreated) sewage that
causes problems.
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bacteria
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A very diferent kind of water pollution may result from the
discharge of chemical waste into waterways. Chemical waste
may contain heavy metals, such as lead, cadmium or mercury.
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B14: Organisms and their environment
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Discarded rubbish is another source of water pollution.
Some of the worst problems arise from plastics.
One big problem with plastics is that most of them are
non-biodegradable. This means that decomposers cannot
break them down. When a plastic item is thrown away,
it does not rot. Discarded plastic objects just accumulate
(Image B14.05).
QuEStiONS
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These substances are very poisonous (toxic) to
living organisms, because heavy metals stop
enzymes from working. If they get into streams,
rivers or the sea, they may kill almost every living thing
in that area of water.
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B14.10 List two substances that can cause
eutrophication if they get into waterways.
B14.12 Explain why throwing away a plastic bag is likely
to cause more harm to the environment than
throwing away a paper bag.
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B14.11 Eutrophication reduces the concentration of a
dissolved gas in a river or lake. Name this gas.
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Explain the diference between each of the following pairs of terms, giving examples where you can:
producer, consumer
primary consumer, secondary consumer
food chain, food web
a
b
c
d
Why do living organisms need carbon?
Explain how carbon atoms become part of a plant.
What happens to some of these carbon atoms when a plant respires?
Explain the role of decomposers in the carbon cycle.
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End-of-chapter questions
1
■
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how deforestation afects the environment
the sources and efects of pollution by chemical waste,
discarded rubbish, untreated sewage and fertilisers
about eutrophication.
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how food chains and food webs describe energy flow
between living organisms
how energy is lost in the transfer between
trophic levels
about the carbon cycle
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You should know:
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Summary
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Image B14.05 Non-biodegradable plastics never rot away.
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ticks
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5% of this energy
stored in tissue
60% of this energy passes
through the wildebeest
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[1]
[3]
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b
[1]
[1]
State the source of the energy in the food eaten by the ox.
State the form in which the energy is present in the carbohydrate eaten by the ox.
Name the process that makes the remaining 35% of the energy in the food available
to the ox.
State three ways in which the energy may be used within the ox.
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energy
contained
within food
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The diagram below shows what happens to energy as it passes through a herbivorous mammal (an ox).
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Cambridge IGCSE Combined and Co-ordinated Sciences
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The graph shows the amount of dissolved oxygen in
the water of a river in a city.
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10
0
1890
1930
Year
1950
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1910
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1970
[2]
[4]
[2]
[2]
[1]
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20
Give two ways in which water obtains dissolved oxygen.
Explain how pollution by sewage causes dissolved oxygen levels to decrease.
Suggest why dissolved oxygen levels in the river:
i decreased until 1948
ii have increased since the 1950s.
What efect would you expect a decrease in dissolved oxygen to have on the fish population
in the river?
Apart from afecting the levels of dissolved oxygen, what other harmful efects can the
discharge of untreated sewage into rivers have?
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d
60
Degree of
saturation 50
with
dissolved 40
oxygen / % 30
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b
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80
70
In the 19th century, sewage from the city drained
directly into the river. At the beginning of the
20th century sewage treatment works were
installed, which removed some of the
organic material from the sewage before it
entered the river. These sewage treatment
works have gradually become more eficient.
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[Cambridge O Level Biology 5090 Paper 21 Q1 June 2010]
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[1]
[3]
Draw a food web to show the feeding relationships of the organisms in the diagram.
Explain why there must always be fewer oxpeckers than ticks in this food web.
Pr
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c
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The bird on the ox’s back is an oxpecker that feeds both on blood-sucking parasites
(ticks) living on the ox, and on blood from the ox’s wounds.
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[2]
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Nitrogen is used in the manufacture of ammonia and
fertilisers in the Haber process. Liquid nitrogen is used
in cryogenics (the storing of embryos and other types of
living tissue at very low temperatures). Nitrogen is also
sometimes used where an unreactive gas is needed to
keep air away from certain products; for example, it is used
to fill bags of crisps (chips) to ensure that the crisps do not
get crushed or go rancid as a result of contact with oxygen
in the air.
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Carbon dioxide is an important part of the air but makes
up only about 0.04% of it. The carbon dioxide which is
used by humans is not usually obtained from the air.
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The atmosphere of the Earth has developed over geological
time and various changes have taken place (Figure C1.01).
The most significant of these changes was the introduction
of oxygen into the atmosphere by the appearance of
photosynthesising life forms. Clean air has the following
approximate composition: nitrogen 78%, oxygen 21%,
argon 0.9% and other gases (including carbon dioxide,
water vapour, neon and other noble gases) 0.1%
(see Table C1.01).
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Uses of the gases of the air
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C1.01 the atmosphere
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■
the composition and uses of the gases in the air
the sources of air pollution; particularly carbon monoxide (CO), sulfur dioxide (SO2) and the oxides of nitrogen (NOx)
the problems of air pollution, and their solution
‘greenhouse gases’ and climate change
water treatment
metal ores and limestone
fossil fuels and the problems they cause.
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This chapter covers:
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C1
Planet Earth
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EARLy vOLCANiC AtMOSPHERE
reacted with early oxygen
carbon dioxide
CO2
y
condensed as
the Earth
cooled down
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CO2 dissolved in oceans,
then concentrated
into the shells of sea
creatures as
calcium carbonate
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plants
(photosynthesis)
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reacted with oxygen
+
denitrifying bacteria
in the soil
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OuR AtMOSPHERE NOW
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—(a)
(a)
All the other gases in the air make up 0.06% of the total.
The biggest single use of oxygen is in the production of
steel from cast iron. It is also used in oxyacetylene torches
to produce the high-temperature flames needed to cut
and weld metals. In hospitals, oxygen in cylinders is used
to help the breathing of sick people.
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ACtivity C1.01
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Table C1.01 The composition of the air.
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Estimating the amount of oxygen in air
This is a demonstration of the reduction in volume when
air is passed over heated copper.
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Argon and other noble gases are used in diferent types
of lighting. Argon is used to ‘fill’ light bulbs to prevent
the tungsten filament burning away (Image C1.02).
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—(a)
helium
At the beginning of each chapter you will find a list of the
topics in the chapter that you need to know about. This is
material which is included in the syllabus and therefore
could appear in examination questions.
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78
neon
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0.9
nitrogen
This chapter sets the chemistry you will study in context.
The Earth (Image C.01) is the only source we have for all
of the chemicals we use. The air in our atmosphere, the
water in our seas and lakes and the chemicals in the rocks
of the Earth’s crust provide us with all that we need.
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argon
—(a)
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krypton
oxygen
—(a)
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xenon
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carbon dioxide (sublimes)
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Proportion in mixture / %
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nitrogen
N2
(78%)
oxygen
O2
(21%)
Figure C1.01 The development of the Earth’s atmosphere.
Gas
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oceans
es
some carbon
trapped as
fossil fuels
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sedimentary rocks
such as limestone
or chalk
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steam
H 2O
ammonia
NH3
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methane
CH4
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TIP
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Many gases are accidentally or deliberately released into the
air. Some are harmless but many create problems for the
environment. The main source of ‘problem’ gases is the burning
of fossil fuels. Most countries produce electricity by burning
coal or oil. Both these fuels are contaminated with sulfur.
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When these fuels are burnt in power stations to generate
energy, the sulfur content reacts with oxygen to produce
sulfur dioxide:
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+
SO2
O2
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Limestone buildings, statues, etc., are worn away.
■ Lakes are acidified, and metal ions (for example,
Al3+ ions) that are leached (washed) out of the soil
damage the gills of fish, which may die.
■ Nutrients are leached out of the soil and from leaves.
Trees are deprived of these nutrients. Aluminium
ions are freed from clays as aluminium sulfate, which
damages tree roots. The tree is unable to draw up
enough water through the damaged roots, and it dies.
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There are numerous efects of acid rain.
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Image C1.02 Filament light bulbs contain argon, which
does not react with the hot tungsten filament.
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chemical reactions in the
air and in the clouds
nitrogen oxides,
hydrocarbons
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sulfur dioxide,
nitrogen oxides
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It does not react with tungsten even at very high
temperatures. The other noble gases are used in
advertising signs because they glow with diferent colours
when electricity flows through them.
effects on trees
and buildings
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rain
run-off
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effects on
soil chemistry
effects on water chemistry
and water biology
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Figure C1.02 The formation of acid rain.
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Oxides of nitrogen (NOx) (for example, nitrogen dioxide, NO2)
are also produced when air is heated in furnaces. These gases
dissolve in rainwater to produce ‘acid rain’ (Figure C1.02).
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sulfur dioxide
sulfur + oxygen
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Pollution of the air
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If you are asked for a use of oxygen, ‘breathing’ is not
considered to be a correct answer because it is air rather than
oxygen that we breathe. You need to give a use of pure oxygen.
Image C1.01 A satellite image over Africa: one view of the
‘blue marble’. This image emphasises the presence of water
on the planet surface, and in the atmosphere.
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C1: Planet Earth
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This is a particular problem in diesel-powered cars,
as the operating temperature is significantly higher.
Because of the lack of oxygen in the enclosed space of
an engine, the fuel does not usually burn completely
and carbon monoxide (CO) is formed. This toxic gas
is formed from the incomplete combustion of the
hydrocarbon fuel.
• Nitrogen dioxide causes acid rain and can combine
with other gases in very hot weather to cause
photochemical smog. This contains low-level ozone
and is likely to cause breathing problems, especially in
people with asthma.
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The dangers of these pollutants are as follows.
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The wind can carry acid rain clouds away from the
industrialised areas, causing the pollution to fall on
other countries.
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Image C1.03 Fumes from a car exhaust.
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In a car fitted with a catalytic converter, the exhaust gases
are passed through a heated ‘honeycombed’ bed of the
transition metal catalyst and the potential pollutants take
part in several diferent reactions, converting them to
carbon dioxide and nitrogen.
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Petrol (gasoline) and diesel for use in road
transport have most of their sulfur removed when they
are refined, producing low sulfur fuels. Sulfur dioxide is
not a serious problem with motor vehicles but the other
contents of vehicle exhaust fumes (Image C1.03) can cause
problems. Nitrogen dioxide, for example, is still produced.
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There are solutions to some of these problems. Catalytic
converters can be attached to the exhaust systems of
cars (Figure C1.03). These convert carbon monoxide and
nitrogen dioxide into carbon dioxide and nitrogen.
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One way to remedy the efects of acid rain is to add lime
to lakes and the surrounding land to decrease the acidity.
The best solution, however, is to prevent the acidic gases
from being released in the first place. ‘Scrubbers’ are fitted
to power station furnaces. In these devices, the acidic
gases are passed through an alkaline substance such as
lime (calcium oxide). This removes the acids, making the
escaping gases much less harmful. In many countries acidic
gases from power stations are still a serious problem.
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• Carbon monoxide is a highly toxic gas. It combines with
the haemoglobin in blood and stops it from carrying
oxygen. Even very small amounts of carbon monoxide
can cause dizziness and headaches. Larger quantities
cause death.
+
2NO2
N2
+
O2
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catalytic converter
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exhaust gases: carbon dioxide,
water and nitrogen
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exhaust gases: unburnt fuel, carbon monoxide
and nitrogen oxides, with carbon dioxide,
water and nitrogen
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2NO
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2O2
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N2
+ 2CO2
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nitrogen dioxide
N2
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nitrogen + oxygen
2CO2
2NO + 2CO
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The high temperature inside the engine’s cylinders causes
the nitrogen and oxygen in the air to react together:
O2
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2CO +
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Figure C1.03 A catalytic converter changes harmful exhaust gases into safer gases.
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C1: Planet Earth
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global warming
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acid rain
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methane, CH4
carbon dioxide, CO2
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Try to keep these diferent atmospheric pollution
problems clear and distinct in your mind rather than
letting them merge together into one (confused?)
problem. They each have distinct causes and
clear consequences.
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• The surface temperature of the Earth will increase.
Deserts will spread and millions of people will have
less water.
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There are two gases in Figure C1.04, carbon dioxide
and methane, which are not in the list of pollutants
given so far. These gases, together with water vapour
and oxides of nitrogen, are causing global warming
due to the ‘greenhouse efect’. The Earth is warmed by
the Sun but this heat would quickly escape if it were
not for our atmosphere. It is always colder on a
clear night because there are no clouds to keep the
heat in. Some gases are better at keeping heat in
than others; if there is too much of these gases in the
atmosphere, the Earth gets warmer and this causes
problems (Figure C1.05).
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Global warming and the ‘greenhouse efect’
• Severe weather events will increase in frequency, and
hurricanes and flooding will become more common.
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Figure C1.04 summarises the efects of the main pollutants
of the air.
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Carbon dioxide enters the air through respiration
and burning and it is removed by plants during
photosynthesis. Burning more fuel and cutting down
the forests increase the problem. Burning less
fossil fuel and planting more trees would help to solve it.
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• In some areas it may become easier to grow
food crops but in others it will certainly become
more dificult.
Carbon dioxide and methane are the two main problem
gases; methane is around 20 times more efective at
stopping heat escaping than carbon dioxide is.
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The raised levels of greenhouse gases in the atmosphere
since the Industrial Revolution is giving rise to additional
problems in the environment, including climate change.
Some of the problems global warming (or the ‘enhanced
greenhouse efect’) will cause are listed below.
• Glaciers and polar ice will melt. This will cause a rise in
sea level, and low-lying land will be flooded.
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Figure C1.05 The greenhouse efect.
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Burning fossil fuels, forest fires, industry and
human activities produce various ’greenhouse
gases‘. As these increase, more and more of the
Sun’s energy is trapped. The Earth warms up.
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Figure C1.04 A summary of various atmospheric pollution
problems caused by human activity.
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EARTH
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sulfur dioxide, SO2
carbon monoxide, CO
unburnt hydrocarbon fuels, HC
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oxides of nitrogen, NOx
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photochemical smog
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Some energy is
radiated back into
space as light
and heat.
energy
Some energy is
radiated
absorbed in the
by the Sun
atmosphere.
Energy radiated
by the Sun
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Methane is a product of the anaerobic decay of organic
matter and is produced in large quantities in rice paddy
fields and landfill rubbish sites. It is also produced by the
digestive systems of animals, ranging from cattle to termites.
Pr
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C1.03
What are the problems caused by acid rain?
C1.04
What is photochemical smog and why is it a problem?
C1.05
How does carbon monoxide stop the blood from
carrying oxygen?
C1.06
Why are light bulbs filled with argon?
C1.07
What is the ‘greenhouse efect’?
C1.08
What does a catalytic converter do to the exhaust
gases from a car?
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In some parts of the world, seawater is made drinkable
by desalination (taking the salt out). This can be done
by distillation or by forcing the water through special
membranes using high pressures (reverse osmosis).
Desalination is particularly important in countries such as
Saudi Arabia.
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C1.02 Water treatment
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precipitators
to clear solid
particles
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storage reservoir
pump
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treated with a small
amount of ozone to
disinfect the water
chlorine
microstrainers
y
activated carbon
granules absorb
some of the
chemicals
op
a small amount
of chlorine is used
to disinfect water
ev
ie
id
g
service reservoir
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br
am
w
e
U
main ozone
pumps
C
ev
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ve
rs
C
ity
Pr
op
y
es
s
screens for straining
floating rubbish
R
drinking water
es
s
Figure C1.06 Purifying water for the domestic and industrial supply.
-C
rapid gravity
sand filters
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id
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river water
pumping
station
br
am
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ve
U
river
-C
Our water supply is very important. Not only is it used in
the home, as shown in Figure C1.07, but it is also used in
large quantities by industry. Most of the water used by
industry is utilised as a solvent for other substances, to
cool down reactions or to transfer heat from one part of a
factory to another.
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182
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op
Pr
y
Water is essential to life but it can also carry disease.
Polluted water kills many millions of people every year.
It is important that the water we drink is treated to
make it safe, and even more important that sewage
(human and animal waste) is treated before being allowed
back into rivers used for drinking water.
w
w
ev
ie
Figure C1.06 shows a modern water treatment
process. The main diference from the simple treatment is
in the use of ozone to remove pesticides and some other
dissolved substances which can cause health problems.
The water is still not totally pure as it contains some
dissolved solids. Some of these, such as calcium salts,
can aid health, whereas others, such as nitrate fertilisers,
can be harmful.
ve
rs
ity
How do the gases responsible for acid rain get
into the atmosphere?
C
op
C1.02
R
ev
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am
br
id
Which gases contribute most significantly to acid rain?
y
C1.01
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QuEStiONS
w
ge
Water from rivers and lakes, and from underground,
can contain dissolved salts, solid particles and bacteria.
The water purification process is designed to remove
the last two of these. At its simplest, water treatment
involves filtering the water to remove solid particles
and adding chlorine to kill any bacteria that could
cause disease.
Copyright Material - Review Only - Not for Redistribution
the main dose
of ozone to
break down
pesticides and
other materials
ve
rs
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C
15 dm3
w
ev
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ev
ie
drinking
12 dm3
10 dm3
This experiment is designed to show that seawater
contains a mixture of diferent salts.
ie
-R
s
es
C1.11
Why is distillation of seawater an expensive
way of making drinking water?
Pr
Why is chlorine added to water?
1 Place 200 cm3 of seawater in a 250 cm3 beaker.
2 Heat and boil the seawater.
ity
4 Allow to cool and let any solids settle.
ni
op
y
ve
5 Pour the clear liquid into a 100 cm3 beaker, leaving the
solids behind.
ge
C
U
6 Add a few drops of dilute hydrochloric acid to the solids
let behind and observe what happens.
ev
id
ie
w
7 Put the 100 cm3 beaker on the tripod and gauze and
heat the liquid until another solid appears. This will
occur when about 30–40 cm3 of liquid remains.
s
-R
8 Carefully filter the liquid into a conical flask.
es
-C
am
br
Chemicals from seawater
Skills:
A03.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
A03.3 Make and record observations, measurements
and estimates
A03.4 Interpret and evaluate experimental
observations and data
Pr
op
y
9 Wash out the 100 cm3 beaker and pour the filtrate into
the beaker.
10 Boil the liquid again until there is almost none let.
11 Let it cool and note what you observe.
ity
Questions
ni
ve
rs
A1 What evidence is there that seawater is a mixture of salts?
A2 What gas is likely to have been given of when
hydrochloric acid is added to the solids first collected?
Take care with hot apparatus and solutions.
A3 What does this tell you about the identity of these solids?
U
op
y
Wear eye protection throughout.
A4 Search the internet to try to find information about the
solubilities of sodium chloride and calcium sulfate –
two common compounds present in seawater. Use this
information to predict the possible identity of the final
solid let at the end of your experiment.
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s
es
am
br
ev
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id
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w
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C
The sea is mainly water but there are lots of other things in
it too. The most common substance in seawater is sodium
chloride, or common salt. Other substances in it include
calcium sulfate, magnesium sulfate and tiny amounts of
metals such as copper and iron.
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C
w
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beaker with first solids
3 Stop heating when about 60–70 cm3 of liquid remains.
Solid will be precipitated during this evaporation process.
rs
w
ie
ev
R
ev
tripod
Bunsen burner
ev
id
br
am
-C
y
C1.10
op
C
Why is water filtered before other
treatments?
ACtivity C1.02
beaker
seawater
gauze
w
ge
U
R
ni
C
op
Figure C1.07 The main uses of water in a UK home.
The numbers show how much water is used on average
per person for each activity every day.
!
96.5% water
3 dm3
QuEStiONS
R
100 dm3
seawater
2.6% sodium chloride
0.3% magnesium chloride
0.2% magnesium sulfate
0.1% calcium sulfate
0.1% potassium chloride
0.01% potassium bromide
small amounts of most
other elements
dishwashing gardening cooking
20 dm3
C1.09
3.5% dissolved
minerals
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op
-R
washing
clothes
y
-C
55 dm3
Pr
es
s
65 dm3
am
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id
personal
washing
w
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toilet
C
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op
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C1: Planet Earth
Copyright Material - Review Only - Not for Redistribution
183
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Limestone
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C1.03 the Earth’s crust
op
y
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Pr
es
s
-C
From this thin layer we get the soil we need for growing
crops, the ores from which metals are extracted, the rocks
we need for building, and the fossil fuels we use.
Limestone is an important resource from which a useful
range of compounds can be made. Figure C1.08 shows
some of the important uses of limestone and the related
compounds quicklime and slaked lime. The reactions
involved in producing these compounds can be imitated in
the laboratory (Figure C1.09).
-R
am
br
id
Only the top layer of the Earth is used to obtain the
chemicals we need. This layer is known as the Earth’s crust
and its thickness varies from about 5 km to about 50 km.
A piece of calcium carbonate can be heated strongly for
some time to produce lime (quicklime, calcium oxide).
The piece of lime is allowed to cool and then a few drops of
water are added. The solid flakes and expands, crumbling
into ‘slaked lime’. This reaction is strongly exothermic.
If more water is added, an alkaline solution (limewater)
is obtained.
ni
C
op
y
Rocks can be used for building and for the extraction of
useful chemicals other than metals. The most useful of
these is limestone.
w
ev
-R
s
Pr
es
LiMEStONE
rs
C
w
neutralise acidic soil
and lakes affected
by acid rain
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ve
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ni
U
id
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w
ge
C
Figure C1.08 Some of the uses of limestone (calcium carbonate).
ev
pieces of limestone
(calcium carbonate)
dropper with water
Pr
op
y
es
s
-C
-R
am
br
concrete
cement
water +
sand
mortar
paper
(used to whiten
and provide bulk)
ity
op
glass
gravel + sand
+ water
op
am
y
-C
heat with sand and
sodium carbonate
ev
wire support
steam
cool
id
g
e
Bunsen flame
ie
quicklime
br
ev
Limestone changes into
quicklime (calcium oxide).
heatproof
mat
Slaked lime is
formed from
quicklime.
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am
y
dropper
with water
C
U
R
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limestone
w
ie
w
ni
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C
ity
Nothing happens
with limestone.
op
R
steel (limestone removes
impurities in furnace)
ie
buildings
and roads
br
id
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U
R
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w
C
ve
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ity
Metal ores are rocks that have a relatively high concentration
of a mineral containing a metal. For more details of ores and
obtaining metals from them see Chapters C8 and C9.
184
C
U
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Cambridge IGCSE Combined and Co-ordinated Sciences
s
es
-C
Figure C1.09 The formation of quicklime and slaked lime in the laboratory.
Copyright Material - Review Only - Not for Redistribution
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C1: Planet Earth
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The Earth’s resources
y
thermal decomposition of calcium carbonate
This activity illustrates some of the chemistry of limestone
(calcium carbonate) and other materials made from it. The
experiment demonstrates the ‘limestone cycle’.
A worksheet is included on the CD-ROM.
Other resources should last forever but, if we misuse them,
problems arise. Clean air can be lost if we pollute it, as can
fresh water. Even the energy from the Sun and the energy
in the wind and waves can be adversely afected by the
pollution causing the ‘greenhouse efect’.
C
op
ni
C1.13
ev
id
es
Pr
y
ity
that the air is composed predominantly of nitrogen and
oxygen, but that other gases have major roles to play too
about the major atmospheric pollution problems that
are changing the nature of our world, including global
climate change and acid rain
that global warming is caused by an increase in the
atmosphere of certain ‘greenhouse gases’ such as
carbon dioxide and methane
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■
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s
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■
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nitrogen............%
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oxygen............%
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s
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id
g
Copy and complete the chart by labelling the percentages of nitrogen, oxygen and other gases.
Name one of the other gases that exists in unpolluted air.
br
i
ii
am
a
other gases............%
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C
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Percentage in
unpolluted air
es
C
The bar chart shows the approximate
composition of clean air.
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op
y
End-of-chapter questions
1
how the availability of clean fresh water is one of the
major problems in the world
that limestone, one of these mineral resources,
has a range of uses, from the making of cement
and concrete to the extraction of iron in the
blast furnace.
op
w
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ev
185
y
op
C
You should know:
■
What makes a gas a ‘greenhouse gas’?
s
-C
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am
C1.14
Summary
■
What is the diference between lime and
slaked lime?
ie
What makes an ore diferent from any other
type of rock?
br
C1.12
ge
QuEStiONS
w
U
R
More details about recycling metals can be found in Chapter C9.
y
ve
rs
ity
op
C
w
ev
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-R
Pr
es
s
-C
More detail on the importance of limestone and the
chemicals derived from it can be found in Chapter C9.
This includes the method of making lime industrially.
ACtivity C1.03
Fossil fuels are only one of the Earth’s important resources
but they are a ‘non-renewable’ resource. When we have
used up all of the fossil fuels, they are gone. Metal ores are
also ‘non-renewable’ and so it important that we recycle
the metals we use in order to conserve the limited supplies
of metal ores in the Earth’s crust.
ev
ie
am
br
id
TIP
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[2]
[1]
(continued)
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b
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NO
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id
br
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am
es
s
-C
Describe a chemical test for water. Give the test and the result.
State one use of water in industry.
Water is a good solvent. What do you understand by the term solvent?
Water vapour in the atmosphere reacts with sulfur dioxide, SO2, to produce acid rain.
i State one source of sulfur dioxide.
ii State two adverse efects of acid rain.
iii Calculate the relative molecular mass of sulfur dioxide.
Water from lakes and rivers can be treated to make the water safer to drink.
Describe two of the steps in water purification. For each of these steps, give an explanation of its purpose.
Water is formed when hydrogen burns in air. State the percentage of oxygen present in the air.
ity
rs
op
C
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id
br
Gas
-R
am
es
s
-C
Pr
op
y
15
nitrogen
60
carbon dioxide
15
water vapour
10
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id
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g
am
f
Quantity / %
oxygen
sulfur dioxide
2
How does the quantity of the first three gases difer from
that on Earth?
What problems could be caused on the surface of the planet by the presence of the final two gases listed?
Give a test to show the presence of water in the condensation from the air on a window on a cold day.
Which pollutant gas in the atmosphere has the same efect on the climate as carbon dioxide?
Sulfur dioxide is not present in pure air on Earth. What process can cause sulfur dioxide to
enter the Earth’s atmosphere?
Lime can be used to remove sulfur dioxide from gases entering the atmosphere.
What name is given to the process which removes the sulfur dioxide?
How is lime manufactured from limestone? Write a word equation for the reaction.
-C
C
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b
c
d
e
[4]
[1]
[Cambridge IGCSE Chemistry 0620 Paper 21 Q3 a–e & f(i) June 2011]
The atmosphere of a newly discovered Earth-like planet had
been analysed. It has the composition shown in the table.
a
[1]
[2]
[1]
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f
[2]
[1]
[1]
Pr
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e
3
[3]
Water is present in the atmosphere, in the seas and in ice and snow.
a
b
c
d
186
[2]
y
Write a balanced chemical equation for the formation of nitric oxide (NO) during a thunderstorm.
Nitrogen dioxide (NO2) dissolves and reacts
Sample of rainwater
pH
with rainwater. A student carried out an
pure water obtained in laboratory
7
experiment to investigate what happened to
rainwater collected when no storm had occurred 5
the acidity of rainwater during a thunderstorm.
rainwater collected during thunderstorm
4
His results are shown in the table.
What conclusions can the student make from these results? Include the name of
any new compound formed during the storm.
U
R
2
NO2
NO2
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C
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i
ii
NO
Pr
es
s
-C
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id
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Nitrogen and oxygen exist in the air in the form of the diatomic
molecules, N2 and O2. When lightning passes through the air,
the gaseous compounds nitric oxide, NO, and nitrogen dioxide,
NO2, are formed.
w
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
[1]
[2]
[2]
[1]
[1]
[1]
[2]
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id
Pr
es
s
-C
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ni
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op
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es
s
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Pr
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C
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Pr
ity
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ni
ve
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C
w
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■
s
■
U
■
e
■
id
g
■
br
■
am
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ev
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C
■
-C
■
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■
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■
s
■
es
■
es
■
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■
br
■
am
■
the three states of matter, and changes of state
atoms, molecules and ions
separating and purifying substances
filtration
use of a separating funnel
crystallisation
distillation
paper chromatography
criteria of purity
elements and compounds
atomic theory
the kinetic model and changes of state
difusion
atomic structure and sub-atomic particles
proton (atomic) number and nucleon (mass) number
isotopes
relative atomic mass
the arrangement of electrons in atoms.
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■
187
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op
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This chapter covers:
■
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C2
The nature of matter
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C
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Cambridge IGCSE Combined and Co-ordinated Sciences
Changes in physical state
w
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C2.01 the states of matter
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Melting and freezing
y
The temperature at which a pure substance turns to a liquid
is called the melting point (m.p.). This always happens at one
particular temperature for each pure substance (Figure C2.02).
The process is reversed at precisely the same temperature if a
liquid is cooled down. It is then called the freezing point (f.p.).
The melting point and freezing point of any given substance
are both the same temperature. For example, the melting and
freezing of pure water take place at 0 °C.
U
ni
C
op
C
ve
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op
y
Chemistry is the study of how matter behaves, and of
how one kind of substance can be changed into another.
Whichever chemical substance we study, we find it can
exist in three diferent forms (or physical states) depending
on the conditions. These three diferent states of matter
are known as solid, liquid and gas. Changing temperature
and/or pressure can change the state of a substance.
w
ev
ie
R
Large changes in temperature and pressure can cause
changes that are more dramatic than expansion or
contraction. They can cause a substance to change its
physical state. The changes between the three states of
matter are shown in Figure C2.01. At atmospheric pressure,
these changes can be brought about by raising or lowering
the temperature of the substance.
Pr
es
s
-C
am
br
id
There are many diferent kinds of matter. The word is used
to cover all the substances and materials of which the
Universe is composed. Samples of all of these materials
have two properties in common: they each occupy space
(they have volume) and they have mass.
op
188
Sublimation
A few solids, such as carbon dioxide (‘dry ice’), do not melt
when they are heated at normal pressures. Instead, they turn
directly into gas. This change of state is called sublimation:
the solid sublimes. Like melting, this also happens at one
particular temperature for each pure solid. Iodine is another
solid that sublimes. It produces a purple vapour, but then
condenses again on a cold surface (Image C2.02).
rs
C
ity
The three physical states show diferences in the way
they respond to changes in temperature and pressure.
All three show an increase in volume (an expansion)
when the temperature is increased, and a decrease in
volume (a contraction) when the temperature is lowered.
The efect is much bigger for a gas than for either a solid or
a liquid.
id
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Pr
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matter: anything that has mass and takes up space. There are
three physical states: solid, liquid and gas.
ev
Evaporation, boiling and condensation
has a fixed volume
liquid
has a fixed volume
gas
no fixed volume –
expands to fill
the container
-R
s
Shape
Fluidity
high
has a definite shape
does not flow
moderate to high
no definite shape –
takes the shape of
the container
generally flows
easily(a)
low
no definite shape –
takes the shape of
the container
flows easily(a)
(a)
op
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Liquids and gases are called fluids.
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s
-C
Table C2.01 Diferences in the properties of the three states of matter.
es
ie
Pr
solid
Density
ity
op
y
Volume
C
Physical state
ev
R
If a liquid is let with its surface exposed to the
air, it evaporates. Splashes of water evaporate at
room temperature. Ater rain, puddles dry up!
es
-C
am
br
The volume of a gas at a fixed temperature can easily be
reduced by increasing the pressure on the gas. Gases are
easy to ‘squash’ – they are easily compressed. Liquids, on
the other hand, are only slightly compressible, and the
volume of a solid is unafected by changing the pressure.
Gallium is a metal that has a melting point just above room
temperature. Because of this it will melt in a person’s hand
(Image C2.01).
s
KEy tERM
am
br
ev
id
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w
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The diferent physical states have certain general
characteristics that are true whatever chemical substance
is being considered. These are summarised in Table C2.01.
Copyright Material - Review Only - Not for Redistribution
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rs
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C
op
Freezing: the reverse takes place sharply at
the same temperature.
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freezing or
solidification
U
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br
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solid
Melting: a pure substance melts suddenly
at a particular temperature.
increasing temperature
-R
condensation or
liquefaction
evaporation or
vaporisation
Pr
es
s
melting
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sublimation
Evaporation and condensation take
place over a range of temperatures; boiling
takes place at a specific temperature.
liquid
ni
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-C
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id
gas
Sublimation: a few solids change
directly from solid to gas on heating;
the term sublimation is used for
the change in either direction.
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C2: The nature of matter
es
s
-C
Figure C2.01 Changes of physical state and the efect of increasing temperature at atmospheric pressure. Note that the
direct conversion from gas into solid can also be called reverse sublimation or deposition.
liquid
189
gas
y
C
boiling point
(b.p.)
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melting point
(m.p.)
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op
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C
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solid
Pr
op
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increasing temperature
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Pr
op
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es
s
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br
Figure C2.02 The relationship between the melting point
and boiling point of a substance.
ev
id
ie
decreasing temperature
y
op
When liquids change into gases in this way, the process
is called evaporation. Evaporation takes place from the
surface of the liquid. The larger the surface area, the faster
the liquid evaporates.
ie
id
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C
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Image C2.02 Iodine sublimes. On warming, it produces a
purple vapour, which then condenses again on the cool
part of the tube.
s
es
-C
am
Image C2.01 The metal gallium has a melting point just
above room temperature. It will literally melt in the hand.
-R
br
ev
The warmer the liquid is, the faster it evaporates.
Eventually, at a certain temperature, it becomes hot enough
for gas to form within the liquid and not just at the surface.
Copyright Material - Review Only - Not for Redistribution
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Substance
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gas
–210
–196
ethanol
(alcohol)
liquid
–117
78
water
liquid
0
100
sulfur
solid
115
444
common
salt (sodium
chloride)
solid
801
1465
1083
2600
(a)
ity
The values for the melting point and boiling point of a pure
substance are precise and predictable. This means that we
can use them to test the purity of a sample. They can also
be used to check the identity of an unknown substance.
The melting point can be measured using an electrically
heated melting-point apparatus or the apparatus shown in
Figure C2.03. A small amount of powdered solid is put in a
narrow (capillary) melting-point tube so that it can be heated
easily. A water bath is used to heat the tube; or an oil bath can
be used if melting points above 100 °C need to be measured.
rs
op
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ni
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thermometer
es
-C
stirrer
Pr
op
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ity
melting-point tube
rubber band
ni
ve
rs
C
Figure C2.03 Apparatus for measuring the melting point of
a solid. A water bath can be used for melting points below
100 °C and an oil bath for those above 100 °C.
es
s
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Be careful with temperatures below 0 °C; –100 °C is a
higher temperature than –150 °C.
am
heat
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id
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C
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Remember to practise using melting and boiling point
data to decide whether a particular substance is a solid,
a liquid or a gas at room temperature. These are quite
common questions.
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solid
y
TIP
oil or water
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w
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w
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Pr
if the m.p. is below 20 °C and the b.p. is above 20 °C,
the substance will be a liquid at room temperature.
A substance’s melting and boiling points in relation to
room temperature (taken as 20 °C) determine whether it
is usually seen as a solid, a liquid or a gas. For example,
ev
–78(a)
gas
Table C2.02 The melting and boiling points of some
common chemical substances.
id
A pure substance consists of only one substance. There
is nothing else in it: it has no contaminating impurities.
A pure substance melts and boils at definite temperatures.
Table C2.02 shows the melting points and boiling points of
some common substances at atmospheric pressure.
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solid
Sublimes.
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Pure substances
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nitrogen
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–183
carbon
dioxide
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190
–219
copper
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am
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The boiling point of a liquid can change if the surrounding
pressure changes. The value given for the boiling point is
usually stated at the pressure of the atmosphere at sea
level (atmospheric pressure or standard pressure).
If the surrounding pressure falls, the boiling point falls.
The boiling point of water at standard pressure is 100 °C. On
a high mountain it is lower than 100 °C. If the surrounding
pressure is increased, the boiling point rises. In a pressure
cooker, the boiling point of water is raised to around 120 °C
and food cooks more quickly at this higher temperature.
Boiling
point / °C
gas
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The reverse of evaporation is condensation. This is usually
brought about by cooling. However, we saw earlier that the
gas state is the one most afected by changes in pressure.
It is possible, at normal temperatures, to condense a gas
into a liquid by increasing the pressure, without cooling.
Melting
point / °C
ev
y
op
A volatile liquid is one which evaporates easily and
has a relatively low boiling point.
■ Ethanol (b.p. 78 °C) is a more volatile liquid than
water (b.p. 100 °C).
■
Physical
state at room
temperature
(20 °C)
oxygen
Pr
es
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br
id
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Bubbles of gas appear inside the liquid. This process is
known as boiling. It takes place at a specific temperature,
known as the boiling point (b.p.) for each pure liquid
(Figure C2.02). Water evaporates fairly easily and has a
relatively low boiling point – it is quite a volatile liquid.
Ethanol, with a boiling point of 78 °C, is more volatile than
water. It has a higher volatility than water.
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90
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Temperature / ºC
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gas
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liquid
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0
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Time
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solid
Figure C2.05 The cooling curve for a substance.
The temperature stays constant while the gas condenses,
and while the liquid freezes. A cooling mixture of
ice and salt could be used to lower the temperature
below 0 °C.
The level portions of the curve occur where the gas
condenses to a liquid, and when the liquid freezes.
These experiments show that heat energy is needed
to change a solid into a liquid, or a liquid into a gas.
During the reverse processes, heat energy is given out.
When a solid is melted, or a liquid is boiled, the
temperature stays constant until the process is
complete. The same is true in reverse when a gas
condenses or a liquid freezes.
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It is possible to heat a liquid in the same apparatus until its
boiling point is reached. Again, the temperature stays the
same until all the liquid has boiled. The reverse processes
can be shown if a sample of gas is allowed to cool. This
produces a cooling curve (Figure C2.05).
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op
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Pr
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Figure C2.04 shows how the temperature changes when
a sample of solid naphthalene (a single pure substance)
is heated steadily. The solid melts at precisely 80 °C.
Notice that, while the solid is melting, the temperature
stops rising. It will only begin to rise again when all the
naphthalene has melted. Generally, the heating curve for
a pure solid stops rising at its melting point. The heating
curve for wax, which is a mixture of substances, shows the
solid wax melting over a range of temperatures.
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10
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8
78
–15
Using the melting point apparatus shown in Figure C2.03,
we can follow the temperature of the sample before and
ater melting. These results can then be used to produce
a heating curve (Figure C2.04). Similar apparatus can be
used to produce a cooling curve, but the thermometer
must be placed in a test tube containing the solid
being studied.
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4
6
Time / minutes
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Remember that pure substances have definite, sharp
melting and boiling points. The presence of an impurity
means that these changes will be spread over a range of
temperatures in each case.
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2
Figure C2.04 The heating curves for naphthalene (a pure
substance) and wax (a mixture of substances).
Heating and cooling curves
R
wax
0
lowers the melting point
■ raises the boiling point.
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70
50
The presence of an impurity in a substance:
TIP
naphthalene
80
60
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In addition, the impurity can also reduce the ‘sharpness’
of the melting or boiling point. An impure substance
sometimes melts or boils over a range of temperatures,
not at a particular point.
■
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Temperature / ºC
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Pr
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Seawater is impure water. You can show this if you put
some seawater in an evaporating dish and boil away
the water, because a solid residue of salt is let behind
in the dish. Seawater freezes at a temperature well below
the freezing point of pure water (0 °C) and boils at a
temperature above the boiling point of pure water (100 °C).
Other impure substances show similar diferences.
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The efect of impurities
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C2: The nature of matter
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liquid solvent
In this experiment, you will plot cooling curves for two
diferent substances.
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a solid dissolved in a liquid. Two-thirds of the Earth’s surface
is covered by a solution of various salts in water. The salts are
totally dispersed in the water and cannot be seen. However,
other substances that are not normally solid are dissolved
in seawater. For example, the dissolved gases, oxygen and
carbon dioxide, are important for life to exist in the oceans.
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Alloys are similar mixtures of metals, though we do not
usually call them solutions. They are made by mixing the
liquid metals together (dissolving one metal in the other)
before solidifying the alloy.
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Each mixture must be made from at least two parts, which
may be solid, liquid or gas. There are a number of diferent
ways in which the three states can be combined. In some,
the states are completely mixed to become one single
state or phase – ‘you cannot see the join’. Technically, the
term solution is used for this type of mixture.
Less obvious perhaps, but quite common, are solutions of
one liquid in another. Alcohol mixes (dissolves) completely
with water. Beer, wine and whisky do not separate out into
layers of alcohol and water (even when the alcohol content
is quite high). Alcohol and water are completely miscible:
they make a solution.
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d solid to gas directly
C2.02
What efect does the presence of an impurity have
on the freezing point of a liquid?
C2.03
Sketch a cooling curve for water from 80 °C to
–20 °C, noting what is taking place in the diferent
regions of the graph.
C2.04
What do you understand by the word volatile
when used in chemistry?
C2.05
Put these three liquids in order of volatility, with
the most volatile first: water (b.p. 100 °C), ethanoic
acid (b.p. 128 °C), ethanol (b.p. 78 °C).
op
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There are various ways in which substances in diferent
states can combine. Perhaps the most important idea here
is that of one substance dissolving in another – the idea of a
solution. We most oten think of a solution as being made of
a liquid to solid
c gas to liquid
e
Solutions
Give the names for the following physical changes:
b liquid to gas at a precise temperature
Pr
op
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C2.01
es
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Solid salt dissolves in liquid water to produce a liquid
mixture – a salt solution (Figure C2.06). In general terms,
the solid that dissolves in the liquid is called the solute.
The liquid in which the solid dissolves is called the solvent.
In other types of mixture, the states remain separate. One
phase is broken up into small particles, droplets, or bubbles,
within the main phase. Perhaps the most obvious example of
this type of mixture is a suspension of fine particles of a solid
in a liquid, such as we oten get ater a precipitation reaction.
s
am
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QUESTIONS
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Our world is very complex, owing to the vast range of
pure substances available and to the variety of ways in
which these pure substances can mix with each other.
In everyday life, we do not ‘handle’ pure substances very
oten. The air we breathe is not a single, pure substance –
and we could not live in it if it were! Water would be rather
tasteless if we drank it pure (distilled).
w
R
A worksheet, with a self-assessment checklist, is
included on the accompanying CD-ROM.
Types of mixture
R
solution –
solute particles
cannot be seen
Figure C2.06 When a solute dissolves in a solvent, the
solute particles are completely dispersed in the liquid.
Adaptations of this experiment and details of the use of
it in assessing practical skills AO3.3 and AO3.4 are given
in the Notes on activities for teachers/technicians.
192
dissolving
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Pr
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A03.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
A03.3 Make and record observations, measurements
and estimates
A03.4 Interpret and evaluate experimental
observations and data
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Plotting a cooling curve
Skills:
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solid
solute
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ACTIVITY C2.01
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a
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C2.02 Separating and purifying
substances
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support
The liquid filters
through: it is called
the filtrate.
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A more generally useful method for separating solids from
liquids is filtration (Figure C2.07a). Here the insoluble
material is collected as a residue on filter paper. Filtration is
useful because both phases can be obtained in one process.
The liquid phase is collected as the filtrate. The process
can be speeded up by using a vacuum pump to ‘suck’ the
liquid through the filter paper in a Buchner funnel and flask
(Figure C2.07b). Various large-scale filtration methods are
used in industry. Perhaps the most useful of these are the
filter beds used to treat water for household use.
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s
use some diference in
properties, e.g. density,
solubility, sublimation,
magnetism
br
solid + solid
(powdered mixture)
Method of separation
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Mixture
Pr
op
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suspension of solid in liquid filtration or centrifugation
use a separating funnel or
decantation
C
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liquid + liquid (immiscible)
to obtain solid: use
evaporation (crystallisation)
to obtain liquid: use
distillation
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solution of solid in liquid
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chromatography
Table C2.03 Separating diferent types of mixture.
Mixtures of two immiscible liquids can be separated if the
mixture is placed in a separating funnel and allowed to
stand. The liquids separate into diferent layers. The lower,
denser layer is then ‘tapped’ of at the bottom. This type of
separation is useful in industry. For example, at the base of the
blast furnace the molten slag forms a separate layer on top of
the liquid iron. The two can then be ‘tapped’ of separately.
C
two (or more) liquids mixed fractional distillation
together (miscible)
solution of two (or more)
solids in a liquid
Another method of separating an insoluble solid from a
liquid is centrifugation where the mixture is spun at high
speed in a centrifuge. This causes the solid to be deposited
at the bottom of the centrifuge tube. The liquid can be
carefully decanted of.
Separating immiscible liquids
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ev
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A vacuum pump is connected
to the side-arm flask; it
speeds up the flow of
liquid through the funnel.
es
Pr
op
ev
A Buchner funnel has a
perforated plate, which
is covered by a circle of
filter paper.
Figure C2.07 Filtration separates an insoluble solid
from a liquid.
In some ways these are the easiest mixtures to separate.
Quite oten, just leaving a suspension of a solid in a liquid
to stand achieves a separation – especially if the particles
of solid are large enough. Once the solid has settled to the
bottom, the liquid can be carefully poured of – a process
called decanting.
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br
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Separating insoluble solids from liquids
R
The solid remains in the
filter as the residue.
b
the type of mixture
which substance in the mixture we are most
interested in.
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■
filter funnel
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■
filter paper
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To make sense of the material world around us,
we need methods for physically separating the
many and varied mixtures that we come across.
Being able to purify and identify the many substances
present in these mixtures not only satisfies our curiosity
but is crucial to our well-being and health. There is a
range of physical techniques available to make the
necessary separations (Table C2.03). They all depend in
some way on a diference in the physical properties of
the substances in the mixture.
The most useful separation method for a particular
mixture depends on:
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C2: The nature of matter
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Cambridge IGCSE Combined and Co-ordinated Sciences
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seawater
Pr
water in
ity
The separation of this type of mixture is oten slightly more
complicated because there is no physical separation of the
phases in the original mixture. The methods of separation
usually depend on solubility properties or on diferences in
boiling point (or volatility).
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Figure C2.09 The distillation of seawater.
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C
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Separating a liquid from a solution is usually carried out
by distillation (Figure C2.09). The boiling point of the
liquid is usually very much lower than that of the dissolved
solid. The liquid is more volatile than the dissolved solid
and can easily be evaporated of in a distillation flask. It is
condensed by passing it down a water-cooled condenser,
and then collected as the distillate.
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br
es
Pr
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For example, ethanol boils at 78 °C whereas water boils at
100 °C. When a mixture of the two is heated, ethanol and
water vapours enter the fractionating column. Glass beads
in the column provide a large surface area for condensation.
Evaporation and condensation take place many times as
the vapours rise up the column. Ethanol passes through the
condenser first as the temperature of the column is raised
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ACTIVITY C2.02
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Separating common salt and sand
The aim of this activity is to separate a mixture of salt and
sand. The method uses the diference in solubility of the
two solids and the technique of filtration.
A worksheet, with a self-assessment checklist, is
included on the accompanying CD-ROM.
-C
Separating the liquids from a mixture of two (or more)
miscible liquids is again based on the fact that the liquids
will have diferent boiling points. However, the boiling
points are closer together than for a solid-in-liquid solution
and fractional distillation must be used (Figure C2.10). In
fractional distillation the most volatile liquid in the mixture
distils over first and the least volatile liquid boils over last.
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condenser
pure water
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water out
heat
Separating a solid from solution in a liquid can be carried
out by evaporation or crystallisation. Evaporation gives
only a powder, but crystallisation can result in proper
crystals. Both processes begin by evaporating away
the liquid but, when crystals are needed, evaporation is
stopped when the solution has been concentrated enough.
Figure C2.08 shows how this can be judged and done safely.
The concentrated solution is allowed to cool slowly. The
crystals formed can then be filtered of and dried.
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gauze
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Separating solutions
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boiling water
thermometer
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am
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194
solution in
evaporating basin
Figure C2.08 An evaporation method. This method should
not be used if the solvent is flammable. Instead, use an
electrical heating element and an oil or water bath.
Separations based on diferences in solubility
One very useful way of separating a soluble substance from
a solid mixture is as follows. The mixture is first ground to
a powder. A suitable liquid solvent is added. The solvent
must dissolve one of the solid substances present, but not
the others. The solvent is oten water, but other liquids can
be useful. The mixture in the solvent is then warmed and
stirred. Care must be taken at the warming stage when using
solvents other than water. The warm mixture is then filtered
(Figure C2.07). This leaves the insoluble substances as a
residue on the filter paper, which can be dried. The soluble
substance is in the liquid filtrate. Dry crystals can be obtained
by evaporation and crystallisation (see Figure C2.08).
While the solvent is
evaporating, dip a
glass rod into the
solution from time
to time. When small
crystals form on the
rod, take the solution
off the water bath
and leave it to cool.
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Pr
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The separation of a solid from a mixture of solids depends
largely on the particular substance being purified. Some suitable
diference in physical properties needs to be found. Separations
can be based on diferences in density, magnetic properties
(separating iron objects from other metals in a scrapyard, for
instance), or sublimation. In the laboratory it usually helps if
the mixture is ground to a powder before any separation is
attempted. By far the most important method for separating
this type of mixture is based on diferences in solubility.
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Separating mixtures of solids
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C2: The nature of matter
ACTIVITY C2.03
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thermometer
Distillation of mixtures
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water drips
back into
flask
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solution of
ethanol and
water
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water in
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80 °C
79 °C
Skills:
condenser
Pr
es
s
78 °C
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fractionating
column
(glass beads)
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water out
AO3.3 Make and record observations, measurements
and estimates
In this experiment, several mixtures will be separated
using diferent types of distillation apparatus, including a
microscale distillation apparatus.
ethanol
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electrical heater
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A worksheet is included on the CD-ROM.
Figure C2.10 Separating a mixture of ethanol (alcohol) and
water by fractional distillation.
ie
A drop of concentrated solution is usually placed
on a pencil line near the bottom edge of a strip of
chromatography paper. The paper is then dipped in the
solvent. The level of the solvent must start below the
sample. Figure C2.11 shows the process in action.
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The temperature on the thermometer stays at 78 °C until
all the ethanol has distilled over. Only then does the
temperature on the thermometer rise to 100 °C and the
water distil over. By watching the temperature carefully the
two liquids (fractions) can be collected separately.
substances present in a solution. For example, it can
tell us whether a solution has become contaminated.
This can be very important because contamination of
food or drinking water, for instance, may be dangerous to
our health.
ev
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above its boiling point. Water condenses in the column and
flows back into the flask because the temperature of the
column is below its boiling point of 100 °C.
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AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
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Stage 2
• The solvent moves up the paper, taking
diferent components along at diferent
rates.
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Pr
op
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A
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In fractional distillation, remember that it is the liquid with
the lowest boiling point that distils over first.
C
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Figure C2.11 Various stages during paper chromatography.
The sample is separated as it moves up the paper.
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A
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Separating two or more dissolved solids in solution can be
carried out by chromatography. There are several types
of chromatography, but they all follow the same basic
principles. Paper chromatography is probably the simplest
form to set up and is very useful if we want to analyse the
Stage 3
• The separation of the mixture is complete.
• The diferent components string out
along the paper like runners in a race.
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solvent
front
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TIP
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A
s
the various fractions from petroleum (Section C11.01)
■ the diferent gases from liquid air.
■
Stage 1
• The solution is spotted and allowed to dry.
The original spot is identified as A.
• The solvent begins to move up the paper by
capillary action.
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Fractional distillation is used to separate any solution
containing liquids with diferent boiling points. The
liquid in the mixture with the lowest boiling point (the
most volatile) distils over first. The final liquid to distil
over is the one with the highest boiling point (the least
volatile). Fractional distillation can be adapted as a
continuous process and is used industrially to separate:
Copyright Material - Review Only - Not for Redistribution
195
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are carried with it and begin to separate. The substance that
is most soluble moves fastest up the paper. An insoluble
substance would remain at the origin. The run is stopped just
before the solvent front reaches the top of the paper.
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The distance moved by a particular spot is measured and
related to the position of the solvent front. The ratio of
these distances is called the Rf value, or retention factor.
This value is used to identify the substance:
Pr
es
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Many diferent solvents are used in chromatography.
Water and organic solvents (carbon-containing solvents)
such as ethanol, ethanoic acid solution and propanone
are common. Organic solvents are useful because they
dissolve many substances that are insoluble in water.
When an organic solvent is used, the process is carried out
in a tank with a lid to stop the solvent evaporating.
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Cambridge IGCSE Combined and Co-ordinated Sciences
Originally, paper chromatography was used to separate
solutions of coloured substances (dyes and pigments)
since they could be seen as they moved up the paper.
However, the usefulness of chromatography has
been greatly increased by the use of locating agents
(Figure C2.12). These mean that the method can also be
used for separating substances that are not coloured.
The paper is treated with locating agent ater the
chromatography run. The agent reacts with the samples to
produce coloured spots.
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C
Paper chromatography is one test that can be used to check
for the purity of a substance. If the sample is pure, it should
only give one spot when run in several diferent solvents. The
identity of the sample can also be checked by comparing its
Rf value to that of a sample we know to be pure.
es
s
level reached by
the solvent (the
solvent front)
paper being drawn
through locating agent
Pr
thermometer
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oven
A
U
e
id
g
2 Running the
chromatogram
shallow
dish
solvent
3 Treating with
the locating agent
br
1 Preparing the paper
and spotting on the
samples
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glass
jar
pencil line
and letters
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E
4 Heating the paper
to bring up the
colour of the spots
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M
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G
locating
agent
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M
E
A
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paper coiled
in a cylinder
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cover
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fine tube for
spotting samples
onto the paper
chromatography
paper
5 The developed
chromatogram
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Figure C2.12 Chromatography using a locating agent to detect the spots on the paper. Alternatively, the locating agent can
be sprayed on the paper.
es
ev
Chromatography has proved very useful in the analysis of
biologically important molecules such as sugars, amino
acids and nucleotide bases. In fact, molecules such as
amino acids can be ‘seen’ if the paper chromatogram is
viewed under ultraviolet light.
The purity and identity of substances
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investigation of food dyes by chromatography
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest possible
improvements
This experiment involves testing some food colours with
paper chromatography to find out if they are pure colours
or mixtures of several dyes. These food colours are used in
cake making, for instance, and there is quite a wide range
of permitted colours readily available.
A worksheet is included on the CD-ROM.
Adaptations of this experiment are given in the Notes
on activities for teachers/technicians.
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ACTIVITY C2.04
distance moved by the substance
distance moved by the solvent front
Rf =
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The substances separate according to their solubility in the
solvent. As the solvent moves up the paper, the substances
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The concentration of a solution is the mass of solute
dissolved in a particular volume of solvent, usually 1 dm3.
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The solubility of gases increases with pressure. Sparkling
drinks contain carbon dioxide dissolved under pressure.
They ‘fizz’ when the pressure is released by opening the
container. They go ‘flat’ if the container is let to stand
open, and more quickly if let to stand in a warm place.
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Carbon dioxide is more soluble than either nitrogen or
oxygen. This is because it reacts with water to produce
carbonic acid. The world is not chemically static. Substances
are not only mixing with each other but also chemically
reacting. This produces a world that is continuously
changing. To gain a better understanding of this, we need to
look more deeply into the ‘makeup’ of chemical substances.
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A solution is made up of two parts:
■ the solute: the solid that dissolves
■ the solvent: the liquid in which it dissolves.
C2.06
If we try to dissolve a substance such as copper(II) sulfate
in a fixed volume of water, the solution becomes more
concentrated as we add more solid. A concentrated
solution contains a high proportion of solute; a
dilute solution contains a small proportion of solute.
Pr
Water is the commonest solvent in use, but other
liquids are also important. Most of these other solvents
are organic liquids, such as ethanol, propanone and
trichloroethane. These organic solvents are important
because they will oten dissolve substances that do not
dissolve in water. If a substance dissolves in a solvent, it is
said to be soluble: if it does not dissolve, it is insoluble.
How would you separate the following?
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a water from seawater
b ethanol from an ethanol/water mixture
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What type of substance was chromatography
originally designed to separate?
C2.09
How can we now extend the use of chromatography
to separate colourless substances?
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C2.08
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C2.10
Define the term Rf value in connection
with chromatography.
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What do you understand by the term sublimation?
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c sugar crystals from a sugar solution
C2.07
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QUESTIONS
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the solubility of solids in liquids
Probably the most important and common examples of
mixtures are solutions of solids in liquids.
KEy tERMS
the solubility of gases in liquids
Unlike most solids, gases become less soluble in water
as the temperature rises. The solubility of gases from the
air in water is quite small, but the amount of dissolved
oxygen is enough to support fish and other aquatic life.
Interestingly, oxygen is more soluble in water than nitrogen
is. So when air is dissolved in water, the proportions of the
two gases become 61% nitrogen and 37% oxygen. This is
an enrichment in life-supporting oxygen compared to air
(78% nitrogen and 21% oxygen).
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A closer look at solutions
The solubility of most solids increases with temperature. The
process of crystallisation depends on these observations.
When a saturated solution is cooled, it can hold less solute
at the lower temperature, and some solute crystallises out.
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The process of purification is of crucial importance in
many areas of the chemical industry. Medicinal drugs
(pharmaceuticals) must be of the highest possible purity. Any
contaminating substances, even in very small amounts, may
have harmful side efects. Coloured dyes (food colourings) are
added to food and drinks to improve their appearance. The
colourings added need to be carefully controlled. In Europe
the permitted colourings are listed as E100 to E180. Many dyes
that were once added are now banned. Even those which
are permitted may still cause problems for some people.
The yellow colouring tartrazine (E102) is found in many drinks,
sauces, sweets and snacks. To most people it is harmless, but
in some children it appears to cause hyperactivity and allergic
reactions, for example asthma. Even where there is overall
government regulation, individuals need to be aware of how
particular foods afect them.
If we keep adding more solid, a point is reached when no
more will dissolve at that temperature. This is a saturated
solution. To get more solid to dissolve, the temperature
must be increased. The concentration of solute in a saturated
solution is the solubility of the solute at that temperature.
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Probably the most generally used tests for purity are
measurements of melting point or boiling point. As we saw
earlier, impurities would lower the melting point or raise the
boiling point of the substance. They would also make these
temperatures less precise. These temperatures have been
measured for a very wide range of substances. The identity
of an unknown substance can be found by checking against
these measured values for known pure substances.
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C2: The nature of matter
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Cambridge IGCSE Combined and Co-ordinated Sciences
Chemical reactions and physical changes
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C2.03 Atoms and molecules
Substances can mix in a variety of ways, and they can
also react chemically with each other. In a reaction, one
substance can be transformed (changed) into another.
Copper(II) carbonate is a green solid, but on heating it
is changed into a black powder (Image C2.03). Closer
investigation shows that the gas carbon dioxide is also
produced. This type of chemical reaction, where a
compound breaks down to form two or more substances,
is known as decomposition.
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Elements and compounds
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KEy tERMS
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What are ‘pure substances’?
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Decomposition can also be brought about by
electricity. Some substances, although they do not
conduct electricity when solid, do conduct when they are
melted or in solution. In the process of conduction, they
are broken down into simpler substances. Thus, lead(II)
bromide, which is a white powder, can be melted. When
a current is passed through molten lead(II) bromide, a
silver-grey metal (lead) and a brown vapour (bromine)
are formed. Neither product can be split into any
simpler substances.
Figure C2.13 summarises what we now know about matter
in simple terms. Elements are the ‘building blocks’ from
which the Universe is constructed. There are over 100
known elements, but most of the Universe consists of
just two. Hydrogen (92%) and helium (7%) make up most
of the mass of the Universe, with all the other elements
contributing only 1% to the total. The concentration, or
‘coming together’, of certain of these elements to make
the Earth is of great interest and significance. There are
a total of 94 elements found naturally on Earth but just
eight account for more than 98% of the mass of the Earth’s
crust. Two elements, silicon and oxygen, which are bound
together in silicate rocks, make up almost three-quarters
of the crust. Only certain elements are able to form the
complex compounds that are found in living things.
For example, the human body contains 65% oxygen,
18% carbon, 10% hydrogen, 3% nitrogen, 2% calcium and
2% of other elements.
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The opposite type of reaction, where the substance
is formed by the combination of two or more other
substances, is known as synthesis. For example, if a
piece of burning magnesium is plunged into a gas jar of
oxygen, the intensity (brightness) of the brilliant white
flame increases. When the reaction has burnt out, a white
ash remains (Image C2.04). The ash has totally diferent
properties from the original silver-grey metal strip
and colourless gas we started with. A new compound,
magnesium oxide, has been formed from magnesium
and oxygen.
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There are two types of pure substance – elements and
compounds:
■ elements: substances that cannot be chemically broken
down into simpler substances
■ compounds: pure substances made from two, or more,
elements chemically combined together
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MIXTURES
More than one substance
present; substances may be in
different physical states (phases)
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Image C2.03 Heating copper(II) carbonate.
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Figure C2.13 Schematic representation of the diferent
types of matter, including elements and compounds.
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COMPOUNDS
Made from elements
chemically bonded
together
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ELEMENTS
Cannot be divided
into simpler
substances
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PURE SUBSTANCES
Only one substance
present; no impurities
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MATTER
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C2: The nature of matter
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When a mixture forms...
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Elements and compounds mix and react to produce
the world around us. They produce massive objects
such as the ‘gas giants’ (the planets Jupiter and Saturn),
and tiny highly structured crystals of solid sugar.
How do the elements organise themselves to give this
variety? How can any one element exist in the three
diferent states of matter simply through a change
in temperature?
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Our modern understanding is based on the atomic
theory put forward by John Dalton in 1807. His theory
reintroduced the ideas of Democritus (460–370 BCE) and
other Greek philosophers who suggested that all matter
was infinitely divided into very small particles known
as atoms. These ideas were not widely accepted at the
time. They were only revived when Dalton developed
them further and experimental observations under
the microscope showed the random motion of dust
particles in suspension in water or smoke particles in air
(Brownian motion).
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the final product of the reaction). The general diferences
between making a mixture of substances and forming a
new compound are shown in Table C2.04.
Atomic theory
Another synthesis reaction takes place between powdered
iron and sulfur. The two solids are finely ground and well
mixed. The mixture is heated with a Bunsen burner. The
reaction mixture continues to glow ater the Bunsen
burner is removed. Heat energy is given out. There has
been a reaction and we are let with a black non-magnetic
solid, iron(II) sulfide, which cannot easily be changed back
to iron and sulfur. This example also illustrates some
important diferences between a mixture (in this case the
powders of iron and sulfur) and a compound (in this case
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the compound cannot
easily be separated into
its elements
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the properties of the
new compound are very
diferent from those of the
elements in it
Table C2.04 The diferences between mixtures and
pure compounds.
new chemical substance(s) are formed
■ usually the process is not easily reversed
■ energy is oten given out.
■
These characteristics of a chemical reaction contrast
with those of a simple physical change such as melting or
dissolving. In a physical change the substances involved
do not change identity. They can be easily returned to their
original form by some physical process such as cooling or
evaporation. Sugar dissolves in water, but we can get the
solid sugar back by evaporating of the water.
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the properties of the
substances present remain
the same
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In a chemical reaction:
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the composition of the
new compound is always
the same
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Although many other reactions are not as spectacular as
this, the burning of magnesium shows the general features
of chemical reactions.
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the composition of the
mixture can be varied
the substances in the
mixture can be separated
by physical methods such
as filtration, distillation or
magnetic attraction
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the substances are simply the substances chemically
mixed together; no reaction react together to form a
takes place
new compound
Image C2.04 Burning magnesium produces a brilliant
white flame.
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When a compound
forms...
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calcium
Ca
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Cl
nitrogen
N
natrium
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potassium
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iron
lead
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phosphorus
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gold
Na
P
kalium
K
ferrum
Fe
plumbum
Pb
argentum
Ag
aurum
Au
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Table C2.05 The symbols of some chemical elements.
Pr
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use symbols, and how they can be combined to show the
formulae of complex chemical compounds.
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The kinetic model of matter
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The idea that all substances consist of very small particles
begins to explain the structure of the three diferent states
of matter. The kinetic theory of matter describes these
states, and the changes between them, in terms of the
movement of particles.
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The main points of the kinetic model
All matter is made up of very small particles
(diferent substances contain diferent types of
particles – such as atoms or molecules).
■ The particles are moving all the time (the higher
the temperature, the higher the average energy of
the particles). In a gas, the faster the particles are
moving, the higher the temperature.
■ The freedom of movement and the arrangement
of the particles is diferent for the three states of
matter (Figure C2.14).
■ The pressure of a gas is produced by the atoms
or molecules of the gas hitting the walls of the
container. The more oten the particles collide with
the walls, the greater the pressure.
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■
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Image C2.05 An ‘atomic logo’ produced by xenon atoms on
a nickel surface ‘seen’ using scanning tunnelling microscopy.
Cu
chlorine
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cuprum
sodium
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carbon
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The symbol for an element consists of one or two letters.
Where the names of several elements begin with the
same letter, the second letter of the name is usually
included in lower case (Table C2.05). As more elements
were discovered, they were named ater a wider range of
people, cities, countries and even particular universities.
We shall see in Chapter C3 how useful it is to be able to
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H
He
copper
Dalton suggested that each element should have its own
symbol – a form of chemical shorthand. He could then
write the formulae of compounds without writing out the
name every time. Our modern system uses letters taken
from the name of the element. This is an international
code. Some elements have been known for a long time
and their symbol is taken from their Latin name.
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Symbol
helium
Certain parts of the theory may have needed to change
as a result of what we have discovered since Dalton’s time.
However, Dalton’s theory was one of the great leaps of
understanding in chemistry. It meant that we could explain
many natural processes. Whereas Dalton only had theories
for the existence of atoms, modern techniques (such as
scanning tunnelling microscopy) can now directly reveal
the presence of individual atoms. It has even been possible
to create an ‘atomic logo’ (Image C2.05) by using individual
atoms, and it may soon be possible to ‘see’ a reaction
between individual atoms.
A chemical language
200
Latin name
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hydrogen
a pure element is composed of atoms
■ the atoms of each element are diferent in
size and mass
■ atoms are the smallest particles that take part in a
chemical reaction
■ atoms of diferent elements can combine to make
molecules of a compound.
■
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Element
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Dalton suggested that:
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Cambridge IGCSE Combined and Co-ordinated Sciences
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-R
It’s important to realise that even in a liquid, the particles
are still close together, although they can move around
and past each other.
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The idea that fluids are made up of moving particles helps
us to explain processes involving difusion.
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melting
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Melting: the temperature
stays constant.
The energy put in is used
to overcome the forces
holding the lattice together.
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B
liquid
In regions A, B and C the
temperature rises with heating.
The energy of the particles
increases and they vibrate or
move faster. In region B, the rate
of evaporation increases with
temperature.
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A
solid
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Time
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Reversing the experiment gives a cooling curve.
The temperature stays constant during condensation
and freezing – energy is given out. Condensation and
freezing are exothermic processes. Melting, evaporation
and boiling are endothermic processes.
Figure C2.15 Energy changes taking place during heating
and cooling.
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C
gas
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Temperature
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Figure C2.14 is a summary of the organisation of the particles
in the three states of matter, and helps to explain their
diferent overall physical properties. The highly structured,
ordered microscopic arrangements (lattices) in solids can
produce the regular crystal structures seen in this state. The
ability of the particles to move in the liquid and gas phases
produces their fluid properties. The particles are very widely
separated in a gas, but are close together in a liquid or
solid. The space between the particles can be called the
intermolecular space (IMS). In a gas, the intermolecular
space is large and can be reduced by increasing the external
pressure – gases are compressible. In liquids, this space is
very much smaller – liquids are not very compressible.
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Boiling: the temperature stays
constant. The energy put in
makes the particles move faster
and overcomes the forces
holding the liquid together.
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Solid
The particles in a solid are:
• packed close together
• in a regular
arrangement or lattice
• not able to move freely,
but simply vibrate in
their fixed positions.
Figure C2.14 Applying the kinetic model to changes in
physical state.
R
Dissolving
A potassium manganate(VII) crystal is placed at the bottom
of a dish of water. It is then let to stand. At first the water
around the crystal becomes purple as the solid dissolves
(Image C2.06). Particles move of the surface of the crystal
into the water. Eventually the crystal dissolves completely
and the whole solution becomes purple. The particles
from the solid become evenly spread through the water.
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Liquid
The particles in a liquid are:
• closely packed together
• in an irregular arrangement
• able to move around
past each other.
When the temperature is raised, the particles gain energy and
vibrate more strongly; the particles occupy more space – this
causes the solid to expand.
Eventually the particles have enough energy to break the
forces holding the lattice together, and they can move around
– the solid melts.
The way the particles in the three states are arranged
also helps to explain the temperature changes when a
substance is heated or cooled. Figure C2.15 summarises
the energy changes taking place at the diferent stages of a
heating-curve or cooling-curve experiment.
Difusion in fluids
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evaporation and boiling
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On heating, the particles move faster and the liquid expands.
In the liquid, some particles have enough energy to escape
from the surface – evaporation takes place. As the
temperature rises, more particles have enough energy to
escape – evaporation is faster at higher temperatures.
At the boiling point, the particles have enough energy to
break the forces attracting them together – the particles
move very fast and separate from each other – the liquid boils.
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TIP
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Gas
The particles in a gas are:
• arranged totally
irregularly
• spread very far apart
compared to solids and
liquids
• able to move randomly.
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C2: The nature of matter
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Image C2.06 The difusion of potassium manganate(VII) in
water as it dissolves.
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Cambridge IGCSE Combined and Co-ordinated Sciences
C
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A few drops of liquid bromine are put into a gas jar and
the lid is replaced. Ater a short time the jar becomes full
of brown gas. Bromine vaporises easily and its gas will
completely fill the container (Image C2.07). (Note that
bromine is useful to illustrate the process of difusion as
its vapour is coloured. However, its use is now prohibited
for student use in UK schools, and examination questions
on its use will not be set.) Gases difuse to fill all the space
available to them. Difusion is important for our ‘sensing’ of
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cotton wool
soaked in
ammonia
solution
white smoke
forms here
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difusion: the process by which diferent fluids mix as a result
of the random motions of their particles
■ Difusion involves the movement of particles from a
region of higher concentration towards a region of lower
concentration. Eventually the particles are evenly spread –
their concentration is the same throughout.
■ It does not take place in solids.
■ Difusion in liquids is much slower than in gases.
Heavier particles move more slowly than lighter
particles at the same temperature; larger molecules
difuse more slowly than smaller ones.
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■
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Three important points derived from kinetic theory
are relevant here:
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cotton wool
soaked in
hydrochloric
acid
Figure C2.16 Ammonia and hydrochloric acid fumes difuse
at diferent rates.
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KEy tERM
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The key idea about difusion is the idea of particles
spreading to fill the space available to the molecules.
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TIP
glass tubing
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the world around us. It is the way smells reach us, whether
they are pleasant or harmful.
Not all gases difuse at the same rate. This is shown by the
experiment in Figure C2.16. The ammonia and hydrochloric
acid fumes react when they meet, producing a white
‘smoke ring’ of ammonium chloride. The fact that the ring
is not formed halfway along the tube shows that ammonia,
the lighter molecule of the two, difuses faster.
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The difusion of gases
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Image C2.07 Bromine vapour difuses (spreads) throughout
the container to fill all the space.
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Whether a solid begins to break up like this in a liquid
depends on the particular solid and liquid involved. But
the spreading of the solute particles throughout the
liquid is an example of difusion. Difusion in solution is
also important when the solute is a gas. This is especially
important in breathing! Difusion contributes to the
movement of oxygen from the lungs to the blood, and of
carbon dioxide from the blood to the lungs.
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Methane, CH4
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ACtivity C2.05
Hydrogen chloride, HCl
Figure C2.17 Simple compounds consisting of molecules
made up of atoms of two diferent elements.
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investigating difusion – a demonstration
Skills:
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AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of
instructions where appropriate)
QUESTIONS
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Define a compound.
C2.13
Summarise the diferences between the three
states of matter in terms of the arrangement of
the particles and their movement.
C2.14
Which gas difuses faster, ammonia or hydrogen
chloride? Briefly describe an experiment that
demonstrates this diference.
C2.15
Which gas will difuse fastest of all?
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C2.04 the structure of the atom
Details of other demonstrations and experiments
on difusion are given in the Notes on activities for
teachers/technicians.
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Atomic structure
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How can atoms join together to make molecules?
What makes certain atoms more ready to do this?
Why do hydrogen atoms pair up but helium atoms
remain single?
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The behaviour of some gaseous elements (their difusion
and pressure) shows that they are made up of molecules,
not separate atoms. This is true of hydrogen (H2), nitrogen
(N2), oxygen (O2) and others. But, as we discussed earlier
in this section, Dalton had originally introduced the
idea of molecules to explain the particles making up
compounds such as water, carbon dioxide and methane.
Molecules of these compounds consist of atoms of
diferent elements chemically bonded together. Water is
made up of two atoms of hydrogen bonded to one atom
of oxygen, giving the formula H2O. Methane (CH4) has one
atom of carbon bonded to four atoms of hydrogen, and
hydrogen chloride (HCl) has one atom of hydrogen and
one atom of chlorine bonded together. Models of these
are shown in Figure C2.17.
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br
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To find answers to questions like these, we need first
to consider the structure of atoms in general. Dalton
thought they were solid, indivisible particles. But research
since then has shown that atoms are made up of various
sub-atomic particles. J. J. Thomson discovered the
electron (in 1897) and the proton. Crucial experiments
were then carried out in Rutherford’s laboratory in
Manchester in 1909 that showed that the atom is largely
empty space. Rutherford calculated that an atom is mostly
space occupied by the negatively charged electrons,
surrounding a very small, positively charged nucleus.
The nucleus is at the centre of the atom and contains
almost all the mass of the atom. By 1932, when the
neutron was discovered, it was clear that atoms consisted
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Define an element.
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Worksheets are included on the accompanying
CD-ROM for both the teacher demonstration and a
microscale version of the experiment which could be
carried out by students.
Atoms and molecules
ev
C2.11
C2.12
s
This is the classic demonstration of the difusion of gases in
which ammonia and hydrogen chloride meet in a long tube.
The demonstration shows how the progress of the gases
can be tracked using indicator. Measurements can be made
to give an estimate of the rate of difusion of the two gases.
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AO3.4 Interpret and evaluate experimental
observations and data
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AO3.3 Make and record observations, measurements
and estimates
R
Water, H2O
ev
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The pressure of a gas is the result of collisions
of the fast-moving particles with the walls of
the container.
■ The average speed of the particles increases with
an increase in temperature.
■
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Cambridge IGCSE Combined and Co-ordinated Sciences
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ACtivity C2.06
Pr
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Questions
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A1 What was remarkable about the structure of the atom
suggested by the Geiger–Marsden experiments?
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A2 What is it about the nature of the neutron that made it
the last of the particles to be discovered?
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br
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Pr
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U
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op
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Imagine this size comparison. If the atom were the size of a
football stadium, the nucleus (at the centre-spot) would be
the size of a pea!
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The next simplest atom is that of helium. This has two
protons and two neutrons in the nucleus, and two orbiting
electrons (Figure C2.18).
oxygen
O
calcium
Ca
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neutron
Cu
64
electron
Au
197
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40
U
proton
Location in
atom
+1
in nucleus
1
0
in nucleus
1
(negligible)
1840
–1
outside
nucleus
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Table C2.07 Properties of the sub-atomic particles.
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Table C2.06 The relative atomic masses of some elements.
Relative
charge
1
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br
gold
1
16
e
copper
Sub-atomic Relative
particle
mass
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H
12
op
hydrogen
The next, lithium, has three protons, four neutrons and
three electrons. The arrangements in the following
atoms get more complicated with the addition of more
protons and electrons. The number of neutrons required
to hold the nucleus together increases as the atomic size
increases. Thus, an atom of gold consists of 79 protons (p+),
118 neutrons (n0) and 79 electrons (e–).
C
C
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carbon
Relative atomic
mass
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ve
rs
Atomic symbol
C
Element
R
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br
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Protons and neutrons have almost the same mass.
1
of the mass
Electrons have virtually no mass at all (
1840
of a proton). The other important feature of these particles
is their electric charge. Protons and electrons have equal
and opposite charges, while neutrons are electrically
neutral (have no charge). The characteristics of these three
sub-atomic particles are listed in Table C2.07.
A single atom is electrically neutral (it has no overall
electric charge). This means that in any atom there must
be equal numbers of protons and electrons. In this way the
total positive charge on the nucleus (due to the protons)
is balanced by the total negative charge of the orbiting
electrons. The simplest atom of all has one proton in its
nucleus. This is the hydrogen atom. It is the only atom
that has no neutrons; it has one proton and one electron.
Atoms of diferent elements are increasingly complex.
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C
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Sub-atomic particles
C
op
ni
A single atom is so small that it cannot be weighed on a
balance. However, the mass of one atom can be compared
with that of another using a mass spectrometer. The
element carbon is chosen as the standard. The masses
of atoms of all other elements are compared to the mass
of a carbon atom. This gives a series of values of relative
atomic mass for the elements. Carbon is given a relative
atomic mass of exactly 12, which can be written as
carbon-12. Table C2.06 gives some examples of the values
obtained for other elements. It shows that carbon atoms
are 12 times as heavy as hydrogen atoms, which are the
lightest atoms of all. Calcium atoms are 40 times as heavy
as hydrogen atoms.
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Modern methods such as scanning tunnelling microscopy
have allowed us to ‘see’ individual atoms in a structure.
However, atoms are amazingly small! A magnification of
100 million times is necessary to show the stacking pattern
of the atoms that make up a gold bar.
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Measuring the size of atoms
204
Discovering the structure of the atom
Skills:
Research skills ICT skills
The discovery of the nature of the sub-atomic particles
that make up all atoms took place in a relatively short
space of time around the beginning of the 20th century.
Investigate this key period in the history of science using
library and internet sources. Devise a PowerPoint or
poster presentation on the significant discoveries and the
scientists involved. Key scientists to research are
J. J. Thomson, Ernest Rutherford and James Chadwick.
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am
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id
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of three sub-atomic particles – protons, neutrons and
electrons. These particles are universal – all atoms are
made from them. The atom remains the smallest particle
that shows the characteristics of a particular element.
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4
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This is the atomic
number (proton number).
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br
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number of electrons = number of protons
= atomic (proton) number
■ number of neutrons
= nucleon number – proton number
=A–Z
■
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s
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Pr
Table C2.08 shows the numbers of protons, neutrons and
electrons in some diferent atoms. Note that the rules
apply even to the largest, most complicated atom found
naturally in substantial amounts.
ity
op
C
Only hydrogen atoms have one proton in their nuclei.
Only helium atoms have two protons. Indeed, only
gold atoms have 79 protons. This shows that the number
of protons in the nucleus of an atom decides which
element it is. This very important number is known as the
proton number (or atomic number, given the symbol Z)
of an atom.
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Remember that you can use the Periodic Table you have in
the exam for information on these numbers for any atom.
Magnesium is the twelth atom in the table, so it must
have 12 protons and 12 electrons in its atoms.
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id
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Isotopes
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Pr
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TIP
br
Protons alone do not make up all the mass of an atom.
The neutrons in the nucleus also contribute to the total
mass. The mass of the electrons can be regarded as so
small that it can be ignored. Because a proton and a
neutron have the same mass, the mass of a particular
atom depends on the total number of protons and
neutrons present. This number is called the nucleon
number (or mass number, given the symbol A) of
an atom.
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Measurements of the atomic masses of some elements
using the mass spectrometer were puzzling. Pure samples
of elements such as carbon, chlorine and many others were
found to contain atoms with diferent masses even though
they contained the same numbers of protons and electrons.
The diferent masses were caused by diferent numbers of
neutrons in their nuclei. Such atoms are called isotopes.
id
g
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ie
Remember that it is just the number of neutrons in the
atoms that is the diference between isotopes. They have
the same number of protons and electrons.
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If these two important numbers for any atom
are known, then its sub-atomic composition can be
worked out.
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TIP
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The atomic number Z and mass number A of an atom of
an element can be written alongside the symbol for that
element, in the general way as ZAX. So the symbol for an
atom of lithium is 73Li. The symbols for carbon, oxygen and
uranium atoms are 126 C, 168O and 238
92U.
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He
These two relationships are useful:
Proton (atomic) number and
nucleon number
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2
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R
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op
We say the charges balance. The atom has no overall electrical charge.
R
This is the symbol
for helium.
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w
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A helium atom has these charged particles in it:
2 protons charge +2 these charges
2 electrons charge –2 cancel out
A helium atom has:
2 protons mass 2 units
2 neutrons mass 2 units
2 electrons with hardly any mass
So a helium atom has a total mass of:
2 + 2 = 4 units
ev
This is the mass number,
the number of protons
and neutrons together.
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rs
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op
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Pr
es
s
The neutron has no electrical charge
and a mass of one unit.
Figure C2.18 The structure of a helium atom.
R
proton (atomic) number (Z)
= number of protons in the nucleus
■ nucleon (mass) number (A)
= number of protons + number of neutrons
■
-R
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+
For proton number and nucleon number we have:
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electron
This has one negative electrical charge (–1).
It has hardly any mass.
The proton + has one positive charge
(+1) and a mass of one unit.
+
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nucleus made
of protons +
and neutrons
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C2: The nature of matter
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205
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Neutrons (A – Z)
1
He
2
4
2
2
2
Li
3
7
3
4
3
beryllium
Be
4
9
4
5
4
carbon
C
6
12
6
6
6
O
8
16
8
8
8
sodium
Na
11
23
11
12
11
calcium
Cl
20
40
20
20
20
gold
Au
79
197
79
118
79
uranium
U
92
238
92
146
92
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tritium(a)
1
1
2
1
3
1
ge
y
Ne
10 protons
12 neutrons
10 electrons
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id
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am
-C
37
17
Pr
es
Cl
17 protons
20 neutrons
17 electrons
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C
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Tritium and carbon-14 atoms are radioactive isotopes because their nuclei are unstable.
op
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KEy tERM
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Table C2.09 Several elements that exist as mixtures of isotopes.
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isotopes: atoms of the same element which have the same proton number but a diferent nucleon number
■ The atoms have the same number of protons and electrons, but diferent numbers of neutrons in their nuclei.
■ Isotopes of an element have the same chemical properties because they have the same electron structure.
■ Some isotopes have unstable nuclei; they are radioisotopes and emit various forms of radiation.
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35
17
(a)
C
6 protons
8 neutrons
6 electrons
neon-21 (0.3%)
22
10
Ne
10 protons
11 neutrons
10 electrons
chlorine-37 (25%)
Ne
10 protons
10 neutrons
10 electrons
chlorine-35 (75%)
Cl
17 protons
18 neutrons
17 electrons
14
6
21
10
20
10
Chlorine
13
6
C
6 protons
7 protons
6 electrons
neon-21 (0.3%)
rs
C
6 protons
6 protons
6 neutrons
neon-20 (90.5%)
Neon
H
1 proton
2 neutrons
1 electron
carbon-14(a) (trace)
w
12
6
H
1 proton
1 neutrons
1 electron
carbon-13 (1.1%)
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-C
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C
Carbon
ie
deuterium (0.01%)
H
1 proton
0 neutrons
1 electron
carbon-12 (98.9%)
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206
Isotopes
hydrogen (99.99%)
Pr
am
br
id
Element
Hydrogen
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Table C2.08 The sub-atomic composition and structure of certain atoms.
C
-C
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C
-R
0
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id
1
oxygen
R
w
Protons (Z)
Outside the nucleus:
Electrons (Z)
1
lithium
w
Inside the nucleus
1
helium
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Mass
number, A
H
hydrogen
R
Atomic
number, Z
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Symbol
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Atoms
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
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The fact that there is more of the lighter isotope moves
the value lower than 36. The actual value is 35.5. The
relative atomic mass of chlorine can be calculated by
finding the total mass of 100 atoms:
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QuEStiONS
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What are the relative masses of a proton, neutron
and electron, given that a proton has a mass of 1?
C2.18
What is the diference in terms of sub-atomic
particles between an atom of chlorine-35 and an
atom of chlorine-37?
Pr
y
ity
op
rs
C
w
U
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op
y
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C
The aurora borealis (Image C2.08) is a spectacular
display seen in the sky in the far north (a similar
phenomenon – the aurora australis – occurs in the
far south). It is caused by radiation from the Sun moving
the electrons in atoms of the gases of the atmosphere.
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Pr
op
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es
s
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am
-C
Because there are several isotopes of carbon, the standard
against which all atomic masses are measured has to be
defined precisely. The isotope carbon-12 is used as the
standard. One atom of carbon-12 is given the mass of
12 precisely. From this we get that 1 atomic mass unit
1
(a.m.u.) = × mass of one atom of carbon-12.
12
The existence of isotopes also explains why most relative
atomic masses are not whole numbers. But, to make
calculations easier, in this book they are rounded to the
nearest whole number. There is one exception, chlorine,
where this would be misleading. Chlorine contains two
isotopes, chlorine-35 and chlorine-37, in a ratio of 3 : 1
(or 75% : 25%). If the mixture were 50% : 50%, then the
relative atomic mass of chlorine would be 36.
-R
Image C2.08 The aurora borealis, or northern lights, as
seen from Finland.
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C2.17
C2.05 Electron arrangements
in atoms
relative atomic mass (Ar): the average mass of naturally
occurring atoms of an element on a scale where the carbon-12
atom has a mass of exactly 12 units
ie
How many protons, neutrons and electrons are
there in an atom of phosphorus, which has a
proton number of 15 and a nucleon number of 31?
ie
C2.16
w
R
ev
R
3550
= 35.5
100
C
op
w
ev
ie
Then,
Ar(Cl) = 35.5
Most elements exist naturally as a mixture of isotopes.
Therefore, the value we use for the atomic mass of an
element is an average mass. This takes into account the
proportions (abundance) of all the naturally occurring
isotopes. If a particular isotope is present in high
proportion, it will make a large contribution to the average.
This average value for the mass of an atom of an element is
known as the relative atomic mass (Ar).
KEy tERM
ev
= 3550
Thus, for chlorine:
Tritium and carbon-14 illustrate another diference in
physical properties that can occur between isotopes,
as they are radioactive. The imbalance of neutrons and
protons in their nuclei causes them to be unstable so
the nuclei break up spontaneously (that is, without any
external energy being supplied), emitting certain types of
radiation. They are known as radioisotopes.
Relative atomic masses
R
mass of 100 atoms = (35 × 75) + (37 × 25)
average mass of one atom =
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Pr
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The isotopes of an element have the same chemical
properties because they contain the same number of
electrons. It is the number of electrons in an atom that
decides the way in which it forms bonds and reacts with
other atoms. However, some physical properties of the
isotopes are diferent. The masses of the atoms difer and
therefore other properties, such as density and rate of
difusion, also vary. The modern mass spectrometer shows
that most elements have several diferent isotopes that
occur naturally. Others, such as tritium – an isotope of
hydrogen (Table C2.09) – can be made artificially.
C
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C2: The nature of matter
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207
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Figure C2.20 Possibly the most versatile atom in the
Universe – the carbon-12 atom.
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C
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• Electrons are in orbit around the central nucleus
of the atom.
a sub-atomic picture can be drawn. Figure C2.20 shows
such a picture for perhaps the most versatile atom in the
Universe, an atom of carbon-12. Studying the organisation
of the electrons of an atom is valuable. It begins to explain
the patterns in properties of the elements that are the
basis of the Periodic Table. This will be discussed in
Chapter C3.
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• The electron orbits are called shells (or energy levels)
and have diferent energies.
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• Shells which are further from the nucleus have
higher energies.
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C
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• The second and subsequent shells can hold eight
electrons to give a stable (noble gas) arrangement
of electrons.
How many electrons are there in the outer shells
of the atoms of the noble gases, argon and neon?
C2.22
Carbon-12 and carbon-14 are diferent isotopes of
carbon. How many electrons are there in an atom
of each isotope?
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TIP
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C2.21
You can see from these elements that the number of
outer electrons in an atom is the same as the number
of the group in the Periodic Table that the element is in.
The number of shells of electrons in an atom tells you the
period (row) of the element in the table. We will look at
this further in the next chapter.
es
s
-R
br
am
What is the electron arrangement of a calcium
atom, which has an atomic number of 20?
Make sure that you remember how to work out the
electron arrangements of the first 20 elements and
can draw them in rings (shells) as in Figure C2.21. Also
remember that you can give the electron arrangement or
electronic structure simply in terms of numbers: 2,8,4 for
silicon, for example.
Third energy level.
Eight electrons can
fit into this level to
give a stable
arrangement.
Figure C2.19 Bohr’s theory of the arrangement of electrons
in an atom.
-C
C2.20
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-C
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R
nucleus made of
protons and neutrons
What are the maximum numbers of electrons
that can fill the first and the second shells (energy
levels) of an atom?
ie
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id
When the two essential numbers describing a particular
atom are known, the numbers of protons and neutrons,
Second energy level.
Eight electrons can
fit into this level.
C2.19
C
U
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Other evidence was found that supported these ideas of
how the electrons are arranged in atoms. The number and
arrangement of the electrons in the atoms of the first 20
elements in the Periodic Table are shown in Table C2.10.
First or lowest energy
level. Only two electrons
can fit into this level.
QUESTIONS
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Pr
op
• The first shell can hold only two electrons.
208
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• The shells are filled starting with the one with
lowest energy (closest to the nucleus).
op
R
ev
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w
A simplified version of Bohr’s theory of the arrangement
of electrons in an atom can be summarised as follows (see
also Figure C2.19):
nucleus contains
6 protons and
6 neutrons
w
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op
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Pr
es
s
-C
In 1913, Niels Bohr, working with Rutherford in Manchester,
developed a theory to explain how electrons were
arranged in atoms. This theory helps to explain how the
colours referred to above come about.
6 electrons
outside
nucleus
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am
br
id
ge
Similar colour efects can be created in a simpler way in the
laboratory by heating the compounds of some metals in a
Bunsen flame (see Section C8.01). These colours are also
seen in fireworks. The colours produced are due to electrons
in the atom moving between two diferent energy levels.
C
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Cambridge IGCSE Combined and Co-ordinated Sciences
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2
●●
lithium
Li
3
●●
●
2,1
beryllium
Be
4
●●
●●
2,2
B
5
●●
●●●
2,3
fluorine
F
Pr
es
s
2,4
7
●●
●●●●●
2,5
8
●●
●●●●●●
9
●●
●●●●●●●
10
●●
●●●●●●●●
y
O
●●●●
2,6
C
op
oxygen
●●
2,7
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nitrogen
2
6
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-C
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op
C
carbon
-R
He
2,8
sodium
Na
11
●●
●●●●●●●●
●
2,8,1
magnesium
Mg
12
●●
●●●●●●●●
●●
2,8,2
aluminium
Al
13
●●
●●●●●●●●
●●●
2,8,3
silicon
Si
14
●●
●●●●●●●●
●●●●
2,8,4
P
15
●●
●●●●●●●●
●●●●●
2,8,5
sulfur
S
16
●●●●●●●●
●●●●●●
2,8,6
chlorine
Cl
17
●●
●●●●●●●●
●●●●●●●
2,8,7
argon
Ar
18
●●
●●●●●●●●
●●●●●●●●
potassium
K
19
●●
●●●●●●●●
●●●●●●●●
●
2,8,8,1
calcium
Ca
20
●●
●●●●●●●●
●●●●●●●●
●●
2,8,8,2
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19K
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-C
2,8,8
potassium
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am
br
Table C2.10 The electron arrangements of the first 20 elements.
sodium
11Na
3Li
y
op
w
We can write this:
[2,8,8,1]
es
s
Figure C2.21 Diferent ways of showing electron structure.
-C
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We can write this:
[2,8,1]
ev
am
br
We can write this:
[2,1]
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lithium
ity
Pr
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C
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id
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es
Pr
ity
rs
●●
ve
br
am
-C
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Ne
id
neon
C
w
1
●
phosphorus
ie
Fourth shell Electron
configuration
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w
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ie
R
Second shell Third shell
1
boron
ev
First shell
H
helium
R
Atomic
number, Z
ge
hydrogen
Symbol
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br
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Elements
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C2: The nature of matter
Copyright Material - Review Only - Not for Redistribution
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■
Pr
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Pr
The word particle can be used to describe a speck of dust, a molecule, an atom or an electron.
How can we avoid confusion in using the word particle?
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Describe the arrangement and movement of the particles in a solid.
Describe how you could separate the sand from a mixture of sand and salt. Give full details
of how this is carried out.
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Sand and salt (sodium chloride) are both solids.
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2
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b
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Substances can be categorised in two ways: as an element, mixture or compound or as a solid,
liquid or gas. Which of these methods is of most use to a chemist?
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End-of-chapter questions
1
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Pr
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■
their properties are very diferent from those of the
elements they are made from
how each element is made from atoms and that atoms
can join together to make the molecules either of an
element or of a compound
how the atoms of the elements are made up of
diferent combinations of the sub-atomic particles –
protons, neutrons and electrons
the electrical charges and relative masses of these
sub-atomic particles
how, in any atom, the protons and neutrons are
bound together in a central nucleus, and the
electrons ‘orbit’ the nucleus in diferent energy levels
(or shells)
that the number of protons in an atom is defined as
the proton (atomic) number (Z) of the element
that the nucleon (mass) number (A) is defined
as the total number of protons and neutrons in
any atom
how isotopes of the same element can exist
which difer only in the number of neutrons in
their nuclei
how the electrons in atoms are arranged in diferent
energy levels that are at diferent distances from the
nucleus of the atom
how each energy level has a maximum number of
electrons that it can contain, and that the electrons fill
the shells closest to the nucleus first.
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■
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that there are three diferent physical states in which a
substance can exist
about the diferent changes in state that can take
place, including sublimation, where the liquid
phase is bypassed
how these changes of state can be produced by
changing conditions of temperature and/or pressure
how the kinetic model describes the idea that the
particles of a substance are in constant motion
and that the nature and amount of motion of these
particles difer in a solid, liquid or gas
how changing physical state involves energy
being absorbed or given out, the temperature of
the substance staying constant while the change
takes place
how pure substances have precise melting and boiling
points – their sharpness can be taken as an indication
of the degree of purity of the substance
that diferent separation methods – such as filtration,
distillation and chromatography – can be used to
purify a substance from a mixture
how pure chemical substances can be either elements
or compounds
that elements are the basic building units of the
material world – they cannot be chemically broken
down into anything simpler
how compounds are made from two or more
elements chemically combined together, and that
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You should know:
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Summary
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
[2]
[3]
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The diagram below shows the apparatus used to separate ethanol and water from a mixture
of ethanol and water.
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C2: The nature of matter
water out
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fractionating
column
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ethanol
and water
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water in
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heat
distillation
flask
higher
lower
solid
volatile
vapour
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Pr
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Fractional
is used to separate a mixture of water and ethanol. The temperature
at the top of the fractionating column is
than the temperature at the bottom.
The more
liquid evaporates and moves further up the column. It eventually
reaches the
where the
changes to a liquid.
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water vapour in air
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ice
[1]
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Pr
es
Which of the materials named in the diagram best fits the following statement describing the organisation
of the particles present?
‘The particles are able to move, are randomly arranged and are closely packed.’
Name the processes by which water molecules in the sea become:
■ water molecules in the air
■ water molecules in the ice.
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a
[5]
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seawater
211
[Cambridge IGCSE Chemistry 0620 Paper 21 Q3 c, d November 2012]
The diagram below shows an iceberg floating in the sea.
air
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heavy
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crystallisation
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condenser
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3
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Write out and complete the following sentences about this separation using words from
the list below.
Copyright Material - Review Only - Not for Redistribution
[2]
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Stearic acid is a solid at room temperature.
The diagram below shows the apparatus used for finding the melting point of stearic acid.
The apparatus was heated at a steady rate and the temperature recorded every minute.
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4
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Cambridge IGCSE Combined and Co-ordinated Sciences
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State the names of the pieces of apparatus labelled A, B.
Suggest why the water needs to be kept stirred during this experiment.
A graph of temperature of stearic acid against time of heating is shown below.
br
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am
100
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60
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6
Time / minutes
8
12
es
s
i What was the temperature of the stearic acid ater 3 minutes heating?
ii Use the information on the graph to determine the melting point of stearic acid.
Describe the arrangement and motion of the particles in liquid stearic acid.
A sample of stearic acid contained 1% of another compound with a higher relative molecular mass.
i Which one of the following statements about this sample of stearic acid is correct?
Its density is exactly the same as that of pure stearic acid.
Its boiling point is the same as that of pure stearic acid.
Its melting point is diferent from pure stearic acid.
Its melting point is the same as that of pure stearic acid.
ii Describe one area of everyday life where the purity of substances is important.
[1]
[1]
[2]
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[1]
[1]
[Cambridge IGCSE Chemistry 0620 Paper 21 Q1 a, b(i), c–e June 2012]
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C
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10
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4
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2
-C
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20
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40
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Temperature / ºC
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80
212
[2]
[1]
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heat
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water
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stirrer
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B
stearic acid
a
b
c
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A
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State the name of the central part of the atom, labelled X.
Which one of these statements about helium and argon is correct?
Argon has an incomplete inner shell of electrons.
An atom of argon has 16 electrons.
Helium has a complete outer shell of electrons.
Helium has an incomplete outer shell of electrons.
iii How many protons are there in an atom of argon?
iv The symbol for a particular isotope of helium is written as 42He.
Write a similar symbol for the isotope of argon which has 16 neutrons.
Argon is a liquid at a temperature of –188 °C.
Complete the diagram below to show how the atoms of argon are arranged at –188 °C.
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[2]
[Cambridge IGCSE Chemistry 0620 Paper 21 Q3 November 2010]
2
2
3
4
3
R
1
0
S
4
5
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ity
1
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4
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Which particles named in the table are not classed as nucleons?
Explain which one of the atoms, P, Q, R and S, has a nucleon number (mass number) of four.
Explain why all atoms are electrically neutral, having no overall electrical charge.
Which of these atoms is an atom of hydrogen?
What would be the arrangement of electrons in an atom of S?
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Pr
2
Q
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Electrons
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Neutrons
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Protons
P
C
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[1]
The table below shows the numbers of protons, neutrons and electrons in four atoms, P, Q, R and S.
Atom
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[1]
[1]
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C
represents one atom of argon
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6
R
[1]
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argon
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Pr
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X
helium
c
[1]
State one use of helium.
The atomic structures of helium and argon are shown below.
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Helium and argon are noble gases.
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C2: The nature of matter
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[1]
[1]
[2]
[1]
[1]
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■
the structure of the Periodic Table
metals and non-metals in the Periodic Table
electron arrangement in the Periodic Table
trends in Group I – the alkali metals
trends in Group VII – the halogens
the noble gases
trends across a period
the transition elements
bonding in metals
bonding in covalent compounds
bonding in ionic compounds
formulae and names of ionic compounds
formulae and names of covalent compounds
alloys and their uses
the nature of ionic crystals
the structure of metal crystals and alloys
the nature of giant covalent structures
the diferent forms of carbon
macromolecules.
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This chapter covers:
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C3
Elements and compounds
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C3: Elements and compounds
Mendeleev’s great achievement lay in predicting the
properties of elements that had not yet been discovered.
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the elements are arranged in order of increasing
proton number (atomic number)
■ the vertical columns of elements with similar
properties are called groups
■ the horizontal rows are called periods.
■
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The chemical properties of metals and non-metals are
also very diferent, as is the type of bonding present in
their compounds. The distinction is therefore a very
important one.
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C
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The Periodic Table does not list substances such as steel,
bronze and brass, which in everyday terms we call metals
and which share the properties listed for metals. They are
not elements! They are in fact alloys, mixtures of elements
(usually metals) designed to have properties that are
useful for a particular purpose.
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There are 94 naturally occurring elements. Some are very
rare. Francium, for instance, has never been seen. The
radioactive metals neptunium and plutonium, which we
make artificially in quite large amounts, occur only in very
small (trace) quantities naturally. Most of the elements
(70) can be classified as metals. Together they form a
group of elements whose structures are held together by a
particular type of bonding between the atoms. The metals
have a number of physical properties that are broadly the
same for all of them (Table C3.01).
C
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Metals and non-metals
Image C3.01 Mendeleev’s early Periodic Table carved on
the wall of a university building in St Petersburg, with a
statue of Mendeleev in front.
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The main distinction in the table is between metals
and non-metals. Metals are clearly separated from
non-metals. The non-metals are grouped into the
top right-hand region of the table, above the
thick stepped line in Figure C3.01. One of the first
uses of the Periodic Table now becomes clear.
Although we may never have seen a sample of the
element hafnium (Hf), we know from a glance at the
table that it is a metal. We may also be able to predict
some of its properties.
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In the Periodic Table:
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All modern versions of the Periodic Table are based on
the one put forward by Mendeleev. An example is given in
Figure C3.01.
Pr
es
s
-C
Building up the modern Periodic Table has been a
major scientific achievement. The first steps towards
working out this table were taken long before anyone had
any ideas about the structure of atoms. But, although
they were partly successful, these groupings were
limited or flawed. The breakthrough came in 1869 when
Mendeleev put forward his ideas of a periodic table. In
his first attempt he used 32 of the 61 elements known at
that time (Image C3.01). He drew up his table based on
atomic masses, as others had done before him. But his
success was mainly due to his leaving gaps for possible
elements still to be discovered. He did not try to force the
elements into patterns for which there was no evidence.
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C3.01 the Periodic table –
classifying the elements
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215
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9
12
Na
Mg
Sodium
Magnesium
23
19
24
20
21
22
23
24
25
26
28
Group VIII/0
Group VII
Group VI
Group V
Group III
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27
Group IV
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Group II
Beryllium
7
11
2
He
Helium
29
30
5
6
7
8
9
4
10
B
C
N
O
F
Ne
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
11
13
12
14
14
15
16
16
19
17
20
18
Al
Si
P
S
Cl
Ar
Aluminium
Silicon
Phosphorus
Sulfur
Chlorine
Argon
27
31
28
32
31
33
32
34
35.5
35
40
36
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Krypton
40
38
45
39
48
40
51
41
52
42
55
43
56
44
59
45
59
46
64
47
65
48
70
49
73
50
75
51
79
52
80
53
84
54
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Technetium
Ruthenium
Rhodium
Palladium
Silver
Cadmium
Indium
Tin
Antimony
Tellurium
Iodine
Xenon
86
55
88
56
89
91
72
93
73
96
74
–
75
101
76
103
77
106
78
108
79
112
80
115
81
119
82
122
83
128
84
127
85
131
86
Radium
–
–
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Tungsten
Rhenium
Osmium
Iridium
Platinum
Gold
Mercury
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
184
186
190
192
195
197
201
204
207
209
–
–
–
181
58
59
60
61
62
63
64
65
66
67
68
69
70
71
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Lanthanum
Cerium
Praseodymium
Neodymium
Promethium
Samarium
Europium
Gadolinium
Terbium
Dysprosium
Holmium
Erbium
Thulium
Ytterbium
Lutetium
175
103
139
89
140
90
141
91
144
92
–
93
150
94
152
95
157
96
159
97
163
98
165
99
167
100
169
101
173
102
Ac
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
Actinium
Thorium
Protactinium
Uranium
Neptunium
Plutonium
Americium
Curium
Berkelium
Californium
Einsteinium
Fermium
Mendelevium
Nobelium
Lawrencium
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
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Francium
Ac
to
Lr
Tantalum
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Ra
Ta
178
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Fr
Hf
Hafnium
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137
88
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Ba
Barium
133
87
s
Cs
Caesium
La
to
Lu
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Ca
Calcium
39
37
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K
Potassium
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Period 7
1
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Period 6
1
H
Hydrogen
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Period 5
= relative atomic mass
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Period 4
b
Be
Lithium
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Period 3
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4
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Period 2
Name
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3
Li
= atomic number
X = symbol
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X
Pr
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Period 1
Key:
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Group I
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Cambridge IGCSE Combined and Co-ordinated Sciences
Pr
The reactive metals: Group I – the alkali
metals; Group II – the alkaline earth metals
ity
The metalloids: includes semiconductors,
e.g. silicon and germanium
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Non-metals
They are solids or gases (except for bromine, which is a
liquid) at room temperature.
Their melting and boiling points are oten low.
They are generally poor thermal conductors.
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Their shape can be changed by hammering (they are malleable). Most non-metals are brittle when solid.
They can also be pulled out into wires (they are ductile).
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They vary in colour.
They oten have a dull surface when solid.
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They are grey in colour (except gold and copper).
They can be polished.
They are not sonorous.
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Electrical conductivity is usually taken as the simplest test of whether a substance is metallic or not.
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They usually make a ringing sound when struck
(they are sonorous).
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Table C3.01 Comparison of the physical properties of metals and non-metals.
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They are good conductors of heat.
Most non-metals are soter than metals (but diamond is
very hard). Their densities are oten low.
They are poor conductors of electricity (except graphite,
a form of carbon). They tend to be insulators.
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All metals are good conductors of electricity.(a)
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They are usually hard and dense.
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They are usually solids (except for mercury, which is a liquid)
at room temperature.
Their melting and boiling points are usually high.
ev
The noble gases: very unreactive
Figure C3.01 The Periodic Table, showing the major regions. (Except for chlorine, the relative atomic masses are given to
the nearest whole number.)
Metals
R
The non-metals: includes Group VII –
the halogens
The ‘poor’ metals
ve
The transition elements: hard, strong and
dense metals
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Elements in Groups I to 0 are sometimes known as the main-group elements.
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TIP
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ACtivity C3.01
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If asked to say how you would test to see whether an
element was a metal or a non-metal, the key test is
electrical conductivity. Describe the setting up of a
simple circuit using a battery and a light bulb, and then
connect in a sample of the element and see if the bulb
lights up (Figure C3.02).
C
op
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The other properties which are most clearly those of a
metal are malleability and ductility. These, and electrical
conductivity, are the properties where there are fewest
exceptions.
battery
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KEy tERMS
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Figure C3.02 Testing the electrical conductivity of a
possible metal.
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Groups and periods in the Periodic Table
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metal: an element that conducts electricity and is malleable
and ductile
non-metal: an element that does not conduct electricity well
and is neither malleable nor ductile
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The change from metallic to non-metallic properties in the
elements is not as clear-cut as suggested by drawing the
line between the two regions of the Periodic Table.
The elements close to the line show
properties that lie between these
extremes. These elements are now
oten referred to as metalloids (or
semi-metals). Such elements have
some of the properties of metals and
others that are more characteristic
of non-metals. There are eight
elements that are called metalloids.
They oten look like metals, but are
Image C3.02
brittle like non-metals. They are
A sample of the
neither conductors nor insulators,
element silicon,
but make excellent semiconductors. the basis of the
The prime example of this type of
semiconductor
element is silicon (Image C3.02).
industry.
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The noble gases, in Group VIII/0 on the right-hand side
of the table, are the least reactive elements in the table.
However, the group next to them, Group VII which are also
known as the halogens, and the group on the let-hand
side of the table, Group I or the alkali metals, are the most
reactive elements. The more unreactive elements, whether
metals or non-metals, are in the centre of the table.
C
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The Periodic Table allows us to make even more
useful subdivisions of elements than simply deciding
which are metals and which are non-metals. The elements
present in Groups I to VIII/0 of the table are sometimes
known as the main-group elements. These vertical
groups show most clearly how elements within the same
group have similar chemical and physical properties.
Some of these groups have particular names as well
as numbers. These are given in Figure C3.01. Between
Groups II and III of these main groups of elements is a
block of metals known as the transition elements (or
transition metals). The first row of these elements occurs
in Period 4. This row includes such important metals as
iron, copper and zinc.
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sodium
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testing metals and non-metals
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
The key test to distinguish between metals and nonmetals is electrical conductivity. A simple circuit is set up
using either a light bulb or an ammeter. Power is supplied
by batteries or a power pack. Examine a range of solid
elements and alloys including magnesium, zinc, tin, iron,
nickel, roll sulfur, graphite, brass and solder.
A worksheet is included on the CD-ROM.
Non-metals are a less uniform group of elements. They show
a much wider range of properties. This reflects the wider
diferences in the types of structure shown by non-metals.
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He
2,1
2,2
2,3
2,4
2,5
2,6
2,7
2,8
Li
Be
B
C
N
O
F
Ne
2,8,1
2,8,2
2,8,3
2,8,4
2,8,5
2,8,6
2,8,7
2,8,8
Na
Mg
Al
Si
P
S
Cl
Ar
2,8,8,1 2,8,8,2
K
Ca
5
Sr
6
Ba
7
Ra
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4
U
an argon atom
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The electron arrangements of atoms are linked to
position in the Periodic Table.
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There are links between the organisation of
particles in the atom and the regular variation in
properties of the elements in the Periodic Table.
This means that we can see certain broad trends in
the table (Figure C3.04). These trends become most
obvious if we leave aside the noble gases in
Group VIII/0. Individual groups show certain ‘group
characteristics’. These properties follow a trend in
particular groups. The awareness of these broad trends
(Figure C3.04) means that the properties of any one
element in a group can be predicted from data and
observations about the other elements in that group.
The trends in Groups I and VII are described in more
detail in the following sections. However, even for, say,
Group IV, it is possible to see that the elements lower in
the group will be more metallic since metallic character
increases down a group.
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H
Elements in the same group have the same
number of electrons in their outer shell.
■ For the main-group elements, the number of
the group is the number of electrons in the
outer shell.
■ The periods also have numbers. This number
shows us how many shells of electrons the
atom has.
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VIII/0
■
It is the outer electrons of an atom that are mainly
responsible for the chemical properties of any element.
Therefore, elements in the same group will have
similar properties.
Certain electron arrangements are found to be more
stable than others. This makes them more dificult to
break up. The most stable arrangements are those of
the noble gases, and this fits in with the fact that they
are so unreactive.
VII
Figure C3.03 The relationship between an element’s
position in the Periodic Table and the electron
arrangement of its atoms.
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This link between electron arrangement and
position in the Periodic Table shows itself in the
essential nature of the element as a metal or a non-metal.
Elements in Groups I to III, with low numbers of electrons
in there outer shell, are metals. These elements
can lose their outer electrons relatively easily,
contributing them to the ‘sea of electrons’ that forms
the metallic bond. In contrast, elements with higher
numbers of outer electrons (Groups IV to VII)
form covalent bonds between the atoms and are
therefore non-metals.
VI
a potassium atom
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-C
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There is a clear relationship between electron
arrangement and position in the Periodic Table
for the main-group elements. The elements in Group II
have two outer electrons. The elements in Period 3
have three shells of electrons. A magnesium atom
has two electrons in its third, outer shell, and is in
Group II. An argon atom has an outer shell
containing eight electrons – a very stable arrangement –
and is in Group VIII/0. A potassium atom has one
electron in its fourth, outer shell, and is in Group I and
Period 4.
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1
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When the first attempts were made to construct a Periodic
Table, nobody knew about the structure of the atom. We
can now directly link the properties of an element with
its position in the table and its electron arrangement
(Figure C3.03). The number of outer electrons in the atoms
of each element has been found. Elements in the same
group have the same number of outer electrons. We also
know that, as you move across a period in the table, a shell
of electrons is being filled.
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PERIODS
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Electron arrangement and the
Periodic Table
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If you are asked a question about an element in the
Periodic Table, use the table at the back of the examination
paper to help you answer it.
GROUPS
III
IV
II
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transition elements
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caesium – the most
reactive metal
available in useful
amounts
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atoms getting larger,
more metallic
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metals getting
more reactive
densities and melting
points increase down
any group
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What is the name of the most reactive non-metal?
C3.02
How many elements are there in Period 1?
C3.03
Where in the Periodic Table will the largest atom
be found?
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C3.01
C3.04
Pr
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Sort the following properties into those
characteristic of a metal, and those typical of a
non-metal.
is an insulator
can be beaten into sheets
gives a ringing sound when hit conducts heat
has a dull surface
conducts electricity
219
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Image C3.03 The alkali metals are all sot and can be cut
with a knife. This is a sample of lithium.
y
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only 17 atoms of francium in existence on Earth at any one
moment in time.
What is the similarity in the electron arrangement
in the noble gases?
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Figure C3.04 General trends in the Periodic Table, leaving aside the noble gases in Group VIII/0.
QuEStiONS
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The physical properties of the alkali metals also change as
we go down the group. The melting points become lower
while the density of the metals increases.
Group I – the alkali metals
The alkali metals (Group I) are the most reactive
metals that occur. They are known as the alkali
metals because they react vigorously with water to
produce hydrogen and an alkaline solution.
The most reactive non-metals are the halogens in
Group VII of the table (Figure C3.05). In contrast with
Group I, here reactivity decreases down the group.
For example, fluorine is a dangerously reactive,
pale yellow gas at room temperature. There is a steady
increase in melting points and boiling points as we go
down the group, and the elements change from gases to
solids as the atomic number increases. Interestingly, the
lowest element in this group is also a highly radioactive
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Group VII – the halogens
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The metals in Group I are oten called the alkali metals.
They are sot solids with relatively low melting points and
low densities (Image C3.03). They are highly reactive and
are stored in oil to prevent them reacting with the oxygen
and water vapour in the air. When freshly cut with a knife,
all these metals have a light-grey, silvery surface, which
quickly tarnishes (becomes dull). Reactivity increases
as we go down the group. All Group I metals react with
water to form hydrogen and an alkaline solution of the
metal hydroxide. The reactions range from vigorous in
the case of lithium to explosive in the case of caesium.
You might predict that francium, at the bottom of Group I,
would be the most reactive of all the metals. However, it is
highly radioactive and very rare because it decays with a
half-life of 5 minutes. It has been estimated that there are
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C3.02 trends in groups
R
fluorine – the most
reactive non-metal
atoms getting smaller, less metallic
non-metals getting
more reactive
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Chlorine (Cl2)
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• dense pale-green gas
• smelly and poisonous
• occurs as chlorides, especially
sodium chloride in the sea
• relative atomic mass .
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and rare element, astatine. The actual properties of
astatine remain a mystery to us, but we could make a
good guess at some of them. The suggestion would be
that astatine would be a black solid, non-metallic but with
some metallic character. We would not expect astatine to
vapourise as easily as iodine does.
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• grey solid with purple vapour
• smelly and poisonous
• occurs as iodides and iodates in
some rocks and in seaweed
• relative atomic mass
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The chemical reactivity of the halogens
HClO
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Image C3.04 Bromine is displaced by chlorine from a
colourless solution of potassium bromide.
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hypochlorous acid
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+
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HCl
hydrochloric acid
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Cl2 + H2O
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Fluorine and chlorine are very reactive. Chlorine dissolves
in water to give an acidic solution. This mixture is called
chlorine water and contains two acids:
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There are gradual changes in properties between the
halogens (see Figure C3.05). As you go down the group, the
boiling points increase. Also there is a change from gas to
liquid to solid. The intensity of the colour of the element
also increases, from pale to dark. Following these trends,
it should not surprise you to know that fluorine is a pale
yellow gas at room temperature.
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■
■
R
iodine (i2)
Figure C3.05 The general properties of some of the
halogens (Group VII).
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They are all poisonous and have a similar
strong smell.
They are all non-metals.
They all form diatomic molecules (for example
Cl2, Br2, I2).
They all have a valency (combining power of an atom
or group of atoms) of 1 and form compounds with
similar formulae, for example hydrogen chloride (HCl),
hydrogen bromide (HBr), hydrogen iodide (HI).
Their compounds with hydrogen are usually
strong acids when dissolved in water, for example
hydrochloric acid (HCl), hydrobromic acid (HBr),
hydriodic acid (HI).
They each produce a series of compounds with
other elements: chlorides, bromides and iodides.
Together these are known as halides.
The halogens themselves can react directly with
metals to form metal halides (or salts).
They all form negative ions carrying a single charge,
for example chloride ions (Cl–), bromide ions (Br–),
iodide ions (I–).
• deep-red liquid with
red-brown vapour
• smelly and poisonous
• occurs as bromides, especially
magnesium bromide in the sea
• relative atomic mass
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Common properties of the halogens
■
220
Bromine (Br2)
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The halogen family found in Group VII of the Periodic Table
shows clearly the similarities of elements in the group.
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iodides
bleaches easily
bleaches slowly
bleaches very slowly
—
no reaction
no reaction
displaces bromine, e.g.
Cl2 + 2KBr → 2KCl + Br2
—
no reaction
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2KCl +
I2
yellow-brown
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colourless
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Chlorine will also displace iodine from potassium iodide:
Cl2 + 2KI
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If you are asked to put elements from a group in order
of reactivity, you must be very careful when reading the
question to see whether the answer should be in order of
increasing or decreasing reactivity.
op
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The atoms of the noble gases do not combine with each
other to form molecules or any other form of structure.
Their melting points and boiling points are extremely low
(Image C3.05). Helium has the lowest melting point of
any element, and cannot be solidified by cooling alone
(pressure is needed also). All these properties point to the
atoms of the noble gases being particularly stable.
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When Mendeleev first constructed his table, part of his
triumph was to predict the existence and properties of
some undiscovered elements. However, there was no
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All the noble gases are present in the Earth’s atmosphere.
Together they make up about 1% of the total, though
argon is the most common. These gases are particularly
unreactive. They were sometimes referred to as the inert
gases, meaning they did not react at all. However, since the
1960s, some compounds of xenon and krypton have been
made and their name was changed to the noble gases.
The uses of the noble gases depend on this unreactivity.
Helium is used in airships and balloons because it is
both light and unreactive. Argon is used to fill light bulbs
because it will not react with the filament even at high
temperatures. The best known use of the noble gases is,
perhaps, its use in ‘neon’ lights . The brightly coloured
advertising lights work when an electric discharge takes
place in a tube containg a little of a noble gas. Diferent
gases give diferent colours.
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2KCl + Br
colourless
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The displacement reactions shown in the lower part
of Table C3.02 demonstrate the order of reactivity of the
three major halogens. For example, if you add chlorine to
a solution of potassium bromide, the chlorine displaces
bromine (Image C3.04). Chlorine is more reactive than
bromine, so it replaces it and potassium chloride is
formed. Potassium bromide solution is colourless. It turns
orange when chlorine is bubbled through it:
indication that a whole group of elements (Group VIII/0)
remained to be discovered! Because of their lack of
reactivity, there was no clear sign of their existence.
However, analysis of the gases in air led to the discovery
of argon. There was no suitable place in the table for an
individual element with argon’s properties. This pointed
to the existence of an entirely new group! In the 1890s,
helium, which had first been detected by spectroscopy
of light from the Sun during an eclipse, and the other
noble gases in the group (Group VIII/0) were isolated. The
radioactive gas radon was the last to be purified, in 1908.
One man, William Ramsay, was involved in the isolation of
all the elements in the group. He was awarded the Nobel
Prize for this major contribution.
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Chlorine water acts as an oxidising agent – hypochlorous
acid can give up its oxygen to other substances. It also acts
as a bleach because some coloured substances lose their
colour when they are oxidised. This reaction is used as the
chemical test for chlorine gas. Damp litmus or Universal
Indicator paper is bleached when held in the gas. The
halogens become steadily less reactive as you go down the
group. Table C3.02 gives some examples of the reactivity of
the halogens.
Cl2 + 2KBr
—
displaces iodine, e.g.
Br2 + 2KI → 2KBr + I2
displaces iodine, e.g.
Cl2 + 2KI → 2KCl + I2
Table C3.02 Some reactions of the halogens.
Iodine
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chlorides
Chlorine
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Reaction with
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C3.03 trends across a period
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The vertical groups of elements show similar properties,
but following a period across the table highlights the
trend from metallic to non-metallic properties. This can be
explored by looking across a period. The first period of the
table contains just two elements, hydrogen and helium,
both of which are distinctive in diferent ways. The final
period in the table is as yet incomplete. Each of the five
remaining periods of elements starts with a reactive alkali
metal and finishes with an unreactive, non-metallic, noble
gas. In Period 3, for example, from sodium to argon, there
appears to be a gradual change in physical properties
across the period. The change in properties seems to
centre around silicon; elements before this behave as
metals and those ater it as non-metals (Figure C3.06).
id
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The changeover in properties is emphasised if we
look at Group IV as well. As we go down this group,
the change is from non-metal to metal. The metalloids,
silicon and germanium, are in the centre of the group
(Figure C3.06).
Pr
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atomic size decreasing
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IV
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Al
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Mg
Si
S
Cl
Ar
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metals
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metalloids
Sn
All elements except
Cl and Ar are solids
at room temperature.
non-metals
s
Pb
Figure C3.06 The changes in properties of the elements in
Period 3 and in Group IV.
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The transition elements
What is the name of the alkali formed when
potassium reacts with water?
Write a word equation for the reaction between
lithium and water.
C3.08
Give a use and a test for chlorine.
C3.09
Which halogen(s) will displace bromine from a
solution of potassium bromide?
y
If we look at Period 4 in the Periodic Table, we see that
there is a whole ‘block’ of elements in the centre of the
table. This block of elements falls outside the main groups
of elements that we have talked about so far. They are
best considered not as a vertical group of elements
but as a row or block. They are usually referred to
as the transition elements (or transition metals).
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C3.07
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QuEStiONS
C3.06
Na
gases
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The elements of Group VIII/0 are between the two
most reactive groups of elements (Groups I and VII).
Indeed, it is their closeness to this group with stable
electron arrangements that makes the alkali metals
and the halogens so reactive. They can fairly easily
achieve a noble-gas electron structure. Group VII elements
gain or share electrons and Group I elements lose
electrons to reach a noble-gas electron arrangement.
3
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atomic size decreasing
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The electron arrangements of the atoms of the
noble gases are very stable.
■ This means that they do not react readily with
other atoms.
■ In many situations where atoms of other elements
bond or react chemically, they are trying to achieve
that stable arrangement of electrons found in the
noble gases.
■
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Image C3.05 A small piece of rapidly melting ‘argon ice’.
The melting point is –189 °C.
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C3: Elements and compounds
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One important feature of transition metals is that their
compounds are oten coloured (Image C3.07).
Two of their distinctive properties:
Many of their compounds are coloured.
■ They oten show more than one valency (variable
oxidation state) – they form more than one type
of ion. For example, iron can form compounds
containing iron(II) ions (Fe2+) or iron(III) ions (Fe3+).
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C3.11
Which metal has the highest melting point in
Period 3?
C3.12
Which metal is the sotest and least dense in
Period 3?
C3.13
What is the formula of chlorine?
C3.14
Which of the elements in Period 3 has the highest
melting point?
Why is copper(II) sulfate blue?
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C3.15
y
In which direction does the change in
element type run, when going across a period
from let to right?
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These general properties mean that the transition metals
are useful in a number of diferent ways. In addition,
there are particular properties that make these metals
distinctive and useful for more specific purposes.
C3.10
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Many familiar objects are made from transition metals.
Image C3.06 shows a range of these: steel nails, chrome
bottle stopper, copper pipe joints, iron horseshoe magnet,
cupro-nickel coins (a mix of 75% copper, 25% nickel) and
copper-plated steel coins.
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QuEStiONS
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Their properties make them among the most useful
metallic elements available to us (Image C3.06).
They are much less reactive than the metals in
Groups I and II. Many have excellent corrosion resistance,
for example chromium. The very high melting point
of tungsten (3410 °C) has led to its use in the filaments
of light bulbs.
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Image C3.06 Some everyday objects made from
transition metals.
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Coloured transition
metal salts dissolve to
give coloured solutions.
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They are hard and strong.
■ They have high density.
■ They have high melting and boiling points.
■
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General features of transition metals (or transition
elements)
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Image C3.07 a Many of the compounds of transition metals are coloured; b when dissolved, they give coloured solutions.
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Bonding in metals
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Metal atoms have relatively few electrons in their outer
shells. When they are packed together, each metal atom
loses its outer electrons into a ‘sea’ of free electrons (or
mobile electrons). Having lost electrons, the atoms are
no longer electrically neutral. They become positive
ions because they have lost electrons but the number of
protons in the nucleus has remained unchanged.
op
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Earlier we saw that some elements are not simply made
up of separate atoms individually arranged. Elements
such as oxygen (O2) and hydrogen (H2) consist of diatomic
molecules. Indeed, the only elements that are made up
of individual atoms moving almost independently of each
other are the noble gases (Group VIII/0). These are the
elements whose electron arrangements are most stable
and so their atoms do not combine with each other.
Therefore the structure of a metal is made up of positive
ions packed together. These ions are surrounded by
electrons, which can move freely between the ions.
These free electrons are delocalised (not restricted to
orbiting one positive ion) and form a kind of electrostatic
‘glue’ holding the structure together (Figure C3.07). In an
electrical circuit, metals can conduct electricity because
the mobile electrons can move through the structure,
carrying the current. This type of bonding (called metallic
bonding) is present in alloys as well. Alloys such as solder
and brass, for example, will conduct electricity.
s
ion: a charged particle; an atom that has lost or gained electrons
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Metal atoms more easily lose electrons than gain
them. So, they become positive ions. In doing so,
they achieve a more stable electron arrangement,
usually that of the nearest noble gas.
Bonding in the elements
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Metallic elements are held together by metallic
bonding, which results in metallic lattices.
■ Non-metallic elements are held together by
covalent bonding or exist as separate atoms (the
noble gases). Covalent bonding results in simple
molecules or giant molecular lattices.
■
ie
id
g
Bonding in non-metals
ev
Hydrogen normally exists as diatomic molecules (H2). Two
atoms bond together by sharing their electrons. The orbits
overlap and a molecule is formed (Figure C3.08).
es
s
-R
br
am
-C
w
ie
ev
R
KEy tERM
es
-C
am
Most of the elements do form structures. Their atoms are
linked by some type of bonding. Most elements are metals.
The structures in this case are held together by metallic
bonding. The non-metallic elements to the right of the
Periodic Table are held together by covalent bonding. Both
these types of bonding use the outer electrons in some way.
-R
br
ev
id
ie
w
ge
C
U
ni
op
y
ve
w
ie
ev
-R
ity
Bonding in the elements
R
s
Pr
y
es
-C
am
br
ev
id
Chemical bonding involves the outer electrons of each
atom. As we examine a range of substances, we shall
see that, whatever type of bonding holds the structure
together, it is the outer electrons that are used. The
diversity of the material world is produced by the diferent
ways in which atoms can join together.
y
y
op
C
Figure C3.07 Metallic bonding – the metal ions are
surrounded by a ‘sea’ of mobile electrons.
Simple compounds such as water, ammonia and methane
begin to show the variety that can be achieved when the atoms
of elements combine together. Water is formed from hydrogen
and oxygen. Each water molecule contains two hydrogen
atoms bonded to an oxygen atom. In fact, the formula of water
(H2O) is perhaps the best-known chemical formula.
w
ev
ie
-R
electron
We live on the ‘water planet’. The surface of the Earth is
distinctive because so much of it is covered with water.
From space, it is the blue colours of water in seas and
oceans and the white of the moisture-laden clouds that
distinguish the Earth from other planets. The Earth is
unique in being the only planet in our solar system where
conditions allow water to exist in all three states of matter.
R
ev
ie
am
br
id
ge
positive
metal ion
C
C3.04 Chemical bonding in
elements and compounds
w
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
H
C
w
Pr
es
s
-C
two hydrogen atoms
op
y
hydrogen molecule (H2)
ni
C
op
displayed formula
model
Figure C3.08 The hydrogen molecule is formed by sharing
the electrons from the atoms. A space-filling model can be
used to show the atoms overlapping.
Cl
op
U
ge
C
chlorine molecule
(each chlorine is now 2,8,8)
w
ie
ev
id
br
Cl2
displayed formula
0.19
nm
es
s
-R
am
-C
Cl Cl
b
ity
Pr
op
y
Many non-metallic elements form diatomic molecules.
However, elements other than hydrogen form bonds in
order to gain a share of eight electrons in their outer shells.
This is the number of electrons in the outer shell of all
the noble gases apart from helium. Thus, the halogens
(Group VII) form covalent molecules (Figure C3.09).
Br2
y
op
Figure C3.09 a The formation of the covalent bond in
chlorine molecules (Cl2). Each atom gains a share in eight
electrons in its outer shell. The diagram can be drawn
showing the outer electrons only, because the inner
electrons are not involved in the bonding. b Molecules
of Br2 and I2 are formed in the same way. They are larger
because the original atoms are bigger.
w
e
ev
ie
id
g
es
s
-R
br
am
-C
0.27
nm
C
U
Molecules of hydrogen and the halogens are each held
together by a single covalent bond. Such a single bond
uses two electrons, one from each atom. The bond can be
drawn as a single line between the two atoms.
Note that, when we draw diagrams showing the overlap
of the outer shells, we can show the outer electrons only,
I2
0.22
nm
ni
ve
rs
C
w
ie
Cl
y
ve
ni
ev
R
Cl
two chlorine atoms
(2,8,7)
The bond is formed by the sharing of a pair of
electrons between two atoms.
■ Each atom contributes one electron to each bond.
■ Molecules are formed from atoms linked together
by covalent bonds.
ev
+
Cl
Features of covalent bonding
■
R
225
rs
C
w
ie
a
ity
op
Pr
y
es
s
-C
am
Through this sharing, each atom gains a share in
two electrons. This is the number of electrons in the outer
shell of helium, the nearest noble gas to hydrogen. (Remember
that the electron arrangement of helium is very stable; helium
atoms do not form He2 molecules.) Sharing electrons like this
is known as covalent bonding. It has been shown that in a
hydrogen molecule, the electrons are more likely to be found
between the two nuclei. The forces of attraction between the
shared electrons and the nuclei are greater than any repulsive
forces. The molecule is held together by the bond.
-R
br
ev
id
ie
w
ge
U
R
The non-metals in the middle of the main-group elements,
for example carbon and silicon, do not form simple
molecules. They exist as giant molecular structures held
together by single covalent bonds. In these structures, the
atoms are joined to each other in an extensive network
or giant covalent (molecular) lattice (see Figure C3.34).
Such structures are very strong because all the atoms are
interlinked by strong covalent bonds. The structure of the
carbon atoms in diamond is a three-dimensional lattice
structure in which each carbon atom is joined to four
others by strong covalent bonds. A similar structure exists
in silicon, an important element in the electronics industry.
y
C
ve
rs
ity
H H
w
ev
ie
ev
ie
H
H
When molecules of oxygen (O2) or nitrogen (N2) are formed,
more electrons have to be used in bonding if the atoms
are to gain a share of eight electrons. These molecules
are held together by a double bond (O2) or a triple bond
(N2) (Figure C3.10). Note that the structure of oxygen is
not required for the syllabus, but is included here as an
example of a double covalent bond, which you will need to
be able to draw for carbon dioxide later.
-R
+
H
because the inner electrons are not involved in the bonding.
Each atom gains a share in eight electrons in its outer shell.
a shared pair of electrons
makes a covalent bond
am
br
id
ge
U
ni
op
y
C3: Elements and compounds
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
N N
am
br
id
N
N
N
-R
displayed formula
A hydrogen atom has
just one electron in its
first energy level.
Pr
es
s
-C
nitrogen, N2
If the two atoms share one pair of electrons:
... hydrogen can fill
its first energy
level...
ni
Diferent elements combine together to form the vast
range of compounds that make up our world. They vary
from inert and heat-resistant ceramic materials to high
explosives, and from lethal poisons to the molecules of life.
All depend on the means of chemical bonding. Two major
types of bond hold compounds together. The first is
covalent bonding, which, as we have seen, involves sharing
electrons between atoms. However, the behaviour of
metal plus non-metal compounds arises from a diferent
type of bonding. Here electrons are transferred from one
atom to another. This transfer of electrons between atoms
produces oppositely charged particles known as ions.
The formation of these ions results in a diferent type of
chemical bonding: ionic bonding.
ie
-R
s
es
a shared pair
of electrons
Pr
ity
We can also draw the molecule like this:
rs
H
Cl
y
ve
w
op
Figure C3.11 Hydrogen and chlorine atoms share a pair of
electrons to form a molecule of hydrogen chloride.
ni
Non-metal plus non-metal compounds are held
together by covalent bonding, which results in the
simple molecules which make up the compound.
■ Metal plus non-metal compounds are held
together by ionic bonding between positive and
negative ions.
■ In some elements and compounds (C and SiO2,
for example) covalent bonding can result in the
formation of giant covalent lattices.
■ In ionic bonding the ions are held together
by electrostatic forces of attraction, forming
giant ionic lattices in the solid crystal.
U
■
w
ge
C
ie
ev
Bonding in compounds
R
br
ev
id
ie
The formation of hydrogen chloride (HCl) involves the two
atoms sharing a pair of electrons (Figure C3.11).
s
es
Pr
ity
ni
ve
rs
y
op
-R
s
es
am
br
ev
ie
id
g
w
e
C
In covalent compounds, bonds are again made by
sharing electrons between atoms. In simple molecules,
the atoms combine to achieve a more stable
arrangement of electrons, most oten that of a noble gas.
-C
In each case, the atoms achieve a share in the same number
of electrons as the noble gas nearest to that element in the
Periodic Table. In all but the case of hydrogen, this means a
share of eight electrons in their outer shell.
Earlier we saw that multiple covalent bonds can exist in
molecules of the elements oxygen and nitrogen. They can
exist in compounds too. The carbon dioxide molecule is held
together by double bonds between the atoms (Figure C3.13).
This figure also shows some other examples of bonding in
compounds that you will meet again in Chapter C10.
U
Covalent compounds
The examples shown in Figure C3.12 illustrate diferent
ways of representing this sharing. They also show how the
formula of the compound corresponds to the numbers of
each atom in a molecule.
-R
am
-C
op
y
C
w
ie
ev
R
This is a
molecule
of hydrogen
chloride.
ev
id
br
am
-C
y
op
C
226
... and chlorine can
fill its third energy
level.
w
ge
U
R
ev
ie
w
Chemical bonding in compounds
A chlorine atom has
seven electrons in its
third energy level.
y
C
ve
rs
ity
op
y
Figure C3.10 The structure of nitrogen (N2) molecules
involves multiple covalent bonding. A nitrogen molecule
contains a triple bond. The nitrogen molecule is the
drawing required for the syllabus.
C
op
N
ev
ie
w
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
H
y
H
H
C
H
C
-R
H
four hydrogen carbon atom
atoms (1)
(2,4)
C
H
H
displayed formula
H
H
methane molecule
Each hydrogen now shares two electrons with carbon.
3
H
H
N
+
N
ve
rs
ity
op
ev
ie
+
ammonia (NH3)
C
H
H
Pr
es
s
am
br
id
-C
H
w
H
methane (CH4)
4
C
ge
U
ni
op
y
C3: Elements and compounds
N
H
H
H
three hydrogen nitrogen atom
atoms (1)
(2,5)
N
H
H
H
H
H
displayed formula
y
ev
ie
w
ammonia molecule
Hydrogen and nitrogen both fill their outer shells by sharing electrons.
H
H
O
w
H
oxygen atom
(2,6)
displayed formula
water molecule
Hydrogen and oxygen both fill their outer shells by sharing electrons.
+
H
Cl
H
H
Cl
Cl
carbon dioxide (CO2)
hydrogen chloride
molecule
displayed formula
Pr
op
y
one hydrogen chlorine atom
atom (1)
(2,8,7)
es
s
Cl
-R
hydrogen chloride
(HCl)
-C
O
ie
two hydrogen
atoms (1)
H
H
O
H
ge
id
br
am
H
O
C
op
+
ev
H
U
R
2
ni
water (H2O)
227
O
C
O
y
C
carbon dioxide molecule (CO2)
op
carbon atom
C
two oxygen
atoms
w
ge
U
R
+
O
ni
ev
ve
ie
w
rs
C
ity
O
O C O
s
-C
displayed formula
-R
am
br
ev
id
ie
model
Pr
AO3.4 Interpret and evaluate experimental
observations and data
In this activity, you will make models of simple molecular
structures of certain elements and compounds to
demonstrate the importance of single, double and triple
covalent bonds in molecules.
The modelling can be extended to show the processes of
bond breaking and bond making that take place during
a chemical reaction. This serves as an introduction to
balancing chemical equations.
A worksheet is included on the CD-ROM.
ni
ve
rs
ity
ACtivity C3.02
-R
s
es
am
br
ev
ie
id
g
w
e
C
U
op
y
Modelling the bonding in covalent substances
Skills:
A03.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
-C
R
ev
ie
w
C
op
y
es
Figure C3.12 Examples of the formation of simple covalent molecules. Again, only the outer electrons of the atoms are shown.
More complex examples are shown in Figure C3.13.
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
w
H
C
+
H
H
Pr
es
s
ethene molecule (C2H4)
ve
rs
ity
op
C
H
w
six hydrogen
atoms
y
C
op
ev
br
H
C
H
am
H
O
+
ie
ge
+
id
H
C
w
ni
U
R
H
two carbon
atoms
H
H
C
C
H
H
O
H
ethanol molecule (C2H5OH)
rs
ni
op
y
ve
C
ev
ie
w
HH
H C C OH
HH
ity
op
Pr
y
es
s
-C
oxygen
atom
-R
ev
ie
displayed formula
H
displayed formula
C
U
R
C
HC CH
H
H
H
id
ie
w
ge
Figure C3.13 The formation of ethene and ethanol molecules, showing the outer electrons only.
Ball-and-stick models can be used to show the structure.
ev
A common example of a compound that involves ionic
bonding is sodium chloride (Figure C3.14). Each of the sodium
atoms, which have an electron arrangement of 2,8,1, loses its
one outer electron to form a sodium ion (Na+) (Figure C3.15).
C
The electrons involved in the formation of
ions are those in the outer shell of the atoms.
■ Metal atoms lose their outer electrons to become
positive ions. In doing so they achieve the more
stable electron arrangement of the nearest noble gas.
■ Generally, atoms of non-metals gain electrons to
become negative ions. Again, in doing so, they
achieve the stable electron arrangement of the
nearest noble gas to them in the Periodic Table.
op
y
ni
ve
rs
The sodium ion then has the stable electron arrangement
(2,8) of a neon atom – the element just before it in the
Periodic Table. The electron released is transferred to a
chlorine atom. The sodium ion has a single positive charge
because it now has just 10 electrons in total, but there are
still 11 protons in the nucleus of the atom.
ev
ie
id
g
w
e
C
U
The chlorine atoms, electron arrangement 2,8,7, each gain
an electron released from the sodium atoms and they
become chloride ions (Cl–) (Figure C3.16). The chloride ion
(electron arrangement 2,8,8) has the electron arrangement
of an argon atom. The chloride ion has a negative charge
because it has one more electron (18) than there are
protons in the nucleus.
es
s
-R
br
am
-C
ie
w
■
ev
s
ity
Pr
op
y
es
-C
am
Compounds of a metal plus a non-metal generally adopt a
third type of bonding. This involves the transfer of electrons
from one atom to another. This transfer of electrons results
in the formation of positive and negative ions. The oppositely
charged ions are then held together by forces of attraction.
-R
br
Ionic compounds
R
H
C
two carbon atoms
y
-C
four hydrogen
atoms
ethanol (C2H5OH)
228
C
H
H
ev
ie
H
-R
am
br
id
ge
ethene (C2H4)
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
w
[Na]+
Mg
ev
ie
Cl
–
Cl
Cl
y
op
ve
rs
ity
Cl
Cl
y
Figure C3.17 Diagrams showing the formation of ionic
bonds in magnesium oxide and calcium chloride. Again,
only the outer electrons are shown.
ni
U
id
ie
w
ge
Figure C3.15 A sodium atom loses an electron to become
a sodium ion.
br
ev
More complex ionic compounds than those formed
between the alkali metals and the halogens require care in
working out the transfer of a greater number of electrons.
Figure C3.17 shows two examples of such compounds.
-R
am
Features common to ionic bonding
■
ity
The chlorine atom [2,8,7] needs to gain
an electron to make it more stable.
Pr
es
s
-C
y
rs
y
ve
ni
ev
ge
C
U
R
■
■
br
ev
id
ie
–
This is an ion of chlorine [2,8,8] .
-R
■
es
The positive and negative ions in sodium chloride are
held together by the electrostatic attraction between
opposite charges.
■
s
-C
am
Figure C3.16 A chlorine atom gains an electron to become
a chloride ion.
TIP
TIP
op
For the Core syllabus, the examples of ionic bonding
you need to be familiar with are those between Group I
metals and Group VII non-metals – the alkali metals
and the halogens. Try drawing diagrams like the one
in Figure C3.14 for compounds such as lithium fluoride
or potassium bromide. You will see that there is a great
similarity in the diagrams.
id
g
w
e
C
U
Do practise drawing the diagrams for both covalent and
ionic bonding so that you can draw them accurately
in the examination.
ev
ie
When you draw the diagrams of ionic bonding, make sure
you remember to put in the charges outside the brackets
on each ion.
es
s
-R
br
am
-C
y
w
ni
ve
rs
C
ity
Pr
op
y
op
w
ie
■
Metal atoms always lose their outer electrons to
form positive ions.
The number of positive charges on a metal ion is
equal to the number of electrons lost.
Non-metal atoms, with the exception of hydrogen,
always gain electrons to become negative ions.
The number of negative charges on a non-metal ion
is equal to the number of electrons gained.
In both cases, the ions formed have a more stable
electron arrangement, usually that of the noble gas
nearest to the element concerned.
Ionic (electrovalent) bonds result from the
attraction between oppositely charged ions.
w
op
–
C
op
C
w
ev
ie
R
Cl
calcium chloride (CaCl2)
When the sodium atom loses an
electron, it forms a sodium ion.
C
O
[Ca]2+
+
Ca
+
an ion of sodium, Na
+
[2,8]
an atom of sodium
[2,8,1]
ie
[Mg]2+
O
–
Pr
es
s
-C
Figure C3.14 The transfer of electrons from a sodium atom
to a chlorine atom to form ions.
ev
+
magnesium oxide (MgO)
sodium chloride (NaCl)
R
2–
-R
+
am
br
id
Na
ge
U
ni
op
y
C3: Elements and compounds
Copyright Material - Review Only - Not for Redistribution
229
ve
rs
ity
–
+
+
–
+
–
+
+
2–
C
w
y
ev
id
ie
w
ge
U
Figure C3.19 Three examples of negatively charged
ionic groups and a positively charged ionic group.
The numbers of atoms and the overall charge carried by
each group of atoms are shown. Note that you are not
expected to know the shapes of these ions; the diagrams
are simply to show you why the formulae are as they are
listed in Table C3.03.
br
es
Pr
y
rs
op
y
ve
ni
w
ie
ev
-R
s
es
Pr
ity
ie
id
g
w
e
C
U
op
y
ni
ve
rs
metallic bonding
■ ionic bonding
■ covalent bonding.
The diferences we observe between the physical
properties (Table C3.04) of ionic and simple covalent
compounds depend on the interactive forces between
the particles present in the compound. The ions making
up an ionic compound interact through the electrostatic
attraction of the full chemical bonding. These forces work
in all directions in the solid and strongly hold the ions
in place in the structure. Simple covalent compounds
are made up of molecules. The full chemical bonding
works within the molecules holding them together.
However, importantly, this bonding does not act between
one molecule and the others around it. The forces
between molecules are just weak intermolecular forces.
Therefore, such substances have lower melting points
and boiling points as it takes less energy to separate the
molecules from each other than to separate the ions in an
ionic compound.
C
U
ge
id
br
am
-C
op
y
C
-R
s
es
am
br
ev
The types of structure based on these methods of
bonding are summarised in Figure C3.20.
-C
w
ie
Through our discussion of elements and
compounds we have seen that there are three major
types of chemical bonding:
■
The physical properties of ionic and
covalent compounds
Knowledge of how atoms combine to make diferent types
of structure helps us begin to understand why substances
have diferent physical properties. Table C3.04 shows
the broad diferences in properties of ionic and simple
covalent compounds.
ity
op
C
w
ie
ev
R
s
-R
am
-C
The ionic compounds mentioned so far have been
made from simple ions, for example Na+, K+, Mg2+, Cl–, O2–.
However, in many important ionic compounds the metal
ion is combined with a negative ion containing a group
of atoms (for example, SO42−, NO3−, CO32−). These ionic
groups are made up of atoms covalently bonded together.
These groups have a negative charge because they have
gained electrons to make a stable structure. Examples
of such ions are shown in Figure C3.19. In addition to
these negative ionic groups, there is one important ionic
group that is positively charged, the ammonium ion, NH4+
(Figure C3.19). Table C3.03 gives a summary of some simple
ions and ionic groups.
The formulae of compounds involving the ions listed in this
table can be worked out by remembering that the overall
change of a formula is zero. The total positive charge must
equal the total negative charge.
ev
one nitrogen + four
hydrogens, with
overall charge of 1+
C
op
ni
ev
ie
R
Ionic compounds (such as sodium chloride) are solids
at room temperature. The ions arrange themselves into
a regular lattice (Figure C3.18). In the lattice, each ion is
surrounded by ions of the opposite charge. The whole
giant ionic structure is held together by the electrostatic
forces of attraction that occur between particles of
opposite charge (see Section C3.06).
Ionic groups
R
+
one sulfur + four
oxygens, with
overall charge of 2–
ve
rs
ity
–
one nitrogen + three
oxygens, with
overall charge of 1–
NH4+
SO42–
Figure C3.18 A giant ionic lattice where each ion is
surrounded by ions of opposite charge.
230
–
one carbon + three
oxygens, with
overall charge of 2–
-R
–
Pr
es
s
-C
y
–
NO3–
2–
+
–
+
–
op
+
–
+
–
CO32–
ev
ie
am
br
id
–
+
–
+
–
w
+
ge
–
+
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
Ionic groups
(+ve)
(+ve)
(–ve)
(+ve)
(–ve)
hydrogen, H+
hydride, H–
ammonium, NH4+
hydroxide, OH–
nitrate, NO3–
silver, Ag+
bromide, Br–
hydrogencarbonate, HCO3–
Pr
es
s
-R
chloride, Cl–
+
calcium, Ca2+
zinc, Zn2+
ev
ie
w
iron(II), Fe2+
ni
copper(II), Cu2+
R
sulfide, S2–
carbonate, CO32–
3+
nitride, N3–
ev
giant
metallic
lattices
metallic
bonding
s
-C
metals
-R
am
br
id
ie
Table C3.03 Some common simple ions and ionic groups.
giant
ionic
lattices
es
Pr
y
op
phosphate, PO43–
w
ge
iron(III), Fe
sulfate, SO42–
U
aluminium, Al3+
3
oxide, O2–
y
magnesium, Mg
ve
rs
ity
y
op
C
iodide, I
2+
–
C
op
-C
potassium, K+
copper(I), Cu
2
ev
ie
sodium, Na+
1
w
Simple non-metallic ions
ge
Simple metal ions
am
br
id
Valency
C
U
ni
op
y
C3: Elements and compounds
COMPOuNDS
op
covalent
bonding
w
C
non-metal +
non-metal(s)
br
ev
ie
ge
separate
atoms
(noble gases)
id
y
ve
giant
molecular
lattices
covalent
bonding
U
R
non-metals
ni
ev
ie
w
rs
C
231
simple
molecules
ity
ELEMENtS
metal +
nonmetal(s)
ionic
bonding
-R
am
Figure C3.20 An overall summary of the bonding in elements and compounds.
es
a sulfur and chlorine
a hydrogen
b
water
c ammonia
d
methane.
ity
c magnesium and nitrogen
y
What force holds the sodium and chlorine
together in sodium chloride?
Why is it true to say that calcium carbonate has
both ionic and covalent bonds?
C3.22
Draw diagrams of the ionic bonding in the following
compounds:
ev
br
s
-R
a magnesium oxide
es
am
b lithium fluoride.
C3.21
e
id
g
Draw diagrams of the covalent bonding in
the following elements and compounds
-C
op
C3.18
a sodium chloride
C
Why is the formula of hydrogen always written
as H2?
U
C3.17
C3.19
Draw diagrams of the ionic bonding in the following
compounds:
w
d zinc and copper
C3.20
ie
w
Pr
(showing the outer electrons only in your
diagrams):
b carbon and oxygen
ie
ev
R
What type of bond would be found between the
following pairs of elements?
ni
ve
rs
C
op
y
C3.16
s
-C
QuEStiONS
Copyright Material - Review Only - Not for Redistribution
b calcium chloride.
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Reason for these properties
They are crystalline solids at room temperature.
There is a regular arrangement of the ions in a lattice. Ions with
opposite charge are next to each other.
They have high melting and boiling points.
Ions are attracted to each other by strong electrostatic forces.
Large amounts of energy are needed to separate them.
-C
-R
am
br
id
ev
ie
w
ge
Properties of typical ionic compounds
y
Properties of simple covalent compounds
Reason for these properties
They are oten liquids or gases at room
temperature.
These substances are made of simple molecules. The atoms are
joined together by covalent bonds.
ni
C
op
ve
rs
ity
C
w
ev
ie
y
In the liquid or solution, the ions are free to move about. They can
move towards the electrodes when a voltage is applied.
op
They conduct electricity when molten or dissolved
in water (not when solid).
They have low melting and boiling points.
The forces between the molecules (intermolecular forces) are only
very weak. Not much energy is needed to move the molecules
further apart.
w
ge
U
R
Water is attracted to charged ions and therefore many ionic solids
dissolve.
Pr
es
s
They are oten soluble in water (not usually soluble
in organic solvents, e.g. ethanol, methylbenzene).
br
ev
id
ie
They are soluble in organic solvents such as ethanol Covalent molecular substances dissolve in covalent solvents.
or methylbenzene (very few are soluble in water).
There are no ions present to carry the current.
-R
am
They do not conduct electricity.
Pr
y
es
s
-C
Table C3.04 The properties of ionic and simple covalent compounds.
For convenience, the same applies to elements such
as phosphorus (P) and sulfur (S). In these cases, the
molecules contain more than three atoms.
ve
The chemical ‘shorthand’ of representing an element by its
symbol can be taken further. It is even more useful to be
able quickly to sum up the basic structure of an element or
compound using its chemical formula.
ni
op
y
The formulae of ionic compounds
Ionic compounds are solids at room temperature, and
their formulae are simply the whole-number ratio of
the positive to negative ions in the structure. Thus, in
magnesium chloride, there are two chloride ions (Cl–) for
each magnesium ion (Mg2+).
w
ge
C
U
R
ev
ie
w
rs
C
ity
op
C3.05 the chemical formulae of
elements and compounds
232
Cu
Zn
Si
Ga
Ge
(P4)
P
(S8)
S
Cl2
Ar
As
Se
Br2
Kr
id
g
e
Ti
simple molecules
-R
s
single atoms
-R
s
es
-C
am
Figure C3.21 The formulae of the elements are linked to
their structure and their position in the Periodic Table.
2−
The size of the charge on an ion is a measure of its valency
(see Table C3.03) or combining power. Mg2+ ions can
combine with Cl– ions in a ratio of 1 : 2, but Na+ ions can
only bond in a 1 : 1 ratio with Cl– ions. This idea of valency
can be used to ensure that you always use the correct
formula for an ionic compound. Follow the examples of
aluminium oxide and calcium oxide below (Figures C3.22
and C3.23), and make sure you understand how this works.
br
giant molecular
lattice
Ne
y
Ca
F2
op
K
O2
C
Al
N2
2+
The formula is MgCl2. The overall structure must be
neutral. The positive and negative charges must balance
each other.
w
Mg
C
total charge
ie
Na
giant metallic
lattice
Cl−
ev
B
He
ni
ve
rs
Be
U
Li
ity
Pr
op
y
C
w
ie
ev
R
H2
Sc
ions present Mg2+ Cl−
es
-C
am
br
Those elements which are made up of individual atoms
or small molecules (up to three atoms covalently
bonded together) are represented by the formula of the
particle present (Figure C3.21). Where elements exist as
giant structures, whether held together by metallic or
covalent bonding, the formula is simply the symbol of the
element (for example Cu, Mg, Fe, Na, K, etc., and C, Si, Ge).
ev
id
ie
The formulae of elements
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
2–
y
Formula for calcium oxide
op
Ca
ve
rs
ity
C
w
2 –
Ca2O2
Simplify the ratio:
Formula
CaO
ni
ev
ie
O
2 +
Write down the charges on the ions
ammonium
nitrate
NH4NO3
NH4+
NO3–
1:1
potassium
sulfate
K 2SO4
K+
SO42–
2:1
calcium
hydrogencarbonate
Ca(HCO3)2
Ca2+
HCO3–
1:2
CuSO4
Cu2+
SO42–
1:1
magnesium
nitrate
Mg(NO3)2
Mg2+
NO3–
1:2
aluminium
chloride
AlCl3
Al3+
Cl–
1:3
ie
ev
s
Table C3.05 The formulae of some ionic compounds.
es
Pr
ity
rs
op
2–
Na2CO3
C
ni
The brackets are not needed if
there is only one ion present.
• simple molecules with a central atom, for example
water, methane, carbon dioxide (Figure C3.26) and
ammonia:
w
ge
U
R
Formula
ie
ev
id
Formula for carbon dioxide
C
Formula
1 +
2–
(NH4)2SO4
Pr
op
y
Write down the charges on the ions
es
s
(SO4)
-R
br
am
-C
(NH4)
Write down the correct ‘symbols’
2
4
Write down the valencies
ity
Formula
ni
ve
rs
C
C2O4
Can simplify:
CO2
y
Figure C3.26 The formula for carbon dioxide.
op
w
TIP
O
Write down the symbols
Figure C3.25 The formula for ammonium sulfate.
Be very careful when writing chemical formulae to get the
symbols of the elements correct. Remember the unusual
symbols: that sodium is Na and not So, for example.
id
g
w
e
C
U
• giant covalent molecules, where the formula is simply
the whole-number ratio of the atoms present in the
giant lattice, for example silica.
ie
Remember that the second letter in any symbol is lower
case, not a capital letter: Na not NA, Cl not CL and Co not
CO, for instance.
ev
The valency of an element in the main groups of the
Periodic Table can be worked out from the group
number of the element. The relationship is shown below.
es
s
-R
br
am
233
y
ve
1 +
Write down the charges on the ions
-C
1:1
The idea of an atom having a valency, or combining power,
can also be applied to working out the formulae of covalent
compounds. Here the valency of an atom is the number
of covalent bonds it can form. The ‘cross-over’ method for
working out chemical formulae can be applied to covalent
compounds in two situations:
(CO3)
Na
Write down the correct ‘symbols’
Formula for ammonium sulfate
ie
Cl
The formulae of covalent compounds
Figure C3.24 The formula for sodium carbonate.
ev
Ratio
–
Na
-R
br
am
-C
y
op
C
ie
w
Formula for sodium carbonate
R
+
NaCl
w
ge
id
The same rules apply when writing the formulae of
compounds containing ionic groups because each of
them has an overall charge (see Table C3.03). It is useful
to put the formula of the ionic group in brackets. This
emphasises that it cannot be changed. For example, the
formula of the carbonate ion is always CO32–. Work through
the examples for sodium carbonate and ammonium
sulfate in Figures C3.24 and C3.25.
Ions present
sodium
chloride
copper(II)
sulfate
U
R
Figure C3.23 The formula for calcium oxide.
ev
Formula
y
Pr
es
s
-C
Figure C3.22 The formula for aluminium oxide.
Write down the correct symbols
Name
-R
Al 2O3
Formula
C
op
am
br
id
O
3 +
Write down the charges on the ions
w
Al
Write down the correct symbols
Table C3.05 summarises the formulae of some important
ionic compounds.
ev
ie
ge
Formula for aluminium oxide
C
U
ni
op
y
C3: Elements and compounds
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
ev
ie
am
br
id
Working out valency
w
ge
systems do aim to be consistent. Some common and
important compounds have historical names that do not
seem to fit into a system. Examples of these include water
(H2O), ammonia (NH3) and methane (CH4). These apart,
there are some basic generalisations that are useful.
For elements in Groups I–IV,
-R
valency = group number
For elements in Groups V–VII,
Pr
es
s
-C
• If there is a metal in the compound, it is named first.
y
valency = 8 – the group number
op
Elements in Group VIII/0 have a valency of 0.
ve
rs
ity
This trend in valency with the group number can be
seen by looking at typical compounds of the elements
of Period 3. You can see that the valency rises to a value
of 4 and then decreases to 0 as we cross the period.
III
IV
V
VI
VII
VIII/0
2
3
4
3
2
1
0
SiCl4
PH3
H2 S
HCl
–
y
ie
• Compounds containing an ionic group usually
containing oxygen) have names that end with –ate; for
example, calcium carbonate (CaCO3), potassium nitrate
(KNO3), magnesium sulfate (MgSO4), sodium ethanoate
(CH3COONa).
ev
id
NaCl MgCl2 AlCl3
br
op
Pr
y
Examples of writing formulae
ity
The method for working out formulae above does not
work for the many covalent molecules that do not have a
single central atom, for example H2O2, C2H6, C3H6, etc. The
formulae of these compounds still obey the valency rules.
However, the numbers in the formula represent the actual
number of atoms of each element present in a molecule of
the compound (Figure C3.27).
op
y
ve
ni
C
ie
w
ge
id
br
-R
am
s
ity
C3.24
y
d Li3N
f
NO
g NO2
h
SO3
Use your Periodic Table to help you give the
formula of each of these compounds:
a silicon chloride
b
c phosphorus chloride
d silicon oxide
s
es
am
Figure C3.27 The structures of hydrogen peroxide (H2O2)
and ethane (C2H6), showing the bonds made.
-C
c K 2O
-R
br
ethane
MgS
ie
H
H
b
ev
H
e Ca(OH)2
U
C
What names would you give these compounds?
a NaI
Each carbon atom
makes four bonds;
each hydrogen
makes one bond.
e
C
C3.23
op
H
id
g
ev
R
H
QuEStiONS
ni
ve
rs
hydrogen peroxide
C
H
es
Each oxygen atom
makes two bonds;
each hydrogen
makes one bond.
O
ie
w
C
H
Pr
op
y
-C
Giving a name to a compound is a way of classifying it.
Not all names are as informative as others, but modern
H
Two important oxidising agents contain polyatomic
negative ions involving metal and oxygen atoms. Their
modern names (potassium manganate(VII) (KMnO4) and
potassium dichromate(VI) (K 2Cr2O7)) include the oxidation
state of the metal. At this stage you will not need to write
equations using these compounds, but you will need to
recognise their names and formulae.
ev
‘What’s in a name?’ – naming chemical
compounds
O
• The names of some compounds use prefixes to tell you
the number of that particular atom in the molecule. This
is useful if two elements form more than one compound;
for example, carbon monoxide (CO) and carbon dioxide
(CO2), nitrogen dioxide (NO2) and dinitrogen tetraoxide
(N2O4), sulfur dioxide (SO2) and sulfur trioxide (SO3).
The names for the important mineral acids are systematic
but are best simply learnt at this stage; for example, sulfuric
acid (H2SO4).
U
R
ev
ie
w
rs
C
234
-R
es
s
-C
am
For example, carbon is in Group IV, so its valency is 4,
and oxygen is in Group VI, so its valency is 8 – 6 = 2.
w
Typical
compound
C
op
II
1
• Compounds containing only two elements have names
ending in –ide; for example, sodium chloride (NaCl),
calcium bromide (CaBr2), magnesium nitride (Mg3N2).
The important exception to this is the hydroxides, which
contain the hydroxide (OH–) ion.
w
I
Valency
ge
Group
U
ni
C
w
ev
ie
R
• Where the metal can form more than one ion, then the
name indicates which ion is present; for example, iron(II)
chloride contains the Fe2+ ion, while iron(III) chloride
contains the Fe3+ ion.
Copyright Material - Review Only - Not for Redistribution
carbon sulfide
ve
rs
ity
w
ev
ie
ve
rs
ity
C
op
y
HNO3
SiCl4
FeSO4
CH4
H2SO4
KEy tERMS
ge
c Give the formulae for the following
compounds:
-R
s
es
Pr
ity
ve
The diagram shows the arrangement of the outer
electrons only in a molecule of ethanoic acid.
y
op
ni
C
w
ie
ge
id
ev
-R
H
Pr
b What is the total number of atoms present in
this molecule?
ity
op
y
ni
ve
rs
c Between which two atoms is there a double
covalent bond?
U
op
y
d How many covalent bonds does each carbon
atom make?
ev
ie
id
g
-R
br
Ethanoic acid will dissolve in methylbenzene.
Would you expect the solution to conduct
electricity? Give a reason for your answer.
s
Image C3.08 A snowflake crystal.
es
am
f
w
e
C
e Would you expect this compound to be a solid
or a liquid at room temperature? Give a reason
for your answer.
-C
C
w
ie
ev
a Name the diferent elements found in this
compound.
s
O
es
H
-C
am
O
C
C
br
H
R
Structures of these diferent types are found all around us.
In some cases, we use and adapt their physical properties
to engineer materials to suit a particular purpose.
U
R
H
w
ev
id
rs
-C
y
op
C
w
ev
ie
C3.26
giant metallic lattice: a lattice of positive ions in a
‘sea’ of electrons
■ giant ionic lattice: a lattice of alternating positive and
negative ions
■ giant covalent (molecular) lattice: a giant molecule
(macromolecule) making the lattice
■ simple molecular substances: consisting of simple
molecules in a lattice held together by weak forces
(Figure C3.28)
■
br
potassium sulfate
aluminium fluoride
iron(III) oxide
calcium nitrate
zinc chloride
ammonia
hydrochloric acid
copper(II) sulfate
sulfur trioxide.
am
i
ii
iii
iv
v
vi
vii
viii
ix
The four diferent types of solid physical structure are:
ie
vi
vii
viii
ix
x
ni
KBr
Al(OH)3
CuCO3
Mg3N2
PCl3
U
R
ev
ie
w
i
ii
iii
iv
v
The hexagonal shapes of snowflake crystals demonstrate
how simple molecules can combine to produce complex
and beautiful solid structures (Image C3.08). The regularity
of a snowflake suggests that the water molecules it contains
are arranged in an organised way. In general, there are three
basic units from which solids are constructed – atoms, ions
and molecules. These diferent particles produce a range
of structures in the solid state, which can be classified into
four broad types.
-R
Pr
es
s
-C
y
b What are the names of the compounds that
have the following formulae?
C
op
C3.06 Metals, alloys and
crystals
ge
a How many atoms of the diferent elements are
there in the formulae of these compounds?
i sodium hydroxide, NaOH
ii ethane, C2H6
iii sulfuric acid, H2SO4
iv copper nitrate, Cu(NO3)2
v sucrose (sugar), C12H22O11
am
br
id
C3.25
C
U
ni
op
y
C3: Elements and compounds
Copyright Material - Review Only - Not for Redistribution
235
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
am
br
id
ev
ie
w
ge
Substances that consist of simple molecules have
relativity low melting points and boiling points.
H
H
H
op
H
H
H
H
H
C
H
C
H
H
C
H
H
C
y
C
op
ni
-R
s
rs
op
y
ve
ni
Particular properties
brass
copper
70%
zinc
30%
harder than pure
copper; ‘gold’ coloured
copper
90%
harder than pure copper
tin
10%
iron
99.7% stronger and harder
0.3% than pure iron
es
Pr
ity
bronze
mild
steel
ni
ve
rs
w
Making alloys with other metals is one of the commonest
ways of changing the properties of metals. Alloys are
formed by mixing the molten metals together thoroughly
and then allowing them to cool and form a solid.
74%
op
C
U
e
18%
nickel
8%
tin
50%
lead
50%
ev
solder
-R
br
harder than pure iron;
does not rust
lower melting point than
either tin or lead
s
Table C3.06 Some important alloys.
es
am
carbon
stainless iron
steel
chromium
id
g
Alloying oten results in a metal that is stronger than the
original individual metals. ‘Silver’ coins are minted from
cupro-nickel alloy, which is much harder than copper itself.
Aluminium is a low-density metal that is not very strong.
-C
ie
Typical
composition
s
am
-C
op
y
C
Alloys
ev
Alloy
-R
br
ev
id
ie
w
ge
C
U
Strength is not the only property to think about when
designing an alloy. For example, solder is an alloy of tin
and lead. It is useful for making electrical connections
because its melting point is lower than that of either of
w
C
w
ie
ev
R
Figure C3.30 shows how the presence of the ‘impurity’
atoms makes it more dificult for the metal ions to slip over
each other. This makes the alloy stronger but more brittle
than the metals it is made from.
ity
op
Pr
The layers of identical ions in a pure metal can be
moved over one another without breaking the structure
(Figure C3.29). This flexibility in the layered structure
means that metals can be beaten or rolled into sheets
(they are malleable). Metals are more malleable when hot,
and steel, for instance, is rolled when hot. They can also be
stretched into wires (they are ductile). The strength of the
metallic bonds means that the metal does not easily break
under these forces. The bonds are strong but not rigid. This
means that metals generally have a high tensile strength.
The mobility of the delocalised electrons in a metal means
that metals conduct electricity very well. Copper is a
particularly good conductor, and most electrical wires
are made from it. For overhead power lines, aluminium is
used, as it is lighter. However, because aluminium is not
strong, a steel core has to be used.
When mixed with 4% copper and smaller amounts of
other elements, it gives a metal (duralumin) that combines
strength and lightness and is ideal for aircrat building.
Other examples of alloys and their properties are given in
Table C3.06.
ie
y
es
-C
am
br
ev
id
ie
w
ge
Figure C3.29 The layers in a metal lattice can slide over
each other.
y
Metal crystals
U
R
ev
ie
w
Figure C3.28 Simple molecular substances have low
melting points.
The idea of the regular packing of metal ions into a lattice
surrounded by a ‘sea’ of mobile electrons helps to explain
many of the physical properties of metals. In most metals,
the packing is as close as possible. This explains why
metals usually have a high density. In some metals the
ions are less closely packed. These metals, for example the
alkali metals, have the lowest densities of all metals. So,
lithium and sodium will float on water.
236
R
The layers of
atoms can carry
on slipping past
each other.
ve
rs
ity
y
H
H
C
Pr
es
s
-C
H
force
-R
This is because there are only weak forces between
the molecules. They don’t conduct electricity.
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
force
applied
here
w
Pr
es
s
y
w
ie
alloy
-R
rs
the two separate metals. Also, steel, which rusts when in
contact with oxygen and water, can be prevented from
doing so when alloyed with chromium and nickel. This
forms stainless steel (see Table C3.06).
y
op
ie
ev
-R
C
U
op
y
ni
ve
rs
Ionic compounds form lattices consisting of positive and
negative ions. In an ionic lattice, the nearest neighbours
of an ion are always of the opposite charge. Thus, in
sodium chloride, each sodium (Na+) ion is surrounded
by six chloride (Cl–) ions (Figure C3.31), and each Cl– ion
is surrounded by six Na+ ions. Overall, there are equal
numbers of Na+ and Cl– ions, so the charges balance.
id
g
w
e
Giant covalent crystals (macromolecules)
ev
ie
Giant covalent crystals are held together by strong covalent
bonds. This type of structure is shown by some elements
(such as carbon, in the form of diamond and graphite), and
also by some compounds (for example, SiO2).
es
s
-R
br
am
The actual arrangement of the ions in other compounds
depends on the numbers of ions involved and on
-C
Disruption of an ionic lattice is also brought about by
water. Many ionic compounds dissolve in water. Water
molecules are able to interact with both positive and
negative ions. When an ionic crystal dissolves, each ion
becomes surrounded by water molecules. This breaks
up the lattice and keeps the ions apart (Figure C3.33).
For those ionic compounds that do not dissolve in water,
the forces between the ions must be very strong.
Ions in solution are able to move, so the solution can
carry an electric current. Ionic compounds can conduct
electricity when dissolved in water. This is also true when
they are melted because, here again, the ions are able to
move through the liquid and carry the current.
ity
C
w
ie
s
es
Pr
op
y
Ionic crystals
ev
Ionic crystals are hard but much more brittle than metallic
crystals. This is a result of the structure of the layers. In a
metallic crystal, the ions are identical and held together
by the mobile electrons. This remains true if one layer is
slid against the next. However, pushing one layer against
another in an ionic crystal brings ions of the same charge
next to each other. The repulsions force the layers apart
(Figure C3.32).
w
ge
id
-C
am
br
It is important that you learn which elements are present
in certain alloys, such as brass, bronze, mild steel and
stainless steel, and you should be familiar with certain key
uses for each alloy. The syllabus gives uses for mild steel
(car bodies and machinery) and stainless steel (chemical
plant and cutlery) – make sure you are aware of these.
R
their sizes. However, it is important to remember that all
ionic compounds are electrically neutral.
C
U
R
ni
ev
ve
ie
w
C
ity
op
Pr
y
es
s
-C
am
br
Figure C3.30 a The positions of atoms in a pure metal
crystal before a force is applied. b Ater the force is applied,
slippage has taken place. The layers in a pure metal can
slide over each other. c In an alloy, slippage is prevented
because the atoms of diferent size cannot slide over
each other.
ev
ge
id
C
op
ni
force
applied
here
U
R
y
ve
rs
ity
op
C
ev
ie
w
c
TIP
intriguing alloys!
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
This activity consists of three sections, each of which
illustrates how the combination of metal elements into
an alloy results in useful and novel properties. The alloys
investigated are solder, Fields metal and nitinol.
A worksheet is included on the CD-ROM.
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pure metal
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b
ACtivity C3.03
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237
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chlorine atom
(Cl )
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e–
sodium ion
(Na+)
Cl –
Na+
unit cell
–
+
–
+
–
+
–
+
+
–
+
–
+
–
+
–
–
+
–
+
–
+
–
+
+
–
+
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–
+
+
–
–
+
+
–
–
+
diamond
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Graphite is a diferent form of carbon that does conduct
electricity (Table C3.07). The carbon atoms are arranged
in a diferent way in the molecular structure of graphite.
They are arranged in flat layers of linked hexagons
(Figure C3.34b). Each graphite layer is a two-dimensional
giant molecule. Within these layers, each carbon atom
is bonded to three others by strong covalent bonds.
Between the layers there are weaker forces of attraction.
The layers are able to slide over each other easily.
This means that graphite feels slippery and can be used
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C
U
The properties of diamond are due to the fact that the
strong covalent bonds extend in all directions through the
whole crystal. Each carbon atom is attached to four others
– the atoms are arranged tetrahedrally (Figure C3.34).
Diamond has a very high melting point and, because the
bonding extends throughout the whole structure, it is
very hard and is used in cutting tools. The bonds are rigid,
however, and these structures are much more brittle than
-C
giant metallic lattices. All the outer electrons of the atoms in
these structures are used to form covalent bonds. There are
no electrons free to move. Diamond is therefore a typical
non-metallic element. It does not conduct electricity.
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-C
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Figure C3.34 a The tetrahedral structure of diamond and
silicon(IV) oxide (silicon dioxide). b The layered structure
of graphite.
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how the layers fit together
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one layer
Figure C3.33 Water molecules form ‘shells’ around metal
(yellow) and non-metal (green) ions. This helps ionic
substances (like sodium chloride, NaCl) to dissolve in water.
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silicon(IV) oxide
Pr
y
Figure C3.32 In ionic crystals, when one layer is forced
to slide against another, repulsions cause the crystal
to fracture.
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–
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+
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–
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–
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+
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–
repulsion
+
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–
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+
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+
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–
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+
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force applied here
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Figure C3.31 The arrangement of the positive and negative ions in a sodium chloride crystal.
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sodium atom
(Na)
R
–
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gains one
electron
238
Cl
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chloride ion
–
(Cl )
loses one
electron
Na+
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C3: Elements and compounds
Appearance
colourless, transparent
crystals that sparkle in light
in jewellery and
ornamental objects
Hardness
the hardest natural
substance
in drill bits, diamond
sot – the layers can slide
saws and glass-cutters over each other – and
solid has a slippery feel
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conducts electricity
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does not conduct electricity
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as electrodes and
for the brushes in
electric motors
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Table C3.07 A comparison of the properties and uses of diamond and graphite.
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Molecular crystals
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A summary of the physical properties of the
diferent types of structure
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C3.27
How does molten sodium chloride conduct
electricity?
C3.28
Why does sodium chloride not conduct when
it is solid?
C3.29
Why can graphite:
a conduct electricity?
b be used as a lubricant?
Why is diamond much harder than graphite?
C3.31
Why do molecular crystals never conduct
electricity?
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C3.33
How is the structure of silicon(IV) oxide similar to
that of diamond?
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Why can metals conduct electricity?
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C3.32
Graphite can be used as a solid lubricant because
molecular layers in graphite can slide over each other.
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C3.30
e
U
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The electrical conductivity of graphite is explained in
terms of the mobile electrons not used in the bonding of
the layers. It is these ‘free’ electrons that are able to move
and carry the current, not those involved in the covalent
bonding of the layers.
C
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s
QuEStiONS
It is important that you can recognise the structures
of diamond and graphite if you are presented with the
diagrams in an exam question. Make sure that you can
describe the essential features of the two structures
and link them to the properties of the two forms.
So you should be able to explain the hardness of diamond
in terms of the strongly bonded three-dimensional
network of the structure.
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The properties of a substance can be related to the type
of structure it has. The four diferent types of structure are
summarised in Figure C3.35.
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TIP
Some non-metals (e.g. iodine and sulfur) and some
covalently bonded compounds exist as solids with low
melting points. In these crystals, molecules of these elements
or compounds are held together by weak intermolecular
forces to form a crystal that is easily broken down by heat.
The molecules are then free to move but, unlike the particles
in an ionic crystal, they have no charge. Neither the liquid nor
the solid forms of these substances conduct electricity.
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ity
The giant structures of diamond and silicon(IV) oxide are
very similar (Figure C3.34a). As a result, they show similar
physical properties. They are both very hard and have
high melting points. Sand and quartz are examples of
silica (silicon(IV) oxide or silicon dioxide, SiO2). The whole
structure of silicon and oxygen atoms is held together
throughout by strong covalent bonds.
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as a lubricant. Pencil ‘lead’ is, in fact, graphite. When we
write with a pencil, thin layers of graphite are let stuck to
the paper. The most distinctive property, however, arises
from the free electrons not used by the layered atoms in
covalent bonding. These electrons can move between
the layers, carrying charge, so that graphite can conduct
electricity in a similar way to metals.
R
in pencils and as
a lubricant
less dense than diamond
(2.25 g/cm3)
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Electrical
conductivity
Uses
dark grey, shiny solid
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more dense than graphite
(3.51 g/cm3)
Properties
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Uses
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Properties
Density
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Graphite
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Diamond
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Cambridge IGCSE Combined and Co-ordinated Sciences
Substances that consist of simple molecules have relatively low melting
points and boiling points.
This is because there are only weak forces between the molecules.
They don’t conduct electricity.
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H
Metals conduct heat and electricity because their structures contain
delocalised (free) electrons. The layers of atoms in metals are able to slide
over each other. This is why we can bend and shape metals.
H
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the ‘sharing’ of electrons between atoms to form
stable molecules
how covalent bonding produces two types of
structure – simple molecules and giant covalent
(macromolecular) structures
that electrostatic forces of attraction between positive
and negative ions are the basis of ionic bonding in
compounds between metals and non-metals
how the physical properties of a substance are related
to the type of bonding present
that diamond and graphite are two diferent forms of
carbon with diferent giant covalent structures and
distinctly diferent properties
that alloys can be made to show properties that are
adapted to a particular purpose; for example, strength
(steel), resistance to corrosion (stainless steel) or low
melting point
about metallic bonding in which the closely packed
metal atoms lose their outer electrons into a ‘sea’ of
mobile electrons
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■
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■
H
H
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■
H
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how the Periodic Table lists the elements of the
Universe in order of increasing proton number
about the diferent characteristics of metallic and nonmetallic elements
how the Periodic Table is divided into vertical groups
and horizontal periods, with clear trends in properties
as we move down a group or across a period
that certain groups, such as the alkali metals (Group I)
and the halogens (Group VII), have distinctive
names and contain the most reactive metals and
non-metals respectively
how the structures of all substances are made up of
atoms, ions or molecules
about the three main types of bonding that hold these
structures together:
●
metallic bonding
●
ionic bonding
●
covalent bonding
about covalent bonding, which occurs in some
elements and non-metallic compounds and involves
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You should know:
■
H
H
C
Compounds made from ions are called ionic compounds. The ions are
arranged in a giant lattice. Ionic compounds have very high melting
points and boiling points.
When they are dissolved in water or melted, they can conduct electricity.
This is because their ions are free to move about and carry the current.
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Summary
■
H
H
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Figure C3.35 Summary of the diferent types of structure.
240
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Atoms that share electrons can form giant covalent structures called
macromolecules. These have very high melting points because their
atoms are linked together with strong covalent bonds.
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copper
sulfur
Use of element
id
Element
chlorine
potassium
U
aluminium
helium
sterilising drinking water
Melting
point/ °C
Electrical
conductivity
Reaction
with water
Pr
A
–39
B
none
–220
very low
reacts quickly
C
–112
very low
none
D
181
high
reacts quickly
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high
241
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Element
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The following table shows properties of four elements A, B, C and D.
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C
Use the information in the table to suggest which of the elements A, B, C and D could be:
i non-metals,
ii an element in Group 0 of the Periodic Table,
iii an element in Group I of the Periodic Table.
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q1 a & b June 2014]
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B
N
O
F
Ne
Pr
Be
[1]
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To which period of the Periodic Table do these elements belong?
Answer these questions using only the elements shown in the diagram. Each element can be
used once, more than once or not at all.
Write down the symbol for the element which:
i has six electrons in its outer shell
ii is a halogen
iii is a metal which reacts rapidly with cold water
iv has two forms, graphite and diamond
v is in Group II of the Periodic Table
vi makes up about 80% of the air.
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Li
ev
[1]
[1]
[1]
The diagram below shows the elements in a period of the Periodic Table.
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2
R
[3]
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b
making food containers
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filling weather balloons
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Select elements from the list below to complete the let-hand column in the table shown.
Each element may be used once, more than once or not at all.
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1
about the diferences in structure and properties
between simple molecular and giant molecular
covalent structures.
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End-of-chapter questions
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■
how the closely packed structure of metals can explain
the characteristic properties of metals, and how one
metal can strengthen another when the two form an alloy
about the nature of ionic lattices and how it gives rise
to the properties of salts
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C3: Elements and compounds
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[6]
(continued)
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Cambridge IGCSE Combined and Co-ordinated Sciences
atoms
electrons
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[2]
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Pr
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s
[Cambridge IGCSE Chemistry 0620 Paper 21 Q1 November 2010]
An element is a substance that is made of atoms which have the same proton number.
Most atoms contain protons, neutrons and electrons.
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a
b
i
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Q 2,8
U
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P 2,8,1
R 2,7
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Use the electronic structures to state and explain the group numbers in the Periodic Table
that contain elements P, Q and R.
P
Group
Q
Group
R
Group
explanation
ii State and explain which of the elements, P, Q or R, is the least reactive.
iii State and explain which one of the elements, P, Q or R, is a good conductor of electricity.
ity
A
2,5
B
2,8,4
C
2,8,8,2
D
2,8,18,8
E
2,8,18,8,1
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2,8,18,18,7
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Choose an element from the list for each of the following descriptions.
i It is a noble gas.
ii It is a sot metal with a low density.
iii It can form a covalent compound with element A.
iv It has a giant covalent structure similar to diamond.
v It can form a negative ion of the type X3−.
Elements C and F can form an ionic compound.
i Draw a diagram that shows the formula of this compound, the charges on the ions and
the arrangement of the valency electrons around the negative ion. Use o to represent an
electron from an atom of C. Use × to represent an electron from an atom of F.
ii Predict two properties of this compound.
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[3]
[2]
ev
[Cambridge IGCSE Chemistry 0620 Paper 31 Q3 June 2009]
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[5]
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Electron distribution
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Element
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The following is a list of the electron distributions of atoms of unknown elements.
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4
[2]
[1]
[1]
[Cambridge IGCSE Combined Science 0653 Paper 22 Q2 a & b June 2012]
C
242
[1]
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Name the element whose atoms do not usually contain any neutrons.
The electronic structures (configurations) of atoms of three elements, P, Q and R,
are shown below.
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protons
of the elements in the Periodic Table are arranged in order of increasing
op
3
neutrons
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The
number of
molecules
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Write out and complete the following sentence using words from the list below.
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III
IV
V
Y
Z
The element represented by X is a solid at room temperature and the elements represented
by Y and Z are gases.
i Suggest one diference, other than physical state at room temperature, between
the properties of elements X and Y.
ii Suggest one diference between the chemical properties of elements Y and Z.
Sodium chloride is a compound of the alkali metal sodium and the halogen chlorine.
i Explain why the elements sodium and chlorine are only ever found combined with other
elements in the Earth’s crust.
ii Describe the changes in electron configuration when sodium atoms (2,8,1) react with
chlorine atoms (2,8,7) to form sodium chloride.
[1]
[2]
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Pr
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[1]
[1]
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Pr
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C
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VII 0
X
Period 2
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VI
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II
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The table below shows Period 2 of the Periodic Table.
I
b
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a
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id
5
C
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C3: Elements and compounds
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■
the diferences between physical and chemical changes
how to write word and chemical equations
the diferent types of chemical reaction
the definition of oxidation and reduction
how to use state symbols in an equation
the writing of ionic equations
electricity and chemistry – conductivity of metals
electrolysis
oxidising agents, reducing agents and redox reactions.
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This chapter covers:
■
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244
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C4
Chemical reactions
Physical change
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But what is a chemical reaction? How does it difer from a
simple physical change?
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C
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The Chinese character for chemistry literally means
‘change study’ (Figure C4.01). Chemistry deals with how
substances react with each other. Chemical reactions
range from the very simple through to the interconnecting
reactions that keep our bodies alive.
-C
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Ice, snow and water may look diferent, but they are all
made of water molecules (H2O). They are diferent physical
forms of the same substance – water – existing under
diferent conditions of temperature and pressure. One
form can change into another if those conditions change.
In such changes, no new chemical substances are formed.
Dissolving sugar in ethanol or water is another example
of a physical change. It produces a solution, but the
substances can easily be separated again by distillation.
ni
ve
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C
ity
C4.01 Chemical reactions and
equations
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Figure C4.01 The Chinese symbols for ‘change’.
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C4: Chemical reactions
This is what we know about physical changes:
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In a physical change, the substances present remain
chemically the same: no new substances are formed.
■ Physical changes are oten easy to reverse. Any
mixtures produced are usually easy to separate.
■
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br
am
The major feature of a chemical change, or reaction, is
that new substance(s) are made during the reaction.
■ Many reactions, but not all of them, are dificult to
reverse.
■ During a chemical reaction, energy can be given
out or taken in:
●
when energy is given out, the reaction
is exothermic
●
when energy is taken in, the reaction
is endothermic.
■ There are many more exothermic reactions than
endothermic reactions.
■
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This is what we know about chemical changes:
Image C4.01 Magnesium burns strongly in oxygen.
-C
Image C4.02 Glow-in-the-dark bracelets. Glow
bracelets are single-use, see-through, plastic tubes
containing isolated chemicals. When the tube is
squeezed, a glass partition keeping the chemicals
apart breaks, and a reaction takes place that
produces chemiluminescence.
During the reaction between nitrogen and oxygen to
make nitrogen monoxide, heat energy is taken in from
the surroundings. The reaction is an endothermic
change. Such reactions are much less common than
exothermic ones.
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R
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When magnesium burns in oxygen (Image C4.01), the
white ash produced is a new substance – the compound,
magnesium oxide. Burning magnesium produces a
brilliant white flame. Energy is given out in the form of
heat and light. The reaction is an exothermic change.
The combination of the two elements, magnesium
and oxygen, to form the new compound is dificult to
reverse. Some other chemical reactions, such as those
in fluorescent ‘glow bracelets’ (Image C4.02), produce
chemiluminescence. They give out energy in the
form of light.
s
am
Chemical change
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C4.01
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QUESTIONS
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Cambridge IGCSE Combined and Co-ordinated Sciences
State whether the following changes are physical
or chemical:
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a the melting of ice
c the sublimation of solid carbon dioxide
C
C4.02
State whether the following changes are
exothermic or endothermic:
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d the dissolving of sugar in water.
Pr
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b the burning of magnesium
w
a the condensation of steam to water
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b the burning of magnesium
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c the addition of concentrated sulfuric acid
to water
ge
d the evaporation of a volatile liquid.
C4.03
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What is the most important thing that shows us
that a chemical reaction has taken place?
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Image C4.03 The decomposition of ammonium
dichromate – the ‘volcano reaction’ – produces heat, light
and an apparently large amount of powder.
rs
op
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C
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Note that, although a large amount of energy is produced
in this reaction, it is not included in the equation.
An equation includes only the chemical substances
involved, and energy is not a chemical substance.
-R
This type of equation gives us some information. But
equations can be made even more useful if we write
them using chemical formulae.
s
es
Pr
We can write out descriptions of chemical reactions, but
these would be quite long. To understand and group
similar reactions together, it is useful to have a shorter way
of describing them. The simplest way to do this is in the
form of a word equation.
ity
Balanced symbol equations
From investigations of a large number of diferent chemical
reactions, a very important point about all reactions has
been discovered. It is summed up in a law, known as the
law of conservation of mass.
op
y
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ve
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C
w
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C
U
This type of equation links together the names of the
substances that react (the reactants) with those of
the new substances formed (the products). The word
equation for burning magnesium in oxygen would be:
KEy tERM
ev
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product
law of conservation of mass: the total mass of all the products
of a chemical reaction is equal to the total mass of all the reactants
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reactants
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magnesium oxide
am
magnesium + oxygen
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ev
water
w
ge
hydrogen + oxygen
br
am
-C
op
y
Word equations
R
The reaction between hydrogen and oxygen is another
highly exothermic reaction. The reaction has been used to
fuel rockets, most notably the now-retired Space Shuttle.
Large tanks beneath the Shuttle contained liquid hydrogen
and oxygen. In 1986, cracked rubber seals on the fuel tanks
of the shuttle Challenger caused a catastrophic explosion
and loss of life. The word equation for this reaction is:
y
ve
w
ie
ev
R
s
ity
op
When some chemical reactions occur, it is obvious that
‘something has happened’. But this is not the case for
others. When a solid explosive reacts to produce large
amounts of gas products, the rapid expansion may
blast the surroundings apart. The volcano reaction, in
which ammonium dichromate is decomposed, gives out
a large amount of energy and produces nitrogen gas
(Image C4.03). Other reactions produce gases much less
violently. The neutralisation of an acid solution with an
alkali produces no change that you can see. However, a
reaction has happened. The temperature of the mixture
increases, and new substances have formed which can be
separated and purified.
C
246
es
Pr
y
-C
C4.02 Equations for chemical
reactions
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b
ev
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H
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a
C
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C4: Chemical reactions
H
Pr
es
s
-C
y
op
ev
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C
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Image C4.04 a A balloon filled with hydrogen and oxygen
b is ignited spectacularly.
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id
-R
s
es
Pr
WORKED EXAMPLE C4.01
ity
rs
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ni
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y
water
Step 2: From this you can write out the word equation:
U
R
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C
magnesium + oxygen
w
ie
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id
-R
s
es
Pr
MgO
Remember that oxygen exists as diatomic
molecules. This equation is not balanced:
there are two oxygen atoms on the let, but
only one on the right.
Step 4: Balance the equation:
ity
2MgO
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ve
rs
id
g
w
e
C
U
op
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We cannot alter the formulae of the substances involved
in the reaction. These are fixed by the bonding in the
substance itself. We can only put multiplying numbers in
front of each formula where necessary.
ev
ie
Chemical reactions do not only involve elements reacting
together. In most reactions, compounds are involved.
For example, potassium metal is very reactive and gives
hydrogen gas when it comes into contact with water.
es
s
-R
br
am
Figure C4.02 The reaction between hydrogen and
oxygen molecules.
-C
Mg + O2
2Mg + O2
This is a balanced equation. The numbers of each type
of atom are the same on both the reactant side and the
H
magnesium oxide
Step 3: Write out the equation using the formulae of
the elements and compounds:
br
am
-C
op
y
C
w
ie
ev
R
Figure C4.03 Summary of the reaction between hydrogen
and oxygen.
Step 1: Make sure you know what the reactants and
products are. For example, magnesium burns
in air (oxygen) to form magnesium oxide.
The symbol equation for the reaction between hydrogen
and oxygen is therefore written:
2H2O
water
2H2O
hydrogen + oxygen
2H2 + O2
What is the balanced equation for the reaction
between magnesium and oxygen?
Each molecule of water (formula H2O) contains only one
oxygen atom (O). It follows that one molecule of oxygen
(O2) has enough oxygen atoms to produce two molecules
of water (H2O). Therefore, two molecules of hydrogen
(H2) will be needed to provide enough hydrogen atoms
(H) to react with each oxygen molecule. The numbers of
hydrogen and oxygen atoms are then the same on both
sides of the equation.
2H2 + O2
H
H
A balanced equation gives us more information about a
reaction than we can get from a simple word equation.
Below is a step-by-step approach to working out the
balanced equation for a reaction.
br
ev
hydrogen + oxygen
H
Writing balanced equations
am
-C
y
op
C
w
ie
Look more closely at Figure C4.02:
O
C
op
ni
U
R
This important law becomes clear if we consider what
is happening to the atoms and molecules involved in a
reaction. During a chemical reaction, the atoms of one
element are not changed into those of another element.
Nor do atoms disappear from the mixture, or appear
from nowhere. A reaction involves the breaking of some
bonds between atoms, and then the making of new
bonds between atoms to give the new products. During
a chemical reaction, some of the atoms present ‘change
partners’, sometimes spectacularly (Image C4.04).
O
O
product side of the equation: four hydrogen atoms and
two oxygen atoms on each side (Figure C4.03).
No matter how spectacular the reaction, this statement is
always true – though it is easier to collect all the products
in some cases than in others!
O
H
O
-R
H
H
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247
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a …Cu + O2
ni
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e …Al + …Cl2
ev
f
-R
C4.03 types of chemical
reaction
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op
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It is possible to distinguish reactions in which complex
compounds are built from simpler substances
(synthesis) from those where the reverse happens
(decomposition).
-R
s
es
ity
Pr
op
y
Synthesis (or direct combination) reactions occur where
two or more substances react together to form just one
product. The reaction between iron and sulfur is an
example of this (Image C4.06):
ni
ve
rs
C
iron + sulfur
Fe +
FeS
ie
id
g
w
e
Heat is required to start the reaction but, once started,
it continues exothermically.
Most synthesis reactions are exothermic. However,
there is one very important synthesis reaction,
which is endothermic: namely photosynthesis.
es
s
-R
br
ev
The only things that you can change when balancing are
the numbers in front of the formulae.
am
iron(II) sulfide
C
U
It is important to remember that you cannot change the
formulae of the substances themselves when balancing
equations. These are fixed by the nature of the atoms and
their bonding.
-C
S
op
w
ie
ev
…Fe3O4 + …H2
Synthesis and decomposition
br
am
-C
2KOH + H2
This equation is now balanced. Check for yourself that
the numbers of the three types of atom are the same on
both sides.
R
…Fe + …H2O
s
es
Pr
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ni
U
This symbol equation needs to be balanced. An even
number of H atoms is needed on the product side,
because on the reactant side the hydrogen occurs as H20.
Therefore, the amount of KOH must be doubled. Then the
number of potassium atoms and water molecules must be
doubled on the let:
TIP
Na2SO4 + …H2O
…AlCl3
There are very many diferent chemical reactions.
To make sense of them, it is useful to try to group
certain types of reaction together. These types do
not cover all reactions; and some reactions, such as
redox reactions, may fit into more than one category.
Organic reactions such as polymerisation have been
let until later chapters.
KOH + H2
2K + 2H2O
…Na2O
d …NaOH + H2SO4
br
w
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K + H2O
…NH3
c …Na + O2
potassium hydroxide + hydrogen
Then:
…CuO
C
op
b N2 + …H2
am
-C
y
op
C
potassium + water
Copy out and balance the following equations:
y
op
C
w
ev
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R
Therefore:
248
a Iron rusts because it reacts with oxygen in the
air to form a compound called iron(III) oxide.
c Sodium reacts strongly with water to give a
solution of sodium hydroxide; hydrogen gas is
also given of.
C4.05
Potassium reacts with water to produce potassium
hydroxide and hydrogen (Image C4.05). All the alkali metals
do this. So, if you know one of these reactions, you know
them all. In fact, you could learn the general equation:
metal hydroxide + hydrogen
Write word equations for the reactions described
below.
b Sodium hydroxide neutralises sulfuric acid to
form sodium sulfate and water.
Pr
es
s
-C
-R
C4.04
y
am
br
id
ev
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w
QuEStiONS
Image C4.05 Potassium reacts strongly with water to
produce hydrogen.
alkali metal + water
C
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U
ni
op
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Cambridge IGCSE Combined and Co-ordinated Sciences
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C
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C4: Chemical reactions
w
ge
Neutralisation and precipitation
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ev
id
br
-R
s
Pr
C6H12O6 +
This type of reaction can be used to prepare insoluble
salts and is also the basis for many analytical tests for both
metal cations and non-metal anions (see Chapter C12).
The limewater test for carbon dioxide depends on
precipitation. Here the insoluble product is calcium
carbonate (Image C4.07). A milky suspension of insoluble
calcium carbonate is formed:
6O2
rs
y
ve
ev
CaO
+
CO2
ev
+ H2O
es
-C
CaCO3
lime
heat
CaCO3
s
am
calcium oxide + carbon dioxide
CO2 + Ca(OH)2
-R
br
id
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w
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C
U
R
ni
op
Decomposition reactions have just one reactant,
which breaks down to give two or more simpler
products. Lime for agriculture and for making cement
is manufactured industrially by the decomposition of
limestone (calcium carbonate):
limestone
precipitation: the sudden formation of a solid, either when
two solutions are mixed or when a gas is bubbled into a
solution
glucose + oxygen
ity
op
C
w
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es
-C
am
KEy tERM
The green pigment chlorophyll is essential for this
reaction because it traps energy from the Sun.
heat
+ water
w
ge
U
R
y
sunlight
chlorophyll
+ 6H2O
calcium carbonate
salt
Precipitation reactions involve the formation of an
insoluble product.
This reaction is essential for life on Earth. It takes
place in the green leaves of plants and requires energy
from sunlight. It is a photochemical reaction. Small
molecules of carbon dioxide and water are used to make
the larger molecule glucose:
6CO2
CuSO4 + H2O
C
op
y
acid + base
Image C4.06 The synthesis reaction between
iron and sulfur.
carbon dioxide + water
Neutralisation reactions involve acids. When acids react
with bases or alkalis, their acidity is destroyed. They are
neutralised and a salt is produced. Such reactions are
known as neutralisation reactions. An example is:
H2SO4 + CuO
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ev
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C
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Pr
es
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-C
-R
am
br
id
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Salts are a useful type of chemical compound that we will
meet in detail in Chapter C5. A few salts, mainly chlorides,
bromides and iodides, can be made by synthesis (direct
combination) as mentioned above. The majority have to
be made either by neutralisation or by precipitation.
+
Cl2
y
op
2Ag
C
chlorine
w
+
-R
Image C4.07 Calcium carbonate is precipitated from
limewater by carbon dioxide.
s
es
am
br
Silver bromide and silver iodide behave in the same way.
These photochemical reactions are the basis of film used
in non-digital photography and movie-making.
ie
id
g
2AgCl
silver
ev
light
U
light
e
silver chloride
ni
ve
rs
Decomposition can also be caused by light energy.
For example, silver chloride, a white solid, turns grey in
sunlight because silver metal is formed:
-C
R
ev
ie
w
C
ity
Pr
op
y
These reactions are endothermic. They require heat
energy. Decomposition caused by heat energy is called
thermal decomposition.
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249
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mainly methane. Its complete combustion produces
carbon dioxide and water vapour:
am
br
id
ev
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w
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a
C
U
ni
op
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Cambridge IGCSE Combined and Co-ordinated Sciences
-R
methane + oxygen
ni
C
op
y
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Our bodies need energy to make the reactions that
take place in our cells possible. These reactions
allow us to carry out our everyday activities. We need
energy to stay alive. We get this energy from food.
During digestion, food is broken down into simpler
substances. For example, the carbohydrates in rice,
potatoes and bread are broken down to form glucose.
The combustion of glucose with oxygen in the cells of
our body provides energy:
U
ie
w
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-R
glucose + oxygen
es
Pr
rs
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ni
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C
This reaction is exothermic and is known by a special
name: respiration.
In combustion reactions, the substance involved is
oxidised. Oxygen is added and oxides are formed. Not all
reactions with oxygen produce a great amount of energy.
For example, when air is passed over heated copper, the
surface becomes coated with black copper(II) oxide. There
is no flame, nor is the reaction very exothermic. But it is
still an oxidation reaction (Figure C4.04a):
ev
id
-R
s
es
Pr
op
y
ity
copper
copper(II) oxide
O2
heat
2CuO
+
y
C
e
id
g
w
Combustion reactions are of great importance and can be
very useful or destructive.
ev
ie
copper(II) oxide + hydrogen
br
heat
copper + water
s
-R
During this reaction, the copper(II) oxide is losing oxygen.
The copper(II) oxide is undergoing reduction – it is losing
es
am
heat
op
U
Combustion, oxidation and reduction
-C
oxygen
This process can be reversed, and the copper surface
regenerated, if hydrogen gas is passed over the heated
material. The black coating on the surface turns pink as the
reaction takes place (Figure C4.04b):
2KCl + I2
The combustion of natural gas is an important source
of energy for homes and industry. Natural gas is
+
2Cu
ni
ve
rs
C
w
ie
+ 6H2O
burning: combustion in which a flame is produced
The halogens can be placed in order of reactivity
using displacement reactions. Thus, chlorine gas will
displace iodine from potassium iodide solution. The
colourless solution turns yellow-brown as iodine appears
(Image C4.08b):
Cl2 + 2KI
6CO2
combustion: the reaction of a substance with oxygen causing
the release of energy; it is exothermic and oten involves a flame
br
am
-C
ZnSO4 + Cu
6O2
KEy tERMS
ity
C
Zn + CuSO4
carbon dioxide + water
s
C6H12O6 +
Displacement reactions are useful in working out the
patterns of reactivity of elements of the same type.
A displacement reaction occurs because a more reactive
element will displace a less reactive one from a solution of
one of its compounds.
w
ie
ev
R
ev
id
br
am
-C
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op
Displacement reactions
ev
+ 2H2O
Pr
es
s
-C
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op
C
Image C4.08 Displacement
reactions. a Zinc will displace
copper from copper(II) sulfate
solution, and the colour of the
solution fades as the copper
forms on the zinc surface.
b Chlorine displaces iodine
from a potassium iodide
solution. The colourless
solution turns yellow-brown.
Zinc is a more reactive metal than copper. If a piece of
zinc is placed in a copper(II) sulfate solution, a red-brown
deposit of copper forms on the zinc (Image C4.08a).
The blue colour of the copper(II) sulfate solution fades.
Zinc displaces copper from copper(II) sulfate solution:
R
CO2
2O2
Substances such as methane, which undergo combustion
readily and give out a large amount of energy, are known
as fuels.
w
ev
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b
R
250
+
CH4
carbon dioxide + water
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copper powder
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C
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C4: Chemical reactions
Pr
es
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heat
b
ni
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excess
hydrogen
burning
C
op
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hydrogen in
y
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black copper(II) oxide
C
op
y
-C
-R
air in
w
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heat
-R
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br
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Figure C4.04 a The oxidation of copper to copper(II) oxide. b The reduction of copper(II) oxide back to
copper using hydrogen.
s
Pr
Cu + H2O
Reduction is very important in industry as it provides
a way of extracting metals from the metal oxide ores
that occur in the Earth’s crust. A good example is the
blast furnace for extracting iron from hematite (Fe2O3)
(Chapter C9).
y
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reduction
ni
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C
U
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op
ev
If a substance gains oxygen during a reaction,
it is oxidised.
■ If a substance loses oxygen during a reaction,
it is reduced.
■
op
y
es
s
-C
am
Notice that the two processes of oxidation and reduction
take place together during the same reaction. This is
true for a whole range of similar reactions. Consider the
following reaction:
zinc + carbon monoxide
y
op
U
Again, in this reaction, the two processes occur together.
Since oxidation never takes place without reduction,
it is better to call these reactions oxidation–reduction
reactions or redox reactions.
e
TIP
ev
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id
g
w
Remember that, in the process of acting as a reducing
agent, that substance will itself be oxidised. The reducing
agent will gain the oxygen it is removing from the other
compound. The reverse is true for an oxidising agent.
es
s
-R
br
In this last example, carbon removes oxygen from
zinc oxide. Carbon is an example of a reducing agent.
am
oxidising agent: a substance that will add oxygen to another
substance. The commonest oxidising agents are oxygen (or air),
hydrogen peroxide, potassium manganate(VII) and potassium
dichromate(VI).
Zn + CO
reduction
-C
ev
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ZnO + C
KEy tERM
ni
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C
oxidation
ity
Pr
zinc oxide + carbon
R
-R
br
ev
Some substances are capable of giving oxygen to others.
These substances are known as oxidising agents.
C
C
heat
reducing agent: an element or compound that will remove
oxygen from other substances. The commonest reducing
agents are hydrogen, carbon and carbon monoxide.
ity
op
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oxidation
CuO + H2
KEy tERM
es
-C
oxygen and being reduced. The hydrogen is gaining
oxygen. It is being oxidised:
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251
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Cambridge IGCSE Combined and Co-ordinated Sciences
There are two common examples of oxidation reactions
that we might meet in our everyday lives.
w
ev
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So far, our equations have told us nothing about the
physical state of the reactants and products.
y
C
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op
Chemical equations can be made more useful by including
symbols that give us this information. These are called
state symbols. They show clearly whether a gas is given
of or a solid precipitate is formed during a reaction.
The four symbols used are shown in Table C4.01.
• Rancidity. Oxidation also has damaging efects on
food. When the fats and oils in butter and margarine
are oxidised, they become rancid. Their taste and smell
change and become very unpleasant. Manufacturers
sometimes add antioxidants to fatty foods and oils to
prevent oxidation. Keeping foods in a refrigerator can slow
down the oxidation process. Storage in airtight containers
also helps. Crisp (chip) manufacturers fill bags of crisps
with nitrogen to prevent the crisps being oxidised.
C
op
y
The following examples show how they can be used. They
can show clearly when a gas or a precipitate is produced
in a reaction (the points of particular interest are shown in
bold type). Note that, when water itself is produced in a
reaction, it has the symbol (l) for liquid, not (aq).
id
ie
w
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ni
w
-R
s
Pr
rs
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ni
C
U
Ionic equations
decomposition
combustion
The last two examples above are useful for showing a
further modification in writing equations. This modification
identifies more clearly those particles that are actually
taking part in a particular reaction. These two reactions
involve mixing solutions that contain ions. Only some of
the ions present actually change their status – by changing
either their bonding or their physical state. The other ions
present are simply spectator ions to the change; they do
not take part in the reaction.
w
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br
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neutralisation
oxidation–reduction (redox)
-R
Pr
ity
c magnesium + copper oxide
magnesium oxide + copper
ni
ve
rs
d hydrochloric acid + sodium hydroxide
sodium chloride + water
Meaning
s
solid
e
b magnesium and steam
l
liquid
g
d bromine and potassium iodide solution
aq
op
aqueous solution, i.e. dissolved in water
s
Table C4.01 The state symbols used in chemical equations.
es
am
e zinc and copper sulfate solution.
gas
-R
br
id
g
c calcium and oxygen
C
U
a sodium and water
Symbol
y
Write word and balanced chemical equations for
the reactions between:
-C
ie
w
C
op
y
es
b calcium carbonate
calcium oxide + carbon dioxide
s
am
-C
carbon dioxide + water
w
R
Cu(OH)2(s) + Na2SO4(aq)
Some types of chemical reaction are listed below.
a hexane + oxygen
ev
NaCl(aq) + H2O(l)
ie
w
ie
ev
CuSO4(aq) + 2NaOH(aq)
Which reaction type best describes the
following changes?
R
Mg(NO3)2(aq) + H2(g)
hydrochloric acid + sodium hydroxide
sodium chloride + water
b Explain why an aqueous solution of iodine
does not react with potassium chloride.
C4.08
Mg(s) + 2HNO3(aq)
copper(II) sulfate + sodium hydroxide
copper(II) hydroxide + sodium sulfate
chlorine + potassium bromide
C4.07
magnesium nitrate + hydrogen
HCl(aq) + NaOH(aq)
a Copy and complete the word equation for
the reaction of chlorine with aqueous
potassium bromide.
ity
op
C
252
es
The halogens are a group of elements showing
trends in colour, state and reaction with other
halide ions.
y
C4.06
-C
QuEStiONS
am
br
ev
magnesium + nitric acid
ev
ev
ie
R
State symbols
Pr
es
s
-C
• Corrosion. If a metal is reactive, its surface may be attacked
by air, water or other substances around it. The efect
is called corrosion. When iron or steel slowly corrodes
in damp air, the product is a brown, flaky substance we
call rust. Rust is a form of iron(III) oxide. Rusting weakens
structures such as car bodies, iron railings, ships’ hulls and
bridges. Rust prevention is a major economic cost.
-R
am
br
id
ge
C4.04 A closer look at reactions,
particularly redox reactions
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Redox reactions
w
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a
C
U
ni
op
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C4: Chemical reactions
op
y
Pr
es
s
-C
-R
am
br
id
ev
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Chemists’ ideas about oxidation and reduction have
expanded as a wider range of reactions have been studied.
Look again at the reaction between copper and oxygen:
y
ie
w
ge
id
w
ev
-R
s
es
Pr
ity
rs
C
y
op
ni
C
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ev
-R
This new definition of redox changes increases the
number of reactions that can be called redox reactions.
For instance, displacement reactions where there is no
transfer of oxygen are now included. This is best seen by
looking at an ionic equation. For example:
y
for the precipitation of copper(II) hydroxide, which was
given above, becomes:
U
op
Zn(s) + CuSO4(aq)
C
w
e
ev
ie
id
g
reduction
Zn(s) + Cu2+(aq)
oxidation
es
s
-R
br
am
ZnSO4(aq) + Cu(s)
As an ionic equation this becomes:
Cu(OH)2(s)
This is the essential ionic equation for the precipitation
of copper(II) hydroxide; the spectator ions (sulfate and
sodium ions) have been let out.
-C
Oxidation is the Loss of electrons
Reduction is the Gain of electrons
s
es
Pr
ity
ni
ve
rs
Cu(OH)2(s) + Na2SO4(aq)
Cu2+(aq) + 2OH–(aq)
OiL RiG
w
ge
id
br
am
op
y
C
w
ie
ev
R
Oxidation is the loss of electrons.
Reduction is the gain of electrons.
We can remember this by using the memory aid ‘OIL RIG’:
ve
ie
ev
R
-C
H2O(l)
Applying the same principles to a precipitation reaction
again gives us a clear picture of which ions are reacting
(Figure C4.05).
CuSO4(aq) + 2NaOH(aq)
2CuO
A new, broader definition of oxidation and reduction
can now be put forward.
■
The use of state symbols clearly shows which ions
have not changed during the reaction. They have been
crossed out (like this) and can be let out of the equation.
This leaves us with the essential ionic equation for all
neutralisation reactions:
The equation:
O2
+
It is clear that copper has been oxidised; but what has
been reduced? We can apply the ideas behind ionic
equations to analyse the changes taking place during this
reaction. It then becomes clear that:
■
[H+(aq) + Cl−(aq)] + [Na+(aq) + OH−(aq)]
[Na+(aq) + Cl−(aq)] + H2O(l)
H+(aq) + OH–(aq)
copper(II) oxide
heat
The copper atoms, which clearly were oxidised during the
reaction, have in the process lost electrons. The oxygen
atoms have gained electrons in the process.
br
Writing out all the ions present, we get:
heat
• the oxygen molecules in the gas have split and become
oxide ions (O2–) in the black solid copper(II) oxide.
am
op
y
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The equation given above for neutralising hydrochloric
acid with sodium hydroxide solution is:
NaCl(aq) + H2O(l)
oxygen
C
op
U
R
Figure C4.05 A precipitation reaction in which
two solutions containing ions are mixed: a the overall
reaction, and b the net reaction with the spectator
ions not shown.
HCl(aq) + NaOH(aq)
2Cu
+
• the copper atoms in the metal have become copper
ions (Cu2+) in copper(II) oxide
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b
copper
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Zn2+(aq) + Cu(s)
253
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Cambridge IGCSE Combined and Co-ordinated Sciences
Zinc has lost two electrons and copper has gained them.
This reaction is a redox reaction as there has been both
loss and gain of electrons by diferent elements during
the reaction.
2Cl−(aq) + I2(aq)
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ev
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The ability to conduct electricity is the major simple
diference between elements that are metals and
elements that are non-metals. All metals conduct
electricity, but carbon in the form of graphite is the only
non-metallic element that conducts electricity. A simple
circuit can be used to test whether any solid conducts
or not (Figure C4.06). The circuit is made up of a battery
(a source of direct current), some connecting copper
wires fitted with clips, and a light bulb to show when a
current is flowing. The material to be tested is clipped into
the circuit. If the bulb lights up, then the material is an
electrical conductor.
C
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From the ionic equation we can see that chlorine atoms
have gained electrons to become chloride ions. They have
been reduced. The iodide ions have lost electrons to form
iodine. They have been oxidised.
ie
ev
Explain the meaning of the symbols (s), (l),
(aq) and (g) in the following equation, with
reference to each reactant and product:
s
Pr
i
silver nitrate solution + sodium chloride
solution
silver chloride + silver nitrate solution
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254
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b Write an ionic equation, including state
symbols, for each of the following reactions:
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Na2CO3(s) + 2HCl(aq)
2NaCl(aq) + H2O(I) + CO2(g)
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a
am
C4.09
id
QUESTIONS
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Conductivity in solids – conductors
and insulators
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Cl2(aq) + 2I−(aq)
Pr
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It is on the basis of this definition that chlorine, for
instance, is a good oxidising agent. It displaces iodine
from potassium iodide solution (see Image C4.08). Is this
reaction a redox reaction?
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am
br
id
ge
of the large-scale supply of electricity can be seen in
the pylons and power lines that mark our landscape.
But electricity is also important on the very small scale.
The silicon chip enables a vast range of products to work,
and many people now have access to products containing
electronic circuits – from MP3 players to washing machines.
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C
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ii sodium sulfate solution + barium nitrate
solution
sodium nitrate solution + barium
sulfate
s
Pr
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electrons repelled into
wire from negative
–
terminal of battery
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bulb
s
-R
Figure C4.06 Testing a solid material to see if it conducts
electricity, by whether it lights a bulb.
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carbon rod
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C4.05 Electrolysis
Electricity has had a great efect on our way of living.
Large urban areas, such as Hong Kong, could not
function without the electricity supply. The results
e–
battery
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e–
+
electrons attracted
to positive terminal
of battery
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C
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is the gain of electrons;
is the loss of electrons. During a redox reaction
the oxidising agent
electrons; the
oxidising agent is itself
during
the reaction.
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Copy and complete the following statement:
op
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C4.10
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iii dilute hydrochloric acid + potassium
hydroxide solution
potassium chloride solution + water
iv dilute hydrochloric acid + copper
carbonate
copper chloride solution + water
+ carbon dioxide
For a solid to conduct, it must have a structure that
contains ‘free’ electrons that are able to flow through it.
There is a flow of electrons in the completed circuit. The
battery acts as an ‘electron pump’. Electrons are repelled
(pushed) into the circuit from the negative terminal of
the battery. They are attracted to the positive terminal.
Metals (and graphite) conduct electricity because they
have mobile free electrons in their structure. The battery
‘pumps’ all the free electrons in one direction. Metallic
alloys are held together by the same type of bonding as
the metal elements, so they also can conduct electricity.
Solid covalent non-metals do not conduct electricity.
Whether they are giant molecular or simple molecular
structures, there are no electrons that are not involved in
bonding – there are no free electrons. Such substances are
called non-conductors or insulators (Table C4.02).
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Non-electrolytes
Giant molecular
Simple molecular
sulfuric acid
distilled water
diamond
sulfur
molten lead bromide
ethanol
poly(ethene)
iodine
sodium chloride solution
petrol
hydrochloric acid
parafin
copper(II) chloride solution
molten sulfur
sodium hydroxide solution
sugar solution
brass
C
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graphite
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C
op
If liquid compounds or solutions are tested using
the apparatus in Figure C4.07, then the result
will depend on the type of bonding holding the
compound together.
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KEy tERM
s
es
The conductivity of liquids can be tested in a similar
way to solids, but the simple testing circuit is changed
(Figure C4.07). Instead of clipping the solid material to
be tested into the circuit, graphite rods are dipped into
the test liquid. Liquid compounds, solutions and molten
materials can all be tested in this way. Molten metals,
and mercury, which is liquid at room temperature,
conduct electricity. Electrons are still able to move through
the liquid metal to carry the charge. As in solid metals,
When these liquids conduct, they do so in a diferent
way from metals. In this case, they conduct because the
ions present can move through the liquid; when metals
conduct, electrons move through the metal.
y
op
C
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op
Pr
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Conductivity in liquids – electrolytes and
non-electrolytes
ie
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br
-C
am
There is no chemical change when an electric current is
passed through a metal or graphite. The copper wire is still
copper when the current is switched of!
ev
id
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Ionic compounds will not conduct electricity
when they are solid because their ions are fixed in
position and cannot move. Liquids that conduct
electricity by movement of ions are called electrolytes.
Liquids that do not conduct in this way are called
non-electrolytes.
-R
br
am
+
Pr
op
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es
s
-C
battery
ammeter A
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liquid under test
KEy tERM
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This type of change is called electrolysis and is described
in more detail below.
heat if necessary
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electrolysis: the breakdown of an ionic compound, molten or
in aqueous solution, by the use of electricity
es
s
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br
am
Figure C4.07 The apparatus for testing the conductivity
of liquids.
-C
Pb(l) + Br2(g)
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graphite rod
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PbBr2(l)
+
graphite rod
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–
When electrolytes conduct electricity, chemical change
takes place and the ionic compound is split up.
For example, lead bromide is changed to lead
and bromine:
ity
bulb
If the compound is bonded covalently, then it will
not conduct electricity as a liquid or as a solution.
Examples of such liquids are ethanol, petrol,
pure water and sugar solution (Table C4.03). Ionic
compounds will conduct electricity if they are either
molten or dissolved in water. Examples of these are
molten lead bromide, sodium chloride solution and
copper(II) sulfate solution.
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electrical conductor: a substance that conducts electricity
but is not chemically changed in the process
–
Table C4.03 Some electrolytes and non-electrolytes.
no chemical change takes place when liquid metals
conduct electricity.
Table C4.02 Solid electrical conductors and insulators.
ni
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Pr
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poly(tetrafluoroethene),
PTFE
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steel
poly(chloroethene), PVC
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aluminium
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silver
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Electrolytes
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Conductors Insulators (non-conductors)
copper
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C4: Chemical reactions
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255
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Pr
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■
graphite
cathode
graphite
anode
The two distinct types of electrical conductivity are called
metallic and electrolytic conductivity. They difer from
each other in important ways.
dilute
hydrochloric acid
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copper(II) chromate(VI)
Figure C4.08 A demonstration to show ionic movement
by using a salt solution containing coloured ions. The acid
solution was colourless at the start of the experiment.
electrons flow
■ a property of elements (metals, and carbon as
graphite) and alloys
■ takes place in solids and liquids
■ no chemical change takes place.
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■
ACTIVITY C4.01
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the movement of ions
The conductivity of ionic compounds is explained by the fact
that ions move in a particular direction in an electric field.
This can be shown in experiments with coloured salts.
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This experiment tests which of a series of liquids and
solutions will conduct electricity, i.e. whether they are
electrolytes or non-electrolytes.
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s
Pr
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experiments is not now advised in school laboratories, but
a similar experiment could be carried out using potassium
manganate(VII). In this case the purple colour of the
manganate ions would accumulate at the anode.
op
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positive ions (metal ions or H+ ions) move towards
the cathode; they are known as cations
■ negative ions (non-metal ions) move towards the
anode; they are known as anions.
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■
es
s
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C
U
During electrolysis:
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C
Ater passing the current for a short time, the solution
around the cathode becomes blue. Around the anode the
solution becomes yellow. These colours are produced by
the movement (migration) of the ions in the salt. The positive
copper ions (Cu2+) are blue in solution. They are attracted to
the cathode (the negative electrode). The negative chromate
ions (CrO42–) are yellow in solution. They are attracted to the
anode (the positive electrode). The use of coloured ions in
solution has shown the direction that positive and negative
ions move in an electric field. The use of chromates in
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R
A worksheet, with a self-assessment checklist, is
included on the CD-ROM.
es
-C
am
br
For example, copper(II) chromate(VI) (CuCrO4) dissolves in
water to give a green solution. This solution is placed in
the lower part of a U-tube. A colourless solution of dilute
hydrochloric acid is then layered on top of the salt solution in
each arm, and graphite rods are fitted (Figure C4.08). These
rods carry the current into and out of the solution. They are
known as electrodes. In electrolysis, the negative electrode
is called the cathode; the positive electrode is the anode.
the conductivity of liquids and aqueous
solutions
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
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Pr
ions flow
■ a property of ionic compounds
■ takes place in liquids and solutions (not solids)
■ chemical decomposition takes place.
s
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Electrolytic conductivity:
256
U-tube
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Metallic conductivity:
■
+
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molten salts
solutions of salts in water
■ solutions of acids
■ solutions of alkalis.
■
battery
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br
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In summary, the following substances are electrolytes:
–
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y
A solid ionic compound will not conduct electricity,
because the ions are in fixed positions in a solid; they
cannot move. The electrolyte must be melted or dissolved
in water for it to conduct.
Electrolysis of molten compounds
An electrolytic cell can be used to electrolyse molten
compounds. Heat must be supplied to keep the salt
molten. The electrolysis of lead(II) bromide to form lead
and bromine vapour is summarised diagrammatically in
Figure C4.09. Electrolysis of molten salts is easier if the
melting point of the salt is not too high.
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the electrolytic cell
The apparatus in which electrolysis is carried out is
known as an electrolytic cell. The direct current is
supplied by a battery or power pack. Graphite electrodes
U
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lead atoms are released at the negative electrode
bromine molecules are released at the positive
electrode.
Pb2+ + e– → Pb
But an ion with a charge of 2+ needs to gain two
electrons to become an atom. We have to balance
the half-equation like this:
y
You must use a
fume cupboard.
Pb2+ + 2e– → Pb
U
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heat
In the electrolysis of molten lead bromide:
At the negative electrode, lead ions gain electrons
(e–) to become lead atoms:
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molten
lead
bromide
Pr
strong
heatproof
container
es
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carbon
electrodes
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bromine
vapour
electricity
lead + bromine
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lead bromide
2Br– → Br2 + 2e–
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Br–
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Pb2+
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bromide ion
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lead ion
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Br–
Br
Figure C4.09 The electrolysis of lead(II) bromide.
-C
molten
lead
bromide
one bromine
molecule
(no charge)
C
Br–
two bromide
ions
op
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Each bromide ion needs to lose one electron to
become an atom. Bromine atoms form molecules
containing two atoms. We have to balance
the half-equation like this:
+ electrode
2+
Pb2+
Pb
R
Br– → Br2 + e–
Pr
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electrode –
lead atom
(no charge)
At the positive electrode, bromide ions lose electrons
to form bromine molecules:
-R
bead of lead
metal
es
-C
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at the end of the
experiment
heatproof
mat
two electrons
from the electrode
C
lead ion
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Pr
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It is important to remember that it is the electrons that
move through the wire when a metal conducts. However,
when a salt solution conducts, it is the ions in the solution
that move to the electrodes. They are then discharged at
the electrodes.
R
carry the current into and out of the liquid electrolyte.
Graphite is chosen because it is quite unreactive (inert).
It will not react with the electrolyte or with the products
of electrolysis. Electrons flow from the negative terminal
of the battery around the circuit and back to the positive
terminal. In the electrolyte it is the ions that move to carry
the current.
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C4: Chemical reactions
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two electrons
to the electrode
257
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Cambridge IGCSE Combined and Co-ordinated Sciences
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When a molten ionic compound is electrolysed:
electron
flow
Pr
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U
One of the most important discoveries in industrial
electrolysis was finding suitable conditions for extracting
aluminium from its mineral ore, bauxite. The bauxite ore
is first treated to produce pure aluminium oxide. This
is then dissolved in molten cryolite (sodium aluminium
fluoride). The melting point of the mixture is much
lower than that of pure aluminium oxide. The mixture is
electrolysed between graphite electrodes (Figure C4.11).
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heat
industrial electrolysis of molten compounds
Electrolysis is important industrially because it is the
only method of extraction available for the most reactive
metals. Metals in Groups I and II, and aluminium, are too
reactive to be extracted by chemical reduction using
carbon like other metals. Metals such as sodium and
magnesium are obtained by electrolysis of their molten
chlorides. The metal is produced at the cathode.
Decomposition products
lead bromide, PbBr2
lead (Pb) and bromine (Br2)
Pb2+ + 2e–
sodium chloride, NaCl
sodium (Na) and
chlorine (Cl2)
Na+ + e–
potassium iodide, KI
potassium (K) and iodine (I2)
copper(II) bromide, CuBr2
copper (Cu) and
bromine (Br2)
Anode reactions(a)
Cathode reactions
ity
Electrolyte
Pb
2Br–
Br2 + 2e–
Na
2Cl–
Cl2 + 2e–
K+ + e –
K
Cu2+ + 2e–
C
Cu
2I–
2Br
I2+ 2e–
Br2 + 2e–
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C
Zn
Zn
2+
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am
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Table C4.04 Some examples of the electrolysis of molten salts.
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br
ev
These anode reactions are the sum of the two stages written in the text. The loss of an electron from a negative ion like Cl– can also be written
2Cl– – 2e–
Cl2.
-C
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Cl –
molten zinc
chloride (ZnCl 2)
Figure C4.10 The movement of ions in the electrolysis of a
molten salt, zinc chloride.
Zn
During electrolysis, the flow of electrons continues through
the circuit. For every two electrons taken from the cathode
by a zinc ion, two electrons are set free at the anode by
two chloride ions. So, overall, the electrons released at
the anode flow through the circuit towards the cathode.
During the electrolysis of molten salts, the metal ions,
which are always positive (cations), move to the cathode
and are discharged. Non-metal ions (except hydrogen),
however, are always negative. They are anions and move
to the anode to be discharged.
(a)
Cl –
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Zn2+ + 2e−
at the cathode
Zn
Cl Cl – Zn2+
Cl
The zinc ions (Zn2+) move to the cathode. There, each
zinc ion picks up (accepts) two electrons and becomes
a zinc atom:
258
Cl –
Cl
Table C4.04 shows some further examples of this
type of electrolysis.
Cl2
Cl + Cl
graphite cathode
Cl
Cl + e–
Cl–
Then two chlorine atoms bond together to make a
chlorine molecule:
R
graphite anode
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op
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Figure C4.10 shows the electrolysis of molten zinc chloride.
When the switch is closed, the current flows and chlorine
gas (which is pale green) begins to bubble of at the anode.
Ater a little time, a bead of molten zinc collects at the
cathode. The electrical energy from the cell has caused a
chemical change (decomposition). The cell decomposes
the molten zinc chloride because the ions present move
to opposite electrodes where they lose their charge (they
are discharged). Figure C4.10 shows this movement. The
chloride ions (Cl–) move to the anode. Each chloride ion
gives up (donates) one electron to become a chlorine atom:
at the anode
electron
flow
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the metal is always formed at the cathode
■ the non-metal is always formed at the anode.
■
battery or power pack
–
+
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switch
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C
+
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Pr
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aluminium
oxide
dissolved
in cryolite
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-C
solid crust
forming on
mixture
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d.c. supply
–
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molten
aluminium
C
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Al
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Al3+ + 3e–
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Molten aluminium is attracted to the cathode and collects
at the bottom of the cell:
at the cathode
O2 + 4e–
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259
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At the anode:
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Metals or hydrogen are released at the negative
electrode (cathode).
■ Non-metals (other than hydrogen) are formed at
the positive electrode (anode).
U
Although water is a simple molecular substance, a very
small fraction of its molecules split into hydrogen ions (H+)
and hydroxide ions (OH–):
C
Electrolysis of dilute sulfuric acid solution
As mentioned above, pure water is a very poor conductor of
electricity. However, it can be made to decompose if some
dilute sulfuric acid is added. A cell such as the one shown in
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H+ + OH−
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only a very few
molecules split into ions
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most molecules intact
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H2O
If the ions of a halogen (Cl–, Br– or I–) are present
in a high enough concentration, they will give up
electrons more readily than OH– ions will. Molecules
of chlorine, bromine or iodine are formed. The OH–
ions remain in solution.
■ If no halogen ions are present, the OH– ions will
give up electrons more easily than any other
non-metal anion. Sulfate and nitrate ions are not
discharged in preference to OH– ions. When OH–
ions are discharged, oxygen is formed.
■
The following general principles apply to the electrolysis
of solutions of ionic compounds.
■
Image C4.09 Copper is
quite unreactive so it
can be seen deposited
on the cathode
when copper(II)
sulfate solution is
electrolysed.
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The electrolysis of ionic solutions also produces chemical
change. However, the products from electrolysis of a
solution of a salt may be diferent from those obtained by
electrolysis of the molten salt. This is because water itself
produces ions.
TIP
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(–)
Pr
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Electrolysis of solutions
R
(+)
es
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At the operating temperature of about 1000 °C, the
graphite anodes burn away in the oxygen to give
carbon dioxide. So they have to be replaced regularly.
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2O2–
s
am
br
Oxygen is released at the anodes:
at the anode
The more reactive a metal, the more it tends to
stay as ions and not be discharged. The H+ ions will
accept electrons instead. Hydrogen molecules will
be formed, leaving the ions of the reactive metal,
for example Na+ ions, in solution.
■ In contrast, the ions of less reactive metals, for example
Cu2+ ions, will accept electrons readily and form metal
atoms. In this case, the metal will be discharged,
leaving the H+ ions in solution (Image C4.09).
■
y
Figure C4.11 The industrial electrolysis of molten
aluminium oxide to produce aluminium.
At the cathode:
C
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Not enough ions are produced for pure water to conduct
electricity very well. During electrolysis, however, these
hydrogen and hydroxide ions are also able to move to
the electrodes. They compete with the ions from the acid
or salt to be discharged at the electrodes. But at each
electrode just one type of ion gets discharged.
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graphite
anodes
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graphite lining
(cathode)
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C4: Chemical reactions
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e–
e– –
e
Na+
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OH–
H+
+
–
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d.c. power supply
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flow
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electron
flow
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Cl –
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Figure C4.12 The movement and discharge of ions in the
electrolysis of concentrated sodium chloride solution.
s
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There are four diferent ions present in the solution. The
positive ions (cations), Na+ and H+, flow to the cathode,
attracted by its negative charge. The negative ions
(anions), Cl– and OH–, travel to the anode.
H+ + e–
rs
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hydrogen
C
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Cl–
Pr
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s
Cl + e–
Cl2
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So, overall, pale green chlorine gas bubbles of at
the anode:
y
Figure C4.13 The Hofmann voltameter for the electrolysis
of dilute sulfuric acid.
U
op
2Cl–
Cl2 + 2e–
C
Let behind in solution are Na+ and OH– ions; this is sodium
hydroxide solution. The solution therefore becomes
alkaline. This can be shown by adding indicator to the
solution. These products – hydrogen, chlorine and
sodium hydroxide – are very important industrially as
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Electrolysis of concentrated sodium
chloride solution
A concentrated solution of sodium chloride can be
electrolysed in the laboratory (Figure C4.12).
am
Then two chlorine atoms combine to make a
chlorine molecule:
Cl + Cl
ity
–
cathode
power
supply
-C
H2
At the anode, the Cl– ions are discharged more readily
than the OH– ions:
-C
+
C
op
y
2H+ + 2e–
ev
id
ge
So, overall, hydrogen gas bubbles of at the cathode:
dilute sulfuric acid
platinum
electrodes
w
ie
H2
H+H
U
R
anode
H
Then two hydrogen atoms combine to form a hydrogen
molecule:
ve
ie
ev
oxygen
R
ev
At the cathode, it is the H+ ions that accept electrons, as
sodium is more reactive than hydrogen:
ity
op
Pr
y
es
-C
am
Figure C4.12 or a Hofmann voltameter (Figure C4.13) can be
used to keep the gases produced separate. Ater a short
time, the volume of gas in each arm can be measured and
tested. The gas collected above the cathode is hydrogen.
Oxygen collects at the anode. The ratio of the volumes
is approximately 2 : 1. This experiment is efectively the
electrolysis of water.
260
the electrolysis of concentrated sodium
chloride solution
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
Investigate the products formed when a solution of
sodium chloride is electrolysed. The experiment is
summarised in Figure C4.12.
-R
electrolysis
cell fitted
with
graphite
electrodes
ev
ie
hydrogen
Pr
es
s
e–
e– –
e
ACTIVITY C4.02
w
ge
-C
am
br
id
chlorine
sodium
chloride
solution
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
Electroplating
The fact that an unreactive metal can be coated on to the
surface of the cathode by electrolysis (see Image C4.09)
means that useful metal objects can be ‘plated’ with a
chosen metal. Electroplating can be used to coat one
metal with another.
hydrogen
out
w
ion-exchange
membrane
bubbles
of hydrogen
Pr
es
s
ve
rs
ity
C
op
Na+
w
+
–
anode
sodium hydroxide
solution out
cathode
The most commonly used metals for electroplating are
copper, chromium, silver and tin. To electroplate a metal
object with copper, the object must be made the negative
electrode. The anode is made of pure copper and a
solution of copper(II) sulfate used as the electrolyte.
id
ie
w
ge
U
TIP
The membrane cell has a titanium anode and a
nickel cathode. Titanium is chosen for the anode
as it is not attacked by chlorine. The anode and
cathode compartments are separated by a membrane.
This membrane is selective; it allows Na+ ions and
water to flow through, but no other ions. This means
that the products are kept separate and cannot react
with each other. The Na+ and OH– ions collect in the
cathode compartment. The sodium hydroxide solution is
removed and purified.
U
The anode dissolves away.
■
The electrolyte solution maintains the same
concentration (thus, if it is coloured, the intensity of
the colour stays the same).
w
ie
ev
id
es
s
-R
br
am
-C
ity
Pr
op
y
op
The basic rules for electroplating an object with a
metal M:
e
C
U
For these examples of industrial electrolysis, you will not
be expected to draw a diagram. You will need to be able
to recognise and label a diagram and give the electrode
half-equations.
The object must be made the cathode.
■ The electrolyte must be a solution of a salt of metal M.
■ The anode is made of a strip of metal M.
ie
id
g
■
-R
s
es
am
br
ev
You will also be expected to know the major reasons for
the distinctive aspects of the process.
-C
One purpose of electroplating is to give a
protective coating to the metal underneath; an
example is the tin-plating of steel cans to prevent them
rusting. This is also the idea behind chromium-plating
articles such as car bumpers, kettles and bath taps, etc.
Chromium does not corrode; it is a hard metal that resists
scratching and wear, and it can also be polished to give an
attractive finish.
y
ni
ve
rs
C
w
ie
ev
R
TIP
y
The object thickens as it becomes plated.
■
ge
■
op
rs
ve
ni
ev
Usually the electrodes used in electrolysis are inert
(graphite or platinum). However, in electroplating
the anode is made of the metal to be plated. It is not
inert, and it reacts. Remember the key observations
during electroplating:
C
ity
op
Pr
y
es
the industrial electrolysis of brine
Several diferent types of electrolytic cell have been used
for the electrolysis of brine. The modern membrane
cell (Figure C4.14) is the safest for the environment and
uses the least electricity. Other types of cell use either a
flowing mercury cathode, or a diaphragm (partition) made
from asbestos.
C
w
ie
-R
s
-C
am
br
ev
the basis for the chlor-alkali industry. So the electrolysis
of concentrated brine (salt water) is a very important
manufacturing process.
w
R
ni
C
op
Figure C4.14 The membrane cell for the electrolysis of
concentrated brine. The selective ion-exchange membrane
allows only Na+ ions to pass through it.
R
For electroplating, the electrolysis cell is adapted from
the type usually used. The cathode is the object to be
plated and the anode is made from the metal being used
to plate it. The electrolyte is a salt of the same metal. As
the process proceeds, the anode dissolves away into the
solution, replacing the metal plated on to the object, and
the concentration of the solution remains the same.
y
y
nickel
titanium
ev
ie
ev
ie
OH–
-C
bubbles of
chlorine
Cl –
-R
brine in
am
br
id
ge
chlorine
out
C
U
ni
op
y
C4: Chemical reactions
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261
ve
rs
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ev
ie
am
br
id
QuEStiONS
w
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C
U
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op
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Cambridge IGCSE Combined and Co-ordinated Sciences
An experiment was carried out to investigate
the efect of electricity on molten lead(II)
bromide (PbBr2).
c What colour is the vapour seen at the
positive electrode?
-R
ve
rs
ity
Pb2+ + 2e–
b Why is the reaction taking place at the negative
electrode viewed as a reduction reaction?
C4.14
ni
U
ge
w
R
Use the information in the first table to complete the
second table. The solutions were electrolysed under
exactly the same conditions as the ones above.
A metal object is to be copper plated.
sodium sulfate
oxygen
silver nitrate
oxygen
concentrated sodium chloride
chlorine
copper(II) nitrate
Solution (electrolyte)
Gas given of at the
anode
Substance let in solution
at the end of electrolysis
copper
sulfuric acid
sodium sulfate
silver
nitric acid
hydrogen
sodium hydroxide
oxygen
copper
nitric acid
Gas given of or metal
deposited at the cathode
Substance let in solution
at the end of electrolysis
ity
Pr
es
hydrogen
rs
y
ve
y
op
C
w
ie
ev
oxygen
ni
U
R
silver sulfate
Gas given of or metal
deposited at the cathode
s
oxygen
-C
copper(II) sulfate
ev
Gas given of at the
anode
am
Solution (electrolyte)
-R
br
id
ie
a Which electrode should the object be made?
262
hydrogen
ie
ev
id
br
-R
am
es
Pr
op
y
AO3.4 Interpret and evaluate experimental observations
and data
s
-C
Electrolysis of copper(II) sulfate solution
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.5 Evaluate methods and suggest possible
improvements
ity
This experiment is designed to demonstrate the diferent
products obtained when the electrolysis of copper(II) sulfate
solution is carried out first with inert graphite electrodes and
then with copper electrodes. The use of copper electrodes
illustrates how copper is refined industrially.
op
y
ni
ve
rs
C
ev
ie
id
g
w
e
C
U
A worksheet is included on the CD-ROM.
es
s
-R
br
am
-C
w
ie
ev
sodium nitrate
w
ge
sodium nitrate
ACTIVITY C4.03
R
The tables list the results of the electrolysis
of a number of aqueous solutions using inert
electrodes.
C
op
e What is the alternative name for the
negative electrode?
C4.12
Pb
a Write the equation for the reaction taking place
at the positive electrode.
y
ev
ie
w
d Give one reason why this electrolysis
should be carried out in a fume cupboard.
In the electrolysis of molten lead(II) bromide, the
reaction occurring at the negative electrode was:
op
C
op
y
b Why does solid lead(II) bromide not
allow the passage of electricity?
C4.13
Pr
es
s
-C
a What happens to a compound
during electrolysis?
b Name a solution that could be used as
the electrolyte.
C
C4.11
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
–
C
-R
c In industry, some plastics are electroplated.
Why must the plastic be coated with a thin film
of graphite before plating?
y
-R
am
br
ev
id
ie
w
ge
U
R
ni
electrolyte
C
op
w
b If graphite were used instead of the copper
electrode in a, what change would you notice
to the electrolyte during the experiment?
Y
ev
ie
X
-C
Summary
es
s
You should know:
Pr
rs
es
s
■
Pr
op
y
ni
ve
rs
ity
■
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
R
ev
ie
w
C
■
■
-R
am
op
y
■
-C
■
w
C
id
ie
■
br
■
op
y
ve
ni
■
ge
R
■
■
U
ev
ie
■
■
ity
op
C
w
■
electrons – oxidation being the loss of electrons and
reduction the gain of electrons
about the electrical conductivity of metals and graphite
about the conductivity of ionic compounds when
molten or dissolved in water that results in a chemical
change (electrolysis)
that electrolytic cells consist of positive (anode) and
negative (cathode) electrodes and an electrolyte
about the factors that decide which ions are
discharged at the electrodes
how to write the reactions taking place at the
electrodes as ionic half-equations
about electroplating, which can be used to produce
a protective and/or decorative layer of one metal
on another
how electrolysis is industrially important for the
extraction of very reactive metals such as aluminium
and the production of sodium hydroxide and chlorine.
ev
about the nature of chemical reactions and how they
difer from physical changes
how to represent the changes in a reaction using word
equations and balanced chemical equations
how equations can be made more informative by
including state symbols
how equations for reactions involving ions can be
simplified to include only those ions taking part
in the reaction
about the exothermic or endothermic energy changes
involved in reactions
about the variety of diferent types of chemical
reaction such as combustion, neutralisation, and
displacement reactions
about the importance of oxidation and reduction
reactions (redox)
how the definitions of oxidation and reduction can be
extended to include reactions involving the transfer of
y
■
Which electrode, X or Y, is the metal strip?
ii Is the metal strip an anode or a cathode?
ve
rs
ity
op
y
d.c. supply
i
Pr
es
s
+
ev
ie
w
ge
The apparatus below was used to plate a strip
of metal with copper. One electrode was made
of copper and the other was the metal strip to
be plated.
-C
a
am
br
id
QuEStiONS
C4.15
C
U
ni
op
y
C4: Chemical reactions
Copyright Material - Review Only - Not for Redistribution
263
ve
rs
ity
w
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
am
br
id
-R
A group of students is conducting an experiment investigating the action of heat on solid copper
carbonate and zinc carbonate. The two experiments gave them the results summarised here:
Pr
es
s
-C
y
positive test for carbon dioxide gas given off
heat
black powder
y
C
ni
ev
ie
w
black powder
-R
es
s
-C
Pr
rs
e
ity
op
op
C
w
ie
ev
-R
s
es
y
op
C
e
w
SO3 + O2
ie
ev
SO2 + O3
id
g
B
SO2
-R
s
es
-C
am
br
S + O2
[1]
[1]
[2]
Pr
The equations A and B below show two reactions which lead to the formation of acid rain.
A
[1]
[1]
[2]
[2]
Write an ionic equation for this reaction.
Explain why magnesium is a reducing agent in this reaction.
ni
ve
rs
C
i
ii
w
ni
MgSO4 + Zn
Mg + ZnSO4
ie
ev
3
U
ge
id
br
op
y
e
ity
d
What observation would show that carbon had been produced?
Write a word equation for this reaction.
Which substances have been:
i reduced in this reaction?
ii oxidised in this reaction?
Magnesium oxide reacts with hydrochloric acid to make the salt magnesium chloride and water.
Write the symbol equation for this reaction.
Magnesium sulfate is produced when magnesium is added to zinc sulfate solution.
am
a
b
c
2MgO + C
-C
R
2Mg + CO2
y
ve
When a strip of burning magnesium ribbon is lowered into a gas jar of carbon dioxide, the
following reaction takes place:
U
2
ev
ie
w
C
264
white powder
What evidence is there that a chemical reaction has taken place in both cases?
What is the major and most reliable evidence of a reaction here?
Write word equations for the two reactions.
Write a brief description of what you would see happen if zinc oxide powder were heated
strongly and then allowed to cool down.
Would this change have been a chemical reaction?
y
a
b
c
d
cool down
am
br
yellow powder
ev
id
heat
ie
w
ge
U
R
positive test for carbon dioxide gas given off
zinc carbonate
white powder
R
cool down
ve
rs
ity
op
copper carbonate
green powder
C
op
1
ev
ie
End-of-chapter questions
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
w
ge
C
U
ni
op
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C4: Chemical reactions
Pr
es
s
w
ie
s
es
Pr
y
ity
cathode
cation
[1]
ve
w
anode
rs
op
C
265
Choose a word from the list below which describes the positive electrode.
anion
ie
-R
br
am
-C
negative
electrode
molten zinc
chloride
i
ii
y
op
ni
ev
State the name of the product formed during this electrolysis at
• the negative electrode.
• the positive electrode.
iii Suggest the name of a non-metal which can be used for the electrodes in this electrolysis.
[2]
[1]
w
ge
C
U
R
[3]
[1]
ev
—
positive
electrode
br
ev
id
ie
[Cambridge IGCSE Chemistry 0620 Paper 21 Q8 June 2010]
-R
am
The diagram shows the apparatus used to electrolyse concentrated aqueous sodium chloride.
anode
+
U
op
ie
—
ev
y
ni
ve
rs
cathode
ity
Pr
op
y
concentrated
aqueous sodium chloride
es
s
-C
gases
w
ie
ev
[2]
[2]
[2]
-R
[Cambridge IGCSE Chemistry 0620 Paper 22 Q5 c November 2011]
s
-C
am
br
id
g
w
e
C
Give a description of this electrolysis. In your description include:
a what substance the electrodes are made from and the reason for using this substance
b what you would observe during the electrolysis
c the names of the substances produced at each electrode.
es
C
[1]
y
C
op
U
id
b
c
ge
R
ni
ev
ie
Which three of the following conduct electricity?
aqueous sodium chloride; ceramics; copper;
graphite; sodium chloride crystals; sulfur
State the name given to a substance, such as plastic, which does not conduct electricity.
Molten zinc chloride was electrolysed using the apparatus shown below.
+
R
[2]
Some substances conduct electricity, others do not.
a
5
[2]
[Cambridge IGCSE Chemistry 0620 Paper 21 Q7 a June 2012]
ve
rs
ity
4
w
C
op
y
-C
-R
am
br
id
ev
ie
a Write a word equation for reaction A.
b Which two of the following statements about reaction B are correct?
SO2 is oxidised to SO3;
SO2 is reduced to SO3
O3 is reduced to O2;
O3 is oxidised to O2
c Complete the equation to show how an aqueous solution of sulfuric acid, H2SO4,
is formed from SO3
SO3 +
H2SO4
Copyright Material - Review Only - Not for Redistribution
op
y
ve
rs
ity
ni
C
U
ev
ie
w
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-R
am
br
id
Pr
es
s
-C
y
ni
C
op
y
ve
rs
ity
op
C
w
ev
ie
br
ev
id
ie
w
ge
U
R
es
ity
op
Pr
y
y
op
op
y
ni
ve
rs
C
U
w
ev
ie
id
g
w
e
The easiest way to detect whether a solution is acidic
or not is to use an indicator. Indicators are substances
that change colour if they are put into an acid or alkaline
solution. Two commonly used indicators are litmus and
methyl orange.
s
-R
br
am
-C
C
-R
es
s
C
The word acid was originally applied to substances with
a ‘sour’ taste. Vinegar, lemon juice, grapefruit juice and
spoilt milk are all sour tasting because of the presence
of acids (Image C5.01). These acids are present in animal
and plant material and are known as organic acids
(Table C5.01).
Carbonic acid from carbon dioxide dissolved in water is
present in Coca Cola®, Pepsi® and other fizzy drinks. The
acids present in these circumstances are weak and dilute.
But taste is not a test that should be tried – some acids
would be dangerous, even deadly, to taste!
A number of acids are also corrosive. They can eat their way
through clothing, are dangerous on the skin, and some are
able to attack stonework and metals. These powerful acids
are oten called mineral acids (Table C5.01). Table C5.01
also gives us some idea of how commonly acids occur.
ity
The major acids
ie
■
Pr
op
y
C5.01 What is an acid?
ev
■
w
ni
U
-C
■
■
the acid–base properties of non-metal oxides and metal oxides
neutral and amphoteric oxides
the ions present in acid and alkali solutions
acids and alkalis in the analysis of salts
the preparation of soluble salts by various methods,
including titration
the choice of the method for preparing a particular salt
the preparation of insoluble salts by precipitation.
ie
ve
■
■
ge
■
■
id
■
■
br
■
■
R
■
ev
common acids – their characteristics, where and
how they occur
the pH scale and indicators
the colour changes of useful indicators
the characteristic reactions of acids
the treatment of acid soils and waste water treatment
proton (H+) transfer in aqueous solutions
the characteristic properties of bases and alkalis
am
R
ev
ie
■
rs
w
C
This chapter covers:
es
266
s
-C
-R
am
C5
Acids, bases and salts
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
Litmus is purple in neutral solution. When added to
an acidic solution, it turns red. This colour change of litmus
needs a chemical reaction. The molecules of the indicator
are actually changed in the presence of the acid.
Substances with the opposite chemical efect to acids
are needed to reverse the change, and these are called
alkalis. They turn litmus solution blue. You can also use
litmus paper. This is paper that has been soaked in litmus
solution. It comes in blue and red forms. The blue form of
litmus paper changes colour to red when dipped into acid
solutions. Red litmus paper turns blue in alkali solutions.
Note that litmus just gives a single colour change.
y
es
s
-C
Pr
y
What are indicators?
ity
op
Certain coloured substances (many extracted from
plants) have been found to change colour if added to an
acid solution. This colour change is reversed if the acid is
‘cancelled out’ or neutralised. Substances that do this are
known as indicators. Coloured extracts can be made from
red cabbage or blackberries, but probably the most used
indicator historically is litmus. This is extracted from lichens.
The presence of water is very important in the action of acids
and alkalis. One practical consequence of this is that, when
we use litmus paper to test gases, it must always be damp.
The gas needs to dissolve in the moisture to bring about the
colour change. This is important in your practical work.
ni
op
y
ve
rs
C
w
ie
ev
r
e
acid / base
l
u
e
-R
am
br
ev
id
ie
w
ge
It may seem simple to remember the colour change that
litmus shows in acid and alkali, but it is important. This
simple visual memory aid may help you to remember:
Image C5.01 Citrus fruits have a sour or sharp taste
because they contain acids.
Where found or used
ethanoic acid
CH3COOH
weak
in vinegar
methanoic acid
HCOOH
weak
lactic acid
CH3CCH(OH)
weak
H2CO3
Pr
carbonic acid
ev
weak
in lemons, oranges and other citrus fruits
weak
in fizzy sot drinks
HCl
weak
used in cleaning metal surfaces; found as
the dilute acid in the stomach
nitric acid
HNO3
weak
H2SO4
weak
H3PO4
weak
y
hydrochloric acid
used in making fertilisers and explosives
C
in car batteries; used in making
fertilisers, paints and detergents
w
ie
-R
in anti-rust paint; used in
making fertilisers
s
Table C5.01 Some common acids.
es
am
br
phosphoric acid
ev
id
g
e
sulfuric acid
-C
in sour milk
op
ni
ve
rs
ity
C6H8O7
U
R
ev
ie
w
Mineral acids
citric acid
-R
br
am
-C
C
op
y
COOH
in ant and nettle stings; used in
kettle descaler
s
Organic acids
C
Strong or weak?
ie
Formula
es
Name
id
Type
w
ge
U
R
C
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TIP
U
R
ni
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w
C
ve
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y
Pr
es
s
-C
-R
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br
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C
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C5: Acids, bases and salts
Copyright Material - Review Only - Not for Redistribution
267
ve
rs
ity
red
purple
blue
thymolphthalein
colourless
colourless
blue
methyl orange
red
orange
yellow
am
br
id
The more acidic solutions (for example, battery acid)
turn Universal Indicator bright red. A less acidic solution
(for example, vinegar) will only turn it orange-yellow.
There are also colour diferences produced with diferent
alkali solutions. The most alkaline solutions give a
violet colour.
op
y
Table C5.02 Some common indicator colour changes.
ni
C
op
ACtivity C5.01
Litmus is not the only single indicator that chemists
find useful. Others that have been frequently used are
phenolphthalein and methyl orange. Phenolphthalein is no
longer recommended for use in school laboratories. It can
be replaced by thymolphthalein and the colour change for
that indicator is included in Table C5.02. They give diferent
colour changes from litmus (Table C5.02). These changes
are sometimes easier to ‘see’ than that of litmus.
ie
ev
id
s
es
Pr
op
y
ve
ni
ie
w
rs
C
ity
Another commonly used indicator, Universal Indicator (or
full-range indicator), is a mixture of indicator dyes. The idea
of a Universal Indicator mixture is to imitate the colours
of the rainbow when measuring acidity. Such an indicator
is useful because it gives a range of colours (a ‘spectrum’)
orange
yellow
7
blue
9
10
ie
11
12
13
14
violet
y
op
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
R
ev
ie
w
ni
ve
rs
C
ity
Pr
green
8
strongly alkaline
ev
6
weakly alkaline
-R
5
neutral
s
id
4
op
y
red
3
weakly acidic
es
am
2
-C
1
br
strongly acidic
pH 0
w
ge
C
U
R
ev
-R
br
am
-C
y
op
Universal Indicator
268
Extracting an indicator from red cabbage
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
Dye is extracted from chopped-up red cabbage leaves
(or other coloured plant material) and then tested to
see the colour change when it is added to acidic and
alkaline solutions.
A worksheet is included on the CD-ROM.
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Litmus is not the only single indicator that chemists
find useful. Others that have been frequently used are
phenolphthalein and methyl orange. They give diferent
colour changes from litmus (Table C5.02). These changes
are sometimes easier to ‘see’ than that of litmus.
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Pr
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litmus
depending on the strength of the acid or alkali added
(Image C5.02). When you use Universal Indicator, you
see that solutions of diferent acids produce diferent
colours. Indeed, solutions of the same acid with diferent
concentrations will also give diferent colours.
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Colour in
alkali
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Neutral
colour
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Colour in
acid
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Indicator
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
Image C5.02 How the
colour of Universal Indicator
changes in solutions of
diferent pH values.
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Rainbow fizz!
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
This activity creates a Universal Indicator pH scale in
a boiling tube. Set up a test-tube rack containing the
following:
■ Tube A: a boiling tube containing half a spatula of
sodium hydrogencarbonate
■ Tube B: a test tube containing 5 cm3 of distilled water
■ Tube C: a test tube containing 0.5 cm3 of Universal
Indicator solution
■ Tube D: a test tube containing 5 cm3 of dilute
ethanoic acid
■ Tube E: a test tube containing 5 cm3 of dilute
sulfuric acid
Then follow this sequence, making careful observations at
each stage.
1 Add the water from tube B to the solid in tube A.
2 Then add the indicator solution from tube C to
tube A.
3 Tilt tube A. Very carefully pour the ethanoic acid from
tube D into tube A down the side of the tube. Do not
shake the tube.
4 Finally, add the sulfuric acid from tube E to tube A.
Again, pour this acid very carefully down the side of the
tilted tube A. Do not shake the tube.
A worksheet is included on the CD-ROM.
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The pH of a solution can be measured in several ways.
Universal Indicator papers that are sensitive over the
full range of values can be used. Alternatively, if the
approximate pH value is known, then we can use a
more accurate test paper that is sensitive over a narrow
range. The most accurate method is to use a pH meter
(Image C5.03), which uses an electrode to measure pH
electrically. The pH values of some common solutions are
shown in Table C5.03.
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■ As we move down from 7, the solution is getting
more acidic.
■ Moving up from pH 7, the solution is getting
more alkaline.
ACtivity C5.02
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■
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■
Acids have a pH less than 7.
The more acidic a solution, the lower the pH.
Neutral substances, such as pure water, have a pH of 7.
Alkalis have a pH greater than 7.
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■
It’s very important to remember that the ‘reference point’
when measuring pH is neutrality, pH 7 – the mid-point of
the scale.
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Question
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A1 Explain the colour changes you observe at
each addition.
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Image C5.03 pH meter for testing soil.
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Rules for the pH scale
■
TIP
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The most useful measure of the strength of an acid
solution was worked out by the Danish biochemist Søren
Sørensen. He worked in the laboratories of the Carlsberg
breweries and was interested in checking the acidity of
beer. The scale he introduced was the pH scale. The scale
runs from 1 to 14, and the following general rules apply.
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The pH scale
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C5: Acids, bases and salts
Copyright Material - Review Only - Not for Redistribution
269
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wine
3.5
tomato juice
4.1
1.0
lemon juice
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5.0
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baking soda solution
8.5
toothpaste
9.0
borax solution
9.2
10.5
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14.0
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sodium hydroxide (NaOH)
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12.0
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household ammonia
11.0
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Which acid is present in orange or lemon juice?
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Is a solution acidic, alkaline or neutral if its pH is:
d
3?
Methyl orange is an indicator. What does
this mean?
U
pH higher
than 7, litmus
turns blue
Which solution is more acidic: one with a pH of 4,
or one with a pH of 1?
C5.06
What colour is Universal Indicator in a
sugar solution?
C5.07
What acid is present in vinegar?
c Alkali solution: OH– > H+
H
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Figure C5.01 pH and the balance of hydrogen ions and
hydroxide ions in solution.
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+
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C5.05
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c 8
7
H+
pH lower
than 7, litmus
turns red
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b
11
–
OH
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b Acid solution: H+ > OH–
Pr
C5.02
C5.04
OH–
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What do you understand by the word corrosive?
C
C5.01
a
H+
pH = 7
QuEStiONS
C5.03
a Pure water: H+ = OH–
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Table C5.03 The pH values of some common solutions.
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limewater
R
The hydrogen ions (H+) in acid solutions make
litmus go red.
■ The hydroxide ions (OH–) in alkali solutions make
litmus go blue.
■
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Milk of Magnesia
strongly
alkaline
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Pr
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7.4
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blood
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weakly
alkaline
Alkali solutions also conduct electricity better than
distilled water. All alkalis dissolve in water to produce
hydroxide ions (OH– ions). Therefore, all alkali solutions
contain an excess of OH– ions. An indicator, like
litmus, is afected by the presence of H+ or OH– ions
(Figure C5.01).
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7.0
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pure water, sugar solution
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NEUTRAL
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6.5
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6.5
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rainwater
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270
6.0
U
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urine
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5.6
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acid rain
milk
If we look again at the chemical formulae of some of the
best known acids (Table C5.01), we see that one element
is common to them all. They all contain hydrogen.
If solutions of these acids are checked to see if they
conduct electricity, we find that they all do. Also, they
conduct electricity much better than distilled water does.
This shows that the solutions contain ions. Water itself
contains very few ions. In pure water, the concentrations of
hydrogen ions (H+) and hydroxide ions (OH–) are equal. All
acids dissolve in water to produce hydrogen ions (H+ ions).
Therefore, all acid solutions contain more H+ ions than
OH– ions. The pH scale is designed around the fact that
acid solutions have this excess of hydrogen ions. The term
pH is taken from the German ‘potenz H(ydrogen)’, meaning
the power of the hydrogen-ion concentration of a solution.
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black cofee
weakly
acidic
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3.0
gastric juices
The importance of hydrogen ions
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vinegar
0.0
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2.5
hydrochloric acid (HCl)
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strongly
acidic
C5.02 Acid and alkali solutions
pH
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Substance
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Copyright Material - Review Only - Not for Redistribution
OH–
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C5: Acids, bases and salts
hydrochloric acid
H+(aq) and Cl–(aq)
nitric acid
H+(aq) and NO3–(aq)
H (aq), HSO4 (aq) and
SO42–(aq)
y
Which ions are present in:
a calcium hydroxide solution
b ammonia solution?
C5.11
What is the formula for:
a sulfuric acid
C5.12
NH4+(aq) and OH–(aq)
U
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What statement can we make about the
concentrations of hydrogen ions and
hydroxide ions in water?
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C5.03 Metal oxides and
non-metal oxides
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Acidic and basic oxides
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The sulfuric acid clouds of Venus are the product of great
volcanic activity (Image C5.04). This has thrown out huge
amounts of water vapour and the oxides of sulfur into the
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KEy tERMS
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acid: a substance that dissolves in water to produce a solution that:
■ turns litmus red
■ has a pH lower than 7
■ contains an excess of H+ ions.
alkali: a substance that dissolves in water to produce a
solution that:
■ turns litmus blue
■ has a pH higher than 7
■ contains an excess of OH– ions.
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Image C5.04 Image of the European Space Agency probe
orbiting above the clouds of the Venus atmosphere.
The Venus Express spacecrat was launched to study the
thick atmosphere responsible for the intense greenhouse
efect on the planet.
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Venus, the Earth’s nearest neighbour, is identical in size
and density to the Earth. But Venus has yielded its secrets
reluctantly, because it is veiled in clouds and has an
atmosphere that destroys space probes. Magellan, the
latest space probe to Venus, has looked from a distance.
If it went into the atmosphere, it would meet with thick
clouds of sulfuric acid and temperatures similar to those
in a self-cleaning oven – acid rain with a vengeance!
The probe would not last long!
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When is an acid not an acid, but simply an ‘acid-inwaiting’? Hydrochloric acid is a good example to illustrate
this problem. The gas hydrogen chloride is made up of
covalently bonded molecules. If the gas is dissolved in
an organic solvent, such as methylbenzene, it does not
show any of the properties of an acid. For example, it
does not conduct electricity. However, when the gas is
dissolved in water, a strongly acidic solution is produced.
The acidic oxides of sulfur, phosphorus and carbon listed
in Table C5.05 are similar. They are covalent molecules
when pure, but produce acids when dissolved in water.
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hydrochloric acid?
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ammonia solution
b
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Ca2+(aq) and OH–(aq)
Table C5.04 The ions present in solutions of some acids
and alkalis.
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Which ion is in excess in an alkali solution?
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calcium hydroxide
Thus, in our most useful definition of an acid, the
characteristic properties of an acid are shown when
dissolved in water. Alkalis are also normally used in
aqueous solution. Both acids and alkalis can be used
in dilute or concentrated solutions. If a large volume of
water is added to a small amount of acid or alkali, then
the solution is dilute; using less water gives a more
concentrated solution.
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C5.09
Na (aq) and OH (aq)
sodium hydroxide
The importance of water
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Which element do all acids contain?
–
+
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C
–
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+
C5.08
C5.10
potassium hydroxide K+(aq) and OH–(aq)
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Ions present
sulfuric acid
Alkalis
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Name
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Acids
QuEStiONS
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The ions present in some important acid and alkali
solutions are given in Table C5.04.
Copyright Material - Review Only - Not for Redistribution
271
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Cambridge IGCSE Combined and Co-ordinated Sciences
give solutions that turn litmus paper blue. The metal
oxides produced in these reactions react with acids to
neutralise them – they are said to be basic oxides.
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combustion
spoon
The characteristics of oxides
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sulfur
dioxide
oxygen
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gas jar
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Neutral and amphoteric oxides
planet’s atmosphere. Similar acidic clouds can be made in a
gas jar by lowering burning sulfur into oxygen (Figure C5.02):
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S(s) + O2(g)
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CO2(g)
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When water is added to the gas jars, it dissolves the gases
and gives solutions that turn blue litmus paper red.
Metals burning in oxygen produce solid products. Some
of these dissolve in water to give solutions that turn red
litmus paper blue. You might be able to work out a pattern
in the reactions of some elements with oxygen, as shown
in Table C5.05.
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Of more importance is the unusual behaviour of some
metal oxides. These metal oxides react and neutralise
acids, which would be expected. However, they also
neutralise alkalis, which is unusual.
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The most important examples of metals that have
amphoteric compounds are zinc and aluminium.
The fact that zinc hydroxide and aluminium hydroxide
are amphoteric helps in the identification of salts
of these metals using sodium hydroxide.
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phosphorus
burns with yellow flame
white solid (phosphorus(V) oxide, P2O5) dissolves, turns litmus red
carbon
glows red
colourless gas (carbon dioxide, CO2)
Metals
white solid (sodium oxide, Na2O)
dissolves slightly, slowly turns
litmus red
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colourless gas (sulfur dioxide, SO2)
C
dissolves, turns litmus blue
burns with yellow flame
magnesium
burns with bright white flame white solid (magnesium oxide, MgO)
dissolves slightly, turns litmus blue
calcium
burns with red flame
white solid (calcium oxide, CaO)
dissolves, turns litmus blue
iron
burns with yellow sparks
blue-black solid (iron oxide, FeO)
insoluble
copper
does not burn, turns black
black solid (copper oxide, CuO)
insoluble
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Table C5.05 The reactions of certain elements with oxygen.
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sodium
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dissolves, turns litmus red
burns with bright blue flame
sulfur
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Non-metals
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Efect of adding water and
testing with litmus
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Product
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How it reacts
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Element
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Turning litmus paper red shows that some of these
solutions contain acids. These solutions are the product
of burning non-metals to produce acidic oxides.
Burning metals produces oxides that, if they dissolve,
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C(s) + O2(g)
Water can be thought of as hydrogen oxide. It has a
pH of 7 and is therefore a neutral oxide. It is an exception
to the broad ‘rule’ that the oxides of non-metals are
acidic oxides. Neutral oxides do not react with either
acids or alkalis. There are a few other exceptions to this
‘rule’ (see Figure C5.03). The most important is carbon
monoxide (CO), noted for being poisonous. The ‘rule’ that
most non-metal oxides are acidic remains useful and
important, however.
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SO2(g)
Other burning non-metals (carbon, for example) react in
the same way to produce acidic gases:
272
Non-metals generally form acidic oxides that
dissolve in water to form acidic solutions.
■ Metals form oxides that are solids. If they dissolve in
water, these oxides give alkaline solutions. These
metal oxides neutralise acids and are basic oxides.
■
Figure C5.02 Burning sulfur in a gas jar of oxygen.
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sulfur
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C5: Acids, bases and salts
Zn(OH)2(s)
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zinc hydroxide + sodium hydroxide
sodium zincate + water
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However, this precipitate will re-dissolve if excess sodium
hydroxide is added, because zinc hydroxide is amphoteric:
Na2ZnO2(aq) + 2H2O(I)
Zn(OH)2(s) + 2NaOH(aq)
What colour is the flame when sulfur burns?
C5.14
What colour flame is produced when
magnesium burns?
C5.15
Write the word equation for the reaction
when sulfur burns in oxygen.
C5.16
What is the chemical equation for the reaction in
question C5.15?
C5.17
Write the word equation for magnesium
burning in air.
C5.18
Which oxide of carbon is neutral?
C5.19
Name one amphoteric metal hydroxide and write
the word and symbol equations for its reaction
with sodium hydroxide solution.
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Aluminium salts will give a similar set of reactions. This test
distinguishes zinc and aluminium salts from others, but
not from each other (see Sections C5.07 and C12.01).
C5.04 Acid reactions in
everyday life
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Metal oxides
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s
Basic oxides
e.g. CaO, MgO,
CuO, K2O,
Na2O, FeO,
Fe2O3 etc.
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Soil pH and plant growth
Pr
Neutral
oxides
e.g. H2O,
CO, NO
Acidic oxides
e.g. CO2, SO2,
SO3, NO2, P2O5,
SiO2 etc.
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C
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Figure C5.03 The classification of non-metal and metal oxides.
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KEy tERM
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amphoteric hydroxide (or amphoteric metal oxide): a
hydroxide or metal oxide that reacts with both an acid and an
alkali to give a salt and water
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ACtivity C5.03
br
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salt + water
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They are reacting as acids with the sodium hydroxide and
producing a salt and water as the products.
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zinc hydroxide + sodium hydroxide
sodium zincate (Na2ZnO2) + water
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aluminium hydroxide + sodium hydroxide
sodium aluminate (NaAlO2) + water
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Do notice how these rather unusual salts are named.
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In these last reactions, the zinc hydroxide and aluminium
hydroxide precipitates re-dissolve in excess sodium
hydroxide because they are amphoteric.
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testing the pH of everyday substances
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(induding following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
In this introductory experiment to the ideas of acids and
alkalis, household and everyday products are tested for
their pH using Universal Indicator.
A worksheet is included on the CD-ROM.
A follow-up experiment on neutralising vinegar with
slaked lime or powdered limestone is included on the
Teacher’s Resource CD-ROM.
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TIP
acid + alkali
Plant growth is afected by the acidity or alkalinity of
the soil. Soils with high peat content, or with minerals
such as iron compounds, or with rotting vegetation and
lack of oxygen, tend to be acidic. Their soil pH can reach
as low as pH 4. Soils in limestone or chalky areas are
alkaline – up to pH 8.3. The soil pH is also afected by the
use of fertilisers and the acidity of rainfall. Diferent plants
prefer diferent pH conditions (Table C5.06). Farmers and
gardeners can test the soil pH to see whether it suits the
needs of particular plants.
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Amphoteric
oxides
e.g. ZnO,
Al2O3
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br
Non-metal
oxides
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C5.13
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Zn2+(aq) + 2OH−(aq)
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Zn(OH)2(s) + 2NaCl(aq)
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ZnCl2(aq) + 2NaOH(aq)
QuEStiONS
-R
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If sodium hydroxide solution is added to a solution of a salt
of either of these metals, a white precipitate of the metal
hydroxide is formed. For example:
Copyright Material - Review Only - Not for Redistribution
273
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5.5–6.5
cauliflower, garlic, tomato
5.5–7.5
broad bean, onion, cabbage
and many others
6.0–7.5
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If the soil is too acidic, it is usually treated by ‘liming’.
‘Lime’ here is a loose term meaning either
calcium oxide, calcium hydroxide, or powdered
chalk or limestone (calcium carbonate). These
compounds all have the efect of neutralising the
acidity of the soil. If the soil is too alkaline, it helps
to dig in some peat or decaying organic matter
(compost or manure).
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Image C5.06 Controlled addition of lime to a stream in
Sweden to neutralise the efects of acidity.
-R
QuEStiONS
Ant stings contain methanoic acid. What
household substance could be used to ease the
efect of the sting?
C5.21
Which acid is present in our stomachs, and
why is it there?
C5.22
Indigestion tablets contain antacid. Name two
compounds that we use in these tablets.
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Efluent and waste water treatment
Liquid waste from factories is oten acidic. If such waste
gets into a river, the acid will kill fish and other river life.
Slaked lime is oten added to the waste to neutralise it.
Slaked lime is similarly used to treat streams, rivers and
lakes afected by acid rain (Image C5.06).
Comparing the efectiveness of diferent
antacid tablets
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
This activity involves titrating powdered samples of
antacid tablets with dilute hydrochloric acid.
A worksheet is included on the CD-ROM.
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ACtivity C5.04
Image C5.05 The colour of the flowers of some
types of hydrangea depend on soil pH. Here the flowers
are showing signs of the colour change between
pink and blue.
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C5.20
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Some flowering plants carry their own ‘built-in’ pH
indicator. The flowers of a hydrangea bush are blue when
grown on acid soil and pink when the soil pH is alkaline
(Image C5.05).
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To reduce emissions of sulfur dioxide, many modern factories
and power stations now spray acidic waste gases with jets
of slaked lime in a flue-gas desulfuriser (or ‘scrubber’) to
neutralise them before they leave the chimneys.
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Table C5.06 Preferred soil pH conditions for
diferent vegetables.
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carrot, sweet potato
Pr
es
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5.0–6.5
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chicory, parsley
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4.5–6.0
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potatoes
Preferred pH range
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Vegetables
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
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C5: Acids, bases and salts
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C5.05 Alkalis and bases
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Figure C5.04 This Venn diagram shows the relationship
between bases and alkalis. All alkalis are bases, but not all
bases are alkalis.
-R
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The common alkalis are:
s
-C
• sodium hydroxide solution
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Pr
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• calcium hydroxide solution oten known as limewater
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The four solutions listed above are the alkalis
you will need to know for your course. They are the
commonest, and they are likely to be the only ones
you refer to.
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es
Pr
These solutions contain OH– ions, turn litmus blue and
have a pH higher than 7. The first two are stronger alkalis
than the others.
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C
Most bases are insoluble in water. This makes the few
bases that do dissolve in water more significant. They are
given a special name – alkalis.
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Properties and uses of alkalis
and bases
C
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Alkalis feel soapy to the skin. They convert the oils in
your skin into soap. They are used as degreasing agents
because they convert oil and grease into soluble soaps,
which can be washed away easily. The common uses of
some alkalis and bases are shown in Table C5.07.
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KEy tERM
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alkali: a base that is soluble in water. Alkalis are generally
used in the laboratory as aqueous solutions.
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w
salt + water
acid + base
It is worth making sure that you learn their names and
formulae! And you should do the same for the four
commonest acids you’ll need to know: hydrochloric acid,
sulfuric acid, nitric acid and ethanoic acid.
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A base will neutralise an acid, and in the process a
salt is formed. This type of reaction is known as a
neutralisation reaction. It can be summed up in a
general equation:
op
TIP
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The relationship of alkalis to bases can be summed up
in a mathematical device known as a Venn diagram
(Figure C5.04). In more general terms it is something like
the diference between our immediate family and our
extended family. The bases are the extended family of
compounds. The alkalis are a particular small group within
that extended family.
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• potassium hydroxide solution
• ammonia solution also known as ammonium hydroxide .
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base: a substance that reacts with an acid to form a salt and
water only
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alkalis are soluble bases
(e.g. NaOH, KOH)
ve
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ity
However, among the antacids we use to relieve
indigestion is insoluble magnesium hydroxide,
which also neutralises acids. As we investigate further,
it is found that all metal oxides and hydroxides will
neutralise acids, whether they dissolve in water or not.
Therefore the soluble alkalis are just a small part of
a group of substances – the oxides and hydroxides of
metals – that neutralise acids. These substances are
known as bases. These bases all react in the same way
with acids.
KEy tERM
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bases (e.g. CuO, MgO, CaO,
NaOH, Cu(OH)2) all neutralise acids
Pr
es
s
-C
In Section C5.04 we saw that the efects of acids could be
neutralised by alkalis. Alkalis are substances that dissolve
in water to give solutions with a pH greater than 7 and turn
litmus blue. The solutions contain an excess of hydroxide,
OH–, ions.
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What types of substance are
alkalis and bases?
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275
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Name
Alkalis
sodium hydroxide (caustic soda) NaOH
strong
potassium hydroxide
(caustic potash)
KOH
strong
calcium hydroxide (limewater)
Ca(OH)2
Pr
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in making sot soaps and biodiesel
to neutralise soil acidity and acidic gases produced
by power stations; has limited solubility
NH3(aq) or weak
NH4OH
in cleaning fluids in the home (degreasing agent);
in making fertilisers
CaO
for neutralising soil acidity and industrial waste;
in making cement and concrete
MgO
in antacid indigestion tablets
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magnesium oxide
strong
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calcium oxide
in oven cleaners (degreasing agent); in making soap
and paper; other industrial uses
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id
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op
C
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Bases
ammonia solution
(ammonium hydroxide)
Strong
Where found or used
or weak?
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Type
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Formula
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Table C5.07 Some common alkalis and bases.
The properties of bases, alkalis and antacids can be
summarised as follows.
The reactions of acids
y
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feel soapy to the skin
■ turn litmus blue
■ give solutions with a pH greater than 7
■ give solutions that contain OH– ions.
op
• a reactive metal for example, magnesium or zinc –
Image C5.07)
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• a base or alkali – a neutralisation reaction
Antacids are compounds that are used to neutralise
acid indigestion and include:
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• a metal carbonate or metal hydrogencarbonate .
magnesium oxide and magnesium hydroxide
■ sodium carbonate and sodium hydrogencarbonate
■ calcium carbonate and magnesium carbonate.
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Normally, we use the word ‘salt’ to mean ‘common salt’,
which is sodium chloride. This is the salt we put on our
food, the main salt found in seawater, and the salt used
over centuries to preserve food. However, in chemistry, the
word has a more general meaning.
Give the names of two examples of insoluble
bases and two examples of alkalis.
C5.24
Write word and balanced symbol equations for
the reaction between:
U
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C5.23
C
a sodium hydroxide and hydrochloric acid
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b potassium hydroxide and sulfuric acid.
KEy tERM
Which of the four alkalis in question C5.25 is
only a weak alkali?
salt: a compound made from an acid when a metal takes the
place of the hydrogen in the acid
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C5.26
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Name the four main alkalis.
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C5.25
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QuEStiONS
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One type of product is common to all these reactions.
They all produce a metal compound called a salt. In all
of them, the hydrogen present in the acid is replaced by a
metal to give the salt. The acid from which the salt is made
is oten called the parent acid of the salt.
s
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am
■
All acids can take part in neutralisation reactions. But are
there any other reactions that are characteristic of all acids?
The answer is ‘Yes’. There are three major chemical reactions
in which all acids will take part. These reactions are best
seen using dilute acid solutions. In these reactions, the acid
reacts with:
y
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■
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Alkalis are bases that dissolve in water, and:
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276
Pr
neutralise acids to give a salt and water only
■ are the oxides and hydroxides of metals
■ are mainly insoluble in water.
■
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Bases:
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C5.06 Characteristic reactions
of acids
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a
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C5: Acids, bases and salts
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The salt made depends on the acid:
TIP
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You may be asked a question where you have to suggest a
metal that will react with an acid to give hydrogen. Do not
give any of the very reactive metals, such as calcium, as
an answer. Your answer will be marked as wrong, as this
reaction is unsafe!
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Pr
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■
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Image C5.07 a Magnesium ribbon and b zinc granules,
reacting with hydrochloric acid – giving of hydrogen.
es
Pr
y
This is the neutralisation reaction that we saw in
Section C5.05:
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acid + base
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The reaction of acids with bases and alkalis
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Other examples of salts made from diferent combinations
of acid and base are shown in Table C5.08.
Pr
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NaCl(aq) + H2O(l)
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Salt made with ...
C
Nitric acid (HN03)
Sulfuric acid (H2SO4)
sodium hydroxide (NaOH)
sodium chloride, NaCl
sodium nitrate, NaNO3
sodium sulfate, Na2SO4
potassium hydroxide (KOH)
potassium chloride, KCl
potassium nitrate, KNO3
magnesium oxide (MgO)
magnesium chloride, MgCl2
magnesium nitrate, Mg(NO3)2 magnesium sulfate, MgSO4
copper chloride, CuCl2
copper nitrate, Cu(NO3)2
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copper oxide (CuO)
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Hydrochloric acid (HCl)
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Table C5.08 Some examples of making salts.
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Mg(NO3)2(aq) + H2(g)
Base
ie
sodium hydroxide + hydrochloric acid
sodium chloride + water
NaOH(aq) + HCl(aq)
magnesium nitrate + hydrogen
salt + water
The salt produced by this reaction will again depend on
the combination of reactants used. To make a particular
salt, you choose a suitable acid and base to give a solution
of the salt you want. For example:
It is unsafe to try this reaction with very reactive metals
such as sodium or calcium. The reaction is too violent.
No reaction occurs with metals, such as copper, which are
less reactive than lead. Even with lead, it is dificult to see
any reaction in a short time.
Mg(s) + 2HNO3(aq)
ev
ZnCl2(aq) + H2(g)
Zn(s) + 2HCl(aq)
salt + hydrogen
metal + acid
magnesium + nitric acid
R
zinc chloride + hydrogen
s
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Metals that are quite reactive (not the very reactive ones,
see sections C8.01 and C8.04) can be used to displace the
hydrogen from an acid safely. Hydrogen gas is given of.
The salt made depends on the combination of metal and
acid used:
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am
zinc + hydrochloric acid
The reaction of acids with metals
For example:
hydrochloric acid always gives a chloride
nitric acid always gives a nitrate
■ sulfuric acid always gives a sulfate
■ ethanoic acid always gives an ethanoate.
■
Copyright Material - Review Only - Not for Redistribution
potassium sulfate, K 2SO4
copper sulfate, CuSO4
277
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Cambridge IGCSE Combined and Co-ordinated Sciences
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AO3.3 Make and record observations, measurements
and estimates
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AO3.4 Interpret and evaluate experimental
observations and data
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id
hydrochloric acid in this case
C
All carbonates give of carbon dioxide when they react
with acids. We have seen that this reaction occurs with
efervescent antacid tablets. The result is to neutralise the
acid and produce a salt solution:
rs
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C
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9 Plot a graph with volume of acid added on the
x-axis and temperature on the y-axis.
10 Indicate using colour or a bar chart how the pH
changed during the experiment.
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Questions
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A1 Explain how and why the temperature changed during
the experiment.
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A2 How could the experiment be changed to obtain more
accurate results?
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br
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A worksheet is included on the CD-ROM.
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the reaction between an acid and
an alkali
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
-C
Estimate the volume of acid needed to
neutralise the alkali. Explain how you arrived at
your answer.
The Notes on activities for teachers/technicians
contain details of how this experiment can
be used as an assessment of skill AO3.3,
and ways in which the experiment can be
made more accurate.
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ACtivity C5.05
R
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Pr
CaCl2(aq) + H2O(l) + CO2(g)
NaCl + H2O
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2HCl(aq) + CaCO3(s)
NaOH + HCl
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hydrochloric acid + calcium carbonate
calcium chloride + water + carbon dioxide
6 Stir and record the new temperature and pH.
8 Repeat this process until a total of 20 cm3 of
acid have been added.
salt + water + carbon dioxide
The normal method of preparing carbon dioxide in the
laboratory is based on this reaction. Dilute hydrochloric
acid is reacted with marble chips (calcium carbonate):
5 Using a plastic pipette, add 1 cm3 of
hydrochloric acid to the mixture.
7 Add a further 1 cm3 of acid and again
record the temperature and pH.
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The reaction of acids with carbonates
Pr
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sodium hydroxide in this case
R
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the non-metallic part comes
from the acid
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br
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4 Use a pH chart to record the pH of the
solution.
the metal comes from the base
or alkali
acid + metal carbonate
This activity investigates what happens to pH and
temperature as an acid reacts with an alkali.
3 Place a thermometer in the solution and
record its temperature.
NaCl
278
! Wear eye protection.
2 Add a few drops of Universal Indicator – suficient
to produce an obvious colour.
U
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SODIUM CHLORIDE
AO3.5 Evaluate methods and suggest possible
improvements
1 Measure 10 cm3 of aqueous sodium hydroxide
into a beaker using a measuring cylinder.
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Pr
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It’s useful to realise the origins of a salt because it helps you
predict which salt you get from a particular combination of
acid and base. The cubic crystals of sodium chloride come
from the neutralisation of hydrochloric acid with sodium
hydroxide solution. For example:
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TIP
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TIP
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These analytical tests are very important – particularly
the tests for metal ions that give coloured precipitates.
Also important is the way that we can identify zinc and
aluminium salts using alkali.
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It is important to be able to give word equations for the
reactions in this section.
What are the formulae of sulfuric and
hydrochloric acids?
C5.28
Write word equations for the reaction of
hydrochloric acid with:
ni
C5.27
bung
b copper oxide
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antacid
tablet
s
Pr
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C5.07 Acids and alkalis in
chemical analysis
y
All salts are ionic compounds. They are made up of a
positive metal ion, combined with a negative non-metal
ion. Thus, common salt, sodium chloride, is made up of
sodium metal ions (Na+ ions) and chloride non-metal ions
(Cl− ions). Table C5.09 shows the ions that form certain
important salts.
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Pr
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es
All carbonates will react with acids to give of
carbon dioxide. We can use this as a test to find out if an
unknown substance is a carbonate or not. A piece of rock
that we think is limestone can be checked by dripping
a few drops of vinegar on it. If it ‘fizzes’, then it could
be limestone. A more usual test would be to add dilute
hydrochloric acid to the powdered substance.
Any gas given of would be passed into limewater
(calcium hydroxide solution) to see if it went cloudy.
If the limewater does turn cloudy, the gas is carbon
dioxide, and the substance is a carbonate. Figure C5.05
shows how an antacid tablet can be tested to see if it
contains a carbonate.
ity
Salt
Na+
Cl–
potassium nitrate
K+
NO3–
Cu2+
SO42–
Ca2+
CO32–
Na+
CH3COO–
C
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calcium carbonate
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Negative ion
sodium chloride
copper(II) sulfate
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sodium ethanoate
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s
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Table C5.09 The ions making up certain important salts.
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Positive ion
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In analysis it would be useful to have tests for the metal
ions in salts. We have seen that most metal hydroxides are
insoluble. By adding an alkali to a solution of the unknown
salt we can begin to identify the salt.
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ev
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Tests for metal ions in salts using alkalis
One important part of chemistry is the analysis
of unknown substances to find out what they are.
There is a series of tests that is important for this
(see Section C12.01). Acids and alkalis play an
important part in some of these tests. The chemistry
of these tests is discussed here.
R
ev
limewater
(calcium
hydroxide
solution)
Figure C5.05 Testing an antacid tablet containing a
carbonate as the active ingredient.
The test for carbonates using acid
R
dilute
hydrochloric
acid
es
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Write balanced chemical equations for the
reactions listed in question C5.28.
C
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am
d sodium carbonate.
C5.29
test tube
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a potassium hydroxide
c zinc
delivery tube
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QuEStiONS
These tests come up frequently in exams because they
are so distinctive, so it would be good to learn them.
The ability to tell an iron(II) salt from an iron(III) salt
is important.
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Pr
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Being able to give balanced chemical equation will be
even more useful.
C
op
TIP
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C5: Acids, bases and salts
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279
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Pr
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The test for ammonium salts using alkali
Ammonium salts are important as fertilisers. For example,
ammonium nitrate and ammonium sulfate are used
extensively as fertilisers. These are industrially important
chemicals made by reacting ammonia with nitric acid
or sulfuric acid, respectively. They are salts containing
ammonium ions, NH4+ ions. These salts react with alkali
solutions to produce ammonia gas, which can be detected
because it turns damp red litmus paper blue:
and OH ions combine to form a
precipitate of Fe(OH)2; Na+ and SO42–
ions stay in solution.
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–
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Figure C5.06 a The precipitation of iron(II) hydroxide.
b The precipitation of iron(III) hydroxide. Note the diferent
colour of the precipitates.
ev
id
ammonium nitrate + sodium hydroxide
sodium nitrate + water + ammonia
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C5.31
What colour precipitate is produced when testing
for copper ions with sodium hydroxide solution?
What is the name of this precipitate?
ev
-R
s
es
Which alkali solution must be used to
distinguish between zinc ions and aluminium
ions in solution? What is the observation that
distinguishes between the two?
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Pr
op
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C5.08 Salts
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zinc sulfate + sodium hydroxide
zinc hydroxide + sodium sulfate
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op
A salt is a compound formed from an acid by the
replacement of the hydrogen in the acid by a metal. Salts
are ionic compounds. There is a wide range of types of salt.
Zn(OH)2(s) + Na2SO4(aq)
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Even though the precipitates are all white, the test is still
useful. When an excess of sodium hydroxide is added, the
zinc and aluminium hydroxide precipitates re-dissolve
to give colourless solutions. The calcium hydroxide
precipitate does not re-dissolve.
C
ZnSO4(aq) + 2NaOH(aq)
The importance of salts – an introduction
U
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Sodium chloride is essential for life and is an
important raw material for industries. Biologically, it has a
number of functions: it is involved in muscle contraction;
it enables the conduction of nerve impulses in the
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ev
Write the word equation for the reaction between
a carbonate and hydrochloric acid.
C5.32
White hydroxide precipitates
Certain hydroxide precipitates are white. They are the
hydroxides of calcium, zinc and aluminium. The addition
of sodium hydroxide to a solution of a salt of these metals
produces a white precipitate in each case. For example:
R
C5.30
C
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Fe(OH)2(s) + Na2SO4(aq)
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QuEStiONS
iron(II) sulfate + sodium hydroxide
iron(II) hydroxide + sodium sulfate
FeSO4(aq) + 2NaOH(aq)
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This reaction occurs because ammonia is a more
volatile base than sodium hydroxide. Ammonia is
therefore easily displaced from its salts by sodium
hydroxide. The reaction can be used to test an unknown
substance for ammonium ions. It can also be used to
prepare ammonia in the laboratory.
• Iron(III) salts give a red-brown precipitate of
iron(III) hydroxide.
For example:
NaNO3(aq) + H2O(l) + NH3(g)
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• Iron(II) salts give a light green precipitate of
iron(II) hydroxide.
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• Copper(II) salts give a light blue precipitate of
copper(II) hydroxide.
280
NH4NO3(s) + NaOH(aq)
Pr
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br
Coloured hydroxide precipitates
Some of the hydroxide precipitates are coloured.
As a result, a solution of a salt can be tested by adding
an alkali to it and checking the colour of the precipitate
(Figure C5.06):
ie
Fe(OH)2(s)
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Fe2+
iron(II)
sulfate
solution
Na+
Na+
SO42–
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sodium
hydroxide
solution
Fe2+ 2–
SO4
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Na+ Na+ OH–
OH–
To identify a zinc or aluminium salt, the test needs to be
repeated with ammonia solution. The same white precipitates
of zinc or aluminium hydroxide are produced. However,
with excess ammonia solution it is only the zinc hydroxide
precipitate that re-dissolves, not the aluminium hydroxide.
Therefore we can tell the two apart using ammonia solution.
b
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a
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Method A – Acid plus solid metal,
base or carbonate
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Two things are important in working out a method
of preparation:
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• Stage 2: The excess solid is filtered out.
• Stage 3: The filtrate is gently evaporated to
concentrate the salt solution. This can be done
on a heated water bath (Figure C5.07) or sand tray
(Image C5.08).
w
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• Is the salt soluble or insoluble in water?
rs
ve
• Stage 5: The concentrated solution is cooled to
let the crystals form. The crystals are filtered of and
washed with a little distilled water. Then the crystals
are dried carefully between filter papers.
op
y
glass rod
ie
hydrogen
metal
heat
(ii)
Add an excess of the metal oxide
to the acid. Wait until the solution
no longer turns blue litmus paper red.
es
Pr
A glass rod is
dipped into
the solution and
then taken out
to cool; when
small crystals
form on the rod,
the solution is
ready to remove
from the bath.
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mixture
c
evaporating
basin
y
C
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residue left in
filter paper
(the excess of
the solid reactant)
evaporating
dish
metal carbonate
(iii)
Add an excess of the metal carbonate to
the acid. Wait until no more carbon dioxide is
given off.
s
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C
filter funnel
carbon
dioxide
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id
metal oxide
(i)
Warm the acid. Switch off the Bunsen burner.
Add an excess of the metal to the acid.
Wait until no more hydrogen is given off.
b
glass rod
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dilute acid
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Soluble salts can be made from their parent acid using any
of the three characteristic reactions of acids we outlined
earlier (Section C5.06).
filtrate
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filtrate
(a solution
of the salt)
Crystals form as solution cools;
filter, wash and then dry them.
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am
Figure C5.07 Method A for preparing a soluble salt. a Stage 1: the acid is reacted with either (i) a metal, (ii) a base or (iii) a carbonate.
b Stage 2: the excess solid is filtered out. c Stage 3: the solution is carefully evaporated. d Stage 4: the crystals are allowed to form.
es
ev
• Stage 4: When crystals can be seen forming
(crystallisation point), heating is stopped and the
solution is let to crystallise.
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s
Pr
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es
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am
br
The first point influences the preparation method chosen.
The second point afects how the crystals are handled
at the end of the experiment.
ev
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• Do crystals of the salt contain water of crystallisation?
a
• Stage 1: An excess (more than enough) of the
solid is added to the acid and allowed to react.
Using an excess of the solid makes sure that all the
acid is used up. If it is not used up at this stage,
the acid would become more concentrated when
the water is evaporated later (stage 3).
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While a number of salts can be obtained by mining,
others must be made by industry. Therefore, it is worth
considering the methods available to make salts. Some of
these can be investigated in the laboratory.
C5.09 Preparing soluble salts
Method A is essentially the same whether you
are starting with a solid metal, a solid base or a
solid carbonate. The method can be divided into
four stages (Figure C5.07).
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nervous system; it regulates osmosis (the passage
of solvent molecules through membranes); and it is
converted into the hydrochloric acid that aids digestion
in the stomach. When we sweat, we lose both water and
sodium chloride. Loss of too much salt during sport and
exercise can give us muscle cramp. Isotonic drinks are
designed to replace this loss of water and to restore energy
and the balance of mineral ions in our body.
C
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C5: Acids, bases and salts
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281
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Cambridge IGCSE Combined and Co-ordinated Sciences
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7 Filter of the unreacted solid, collecting the clear blue
solution in a 100 cm3 conical flask A fluted filter paper
can be used to speed up the filtration.
Questions
A1 Write word and balanced chemical equations for the
reaction taking place.
y
C
op
The preparation of magnesium sulfate crystals
(Epsom salts) is included in the Notes on activities for
teachers/technicians.
ge
U
R
9 Pour the hot solution into a clean, dry dish and
watch the crystals grow!
A2 What does the fact that there is some unreacted solid
let ater the reaction tell you about the proportions
of reactants used? Why is it useful that the reaction is
carried out with these proportions?
ni
ev
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w
C
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Pr
es
s
-C
-R
8 Boil the solution for 2–3 minutes.
br
ev
id
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w
Image C5.08 Evaporating of the water to obtain salt crystals.
Here a sand tray is being used to heat the solution carefully.
s
rs
colourless
acid
pipette
w
ge
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id
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alkali
br
-R
burette
es
s
am
-C
conical flask
ity
indicator
U
op
y
ni
ve
rs
ater adding
the indicator
evaporation of the
solution and
crystallisation as
in method A
c
ev
ie
id
g
w
e
C
Figure C5.08 Method B (the titration method) for preparing
a soluble salt. a Stage 1: the burette is filled with acid and
a known volume of alkali is added to the conical flask.
b Stage 2: the acid is added to the alkali until the endpoint is reached. c Stage 3: the solution is evaporated and
crystallised as for method A.
es
s
-R
br
am
-C
end-point has
been reached
tap
Pr
op
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C
w
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b
C
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R
y
a
ni
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Quick and easy copper(II) sulfate crystals
Skill:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
Wear eye protection. Note that sulfuric acid
!
is an irritant at the concentration used in
this activity.
This activity is an adaptation of the larger-scale method of
preparing a soluble salt (see Figure C5.07).
1 Pour 15 cm3 of 2 mol/dm3 sulfuric acid into a
boiling tube.
2 Place the tube in a beaker half-filled with boiling water
from a kettle.
3 Weigh out between 1.8 g and 2.0 g of copper(II) oxide.
4 Add half the copper(II) oxide to the acid in the boiling tube.
Agitate the boiling tube and return it to the hot water.
5 When the solid has dissolved, add the remaining
portion of copper(II) oxide.
6 Keep the tube in the hot water for 5 more minutes,
taking it out occasionally to agitate.
op
ACtivity C5.06
Method B (the titration method) involves the neutralisation
of an acid with an alkali (for example, sodium hydroxide)
or a soluble carbonate (for example, sodium carbonate).
Since both the reactants and the products are colourless,
an indicator is used to find the neutralisation point or
end-point (when all the acid has just been neutralised).
The method is divided into three stages (Figure C5.08).
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282
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op
Pr
y
Always remember to finish your description of a method
of preparing salt crystals with at least the words ‘filter,
wash and carefully dry the crystals’ to cover the final
stages of the preparation.
es
-C
TIP
-R
am
Method B – Acid plus alkali by titration
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C
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C5: Acids, bases and salts
ge
• Stage 1: The acid solution is poured into a burette.
The burette is used to accurately measure the volume
of solution added. A known volume of alkali solution
is placed in a conical flask using a pipette. The pipette
delivers a fixed volume accurately. A few drops of an
indicator (for example, thymolphthalein or methyl
orange, Figure C5.09) are added to the flask.
What colour is the indicator methyl orange in alkali?
C5.34
In the methods of preparing a salt using a solid metal,
base or carbonate, why is the solid used in excess?
C5.35
In such methods, what method is used to remove
the excess solid once the reaction has finished?
C5.36
Name the two important pieces of graduated
glassware used in the titration method of
preparing a salt.
C5.37
Why should the crystals prepared at the end of
these experiments not be heated too strongly
when drying them?
U
R
ni
C
op
y
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op
y
• Stage 2: The acid solution is run into the flask from the
burette until the indicator just changes colour. Having
found the end-point for the reaction, the volume of acid
run into the flask is noted. The experiment is then repeated
without using the indicator. The same known volume of
alkali is used in the flask The same volume of acid as noted
in the first part is then run into the flask. Alternatively,
activated charcoal can be added to remove the coloured
indicator. The charcoal can then be filtered of.
C
w
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C5.33
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Pr
es
s
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QuEStiONS
ie
w
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C5.10 Choosing a method of
salt preparation
id
• Stage 3: The salt solution is evaporated and cooled to
form crystals as described in method A.
br
ev
Soluble salts
The choice of method for preparing a soluble salt (see
Section C5.09) depends on two things:
s
es
Pr
y
op
a
red
Making salts by precipitation
y
op
U
blue
colourless
w
ge
R
thymolphthalein
The reaction between marble chips (calcium carbonate)
and sulfuric acid would be expected to produce a strong
reaction, with large amounts of carbon dioxide being
given of. However, the reaction quickly stops ater a very
short time. This is caused by the fact that calcium sulfate
is insoluble. It soon forms a layer on the surface of the
marble chips, stopping any further reaction.
C
ni
ev
ve
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rs
yellow
methyl orange
y
e
C
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op
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w
KEy tERMS
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precipitation: the sudden formation of a solid, either:
■ when two solutions are mixed, or
■ when a gas is bubbled into a solution
es
s
-R
br
am
Figure C5.09 a The colour changes for the indicators
methyl orange and thymolphthalein. b The actual colours
of methyl orange in acid and alkali.
-C
This reaction emphasises that some salts are insoluble in water
(for example, silver chloride and barium sulfate – precipitations
that are important in analysis). Such salts cannot be made by
the crystallisation methods we have described earlier. They are
generally made by ionic precipitation.
For example, barium sulfate can be made by taking a solution
of a soluble sulfate (such as sodium sulfate). This is added
to a solution of a soluble barium salt (for example, barium
nitrate). The insoluble barium sulfate is formed immediately.
ni
ve
rs
C
ity
Pr
op
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es
s
-C
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br
ev
id
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Add acid until the colour just changes.
b
• Is the metal reactive enough to displace the hydrogen in
the acid? If it is, is it too reactive and therefore unsafe?
• Is the base or carbonate soluble or insoluble? Figure C .
shows a flow chart summarising the choices.
acid
ity
alkali
w
C
-R
am
-C
This titration method is very useful not simply for
preparing salts but also for finding the concentration of a
particular acid or alkali solution (see Section C6.05).
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283
ve
rs
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Does it
react safely ?
Yes
No
Pr
es
s
-C
No
Is the base or
carbonate soluble?
Method A: can prepare
salt by reacting acid with excess
solid, followed by filtration,
e.g. CuSO4. 5H2O
ve
rs
ity
Method B: can use
titration method,
e.g. NaCl, K2SO4, NH4NO3
y
C
Yes
ni
w
ge
U
Figure C5.10 Flow chart showing which method to use for preparing soluble salts. The two methods A and B are described
in the text and in Figures C5.07 and C5.08.
ie
What happens to ions in neutralisation?
id
This solid ‘falls’ to the bottom of the tube or beaker as a
precipitate. The precipitate can be filtered of. It is then
washed with distilled water and dried in a warm oven. The
equation for this reaction is:
ev
An acid can be neutralised by an alkali to produce a salt
and water only, according to the general equation:
-R
br
es
Pr
y
op
ve
op
y
All these compounds are completely ionised, except
for the water produced.
ni
C5.38
C
U
There are two general methods of preparing
soluble salts:
ge
The hydrogen ions from the acid and the hydroxide
ions from the alkali combine to form water molecules.
w
br
ev
id
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Method B – add an excess of base or metal to a
dilute acid and remove the excess by filtration.
method
-R
s
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+
iii word equation.
H
op
C
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water molecule
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g
ii reagent
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e
method
O
ie
-R
ev
H
H
s
KCl + H2O
es
am
br
iii copy and complete the following symbol
equation
+
–
O
b the soluble salt, potassium chloride, from the
soluble base, potassium hydroxide
i
both containing
a few drops of
Universal
Indicator
hydroxide ion
ni
ve
rs
ii reagent
acidic
solution
H+
ity
i
-C
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a the soluble salt, zinc sulfate, from the
insoluble base, zinc oxide
hydrogen ion
Pr
-C
am
For each of the following salt preparations,
choose one of the methods, A or B, name any
additional reagent needed and then write or
complete the equation asked for.
y
w
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ev
QuEStiONS
R
hydrochloric acid + sodium hydroxide
sodium chloride + water
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
rs
C
ity
This equation shows how important state symbols can be – it is
the only way we can tell that this equation shows a precipitation.
ev
salt + water
For example:
Method A – use a burette and an indicator.
R
acid + alkali
s
am
-C
barium nitrate + sodium sulfate
barium sulfate + sodium nitrate
Ba(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2NaNO3(aq)
284
Salt crystals prepared by
evaporation and crystallisation
C
op
op
y
No
w
ev
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R
Method A: can prepare
salt by using excess metal
and acid, followed by filtration,
e.g. MgSO4. 7H2O, ZnCl2
Yes
-R
Does the metal
react with acids ?
ev
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am
br
id
Start
w
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C
U
ni
op
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
alkaline
solution
ve
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C
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C5: Acids, bases and salts
We can show this in the following equation:
H2O(l)
water
y
acid: a molecule or ion that is able to donate a proton (H+ ion)
to a base
base: a molecule or ion that is able to accept a proton
ni
C
op
C
w
U
H+
ions in
hydrochloric
acid
ge
+
TIP
ie
id
–
br
Cl –
It is important to realise that a hydrogen ion (H+) is simply
a proton. Once the single electron of a hydrogen atom has
been removed to form the positive ion, all that is let is the
proton of the nucleus (Figure C5.12).
ev
ev
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R
Na+
O
KEy tERMS
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Pr
es
s
-C
This is the ionic equation for this neutralisation reaction.
The spectator ions (chloride and sodium ions) remain in
solution – which becomes a solution of sodium chloride
(Figure C5.11).
ions in sodium
hydroxide
In these reactions, the acid is providing hydrogen ions to
react with the hydroxide ions. In turn, the base is supplying
hydroxide ions to accept the H+ ions and form water.
This leads to a further definition of an acid and a base in
terms of hydrogen ion (proton) transfer:
-R
hydroxide ions
(in water)
hydrogen ions
(in water)
'spectator ions'
rs
+
O
a hydrogen atom
op
y
H
water
U
ni
ev
H
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C
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Na+
Cl –
R
+
Pr
op
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es
s
-C
-R
am
H
C
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id
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am
Summary
s
ity
■
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■
id
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■
ev
■
es
s
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am
-C
■
that bases are the ‘chemical opposites’ of acids and
they neutralise the efects of acids; alkalis are bases
that dissolve in water
that neutralisation between an acid and a base
produces a salt and water only
that acids have certain characteristic reactions with some
metals to give a salt and hydrogen gas, and with metal
carbonates to give a salt, water and carbon dioxide gas
how salts are produced when the hydrogen in the acid
is replaced by a metal
that salts are prepared in the laboratory by a series of
methods depending on the compound reacted with
the acid
C
C
w
ie
R
ev
■
■
Pr
op
y
es
how all acids contain hydrogen and dissolve in water
to give solutions with a pH below 7
that pH is a measure of the acidity or alkalinity of an
aqueous solution; acids have a pH below 7, alkalis
above 7 and a neutral solution a pH of 7
that indicators change colour depending on the pH
of the solution they are added to; some show a single
colour change (litmus, for example), while Universal
Indicator shows a range of colours depending on the
solution tested
how the dissolved oxides of non-metals usually form
acidic solutions and that metal oxides, if they dissolve,
usually form alkaline solutions
ie
-C
You should know:
■
a hydrogen ion (H+)
(the electron has been lost,
leaving just the proton
of the nucleus)
Figure C5.12 A hydrogen ion is simply a proton.
Figure C5.11 The reaction of the ions when hydrochloric
acid is mixed with sodium hydroxide.
■
w
am
br
id
H+(aq) + OH− (aq)
ev
ie
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By evaporating some of the water, the salt can be
crystallised out. In fact, the same ionic equation can be
used for any reaction between an acid and an alkali.
Copyright Material - Review Only - Not for Redistribution
285
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■
Pr
es
s
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how the neutralisation reaction between
any acid and alkali can be represented by the
ionic equation:
H2O(l)
H+(aq) + OH–(aq)
how some non-metal oxides are neutral, and some
metal oxides and hydroxides are amphoteric.
ev
ie
■
w
ge
-C
■
that acid solutions have an excess of H+ ions, while
alkali solutions have an excess of OH– ions
that the pH of a solution depends on the balance of
the H+ and OH– ion concentrations present; water is
neutral because these concentrations are equal in
pure water
am
br
id
■
C
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Cambridge IGCSE Combined and Co-ordinated Sciences
pH 7
pH 11
b
quicklime
slaked lime
-R
es
Pr
ity
op
C
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w
ge
id
[3]
s
-C
solution of
calcium hydroxide
-R
am
br
ev
flask
es
[Cambridge IGCSE Chemistry 0620 Paper 21 Q (part) November 2010]
Pr
op
y
ity
Which one of the following is a pH value for an acidic solution?
pH 3
pH 9
pH 13
es
s
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br
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id
g
w
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C
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op
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Describe how you would use litmus to test if a solution is acidic.
Acids react with metal carbonates.
i Write a word equation for the reaction of calcium carbonate with hydrochloric acid.
ii Calcium carbonate can be used to treat acidic soil. State one other use of
calcium carbonate.
iii Name one other compound that can be used to treat acidic soil.
am
b
c
pH 7
ni
ve
rs
a
-C
C
burette
Hydrochloric acid and ethanoic acid are both acidic in nature.
w
ie
ev
[1]
y
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R
2
[1]
[3]
s
-C
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C
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hydrochloric
acid
Describe how you would carry out this titration.
R
[1]
Some farmers use calcium hydroxide to control soil acidity.
i Why is it important to control soil acidity?
ii Acid rain can cause soil to become acidic. Describe how acid rain is formed.
Calcium hydroxide reacts with hydrochloric acid.
calcium hydroxide + hydrochloric acid
calcium chloride + water
i State the name of this type of chemical reaction.
ii A dilute solution of calcium hydroxide can be titrated with
hydrochloric acid using the apparatus shown.
am
c
limestone
ev
cement
ie
Which of the following is the common name for calcium hydroxide?
[1]
w
pH 6
ge
pH 3
286
C
op
ni
Which one of the pH values below is alkaline?
U
R
a
d
y
A solution of calcium hydroxide in water is alkaline.
id
1
br
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C
End-of-chapter questions
Copyright Material - Review Only - Not for Redistribution
[1]
[3]
[3]
[1]
[1]
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Hydrochloric acid reacts with iron to form iron(II) chloride and hydrogen.
Complete the equation for this reaction.
Fe +
HCl
FeCl2 +
Pr
es
s
-C
[Cambridge IGCSE Chemistry 0620 Paper 21 Q3 a–d June 2012]
Soluble salts can be made using a base and an acid.
y
3
[2]
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id
d
C
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C5: Acids, bases and salts
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C
op
Complete the method of preparing dry crystals of the soluble salt cobalt(II) chloride-6-water from the
insoluble base cobalt(II) carbonate. The method involves four steps. The first is as follows:
y
[4]
[Cambridge IGCSE Chemistry 0620 Paper 31 Q8 a November 2010]
U
R
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ev
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What are Steps 2, 3 and 4?
C
op
w
Step 1: Add an excess of cobalt(II) carbonate to hot dilute hydrochloric acid.
Ammonium nitrate and ammonium sulfate are both commercially produced fertilisers.
ie
w
ge
damp red
A student is given a white solid and is told
litmus paper
that it is either ammonium nitrate or
ammonium sulfate. She adds sodium
hydroxide solution to some of the solid
contained in a test-tube, and then warms
the mixture gently. The figure shows what
the student observed.
mixture of white
-R
s
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op
287
solid and sodium
hydroxide solution
ve
ni
op
gentle heat
[2]
-R
am
br
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id
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C
U
Explain the observation shown in the figure.
ii The student then makes an aqueous solution of the white solid and adds
hydrochloric acid and barium chloride solution. State what would be observed,
if anything, if the white solid is ammonium nitrate, or if it is ammonium sulfate.
Calcium carbonate is another compound that is sometimes added to soil.
State and explain how calcium carbonate can improve the quality of soil used for crops.
[2]
[2]
es
s
-C
[Cambridge IGCSE Co-ordinated Sciences 0654 Paper 22 Q12 b, c May 2014]
Copy and complete the table.
ity
pH of solution of oxide
i
ii
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op
C
U
neutral
e
Explain the term amphoteric.
Name two reagents that are needed to show that an oxide is amphoteric.
id
g
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basic
ie
ie
acidic
-R
s
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am
-C
[6]
[1]
[2]
[Cambridge IGCSE Chemistry 0620 Paper 31 Q2 November 2009]
br
b
Example
ni
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Type of oxide
ev
C
a
Pr
Oxides are classified as acidic, basic, neutral and amphoteric.
op
y
5
y
rs
C
w
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R
b
R
litmus paper
turns blue
Pr
y
-C
am
br
i
ev
a
id
4
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am
br
id
Pr
es
s
-C
y
ni
C
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C
w
ev
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es
s
-C
Pr
y
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C
■
ve
y
C
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C
w
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C
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In many diferent situations it is important to know not
only what is in a chemical product but also how much
of each substance there is. The fertiliser bags found
around a farm oten carry three numbers (Image C6.01).
The numbers tell the farmer the amounts of the three key
elements present in the fertiliser: that is, the percentages
ev
ie
id
g
The same demands apply in many areas of chemistry.
Environmental chemists need to check levels of
es
s
-R
br
am
-C
ie
ev
of nitrogen (N), phosphorus (P) and potassium (K).
The same idea lies behind the rules controlling the food
industry. For instance, European Union regulations
require all breakfast cereal packets to show the amounts
of various chemical substances (such as protein, fat and
vitamins) present in the cereal.
ity
C6.01 Chemical analysis and
formulae
R
op
ni
U
op
y
■
-R
■
s
■
es
■
ge
■
id
■
br
■
the relative atomic mass of elements
the relative formula mass of compounds
that substances react in fixed proportions by mass
the mole as the ‘accounting unit’ in chemistry
simple calculations involving the mole
calculations involving the mole and reacting masses
calculations involving gases
the concentration of solutions
the titration of acid and alkali solutions.
am
■
-C
R
ev
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w
rs
This chapter covers:
Pr
288
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C6
Quantitative chemistry
Copyright Material - Review Only - Not for Redistribution
ve
rs
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w
1.7 × 10–24
1
carbon-12
2.0 × 10–23
12
fluorine
3.2 × 10–23
19
-R
am
br
id
Pr
es
s
-C
y
Table C6.01 The relative masses of some atoms.
ni
C
op
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op
C
w
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id
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1 carbon atom
mass 12 units
Ar = 12
289
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id
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Pr
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ni
ve
rs
= 0.000 000 000 000 000 000 000 001 7 g
H
hydrogen (H)
Figure C6.01 The relative mass of atoms. Twelve hydrogen
atoms have the same mass as one atom of carbon-12.
A helium atom has the same mass as four hydrogens.
op
This must be taken into account. The relative atomic mass
(Ar) of an element is the average mass of an atom of the
element, taking into account the diferent natural isotopes
of that element (Table C6.02). So most relative atomic
masses are not whole numbers. But in this book, with the
exception of chlorine, they are rounded to the nearest
whole number to make our calculations easier.
C
U
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id
g
es
s
-R
br
am
helium (He)
H
y
It is much more useful and convenient to measure the
masses of atoms relative to each other (Table C6.01).
To do this, a standard atom has been chosen, against
which all others are then compared. This standard atom
is an atom of the carbon-12 isotope, the ‘mass’ of which is
given the value of exactly 12 (Figure C6.01).
-C
H H
element that have diferent masses because they
have diferent numbers of neutrons in the nucleus
(see Section C2.04). The majority of elements have
several isotopes (Figure C6.02).
mass of one hydrogen atom = 1.7 × 10 –24 g
The use of the mass spectrometer first showed the
existence of isotopes. These are atoms of the same
w
C
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He
br
am
-C
op
y
C
w
12 hydrogen atoms
mass 1 unit each
Ar = 1
Pr
y
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s
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br
-C
am
pollutants in the air caused by burning a particular fuel.
Polymer chemists require an estimate of how much
material a new and diferent reaction method will yield.
They need to check on losses through the purification
process. Medical researchers must find a safe dose for
an experimental drug. They must consider its possible
side efects by measuring the amounts of its metabolic
products in cells. A chemical formula or equation not only
tells us what happens but puts ‘numbers’ to it. This is vital
to modern chemistry.
The mass of a single hydrogen atom is incredibly small
when measured in grams (g):
ie
H H
H H H H H
H H H H H
C
C
w
ie
ev
R
Whole-number
ratio
hydrogen
Image C6.01 NPK fertiliser contains the plant nutrients
nitrogen, phosphorus and potassium. The 5–10–5 on the
bag refers to the ratio of these nutrients in the fertiliser:
5% nitrogen; 10% phosphorus; 5% potassium.
We need to be able to predict the amounts of substances
involved in chemical reactions. To do this, we must have a
good understanding of the atom. For some time now we
have been able to use the mass spectrometer as a way of
‘weighing’ atoms.
ev
Mass in grams
ev
ie
ge
Atom
Relative atomic mass
R
C
U
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op
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C6: Quantitative chemistry
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C
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op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Relative formula mass
ge
F
Cl
w
ev
ie
7
-C
35
Kr
83
37
84
86
117
118
119
120
y
U
O
16
fluorine
F
19
sodium
Na
23
magnesium
Mg
aluminium
Al
sulfur
S
chlorine
Cl
op
y
• Sodium chloride: Sodium chloride is an ionic solid.
It contains one chloride ion for each sodium ion
present. The formula unit of sodium chloride is
therefore Na+Cl–. So its relative formula mass is the
relative atomic mass of sodium plus the relative
atomic mass of chlorine:
C
32
35.5
ie
64
br
ev
Except for chlorine, all values have been rounded to the nearest
Mr(NaCl) = 23 + 35.5 = 58.5
op
y
es
s
-C
Table C6.02 The relative atomic masses of some elements.
KEy tERM
Pr
KEy tERM
relative molecular mass: (Mr) of a covalent substance; the
sum of the relative atomic masses of the elements present in a
molecule of the substance.
ni
ve
rs
C
ity
relative atomic mass: (Ar) of an element; the average mass of
naturally occurring atoms of the element on a scale where the
carbon-12 atom has a mass of exactly 12 units
y
If the substance is an ionic compound, this mass is called the
relative formula mass (Mr).
op
ie
w
-R
whole number.
e
C
U
It is important to note that the mass of an ion will be
the same as that of the parent atom. The mass of the
electron(s) gained or lost in forming the ion can be ignored
in comparison to the total mass of the atom.
ev
ie
id
g
w
The practical result of these definitions can be seen by
looking at further examples (Table C6.03).
es
s
-R
br
am
-C
ev
w
27
w
ni
ve
rs
Mr(H20) = (2 × 1) + 16 = 18
am
(a)
Cu
id
copper
• Water: Water is a liquid made up of H2O molecules
(H—O—H). Each molecule contains two hydrogen atoms
and one oxygen atom. So its relative molecular mass
is twice the relative atomic mass of hydrogen plus the
relative atomic mass of oxygen:
24
ge
R
ev
ie
w
C
290
U
am
-C
op
y
oxygen
ie
14
ev
N
nitrogen
Mr(H2) = 2 × 1 = 2
-R
12
s
C
carbon
ity
1
es
id
H
br
hydrogen
• Hydrogen: Hydrogen gas is made up of H2 molecules
(H—H). Each molecule contains two hydrogen atoms.
So its relative molecular mass is twice the relative
atomic mass of hydrogen:
Relative atomic
mass, Ar(a)
Pr
Symbol
ge
Element
R
C
op
ni
w
Figure C6.02 Many diferent elements have more than one
isotope. These bars show the proportions of diferent isotopes
for some elements. Fluorine is rare in having just one.
R
ev
ie
122
124
116
C
Sn
ve
rs
ity
op
y
82
-R
6
Atoms combine to form molecules or groups of ions.
The total masses of these molecules or groups of ions
provide useful information on the way the elements have
combined with each other. The formula of an element
or compound is taken as the basic unit (the formula
unit). The masses of the atoms or ions in the formula are
added together. The mass of a substance found in this
way is called the relative formula mass (Mr). For covalent
elements or compounds, where the substance is made
up of molecules, this mass is also referred to as the
relative molecular mass (Mr). Here we illustrate the
method by calculating the relative formula masses of
three simple substances.
Pr
es
s
Li
am
br
id
19
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
CO2
1C
2O
2
C
=
12
1 × 12
=
12
O
=
16
2 × 16
=
32
44
=
40
1 × 40
=
40
1C
C
=
12
1 × 12
=
12
3O
O
=
16
3 × 16
=
48
y
1S
2 × 14
=
28
=
1
8×1
=
8
=
32
1 × 32
=
32
O
=
16
4 × 16
=
64
ie
ev
-R
132
Mg
=
24
1 × 24
=
24
1S
S
=
32
1 × 32
=
32
4O
O
=
16
4 × 16
=
64
14H
H
=
1
14 × 1
=
14
7O
O
=
16
7 × 16
=
112
s
Pr
ity
op
rs
C
14
S
1Mg
es
MgSO4 · 7H2O
(one Mg2+ ion, one SO42– ion,
seven H2O molecules)
y
ve
246
ni
op
The figure 2 outside the brackets multiplies everything in the brackets; there are two ammonium ions in this formula.
(b)
C
U
R
The 7 means there are seven H2O molecules per MgSO4 formula unit.
ie
w
ge
ev
id
br
AO3.3 Make and record observations, measurements
and estimates
Pr
es
s
-R
am
ity
AO3.5 Evaluate methods and suggest
possible improvements
op
y
Reacting marble chips with acid
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
e
C
U
A worksheet, with a self-assessment checklist, is
included on the CD-ROM.
ev
ie
id
g
w
Follow-up experiment: Is eggshell pure
calcium carbonate? A worksheet on this activity
is included in the Notes on activities for
teachers/technicians.
es
s
-R
br
am
AO3.4 Interpret and evaluate experimental
observations and data
When marble is reacted with acid, it decomposes,
giving of carbon dioxide. This activity is designed to
find the percentage of the mass of marble released as
carbon dioxide.
ni
ve
rs
ACtivity C6.01
-C
w
C
op
y
-C
Pay particular attention to the example of ammonium
sulfate in Table C6.03. This is an example of a formula
that has brackets. Remember to take into account the
number outside the bracket when counting up all the
atoms of a particular type.
ie
C
op
H
40
=
Table C6.03 The relative formula masses of some compounds.
ev
100
w
ni
8H
U
N
id
y
=
2N
w
ie
ev
2×1
(NH4)2SO4
(two NH4+ ions, one SO42– ion)
(a)
TIP
R
1
Ca
br
am
-C
hydrated
magnesium
sulfate(b)
=
ge
ammonium sulfate
H
1Ca
ve
rs
ity
C
w
ev
ie
(a)
R
Pr
es
s
y
CaCO3
(one Ca2+ ion,
one CO32– ion)
op
calcium carbonate
Relative formula
mass, Mr
w
-R
2H
-C
carbon dioxide
H2
Relative
atomic masses
ev
ie
Atoms in
formula
am
br
id
hydrogen
Formula
ge
Substance
C
U
ni
op
y
C6: Quantitative chemistry
Copyright Material - Review Only - Not for Redistribution
291
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
The increase in mass is due to the oxygen that has now
combined with the magnesium. The mass of magnesium
used and the mass of magnesium oxide produced can be
found from the results.
y
WORKED EXAMPLE C6.01
How much magnesium oxide is produced from a
given mass of magnesium?
Here are some results obtained from this experiment:
a mass of empty crucible + lid = 8.52 g
b mass of crucible + lid + magnesium = 8.88 g
y
c mass of crucible + lid + magnesium oxide = 9.12 g
ni
C
op
C
ve
rs
ity
op
Several diferent groups in a class can prepare
magnesium oxide by heating a coil of magnesium
in a crucible (Figure C6.03). The crucible must first be
weighed empty, and then re-weighed with the
magnesium in it. The crucible is then heated strongly.
Air is allowed in by occasionally liting the lid
very carefully. Solid must not be allowed to escape
as a white smoke. Ater a while, the lid may be
taken of and the open crucible heated strongly.
The crucible and products are then allowed to cool
before re-weighing.
w
U
d mass of magnesium (b – a) = 0.36 g
ge
mass of magnesium oxide (c – a) = 0.60 g
id
ie
w
mass of oxygen combined with magnesium =
0.60 – 0.36 = 0.24 g
op
• the more magnesium used, the more oxygen
combines with it from the air and the more magnesium
oxide is produced
ie
• the graph is a straight line, showing that the ratio of
magnesium to oxygen in magnesium oxide is fixed.
A definite compound is formed by a chemical reaction.
heat
Mass of oxygen / g
es
Pr
0.2
0.1
y
TIP
0
U
Make sure you are familar with this type of quantitative
experiment, particularly the need to re-weigh until there
is no further change in mass. This ‘heating to constant
mass’ is a way of making sure that the reaction has
completely finished.
0.1
0.2
0.3
Mass of magnesium / g
id
g
w
e
C
0
0.4
ev
ie
Figure C6.04 A graph of the results obtained from heating
magnesium in air. The graph shows the mass of oxygen
(from the air) that reacts with various masses of magnesium.
es
s
-R
br
am
-C
ev
ie
w
ni
ve
rs
C
ity
op
y
Figure C6.03 Heating magnesium in a crucible.
R
0.3
s
-C
tripod
-R
am
br
ev
id
pipe-clay
triangle
w
ge
C
U
R
The results show that:
magnesium
ribbon
ni
ev
ve
ie
w
rs
crucible
The results of the various experiments in the class can be
plotted on a graph. The mass of oxygen combined with
the magnesium (y-axis) is plotted against the mass of
magnesium used (x-axis). Figure C6.04 shows some results
obtained from this experiment.
y
C
292
ity
op
Pr
y
es
s
-C
-R
am
br
ev
0.60 g of magnesium oxide is produced from heating
0.36 g of magnesium
op
ev
ie
R
ev
ie
Pr
es
s
-C
The idea that compounds are made up of elements
combined in fixed amounts can be shown experimentally.
Samples of the same compound made in diferent ways
always contain the same elements. Also, the masses of the
elements present are always in the same ratio.
-R
am
br
id
w
ge
Compound formation and
chemical formulae
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
ge
C
U
ni
op
y
C6: Quantitative chemistry
w
WORKED EXAMPLE C6.02
y
-R
We have:
0.24 g Mg producing 0.40 g MgO
ve
rs
ity
op
C
so 1 g Mg produces
For example, magnesium oxide always contains 60%
magnesium and 40% oxygen by mass; and ammonium
nitrate always contains 35% nitrogen, 60% oxygen and
5% hydrogen by mass.
y
= 1.67 g MgO
ni
U
R
so 12 g Mg produces 12 × 1.67 g MgO
ge
Similar experiments can be done to show that the water of
crystallisation present in a particular hydrated salt, such
as hydrated copper(II) sulfate (CuSO4 · 5H2O), is always the
same fraction of the total mass of the salt.
ev
id
ie
w
= 20 g MgO
br
-R
am
s
-C
Pr
y
es
ACtivity C6.02
ity
Calculations of quantities like these are a very
important part of chemistry. These calculations show
how there is a great deal of information ‘stored’ in
chemical formulae and equations. The equation for
the reaction between magnesium and oxygen defines
the proportions in which the two elements always
react (Figure C6.05).
y
op
Magnesium reacts with oxygen to form
magnesium oxide. Work out the reacting masses
and the product mass.
ev
id
br
-R
am
-C
2 magnesium
atoms
+
2 oxygen atoms
s
Calculate the formula of magnesium oxide formed
when magnesium is heated in a crucible. Group results
can be processed as shown in the text and compared
with a novel method using a ‘bottle-top crucible’
rather than the conventional apparatus.
es
Mg = 24
O = 16
ity
ni
ve
rs
op
y
48 + 32 → 80
C
U
48 grams of magnesium react with 32 grams of
oxygen to form 80 grams of magnesium oxide.
id
g
w
e
Reacting amounts of substance
Relative formula masses can also be used to calculate the
amounts of compounds reacted together or produced in
reactions. Here is an example.
ev
ie
Figure C6.05 The proportions in which magnesium and
oxygen react are defined by the chemical equation for
the reaction.
es
s
-R
br
am
2 magnesium
atoms
+
2 oxygen atoms
For the product, work out the inner brackets first.
A worksheet is included on the CD-ROM.
-C
→
2MgO
(2 × 24) + (2 × 16) = [(2 × 24) + (2 × 16)]
+ 32 = [48 + 32]
48
+ 32 = 80
48
Pr
op
y
C
w
ie
+ O2 →
2Mg
ie
w
ge
C
U
R
ni
ev
ve
ie
w
rs
C
op
Finding the composition of magnesium oxide
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest
possible improvements
ev
R
0.40
g MgO
0.24
C
op
w
ev
ie
If 0.24 g of magnesium react with 0.16 g of oxygen to
produce 0.40 g of magnesium oxide (Figure C6.04),
how much magnesium oxide (MgO) will be produced
by burning 12 g of magnesium?
Pr
es
s
-C
am
br
id
ev
ie
A particular compound always contains the
same elements.
■ These elements are always present in the same
proportions by mass.
■ It does not matter where the compound is found or
how it is made.
■ These proportions cannot be changed.
■
Copyright Material - Review Only - Not for Redistribution
293
ve
rs
ity
The diagrams represent the structure of
six diferent compounds (A–F).
I–
Na+
U
C
I
Na
Cl
0.16
0.24
0.40
0.16
0.30
0.50
I
0.28
0.46
0.10
0.18
0.20
0.32
–
6
a Write down the correct mass of oxygen
that reacts with the magnesium in the last
four experiments.
b Plot a graph of the mass of oxygen
reacted against the mass of magnesium
used. Draw in the best-fit line for
these points.
rs
y
ve
ni
op
c Comment on what this graph line
shows about the composition of
magnesium oxide.
C
w
ie
Br
ev
br
id
H
F
ACtivity C6.03
-R
s
Pr
op
y
es
-C
Calculate the relative formula masses (Mr) of the
following substances:
a oxygen, O2
b ammonia, NH3
ity
c sulfur dioxide, SO2
e sulfuric acid, H2SO4
op
potassium bromide, KBr
C
U
g copper nitrate, Cu(NO3)2
w
e
h aluminium chloride, AlCl3
es
s
-R
br
ev
ie
id
g
(Relative atomic masses: H = 1, C = 12, N = 14,
O = 16, Al = 27, S = 32, Cl = 35.5, K = 39, Cu = 64,
Br = 80)
am
the efect of varying the quantity of a reactant
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
This investigation uses the reaction between magnesium
and dilute sulfuric acid to study the efect of varying the
amount of one reactant on the amount of product formed.
A worksheet is included on the CD-ROM.
y
ni
ve
rs
d octane, C8Hl8
-C
C
0.38
4
am
E
w
0.22
U
ge
F
F
ie
3
-R
Cl
D
Br
ev
0.10
7
Cl
ie
w
+
I
H
F
R
0.25
8
F
f
0.15
w
Na
H
ev
Na+
B
F
R
I–
–
C
C6.02
Na+
2
s
C
I–
0.04
es
H
Na+
0.10
5
+
br
–
I–
am
H
C
294
I
I–
0.06
Pr
y
C
op
H
-C
A
H
H
H
id
H
Na+
Oxygen
1
ie
ge
Na+
C
I–
Mass/g
Magnesium Magnesium
oxide
ity
R
H
Na+
Experiment
y
I–
ni
w
ev
ie
Na+
ve
rs
ity
c State the simplest formula for each
compound A to F.
C
op
y
Pr
es
s
b What type of bonding is present in
compound B?
C
op
-C
-R
a What type of bonding is present in
compounds A, C, D, E and F?
ev
C6.01
A class of students carry out an experiment
heating magnesium in a crucible (as described
earlier in this section). The table shows the results
of the experiments from the diferent groups in
the class.
ev
ie
C6.03
am
br
id
QUESTIONS
w
ge
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
U
ni
op
y
C6: Quantitative chemistry
op
C
op
w
ie
ev
es
3
4
Pr
2
Mass of iron / g
y
1
y
ie
ev
id
es
s
-R
br
am
-C
Relative formula
mass, Mr
Mass of one mole
(molar mass)
This mass (1 mol) contains
12 g
6.02 × 1023 carbon atoms
56 g
6.02 × 1023 iron atoms
C
12
iron
Fe
56
hydrogen
H2
2×1=2
oxygen
O2
2 × 16 = 32
32 g
water
H2O
(2 × 1) + 16= 18
18 g
6.02 × 1023 H2O molecules
magnesium oxide
MgO
24 + 16 = 40
40 g
6.02 × 1023 MgO ‘formula units’
100 g
6.02 × 1023 CaCO3 ‘formula units’
SiO2
y
op
C
w
40 + 12 + (3 × 16) = 100
28 + (2 × 16) = 60
ev
id
g
CaCO3
2g
60 g
6.02 × 1023 H2 molecules
6.02 × 1023 O2 molecules
6.02 × 1023 SiO2 ‘formula units’
-R
am
silicon(IV) oxide
br
calcium carbonate
e
ni
ve
rs
ity
carbon
U
Pr
op
y
Formula
C
es
s
Table C6.04 Calculating the mass of one mole of various substances.
-C
w
ie
ev
w
has a mass equal to its relative formula mass in grams
■ contains 6.02 × 1023 (the Avogadro constant) atoms,
molecules or formula units, depending on the
substance considered.
■
ge
When carrying out an experiment, a chemist cannot weigh
out a single atom or molecule and then react it with another
one. Atoms and molecules are simply too small. A ‘counting
unit’ must be found that is useful in practical chemistry.
This idea is not unusual when dealing with large numbers of
small objects. For example, banks weigh coins rather than
C
One mole of a substance:
U
R
ni
op
The mole – the chemical counting unit
ie
ev
ve
ie
rs
C
ity
Figure C6.06 Experiments on heating iron with sulfur
show that the two elements react in a fixed ratio by mass
to produce iron sulfide.
Substance
R
One mole of each of these diferent substances contains
the same number of atoms, molecules or formula units.
That number per mole has been worked out by several
diferent experimental methods. It is named ater the
19th-century Italian chemist, Amedeo Avogadro, and is
6.02 × 1023 per mole (this is called the Avogadro constant,
and it is given the symbol L). The vast size of this constant
shows just how small atoms are! For instance, it has
been estimated that 6.02 × 1023 sot-drink cans stacked
together would cover the surface of the Earth to a depth of
200 miles.
-R
s
-C
0
0
Chemists count atoms and molecules by weighing them.
The standard ‘unit’ of the ‘amount’ of a substance is taken
as the relative formula mass of the substance in grams.
This ‘unit’ is called one mole (1 mol) of the substance
(mol is the symbol or shortened form of mole or moles).
The unit ‘moles’ is used to measure amounts of elements
and compounds. The idea becomes clearer if we consider
some examples (Table C6.04).
y
ve
rs
ity
ni
U
ge
id
br
1.0
am
Mass of sulfur / g
1.5
0.5
w
ev
ie
Pr
es
s
y
op
C
w
ev
ie
2.0
R
-R
am
br
id
-C
A particular compound always contains the same
elements. They are always present in a fixed ratio by mass
(Figure C6.06). These two experimental results were of great
historical importance in developing the ideas of chemical
formulae and the bonding of atoms. How can we make the
link between mass ratios and the chemical formula of a
compound? To do this, we need to use the idea of the mole.
2.5
count them – they know that a fixed number of a particular
coin will always have the same mass. The number of sweets
in a jar can be estimated from their mass. Assuming that you
know the mass of one sweet, you could calculate how many
sweets were in the jar from their total mass. How can we
estimate the number of iron atoms in an iron block? Again,
we can try to link mass to the number of items present.
w
ge
C6.02 the mole and chemical
formulae
Copyright Material - Review Only - Not for Redistribution
295
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
w
ge
Calculations involving the mole
-C
-R
number of moles =
1 Write down the formula of the substance; for example,
ethanol is C2H5OH.
2 What is the mass of 0.5 mol of copper(II) sulfate
crystals?
We have: the relative formula mass of hydrated
copper(II) sulfate is:
C
ve
rs
ity
op
y
Pr
es
s
2 Work out its relative formula mass; for example, ethanol
contains two carbon atoms (Ar = 12), six hydrogen
atoms (Ar = 1) and one oxygen atom (Ar = 16). So for
ethanol Mr = (2 × 12) + (6 × 1) + 16 = 46.
y
molar mass of CuSO4 · 5H2O = 250 g/mol
ie
-R
s
es
mass
250 g / mol
mass = 0.5 × 250 = 125g
0.5 mol =
rs
y
ve
This shows that, if we need to calculate the mass of one
mole of some substance, the straightforward way is to
work out the relative formula mass of the substance and
write the word ‘grams’ ater it. Using the above equation it
is possible to convert any mass of a particular substance
into moles, or vice versa. We shall look at two examples.
ni
op
Working out chemical formulae
ev
id
ie
w
ge
C
U
The idea of the mole means that we can now work out
chemical formulae from experimental data on combining
masses. It provides the link between the mass of an element
in a compound and the number of its atoms present.
br
es
Pr
y
C
U
op
ie
ev
e
For giant structures, the formula of the compound is the simplest
whole-number formula – in this example, MgO. Silicon(IV)
oxide is a giant molecular structure. A sample of silicon
oxide with a mass of 10.0 g is found to contain 4.7 g of silicon.
How can we find its formula? This is done in Figure C6.07b.
id
g
w
R
60 g
no. of
moles
The formula of magnesium oxide tells us that 1 mol of
magnesium atoms combine with 1 mol of oxygen atoms.
The atoms react in a 1 : 1 ratio to form a giant ionic
structure (lattice) of Mg2+ and O2– ions.
ni
ve
rs
molar mass of NaOH = 40 g/mol
ity
op
y
-R
s
-C
am
br
ev
ie
40 g/mol
es
C
Mr(NaOH) = 23 + 16 + 1 = 40
s
-C
1 How many moles are there in 60 g of sodium
hydroxide?
We have: the relative formula mass of sodium
hydroxide is:
In the experiment to make magnesium oxide (see Section
6.01), a constant ratio was found between the reacting
amounts of magnesium and oxygen. If 0.24 g of magnesium
is burnt, then 0.40 g of magnesium oxide is formed. This
means that 0.24 g of magnesium combines with 0.16 g of
oxygen (0.40 – 0.24 = 0.16 g). We can now use these results
to find the formula of magnesium oxide (Figure C6.07a).
-R
am
WORKED EXAMPLE C6.03
w
mass
molar mass
Therefore,
ity
molar
mass
w
ie
ev
R
number of moles =
Pr
y
op
no. of
moles
C
296
0.5 mol 250 g/mol
ev
id
br
am
-C
mass
mass
w
ge
U
R
ni
For any given mass of a substance:
mass
number of moles =
molar mass
where the mass is in grams and the molar mass is in
grams per mole. The triangle shown below can be a
useful aid to memory: cover the quantity to be found
and you are let with how to work it out.
Mr(CuSO4 · 5H2O) = 64 + 32+(4 × 16) + (5 × 18) = 250
C
op
w
3 Express this in grams per mole; for example, the molar
mass of ethanol is 46 g/mol.
ev
ie
mass
molar mass
60 g
=
40 g / mol
number of moles = 1.5
ev
ie
am
br
id
You can find the molar mass (mass of one mole) of any
substance by following these steps.
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
Find the number
of moles of
atoms of each
element that
combine.
0.16 g
molar mass
24 g/mol
16 g/mol
number of moles
0.01 mol
0.01 mol
simplest ratio
1
w
ev
ie
y
C
op
ni
0.331 mol
1
2
If we tried to react 5 g of sulfur with 5.6 g of iron, the excess
sulfur would remain unreacted. Only 3.2 g of sulfur could
react with 5.6 g of iron: 1.8 g of sulfur (5.0 – 3.2 = 1.8 g)
would remain unreacted.
w
0.168 mol
am
SiO2
rs
The formula of silicon(IV) oxide is SiO2. It consists of a giant
molecular lattice of covalently bonded silicon and oxygen atoms
in a ratio 1 : 2. Since it is a giant structure, the formula we use for
this compound is SiO2.
The reacting amounts given by an equation can also be
scaled up (that is, use larger amounts). In industry, tonnes
of chemical reactants may be used, but the ratios given
by the equation still apply. The manufacture of lime is
important for the cement industry and agriculture. Lime is
made by heating limestone in lime kilns. The reaction is an
example of thermal decomposition:
op
calcium
carbonate
1 mol
1 mol
1 mol
40 + 12 + (3 × 16)
40 + 16
12 + (2 × 16)
= 100 g
= 56 g
= 44 g
w
CaO
-R
s
CO2
ity
When we write a chemical equation, we are
indicating the number of moles of reactants and
products involved in the reaction.
op
y
ni
ve
rs
This indicates that we need equal numbers of atoms of
iron and sulfur to react. We know that 1 mol of iron (56 g)
and 1 mol of sulfur (32 g) contain the same numbers of
atoms. Reacting these amounts should give us 1 mol of
iron(II) sulfide (88 g). The equation is showing us that:
C
U
This can be scaled up to work in tonnes:
100 tonnes
88 g
ie
32 g
44 tonnes
Similarly, if 10 tonnes of calcium carbonate were
heated, we should expect to produce 5.6 tonnes of lime
(calcium oxide).
ev
1 mol
-R
1 mol
56 tonnes
s
56 g
FeS
es
am
1 mol
S
w
e
id
g
+
br
Fe
-C
+
es
Pr
op
y
FeS
Fe + S
C
w
ie
ev
dioxide
CaCO3
ev
id
br
-C
am
We can now see that the chemical equation for a reaction
is more than simply a record of what is produced.
In addition to telling us what the reactants and products
are, it tells us how much product we can expect from
particular amounts of reactants. When iron reacts with
sulfur, the equation is:
carbon
+
ie
ge
C6.03 the mole and chemical
equations
calcium
oxide
C
U
R
ni
ev
ve
ie
w
C
ity
op
Pr
y
es
s
-C
Figure C6.07 a Calculating the formula of magnesium
oxide from experimental data on the masses of magnesium
and oxygen that react together. b Calculating the formula
of silicon(IV) oxide from mass data.
R
8.8 g
ie
16 g/mol
3.2 g
ev
28 g/mol
FeS
y
U
br
simplest ratio
10.0 – 4.7 = 5.3 g
ge
number of moles
id
molar mass
S
+
5.6 g
O
4.7 g
mass in 10.0 g
Fe
-R
C
op
y
MgO
ev
ie
w
We could use:
1
Si
Formula
The mass of the product is equal to the total mass of the
reactants. This is the law of conservation of mass, which
we met in Chapter C4. Although the atoms have rearranged
themselves, their total mass remains the same. A chemical
equation must be balanced. In practice, we may not want
to react such large amounts. We could scale down the
quantities (that is, use smaller amounts). However, the
mass of iron and the mass of sulfur must always be in the
ratio 56 : 32.
-R
0.24 g
ve
rs
ity
mass combined
Pr
es
s
am
br
id
O
-C
Mg
Formula
R
Find the
simplest
whole-number
ratio.
ge
Find the number
of grams of the
elements that
combine.
C
U
ni
op
y
C6: Quantitative chemistry
Copyright Material - Review Only - Not for Redistribution
297
ve
rs
ity
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Pr
es
s
-C
y
br
y
w
-R
am
s
2Al2O3
3O2
y
C
U
R
ration = 4 mol : 2 mol
mass = ?
w
ge
9.2 g
id
br
ev
Step 1 (the ‘up’ stage): Convert 9.2 g of Al into moles:
-R
am
9.2 g
= 0.34 mol
27 g / mol
es
ity
C
ni
ve
rs
0.34 mol of Al produce 0.17 mol of Al2O3
C
U
w
ie
ev
br
so
iii Why was it necessary to repeat the process
until there was no further change in mass?
es
s
-R
mass of Al2O3 produced = 0.17 × 102 g = 17.3 g
am
Where is the most suitable place to clamp
the tube?
ii Why was the hydrogen passed through for
15 seconds before the gas was lit?
e
mass
102 g / mol
a i
id
g
0.17 mol =
y
Step 3 (the ‘down’ stage): Work out the mass of this
amount of aluminium oxide (the relative formula mass
of Al2O3 is 102):
-C
ev
ie
w
so
R
Hydrogen was passed through the tube for
15 seconds before the escaping gas was lit.
The tube was heated for a few minutes.
The apparatus was then allowed to cool with
hydrogen still passing through. The tube was
re-weighed. The process was repeated until there
was no further change in mass.
Pr
op
y
4 mol of Al produce 2 mol of Al2O3
heat
s
-C
dry hydrogen
from a cylinder
Step 2 (the ‘across’ stage): Use the ratio from the
equation to work out how many moles of Al2O3
are produced:
unused
hydrogen
burning
copper(II) oxide
ie
Then we work through the steps of the ‘footbridge’.
number of moles =
Copper(II) oxide can be reduced to copper
metal by heating it in a stream of hydrogen gas.
Dry copper(II) oxide was placed in a tube which
had previously been weighed empty. The tube
was re-weighed containing the copper(II) oxide
and then set up as in the diagram.
op
ni
ev
+
C6.04
ve
4Al
QUESTIONS
rs
C
To answer this question, we first work out the
balanced equation:
ity
op
Pr
es
-C
y
What mass of aluminium oxide is produced when
9.2 g of aluminium metal reacts completely with
oxygen gas?
w
The limiting reactant is the one that is not in excess – there
will be a smaller number of moles of this reactant present,
taking into account the reacting ratio from the equation.
ev
P4O10
WORKED EXAMPLE C6.04
ie
ie
O
O
In carrying out a reaction, one of the reactants may be
present in excess. Some of this reactant will be let over at
the end of the reaction.
id
R
U
O
O
298
TIP
ge
P
O
P
C
op
O
O
op
P
Figure C6.09 A chemical
‘footbridge’. Following the
sequence ‘up–across–down’
helps to relate the mass of
product made to the mass of
reactant used. The ‘bridge’
can, of course, be used in the
reverse direction.
ni
ev
ie
w
O
P
Remember also to take the balancing numbers into
account in making your calculation (this is called the
stoichiometry of the equation).
ve
rs
ity
C
op
We shall consider an example.
O
Remember to read questions on reacting masses
carefully. If you set out the calculation carefully, using the
equation as we have done here, you will be able to see
which substances are relevant to your calculation.
-R
am
br
id
TIP
We can use the idea of the mole to find reactant or product
masses from the equation for a reaction. There are various
ways of doing these calculations. The balanced equation
itself can be used as a numerical ‘footbridge between the
two sides of the reaction (Figure C6.09).
O
ev
ie
w
ge
Calculating reacting amounts – a chemical
‘footbridge’
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
w
ge
C
U
ni
op
y
C6: Quantitative chemistry
am
br
id
TIP
= .......g
= 47.40 g
E Mass of copper produced (D – A)
F Mass of oxygen in the
copper(II) oxide
= .......g
= .......g
ve
rs
ity
y
op
C
w
Pr
es
s
= 47.72 g
C Mass of copper(II) oxide (B – A)
-C
B Mass of tube + copper(II) oxide
D Mass of tube + copper
ev
ie
Remember that the molar gas volume is given at the
bottom of the Periodic Table you are given in the exam.
The value is given as 24 dm3 at r.t.p. Do not forget that
1 dm3 = 1000 cm3.
= 46.12 g
-R
A Mass of empty tube
i
ev
ie
b The results for the experiment are
given below.
Copy out and complete the results
table above.
number of moles =
y
ii How many moles of copper atoms are
involved in the reaction? (Relative atomic
mass: Cu = 64)
C
op
ni
es
Pr
rs
op
ni
C
U
R
WORKED EXAMPLE C6.05
id
ie
w
ge
If 8 g of sulfur are burnt, what volume of SO2 is
produced?
es
1 mol
ity
sulfur dioxide
SO2(g)
O2(g)
1 mol
24 dm
1 mol
3
24 dm3
ni
ve
rs
We have
8g
32 g / mol
= 0.25 mol
y
number of moles of sulfur burnt =
op
C
U
From the equation:
id
g
w
e
1 mol of sulfur
1 mol of SO2
ev
ie
Therefore:
0.25 mol of sulfur
es
s
-R
br
am
+
32 g
One mole of any gas occupies a volume of
approximately 24 dm3 (24 litres) at room
temperature and pressure (r.t.p.).
■ The molar volume of any gas therefore has the
value 24 dm3/mol at r.t.p.
■ Remember that 1 dm3 (1 litre) = 1000 cm3.
-C
sulfur + oxygen
S(s)
Pr
op
y
C
ie
w
-R
s
-C
am
br
ev
First consider the reaction of sulfur burning in
oxygen.
In a gas, the particles are relatively far apart. Indeed,
any gas can be regarded as largely empty space. Equal
volumes of gases are found to contain the same number of
particles (Table C6.05); this is Avogadro’s law. This leads to
a simple rule about the volume of one mole of a gas.
■
299
y
ev
C6.04 Calculations involving gases
The volume of one mole of a gas
ev
molar
volume
For reactions in which gases are produced, the calculation
of product volume is similar to those we have seen already.
ve
ie
w
no. of
moles
Reactions involving gases
ity
y
op
C
vi Write a word equation for the reaction
and then, using the calculated formula
for copper(II) oxide, write a full balanced
equation for the reaction with hydrogen.
Many reactions, including some of those we have just
considered, involve gases. Weighing solids or liquids is relatively
straightforward. In contrast, weighing a gas is quite dificult. It is
much easier to measure the volume of a gas. But how does gas
volume relate to the number of atoms or molecules present?
R
volume
s
-C
am
iv From the results of the experiment,
how many moles of oxygen atoms have
combined with one mole of copper atoms?
-R
br
ev
id
ie
w
ge
iii How many moles of oxygen atoms are
involved in the reaction? (Relative atomic
mass: O = 16)
v From the results of the experiment, what is
the formula of copper(II) oxide?
volume
molar volume
where the volume is in cubic decimetres (dm3) and the
molar volume is 24 dm3/mol.
U
R
This rule applies to all gases. This makes it easy to convert
the volume of any gas into moles, or moles into volume:
Copyright Material - Review Only - Not for Redistribution
0.25 mol of SO2
ve
rs
ity
am
br
id
ev
ie
So, from the above rule:
volume
molar volume
C
6.02 × 1023 ethane
molecules
y
C
op
ie
y
C
When a chemical substance (the solute) is dissolved in a
volume of solvent, we can measure the ‘quantity’ of solute
in two ways. We can measure either its mass (in grams) or
its amount (in moles). The final volume of the solution is
normally measured in cubic decimetres, dm3 (1 dm3 = 1 litre
or 1000 cm3). When we measure the mass of the solute
in grams, it is the mass concentration that we obtain, in
grams per cubic decimetre of solution (g/dm3).
w
ity
y
op
C
w
ie
ev
U
e
mass or
volume of
gaseous
product
es
s
-R
br
id
g
Colourful tricks can be played with chemical substances.
A simple reaction can produce a ‘water into wine’ colour
change – when two colourless solutions mixed together
produce a wine-coloured mixture. These reactions all take
place in solution, as do many others. The usual solvent
is water. When setting up such reactions, we normally
measure out the solutions by volume. To know how much
of the reactants we are actually mixing, we need to know
the concentrations of the solutions.
But it is more useful to measure the amount in moles, in
which case we get the molar concentration in moles per
cubic decimetre of solution (mol/dm3):
amount of solute
concentration =
volume of solution
For example, a 1 mol/dm3 solution of sodium chloride
contains 58.5 g of NaCl (1 mol) dissolved in water and
made up to a final volume of 1000 cm3. Figure C6.11 shows
how the units are expressed for solutions of difering
concentrations. It also shows how solutions of the same
final concentration can be made up in diferent ways.
ni
ve
rs
C
w
ie
am
24
ie
es
Pr
moles of product
Figure C6.10 An outline of the ‘footbridge’ method for
calculations involving gases.
-C
30
ev
br
-C
op
y
ethane
(C2H6)
-R
2 volumes
ev
R
6.02 × 1023 carbon
dioxide molecules
s
1 volume
use ratio from
the equation
volume
of gas
24
The concentration of solutions
ge
id
2HCl(g)
Cl2(g)
So, if we react 20 cm3 of hydrogen with suficient chlorine,
it will produce 40 cm3 of hydrogen chloride gas.
moles of reactant
44
ev
48 dm3
am
1 volume
+
carbon
dioxide (CO2)
-R
2 mol
3
The volumes of the gases involved are in the same ratio as
the number of moles given by the equation:
H2(g)
6.02 × 1023 oxygen
molecules
op
24 dm
24
s
es
Pr
ity
rs
ve
1 mol
3
U
24 dm
2HCl(g)
Cl2(g)
ni
w
ie
ev
R
hydrogen chloride
hydrogen + chlorine
1 mol
32
w
ge
id
y
op
C
For example:
+
oxygen (O2)
C6.05 Moles and solution
chemistry
br
am
-C
Some important reactions involve only gases. For such
reactions, the calculations of expected yield are simplified by
the fact that the value for molar volume applies to any gas.
H2(g)
6.02 × 1023 hydrogen
molecules
Table C6.05 The molar mass and molar volume of
various gases.
U
R
ni
ev
ie
w
volume
0.25 mol =
24 dm3 / mol
volume of sulfur dioxide = 0.25 × 24 dm3
= 6 dm3 at r.t.p.
The approach used is an adaptation of the ‘footbridge’
method used earlier for calculations involving solids. It is
shown in Figure C6.10.
300
24
-R
24 dm3/
mol
0.25
mol
ve
rs
ity
op
y
volume
Number of particles
Molar
volume /
dm3/mol
2
hydrogen
(H2)
Pr
es
s
-C
number of moles =
Molar
mass /
g/mol
w
ge
Substance
C
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
U
ni
op
y
C6: Quantitative chemistry
w
ge
number of moles in solution
concentration
× volume of solution (in cm3)
=
1000
y
-R
ve
rs
ity
op
C
w
concentration
1000
ie
ev
dissolve to
make 1 dm3
of solution,
concentration
= 2 mol/dm3
dissolve to
make 2 dm3
of solution,
concentration
= 1 mol/dm3
es
rs
y
ve
op
U
R
ni
ev
Step 1: Find out how many moles of NaOH are
present:
C
Calculations using solution concentrations
ge
relative formula mass of NaOH = 23 + 16 + 1 = 40
10
= 0.25 mol
number of moles of NaOH =
40
w
ie
ev
id
br
number of moles
=
Pr
op
y
es
s
This equation can be represented by this triangle:
-C
Step 2: Find the concentration: number of moles
-R
am
number of moles in solution
= molar concentration × volume of solution (in dm3)
0.25 =
ni
ve
rs
volume/
dm3
0.25 × 1000
250
= 1 mol / dm3
ev
ie
id
g
es
s
-R
br
am
concentration
× 250
1000
w
e
C
U
In practice, however, we are usually dealing with solution
volumes in cubic centimetres (cm3). The equation is
therefore usefully adapted to:
-C
concentration
× volume (in cm3)
1000
y
concentration =
op
w
C
ity
moles
ie
ev
In practice, a chemist still has to weigh out a substance in
grams. So questions and experiments may also involve
converting between moles and grams.
Calculate the concentration of a solution of sodium
hydroxide, NaOH, that contains 10 g of NaOH in a
final volume of 250 cm3.
Figure C6.11 Making copper(II) sulfate solutions of
diferent concentrations.
concentration/
mol/dm3
3.0
× 500 = 1.5 mol
1000
WORKED EXAMPLE C6.06
The following equation is useful when working out the
number of moles present in a particular solution:
R
number of moles =
-R
1
Pr
dissolve to
make 2 dm3
of solution,
concentration
= 0.5 mol/dm3
1
ity
1
2
s
ge
id
y
dissolve to
make 1 dm3
of solution,
concentration
= 1 mol/dm3
op
C
w
ie
2
br
am
-C
1
2
We get:
C
op
For example, how many moles of sugar are there in
500 cm3 of a 3.0 mol/dm3 sugar solution?
U
R
2
volume/
cm3
y
2 mol of copper sulfate, CuSO4
ni
1 mol of copper sulfate, CuSO4
moles
w
ev
ie
where concentration is in moles per cubic decimetre, but
volume is in cubic centimetres.
Pr
es
s
-C
am
br
id
ev
ie
The mass concentration of a solution is measured
in grams per cubic decimetre (g/dm3).
■ The molar concentration of a solution is
measured in moles per cubic decimetre (mol/dm3).
■ When 1 mol of a substance is dissolved in water
and the solution is made up to 1 dm3 (1000 cm3),
a solution with a concentration of 1 mol/dm3
is produced.
■
Copyright Material - Review Only - Not for Redistribution
301
ve
rs
ity
C
w
ev
ie
-R
The method uses a further variation of the ‘footbridge’
approach to link the reactants and products (Figure C6.13).
ve
rs
ity
C
op
ni
Wash out
apparatus – repeat
several times,
and take average.
w
op
ie
moles of product
concentration
of product
solution
ev
id
w
ge
use ratio from
the equation
moles of reactant
-R
am
br
concentration
and volume of
reactant
solution
C
U
R
ni
ev
ve
ie
volume
of gas
y
mass or
volume of
gaseous
product
rs
C
ity
op
Pr
y
es
s
-C
-R
am
br
ev
id
ie
w
ge
U
R
Figure C6.12 Summary of the titration method.
302
Run acid into flask
until indicator just
changes colour –
read burette again.
Put acid in burette –
read starting
value.
y
y
op
C
Pipette 25 cm3 of
alkali into a
conical flask –
add indicator.
Make standard
alkali solution
accurately.
w
We shall now look at an example of the type of calculation
that can be carried out.
Pr
es
s
-C
am
br
id
The concentration of an unknown acid solution can be
found if it is reacted with a standard solution of an alkali.
A standard solution is one that has been carefully made up
so that its concentration is known precisely. The reaction is
carried out in a carefully controlled way. The volumes are
measured accurately using a pipette and a burette. Just
suficient acid is added to the alkali to neutralise the alkali.
ev
ie
This end-point is found using an indicator. The method
is known as titration, and can be adapted to prepare a
soluble salt. It is summarised in Figure C6.12.
ge
Acid–base titrations
U
ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
s
mass of
product
Pr
op
y
es
-C
mass of
reactant
y
op
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
R
ev
ie
w
ni
ve
rs
C
ity
Figure C6.13 A summary of the diferent ways in which a balanced equation acts as a ‘footbridge’ in calculations.
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
ge
C
U
ni
op
y
C6: Quantitative chemistry
concentration
× volume (in cm3)
1000
U
So:
number of moles =
w
ie
ev
br
-R
am
s
-C
concentration
× 20.0
1000
2.5 × 10−3 × 1000
20
es
= 0.125 mol / dm3
303
ity
op
Pr
y
concentration
× volume (in cm3)
1000
rs
ve
y
op
U
R
ni
ev
Calculation questions are oten structured for you, so make sure you work your way through the question as far as
you can go.
-R
Pr
In this activity, a hydrochloric acid solution of unknown
concentration is standardised against a solution of
sodium carbonate of known concentration. This is done
using the titration method.
A worksheet is included on the CD-ROM.
ev
ie
id
g
w
e
C
U
op
y
ni
ve
rs
ity
Details of a microscale version of the experiment
are given in the Notes on activities for teachers/
technicians.
es
s
-R
br
am
-C
ev
ie
w
C
op
y
es
Determining the concentration of a
hydrochloric acid solution
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
s
-C
am
br
ev
id
ie
w
ge
C
Always show your working when responding to a calculation question, because you may still get credit even if you make a
mistake in the final stage – it will also help you work out where you went wrong.
ACtivity C6.04
R
2.5 × 10−3 =
concentration of acid =
C
w
2.5 × 10 –3 mol of NaOH neutralise 2.5 × 10 –3 mol of HCl
0.10
× 2.5 = 2.5 × 10−3 mol
1000
TIP
ie
1 mol of NaOH neutralises 1 mol of HCl and so:
The acid solution contains 2.5 × 10 –3 mol in 20.0 cm3.
id
=
1 mol 1 mol
ge
R
=
NaCl + H2O
Step 3: Use the titration value. What is the concentration
of the acid?
ni
number of moles of NaOH
HCl + NaOH
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We have
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C
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Step 1: Use information about the standard
solution. How many moles of alkali are in
the flask?
The equation is:
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The calculation goes like this.
Step 2: Use the chemical equation. How many moles of
acid are used?
Pr
es
s
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A solution of hydrochloric acid is titrated against
a standard sodium hydroxide solution. It is found
that 20.0 cm3 of acid neutralise 25.0 cm3 of 0.10 mol/
dm3 NaOH solution. What is the concentration of the
hydrochloric acid solution?
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WORKED EXAMPLE C6.07
Copyright Material - Review Only - Not for Redistribution
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QuEStiONS
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Cambridge IGCSE Combined and Co-ordinated Sciences
C6.05
Calculate the number of moles of gas there are in the following:
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a 480 cm3 of argon
b 48 dm3 of carbon dioxide
Pr
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C6.06
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c 1689 cm3 of oxygen.
Calculate the volume in cm3 of the following at r.t.p.
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b 0.06 moles of ammonia
c 0.5 moles of chlorine.
Calculate the concentration (in mol/dm3) of the following solutions.
C6.07
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a 1.5 moles of nitrogen
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a 1.0 mol of sodium hydroxide is dissolved in distilled water to make 500 cm3 of solution.
ni
C
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b 0.2 mol of sodium chloride is dissolved in distilled water to make 1000 cm3 of solution.
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c 0.1 mol of sodium nitrate is dissolved in distilled water to make 100 cm3 of solution.
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Summary
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What mass would 3.4 g of ammonia produce?
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[1]
[1]
[1]
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Write a word equation for this reaction.
How many hydrogen atoms are there in the formula for ammonium sulfate?
What is the formula mass of sulfuric acid?
In this reaction, 17 g of ammonia produce 33 g of ammonium sulfate.
br
a
b
c
d
(NH4)2SO4
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2NH3 + H2SO4
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The equation below shows how the fertiliser ammonium sulfate is manufactured.
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End-of-chapter questions
1
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substance and is used to express the amount of a
substance taking part in a reaction
how the balanced chemical equation for a reaction can
be used to calculate the reacting masses of substances
involved and the amount of product formed
that one mole of any gas has a volume of 24 dm3 at
room temperature and pressure (r.t.p.)
how the concentration of a solution can be expressed
in moles per cubic decimetre (mol/dm3) and that
these values are useful in calculating the results of
titration experiments.
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how it has been possible to find the masses of the
atoms of the elements, including isotopes
that these atomic masses are measured relative to a
standard – a carbon-12 atom is fixed as having a mass
of 12 exactly
how the relative atomic mass is the average mass of
an atom of an element
about calculating the relative formula mass as the
sum of all the atomic masses in a formula
that the mole is the unit which contains Avogadro’s
constant number of constituent particles of a
rs
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You should know:
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(Relative atomic masses: H = 1, O = 16, Na = 23, N= 14, Cl = 35.5)
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d 0.8 g of solid sodium hydroxide is dissolved in distilled water to a final volume of 1 dm3.
Copyright Material - Review Only - Not for Redistribution
[2]
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Sulfuric acid is produced industrially by the Contact process. A 1.00 kg sample of
concentrated sulfuric acid contains 98% by mass of sulfuric acid molecules.
Calculate the number of moles of H2SO4 molecules in this 1.00 kg sample of
concentrated sulfuric acid. You should show your working.
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Pr
es
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CaO(s) + H2O(l) → Ca(OH)2(aq)
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ii
The can is designed to hold 168 g of calcium oxide. Calculate how many moles of
calcium oxide this is.
Calculate the mass of water needed to react with the 168 g of calcium oxide.
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The equation shows that one mole of calcium oxide reacts with one mole of water.
A design of self-heating can uses this reaction to heat the contents when necessary.
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=
=
=
= 238 g
=
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(use value from above)
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=
= 119 g
=
[1]
[1]
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ni
To show that cobalt(II) carbonate is in excess
Number of moles of HCl used
Mass of one mole of CoCO3
Number of moles of CoCO3 in 6.0 g of cobalt(II) carbonate
Explain why cobalt(II) carbonate is in excess.
am
[4]
g
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Maximum yield
Number of moles of HCl used
Number of moles of CoCl2 formed
Number of moles of CoCl2 · 6H2O formed
Mass of one mole of CoCl2 · 6H2O
Maximum yield of CoCl2 · 6H2O
s
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CoCl2 + CO2 + H2O
CoCl2 · 6H2O
Pr
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am
CoCO3 + 2HCl
CoCl2 + 6H2O
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[2]
[2]
6.0 g of cobalt(II) carbonate was added to 40 cm3 of hydrochloric acid, concentration 2.0 mol/dm3.
Calculate the maximum yield of cobalt(II) chloride-6-water and show that the cobalt(II) carbonate
was in excess.
3
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[3]
The balanced chemical equation for the exothermic reaction between
calcium oxide (quicklime) and water is:
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C6: Quantitative chemistry
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[Cambridge IGCSE Chemistry 0620 Paper 31 Q8 b November 2010]
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into glass. Forest fires can rage impressively, producing
overpowering waves of heat (Image C7.01). Bringing such
fires under control requires great expertise, and a great
deal of courage!
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Some chemical reactions are capable of releasing vast
amounts of energy. For example, at the end of the
Gulf War in 1991, oil and gas fires in the oilfields were
let burning out of control. The heat given out was
suficient to turn the sand around the burning wells
C
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C7.01 Energy changes in
chemical reactions
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Yet we use similar reactions, under control, to provide heat
for the home and for industry. Natural gas, which is mainly
methane, is burnt under controlled conditions to produce
heat for cooking in millions of homes (Image C7.02).
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exothermic and endothermic reactions
the drawing of energy profiles for exothermic and endothermic reactions
the idea of the activation energy for a reaction
suitable apparatus for experiments
factors afecting the rate of reaction
surface area of reactants
reactant concentration
temperature
the role of catalysts in a reaction
experiments on rates of reaction
collision theory and activation energy
that some reactions are reversible.
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This chapter covers:
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C7
How far? How fast?
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C7: How far? How fast?
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The reaction between methane and oxygen
Such combustion reactions are exothermic reactions.
They give out heat and raise the temperature of
the surroundings.
C
op
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During this reaction, as with all others, bonds are first
broken and then new bonds are made (Figure C7.01).
In methane molecules, carbon atoms are covalently
bonded to hydrogen atoms. In oxygen gas, the atoms
are held together in diatomic molecules. During the
reaction, all these bonds must be broken. Chemical
bonds are forces of attraction between atoms or ions.
To break these bonds requires energy; energy must be
taken in to pull the atoms apart.
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Breaking chemical bonds takes in energy from the
surroundings. This is an endothermic process.
New bonds are then formed: between carbon and oxygen
to make carbon dioxide, and between hydrogen and
oxygen to form water. Forming bonds gives out energy.
am
br
ev
Making chemical bonds gives out energy to the
surroundings. This is an exothermic process.
H
O
e
O
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H
Figure C7.01 The burning of methane
first involves the breaking of bonds in the
reactants. This is followed by the formation of
the new bonds of the products.
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H
O
H
Progress of reaction
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H
O
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bond making
gives out energy
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Energy / kJ
O
H
+
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bond breaking
takes in energy
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Image C7.02 A lighted gas ring on a cooker.
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carbon dioxide + water
CO2(g)
+ 2H2O(g)
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Image C7.01 A forest fire.
methane + oxygen
CH4(g) + 2O2(g)
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Hydrocarbon molecules contain only the elements
carbon and hydrogen (see Section C10.02). Methane is
the simplest hydrocarbon molecule. When it burns, it
reacts with oxygen. The products are carbon dioxide and
water vapour:
Copyright Material - Review Only - Not for Redistribution
307
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Figure C7.03 An energy profile for the reaction between
nitrogen and oxygen. The products are less stable than the
reactants. Energy is taken in from the surroundings.
This is an endothermic reaction.
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Figure C7.02 An energy profile for the burning of methane.
The products are more stable than the reactants.
Energy is given out to the surroundings. This is an
exothermic reaction.
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N2(g)
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The reaction between nitrogen and oxygen
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Endothermic reactions are far less common than
exothermic ones. Here, energy is absorbed from the
surroundings and the temperature of the surroundings
is lowered.
The reaction between nitrogen and oxygen is
endothermic. It is one of the reactions that take place
ACtivity C7.01
Exothermic and endothermic reactions
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
• they are capable of releasing large amounts of energy
as heat.
-C
2NO(g)
EXothermic means that heat EXits the reaction;
ENdothermic means that heat ENters the reaction.
• they are easy to ignite and burn
ie
O2(g)
nitrogen monoxide
When you try to remember the terms exothermic and
endothermic, concentrate on the first letters of the
words involved:
As mentioned earlier, the combustion reactions of fossil
fuels such as oil and gas are exothermic. Indeed, the major
characteristics that make these fuels so useful are that:
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+
TIP
Some bonds are stronger than others. They require
more energy to break them, but they give out more
energy when they are formed.
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nitrogen + oxygen
Here the bonding in the products in weaker than the
reactants. Overall energy is taken in by the reaction
(Figure C7.03).
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The overall change in energy for this exothermic reaction
can be shown in an energy level diagram (or energy profile)
(Figure C7.02). In this reaction, energy is given out because
the bonds in the products (CO2 and H2O) are stronger than
those in the reactants (CH4 and O2). This means that the
products are more stable than the reactants.
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when fuel is burnt in car engines. The equation for this
reaction is:
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Progress of reaction
When methane reacts with oxygen, the total energy
given out is greater than the total energy taken in. So,
overall, this reaction gives out energy – it is an exothermic
reaction. The energy is released as heat.
308
heat
taken
in
N2(g) + O2(g)
CO2(g) + 2H2O(g)
Progress of reaction
2NO(g)
Energy / kJ
heat
given
out
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Energy / kJ
CH4(g) + 2O2(g)
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C7: How far? How fast?
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Experimental thermochemistry
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Pr
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There is always an overall energy change in any chemical
reaction. This activity investigates whether heat is taken
in (endothermic) or given out (exothermic) during three
diferent reactions.
1 Prepare a results table like the one shown below.
3 Add three spatula measures of anhydrous copper(II)
sulfate to the water. Stir with a thermometer. Keep
checking the temperature.
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2 Put 50 cm3 of water into a polystyrene cup. Measure its
temperature and record it in the results table.
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thermometer
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Exper- Temperature/°C
Obser- Exothermic
iment
vations or
Before Ater Change
endothermic
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draught
shield
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spirit
burner
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Figure C7.04 Apparatus for finding the heat of combustion
of ethanol.
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A2 Why is an expanded polystyrene cup used for
these reactions?
Pr
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A1 Which of these reactions are exothermic and which
are endothermic?
A3 How would the temperature change be afected if the
amount of water used was halved from 50 cm3 to 25 cm3?
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ACtivity C7.02
Comparing the energy from diferent fuels
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
TIP
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It is useful to remember that combustion reactions are
always exothermic.
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ethanol
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Details of a related teacher demonstration that results
in the freezing of a beaker to a wooden board are given
in the Notes on activities for teachers/technicians.
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water
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A worksheet is included on the CD-ROM.
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clamp
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metal
calorimeter
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This type of experiment can be useful, though,
for comparing diferent fuels to see which would give
the most heat to warm a known amount of water.
The amount of liquid fuel put into the spirit burner would
need to be controlled. The method could also be adapted
to compare the heat produced by the same mass of
diferent solid fuels.
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5 Allow the solution from step 4 to cool down. Then add
three spatula measures of zinc powder to that solution. Stir
the mixture. Note the maximum temperature and record
your observations in the table, as before (experiment 2).
Results table
The experiment involves heating a known volume of water
with the flame from burning ethanol. The temperature
rise of the water is measured. From this, the heat energy
given to the water by burning a known amount of ethanol
can be worked out. There is a method for working out a
precise value for the heat of combustion of a fuel from this
type of experiment. However, that is currently beyond the
requirements of the syllabus.
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4 In your table, record the maximum temperature
reached. This is the temperature when the reaction
has just finished. Record your other observations too
(experiment 1).
6 Empty and rinse the polystyrene cup and put 50 cm3
of water into it. Then add three spatula measures
of sherbet. Record the temperature as before
(experiment 3) together with your observations.
Heat of combustion
The heat of combustion is the energy change of a
reaction when a substance is burnt. For liquid fuels such as
ethanol, it can be found using a metal calorimeter and a
spirit burner (Figure C7.04).
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! Wear eye protection.
Copyright Material - Review Only - Not for Redistribution
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Cambridge IGCSE Combined and Co-ordinated Sciences
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A more reactive metal will displace a less reactive one from
solutions of its salts.
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AO3.2 Plan experiments and investigations
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AO3.4 Interpret and evaluate experimental
observations and data
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Pr
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This activity compares the energy given out by several
liquid fuels by measuring the mass of each fuel that will
heat a given volume of water to a given temperature.
y
Although the vast majority of reactions are exothermic,
only a few are totally spontaneous and begin without
help at normal temperatures; for example, sodium or
potassium reacting with water. More usually, energy is
required to start the reaction. When fuels are burnt, for
example, energy is needed to ignite them (Figure C7.05).
This energy may come from a spark, a match or sunlight.
It is called the activation energy (given the symbol EA).
It is required because initially some bonds must be broken
before any reaction can take place. Suficient atoms or
fragments of molecules must be freed for the new bonds
to begin forming. Once started, the energy released as new
bonds are formed causes the reaction to continue.
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• a solid carbonate and an acid
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• displacement reactions between a metal and a solution
of a salt of a less reactive metal.
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Energy / kJ
s
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heat of reaction
(heat of combustion)
CO2(g) + 2H2O(g)
Pr
op
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es
Energy changes in metal displacement reactions
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest
possible improvements
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Progress of reaction
Figure C7.05 An energy profile for the burning of methane,
showing the need for activation energy to start the reaction.
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For a chemical reaction to happen, some bonds in the
reactants must first break before any new bonds can be
formed. That is why all reactions have an activation energy.
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CH4(g) + 2O2(g)
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ACtivity C7.03
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activation
energy
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Details of a data-logging version of this experiment
using a temperature sensor are given in the Notes on
activities for teachers/technicians.
Activation energy
This equipment can be used to measure the heat energy
given out during the neutralisation reactions between
acids and alkalis. This energy change is known as the
heat of neutralisation. The method can also be adapted
for reactions involving:
• a solid base and an acid
A worksheet is included on the CD-ROM.
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Heat of neutralisation
Polystyrene is a good heat insulator and is used to
make disposable cups for warm drinks. These cups
can be used as simple calorimeters to measure the
temperature rise of exothermic reactions between
solutions. The solutions are mixed in a polystyrene
cup and the initial temperature is measured quickly.
The mixture is then stirred well with the thermometer.
The temperature is checked oten during the
reaction, and the maximum temperature is recorded.
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A worksheet is included on the CD-ROM.
310
In this activity, you will plan an experiment to see which
combination of metal and solution provided generates the
most heat energy by observing the maximum temperature
rise in each case. The order of heat evolved for the
diferent combinations can be compared with the voltages
generated by electrochemical cells involving the metals.
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AO3.3 Make and record observations, measurements
and estimates
Copyright Material - Review Only - Not for Redistribution
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Is bond breaking an endothermic or an
exothermic process?
C7.03
Why is a polystyrene cup useful for carrying out
thermochemistry experiments with solutions?
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am
Draw a reaction profile for the following reaction,
which is exothermic.
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C7.02
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Which type of reaction takes in heat from
its surroundings?
Zn(s) + CuSO4(aq)
311
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ZnSO4(aq) + Cu(s)
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On 7 May 1915, the British liner Lusitania was sunk of the
south-west coast of Ireland (Image C7.03). The liner was
torpedoed by a German submarine and 1198 passengers
lost their lives. The sinking was accompanied by a second
explosion. This explosion gave possible support to the
idea that the ship was carrying explosives to Britain for
use in the war. The wreck of the Lusitania has now been
Image C7.04 A fireball produced by dropping powdered
flour into a flame.
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C7.02 Rates of reaction
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C7.01
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QuEStiONS
C7.04
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All reactions require some activation energy. For the
reaction of sodium or potassium with water, the activation
energy is low, and there is enough energy available from the
surroundings at room temperature for the reaction to begin
spontaneously. Other exothermic reactions have a higher
activation energy; for example, the burning of magnesium
can be started with heat from a Bunsen burner. Reactions
can be thought of as the result of collisions between atoms,
molecules or ions. In many of these collisions, the colliding
particles do not have enough energy to react, and just
bounce apart, rather like ‘dodgem cars’. A chemical reaction
will only happen if the total energy of the colliding particles is
greater than the required activation energy of the reaction.
C
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C7: How far? How fast?
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This type of explosion can also occur with fine powders
in flour mills (Image C7.04), in mines when dangerous
gases collect, and with dust. Dust particles have a large
surface area in contact with the air. A simple spark can
set of an explosive reaction. For example, powdered
Lycopodium moss piled in a dish does not burn easily –
but if it is sprayed across a Bunsen flame, it produces a
spectacular reaction. Even metal powders can produce
quite spectacular efects (Image C7.05).
y
op
The same idea does have a more positive use. In some modern
coal-fired power stations, powdered coal is burnt instead of
the usual lumps of coal because it burns very eficiently.
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investigated by divers. Evidence suggests that the second
explosion was caused by coal dust exploding in the hold.
If so, this is a dramatic example of explosive combustion.
-R
Explosive reactions represent one end of the ‘spectrum’ of
reaction rates. Other reactions, such as rusting, take place
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Image C7.03 The sinking of the Lusitania.
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Factors afecting the rate of reaction
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between limestone or marble chips (two forms of calcium
carbonate) and dilute hydrochloric acid:
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calcium carbonate + hydrochloric acid
calcium chloride + water + carbon dioxide
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The experiment can be done as shown in Image C7.06.
Using this arrangement, we can compare two samples
of marble chips, one sample (B) being in smaller pieces
than the other (A). The experiment is carried out twice,
once with sample A and once with sample B. In each
experiment the mass of sample used is the same, and the
same volume and concentration of hydrochloric acid is
used. The flask sits on the balance during the reaction.
A loose cotton wool plug prevents liquid spraying out of
the flask but allows the carbon dioxide gas to escape into
the air. This means that the flask will lose mass during the
reaction. Balance readings are taken at regular time
intervals and the loss in mass can be worked out. When
the loss in mass is plotted against time, curves such as
those in Figure C7.06 are obtained.
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Image C7.05 Iron dust ignited in a Bunsen flame.
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• the temperature at which the reaction is carried out
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• The total volume of gas released is the same in both
experiments. The mass of CaCO3 and the amount of acid
are the same in both cases. Both curves flatten out at
the same final volume. Sample B reaches the horizontal
part of the curve (the plateau) first.
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These results show that:
the rate (speed) of a reaction increases when the
surface area of a solid reactant is increased.
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TIP
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It is important that you understand how to interpret the
diferent regions of the graphs obtained.
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If a solid is being reacted with a liquid (or solution), the
greater the surface area, the more the solid is exposed to
the liquid. A good demonstration of this is the reaction
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For reactions involving two solids, grinding the reactants
means that they can be better mixed. The mixed powders
are then in greater contact with each other and are more
likely to react.
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the surface area of solid reactants
Where one or more of the reactants is a solid, the more
finely powdered (or finely divided) the solid(s) are, the
greater is the rate of reaction. This is because reactions
involving solids take place on the surface of the solids. A
solid has a much larger surface area when it is powdered
than when it is in larger pieces.
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• the influence of light on some reactions.
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• The reaction is fastest at the start. This is shown by
the steepness of the curves over the first few minutes.
Curve B is steeper than curve A. This means that gas
(CO2) is being produced faster with sample B. The
sample with smaller chips, with a greater surface
area, reacts faster. Beyond this part of the graph, both
reactions slow down as the reactants are used up
(Figure C7.07).
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• the concentration of the reactants
• the use of a catalyst
There are several important points about the graph,
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• the surface area of any solid reactants
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over much longer time periods. What factors influence the
speed of a reaction? Experiments have been carried out to
study a wide range of reactions, and there seem to be five
major influences on reaction rate:
312
CaCl2(aq) + H2O(l) + CO2(g)
Pr
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CaCO3(s) + 2HCl(aq)
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b
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marble
chips
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balance
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dilute
hydrochloric
acid
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Image C7.06 Apparatus for experiments A and B: the reaction of marble chips with dilute hydrochloric acid.
The loss of carbon dioxide from the flask produces a loss in mass. This is detected by the balance.
2.0
B (small chips)
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5
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AO3.5 Evaluate methods and suggest
possible improvements
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A worksheet is included on the CD-ROM.
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AO3.4 Interpret and evaluate experimental
observations and data
The aim of this activity is to investigate the efect of a
change in surface area on the rate of a reaction,
measuring how the diference in surface area of the
marble chips will influence the rate of the reaction
with hydrochloric acid. The activity follows the rate of
production of carbon dioxide, and involves the evaluation
and planning of methods to collect the gas, measuring
its volume.
U
Reaction rate and surface area; following the
rate of production of a gas
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
-C
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AO3.3 Make and record observations, measurements
and estimates
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The experiment described here uses a balance to study
the rate of a reaction producing a gas. There are other
methods to assess the rate of gas production (see Activity
C7.04, below and later experiments in this chapter).
Chapter 12 discusses ways of collecting gases and these,
particularly the use of a gas syringe, can be used to follow
the volume of gas produced with time (see below).
ACtivity C7.04
6
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3
Time / min
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2
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1
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0
Figure C7.06 The graph shows
the loss in mass against time for
experiments A and B. The reaction
is faster if the marble chips are
broken into smaller pieces (curve B).
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0.5
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A (large chips)
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Loss in mass / g
1.5
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cotton wool
to stop acid
‘spray’
escaping
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C7: How far? How fast?
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Cambridge IGCSE Combined and Co-ordinated Sciences
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the concentration of reactants
Reactions that produce gases are also very useful in
studying the efect of solution concentration on the
reaction rate. The reaction between marble chips and acid
could be adapted for this. Another reaction that can be
used to study this is the reaction between magnesium and
excess dilute hydrochloric acid:
magnesium + hydrochloric acid
magnesium chloride + hydrogen
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
The apparatus is shown in Figure C7.08. As in the previous
experiment, we will compare two diferent experiments,
which we will call C and D. The acid in experiment C is twice
as concentrated as in experiment D. Apart from changing
the concentration of the acid, everything else must stay the
same. So the volume of acid, the temperature and the mass
of magnesium used must be the same in both experiments.
The gas produced in this reaction is hydrogen and is collected
in a gas syringe. The volume of gas produced is measured at
frequent time intervals. We can then plot a graph of volume
of gas collected against time, like that in Figure C7.09.
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no more product
is formed
reaction is
slowing down
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small change in
amount of product
in a large time
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large change in
amount of product
in a small time
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reaction is fastest
at the start
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smaller change in
amount of product
in a larger time
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Amount of product
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reaction
has finished
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Time
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0
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• The curve for experiment C starts of twice as steeply as
for D. This means that the reaction in C is twice as fast as
in experiment D initially. So doubling the concentration
of the acid doubles the rate of reaction.
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gas syringe
magnesium
Again the graph shows some important points.
• The curve for experiment C is steeper than for D. This
shows clearly that reaction C, using more concentrated
acid, is faster than reaction D.
Plunger
moves
out when
reaction
starts.
C
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C
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Figure C7.07 A chemical reaction is fastest at the start.
It slows down as the reactants are used up.
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glass wall divides
flask in two
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• The total volume of hydrogen produced is the same in both
experiments, although experiment C produces it faster.
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C (1 mol / dm3 acid)
40
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40
50
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Time / s
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90
100
110
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Figure C7.09 The graph shows the volume of
hydrogen against time for experiments C and D.
The reaction is faster if the acid solution is more
concentrated (curve C).
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130
s
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D (0.5 mol / dm acid)
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Volume of hydrogen / cm
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60
These results show that:
the rate (speed) of a reaction increases when the
concentration of a reactant in solution is increased.
Pr
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-C
Figure C7.08 Apparatus for experiments C and D: the
reaction of magnesium with dilute hydrochloric acid.
The hydrogen given of can be collected and measured
in a gas syringe.
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excess dilute
hydrochloric acid
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An alternative approach is to use the reaction between
sodium thiosulfate and hydrochloric acid. In this case
(which we shall call experiment E), the formation of a
precipitate is used to measure the rate of reaction.
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Pr
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The previously described experiments (A/B or C/D) could
be altered to study the efect of temperature on the rate of
production of gas.
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sodium thiosulfate + hydrochloric acid
sodium chloride + sulfur + sulfur dioxide + water
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Na2S2O3(aq) + 2HCl(aq)
ACtivity C7.05
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add dilute acid
and start timing
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cross drawn
on paper
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sodium
thiosulfate
solution
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Image C7.07 Apparatus for experiment E: the reaction
between hydrochloric acid and sodium thiosulfate.
This can be studied by following the appearance of the
precipitate. The cross drawn on the paper appears fainter
with time. Time how long it takes for the cross to disappear.
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view from
above the
flask
C
b
the factors afecting reaction rate
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest
possible improvements
Wear
eye protection. Sulfuric acid is corrosive.
!
You must plan an investigation to discover how one chosen
factor afects the rate of a chemical reaction.
MgSO4 + H2
Mg + H2SO4
1 Measure 10 cm3 of 2 mol/dm3 sulfuric acid into a
boiling tube.
2 Add a 5 cm strip of magnesium ribbon and start
a stopclock.
3 When the reaction stops, record the time taken.
4 List the factors that could speed up or slow down
this reaction.
5 Choose one of these factors and plan an investigation
to discover how it afects the rate.
6 Your investigation should produce suficient results to
enable you to draw a graph.
A worksheet is included on the CD-ROM. The Notes on
activities for teachers/technicians contain details of
how this experiment can be used as an assessment of
skills AO3.2 and AO3.5.
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The experiment is shown in Image C7.07. A cross is marked
on a piece of paper. A flask containing sodium thiosulfate
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2NaCl(aq) + S(s) + SO2(g) + H2O(l)
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solution is placed on top of the paper. Hydrochloric
acid is added quickly. The yellow precipitate of sulfur
produced is very fine and stays suspended in the liquid.
With time, as more and more sulfur is formed, the liquid
becomes cloudier and more dificult to see through.
The time taken for the cross to ‘disappear’ is measured.
The faster the reaction, the shorter the length of time
during which the cross is visible. The experiment is carried
out several times with solutions pre-warmed to diferent
temperatures. The solutions and conditions of the
experiment must remain the same; only the temperature
is altered. A graph can then be plotted of the time taken
for the cross to disappear against temperature, like that
shown in Figure C7.10.
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temperature
A reaction can be made to go faster or slower by changing
the temperature of the reactants. Some food is stored in
a refrigerator, because the food ‘keeps better’. The rate of
decay and oxidation is slower at lower temperatures.
C
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C7: How far? How fast?
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QuEStiONS
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Temperature / ºC
a an increase in temperature
b an increase in the surface area of a solid reactant
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The graph shows two important points.
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C7.08
Why does the rate of a chemical reaction slow
down at the end?
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C
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Hydrogen peroxide is a colourless liquid with the formula
H2O2. It is a very reactive oxidising agent. Hydrogen
peroxide decomposes to form water and oxygen:
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hydrogen peroxide
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+ oxygen
O2(g)
We can follow the rate of this reaction by collecting the
oxygen in a gas syringe. The formation of oxygen is very
slow at room temperature. However, the addition of 0.5 g of
powdered manganese(IV) oxide (MnO2) makes the reaction
go much faster (we shall call this experiment F). The black
powder does not disappear during the reaction (Figure C7.11).
Indeed, if the solid is filtered and dried at the end of the
reaction, the same mass of powder remains. If the amount
of MnO2 powder added is doubled (experiment G), the rate of
reaction increases (Figure C7.12). If the powder is more finely
divided (powdered), the reaction also speeds up. Both these
results suggest that it is the surface of the manganese(IV)
oxide powder that is important here. By increasing the
surface area, the rate of reaction is increased. We say that
manganese(IV) oxide is a catalyst for this reaction.
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water
2H2O(l) +
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2H2O2(l)
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the efect of concentration on rate of reaction
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
This microscale experiment investigates the efect of
concentration on the rate of the reaction between sodium
thiosulfate and dilute hydrochloric acid.
A worksheet is included on the CD-ROM.
Details of a scaled-up version of the experiment
are given in the Notes on activities for teachers/
technicians.
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When is a chemical reaction at its fastest?
The decomposition of hydrogen peroxide
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Why is perishable food kept in a refrigerator?
C7.07
• The curve is not a straight line.
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C7.06
C7.03 Catalysts
ACtivity C7.06
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c an increased concentration of a reacting solution
• The cross disappears more quickly at higher
temperatures. The shorter the time needed for the cross
to disappear, the faster the reaction.
C
316
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Figure C7.10 The graph for experiment E. As the
temperature is increased, the time taken for the cross
to disappear is shortened. The reaction speeds up at
higher temperature.
What do we observe happen to the rate of a
chemical reaction in response to the following?
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10
C7.05
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It is important to realise in this experiment that the
shorter the time taken for the cross to disappear, the
faster the reaction has taken place.
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40
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60
TIP
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100
These results show that:
the rate of a reaction increases when the
temperature of the reaction mixture is increased.
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Time for cross to disappear / s
120
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140
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Many catalysts work by providing a surface on which other
molecules or atoms can react. However, others work in
more complex ways. Thus it is wrong to say that catalysts
do not take part in the reaction: some do. But at the end of
the reaction, there is the same amount of catalyst as at the
beginning, and it is chemically unchanged.
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C7: How far? How fast?
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oxygen
b
black powder
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C
F (0.5 g MnO2)
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Volume of oxygen / cm
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3
70
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20
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30
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10
40
60
80 100 120 140 160 180 200 220
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fermentation of sugars
(alcoholic drinks industry)
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vanadium(V)
oxide
enzymes
(in yeast)
Table C7.01 Some examples of industrial catalysts.
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2CO2(g)
Catalyst
sulfuric acid manufacture
(Contact process)
Remember to give a full definition of a catalyst. Include
in your answer the fact that the catalyst itself remains
unchanged at the end of the reaction.
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carbon dioxide
ammonia manufacture (Haber process) iron
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+ O2(g)
Industrial process
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Figure C7.12 Increasing the amount of catalyst increases the
rate of reaction. Here the amount of manganese(IV) oxide
has been doubled in experiment G compared to F.
TIP
carbon monoxide + oxygen
2CO(g)
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Time / s
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Pr
20
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0
0
One way to reduce the polluting efects of car exhaust
fumes is to fit the car with a catalytic converter
(Image C7.08). In many countries these converters are
a legal requirement. Car exhaust fumes contain gases
such as carbon monoxide (CO), nitrogen monoxide
(nitrogen(II) oxide, NO) and unburnt hydrocarbons (HC)
from the fuel which cause pollution in the air. The catalytic
converter converts these to less harmful products such as
carbon dioxide (CO2), nitrogen (N2) and water (H2O).
Some of the reactions that occur are:
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Catalytic converters
G (1 g MnO2)
80
Table C7.01 shows some examples of industrial catalysts.
You should notice that transition elements (see Chapter C8)
or their compounds make particularly good catalysts.
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Figure C7.11 Apparatus for experiments F and G: the
decomposition of hydrogen peroxide to water and oxygen.
The decomposition is very slow at room temperature.
a It can be speeded up by adding a catalyst, manganese(IV)
oxide. b The catalyst is unchanged at the end, and can be
separated from the water by filtration.
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water
90
Catalysts have been found for a wide range of reactions.
They are useful because a small amount of catalyst
can produce a large change in the rate of a reaction.
Also, since they are unchanged at the end of a reaction,
they can be re-used. Industrially, they are very important.
Industrial chemists use catalysts to make everything
from polythene and painkillers, to fertilisers and fabrics.
If catalysts did not exist, many chemical processes
would go very slowly and some reactions would need
much higher temperatures and pressures to proceed at
a reasonable rate. All these factors would make these
processes more expensive, so that the product would cost
much more. If it cost more than people wanted to pay for it,
it would be uneconomic.
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manganese(IV) oxide
Other examples of catalysts
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hydrogen peroxide solution
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catalyst: a substance that increases the rate of a chemical
reaction. The catalyst remains chemically unchanged at the
end of the reaction.
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Cambridge IGCSE Combined and Co-ordinated Sciences
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Collision theory
The importance of surface area in reactions involving
solids helps us understand how reactions take place.
In these cases, reactions can only occur when particles
collide with the surface of a solid. If a solid is broken
into smaller pieces, there is more surface exposed.
This means there are more places where collisions can
take place, and so there is more chance of a reaction
taking place. Iron reacts more readily with oxygen if it is
powdered (Figure C7.13a).
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The process of adsorption is also thought to weaken the
bonds in the reactant molecules. This makes them more
likely to react. Some of the most important examples of
industrial catalysts work in this way, for example iron in the
Haber process, vanadium(V) oxide in the Contact process,
and finely divided nickel where hydrogen is added to
unsaturated hydrocarbons.
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The catalytic converter therefore ‘removes’ polluting
oxides and completes the oxidation of unburnt
hydrocarbon fuel. It speeds up these reactions
considerably by providing a ‘honeycombed’ surface on
which the gases can react. The converter contains a thin
coating of rhodium and platinum catalysts on a solid
honeycomb surface. These catalysts have many tiny pores
which provide a large surface area for the reactions.
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id
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For reactions involving gases, increasing the pressure has
the same efect as increasing the concentration, so the
rate of a reaction between gases increases with pressure
(Figure C7.13c).
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Surface catalysts and collision theory
C
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In a lump of iron,
oxygen can’t get
to most of the atoms.
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Solid catalysts
Diferent chemical reactions need diferent catalysts.
One broad group of catalysts works by adsorbing
molecules on to a solid surface. This process of adsorption
brings the molecules of reactants closer together.
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oxygen molecule
iron atom
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Figure C7.13 (Continued)
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Pr
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The equations for the reactions taking place in the
catalytic converter are quite dificult to remember but
it will help you if you do remember that the reactions
finish back at components that are present in normal
air – carbon dioxide and nitrogen.
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TIP
We can see how these ideas – sometimes referred to
as the collision theory – apply in other situations.
When solutions are more concentrated, the speed of a
reaction is faster. A more concentrated solution means
that there are more reactant particles in a given volume.
Collisions will occur more oten. The more oten they
collide, the more chance the particles have of reacting.
This means that the rate of a chemical reaction will
increase if the concentration of the reactants is increased.
A more concentrated acid reacts more vigorously
with a piece of magnesium ribbon than a dilute one
(Figure C7.13b).
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carbon dioxide + water
hydrocarbons + oxygen
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nitrogen + oxygen
N2(g) + O2(g)
nitrogen monoxide
2NO(g)
Pr
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nitrogen monoxide + carbon monoxide
nitrogen + carbon dioxide
2NO(g) + 2CO(g) N2(g) + 2CO2(g)
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Image C7.08 A catalytic converter can be fitted to a car
exhaust system.
Copyright Material - Review Only - Not for Redistribution
If the iron is in small bits,
the oxygen molecules can
collide with many more
iron atoms. The iron now
has a much bigger
surface area.
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have more energy at the higher temperature. This
increases the chances that a collision will result in bonds
in the reactants breaking and new bonds forming to make
the products. If we look at the reaction between zinc and
hydrochloric acid, we can see how the rate of reaction
changes with changes in collision frequency (Figure C7.14).
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Pr
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Collisions between
particles from the acid
and the magnesium
are more frequent.
ve
rs
ity
y
high pressure
es
s
-C
-R
Collisions between
different molecules
are much more
frequent.
am
Collisions between
different molecules
do not happen very
often.
ev
br
id
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w
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U
R
ni
C
op
low pressure
op
Pr
y
Figure C7.13 The efect of changing conditions on the
frequency of collisions.
ity
When the temperature is raised, a reaction takes place
faster. At higher temperatures, the particles are moving
faster. Again, this means that collisions will occur more
oten, giving more chance of reaction. Also, the particles
TIP
When explaining the efects of concentration and
temperature using the collision theory, remember that
it is collision frequency that is the key factor – don’t talk
vaguely about ‘more collisions’: it is the fact that there are
more frequent collisions of the particles that is important.
y
ve
rs
C
w
ie
a
b
Concentration of acid
c
Temperature
y
op
w
ie
more chance of
particles colliding
br
more zinc exposed
to collisions
more collisions and particles
collide with more energy
ev
id
g
e
C
U
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ni
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rs
C
ity
Pr
op
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es
s
-C
-R
am
br
ev
id
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w
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Surface area of zinc
C
U
R
ni
ev
When solutions are more concentrated, the speed of a
reaction is faster. A more concentrated solution means
that there are more reactant particles in a given volume.
Collisions will occur more oten. The more oten they collide,
the more chance the particles have of reacting. This means
that the rate of a chemical reaction will increase if the
concentration of the reactants is increased (Figure C7.14b).
When the temperature is raised, a reaction takes place
faster. At higher temperatures, the particles are moving
faster. Again, this means that collisions will occur more oten,
giving more chance of reaction. Also, the particles have more
energy at the higher temperature. This increases the chances
that a collision will result in bonds in the reactants breaking
and new bonds forming to make the products (Figure C7.14c).
op
y
op
C
There are not very many
collisions between
particles from the acid
and the magnesium.
ev
ie
w
c
magnesium
ribbon
concentrated acid
am
br
id
dilute acid
ge
particle from
acid
water
molecule
b
C
U
ni
op
y
C7: How far? How fast?
-R
s
es
-C
am
Figure C7.14 The collision theory can be used to explain how various factors afect the rate of the reaction.
Here we use the reaction between zinc and hydrochloric acid as an example.
Copyright Material - Review Only - Not for Redistribution
319
ve
rs
ity
-R
catalysed path = pass route
C
op
ni
y
product(s)
Reaction path
U
R
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w
C
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op
y
reactant(s)
br
ev
id
ie
w
ge
Figure C7.15 The barrier between reactant(s) and product(s) may be so high that it defeats all but the most energetic.
The catalyst’s route is an easy pass through the mountains.
A closer look at activation energy
-R
es
Pr
ity
Which solid catalyst will speed up the
decomposition of hydrogen peroxide?
C7.12
What are the catalysts used in:
rs
What changes in physical conditions are enzymes
particularly sensitive to?
C7.14
Does the presence of a catalyst increase or
decrease the activation energy for a reaction?
C7.15
In terms of the collision theory, explain why the
rate of a reaction increases with:
op
ev
es
s
-R
a an increase in temperature
Pr
ity
b an increase in the surface area of a solid reactant
c an increased concentration of a reacting solution.
C7.04 Reversible reactions
ni
ve
rs
The reversible hydration of salts
op
y
Thermal decomposition of salts such as hydrated
copper(II) sulfate (CuSO4 · 5H2O) results in the dehydration
of the salt:
C
U
CuSO4 · 5H2O(s)
heat
CuSO4(s) + 5H2O(g)
white powder
id
g
w
e
light blue crystals
ev
ie
In this case, the reaction results in a colour change from
blue to white. The physical structure of the crystals is
also destroyed. The water driven of can be condensed
separately (Figure C7.16).
es
s
-R
br
am
-C
C
U
ge
id
C
y
C7.13
w
ve
b the Contact process?
br
am
-C
op
y
We can think of an ‘analogy’ for this. Suppose we are
hiking in the Alps (Figure C7.15). We start on one side of a
mountain and want to get to the other side. We could go
right over the summit of the mountain. This would require
us to be very energetic. What we might prefer to do would
be to find an alternative route along a pass through the
mountains. This would be less energetic. In our analogy,
the starting point corresponds to the reactants and the
finishing point to the products. The route over the top of
the mountain would be the uncatalysed path. The easier
route through the pass would be a catalysed path.
w
ie
What is an enzyme?
C7.11
ni
ev
R
A catalyst increases the rate of reaction by reducing the
amount of energy that is needed to break the bonds.
This reduces the activation energy of the reaction and
makes sure that more collisions are likely to give products.
The rate of the reaction is therefore increased.
ev
C7.10
a the Haber process?
Each reaction has its own diferent value of
activation energy.
■ When particles collide, they must have a combined
energy greater than this activation energy, otherwise
they will not react.
■ Chemical reactions occur when the reactant
particles collide with each other.
■
What is a catalyst?
ie
-C
y
op
C
w
ie
C7.09
s
am
QuEStiONS
Not every collision between particles in a reaction
mixture produces a reaction. We have seen earlier that
a certain amount of energy is needed to begin to break
bonds. This minimum amount of energy is known as the
activation energy of the reaction.
320
R
uncatalysed path =
over-the-hill route
Pr
es
s
Energy
-C
am
br
id
ev
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w
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C
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ni
op
y
Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
ve
rs
ity
C
ev
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w
ge
-R
Image C7.10 The reversible reaction involving
ammonium chloride.
U
R
ni
water
y
ev
ie
w
ice-cold
water
C
op
C
ve
rs
ity
op
y
Pr
es
s
-C
heat
am
br
id
hydrated copper(II) sulfate
U
ni
op
y
C7: How far? How fast?
The decomposition of ammonium chloride is a further
example of this type of change (Image C7.10). When warmed
in a test tube, the white solid decomposes to ammonia and
hydrogen chloride:
NH4Cl(s)
ity
op
NH3(g) + HCl(g)
NH4Cl(s)
ACtivity C7.07
op
y
A reversible reaction involving copper(II) sulfate
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
The water of crystallisation is removed from hydrated copper(II)
sulfate by heating. Condensing the vapour produced in a
second test tube collects the water. The white anhydrous
copper(II) sulfate is then rehydrated and the blue colour returns.
A worksheet is included on the CD-ROM.
y
op
C
U
R
ev
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w
ni
ve
rs
C
ity
Pr
op
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es
s
-C
-R
am
br
ev
id
ie
w
ge
C
U
R
ni
ev
ve
ie
Some reactions, for example the dehydration of
hydrated salts, can be reversed if the conditions
are changed.
id
g
e
QuEStiONS
ev
br
What can this colour change be used as a test for?
s
-R
C7.17
es
-C
am
Image C7.09 Adding water back to dehydrated
copper(ii) sulfate.
What colour change do we see when water is
added to anhydrous copper(II) sulfate powder?
ie
C7.16
w
w
rs
C
NH3(g) + HCl(g)
However, on the cooler surface of the upper part of the
tube, the white solid is re-formed:
Pr
y
es
s
-C
The white anhydrous copper(II) sulfate and the water
are cooled down. Then the dehydration reaction can be
reversed by slowly adding the water back to the powder
(Image C7.09). This reaction is strongly exothermic and the
colour of the powder returns to blue.
-R
am
br
ev
id
ie
w
ge
Figure C7.16 Apparatus for condensing the water vapour
driven of from blue crystals of hydrated copper(II) sulfate
by heating. The change can be reversed by adding the
liquid water back to the white anhydrous copper(II) sulfate.
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321
ve
rs
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ev
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am
br
id
■
ie
w
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rs
1
Sometimes, in chemical factories, ‘runaway reactions’ occur. These are reactions which begin
to take place much too quickly and can cause explosions which are very dangerous. In what ways
can reactions be slowed down?
2
When iron(II) sulfate crystals are heated in a test tube, they change to a white powder and
condensation collects at the top of the tube.
y
op
C
U
ie
id
ev
FeSO4 + 7H2O
br
FeSO4 · 7H2O
w
ge
R
ni
ev
ve
ie
w
C
End-of-chapter questions
Pr
322
ity
op
y
es
s
-C
-R
am
br
ev
id
■
■
how certain reactions can be reversed if the conditions
are changed
that chemical reactions involve the initial breaking
of bonds in the reactants so that new bonds can be
formed, giving rise to products
how the breaking of bonds is an endothermic process
requiring energy, while the making of bonds is an
exothermic process releasing energy
that the activation energy of a reaction is the
minimum energy required to start a particular reaction
that changes which increase the frequency of collision
between reactant particles give rise to an increased
rate of reaction.
y
ve
rs
ity
■
U
R
■
■
ni
ev
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w
C
■
■
Pr
es
s
-C
op
■
how all chemical reactions involve changes
in energy, with most giving out energy to the
surroundings (exothermic)
how some reactions take in energy and
are endothermic
that diferent chemical reactions occur at vastly
diferent rates and that the rate of a particular
reaction can be altered by changing conditions,
including temperature
how some reactions are speeded up by the
presence of a catalyst
that catalysts are significant in several key
industrial processes
y
■
-R
You should know:
C
op
Summary
w
ge
C
U
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op
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Cambridge IGCSE Combined and Co-ordinated Sciences
[3]
ni
ve
rs
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Pr
op
y
-R
s
-C
am
br
ev
ie
id
g
w
e
C
U
op
y
What is the meaning of the symbol
?
Explain how this reaction can be used as a test for water.
es
C
w
ie
CoCl2 + 6H2O
R
ev
COCl2 · 6H2O
e
f
[1]
[2]
[1]
es
s
-C
-R
am
a Write a word equation for the reaction.
b Is the reaction exothermic or endothermic? Explain your answer.
c What colour are iron(II) sulfate crystals?
When water is added to the white iron sulfate, there is a hissing sound as steam is produced
and the iron sulfate changes back to its original colour.
d Explain these observations.
This equation shows a similar reaction.
Copyright Material - Review Only - Not for Redistribution
[1]
[2]
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rs
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3
w
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C
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ni
op
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C7: How far? How fast?
-R
CaCl2 + CO2 + H2O
cotton wool
Pr
es
s
-C
CaCO3 + 2HCl
ev
ie
am
br
id
A student used the apparatus shown below to investigate the rate of reaction of calcium carbonate
with dilute hydrochloric acid.
dilute
hydrochloric acid
ve
rs
ity
C
op
y
calcium carbonate
y
Use the information in the equation to suggest why the mass of the flask and
contents decreases with time.
The graph shows how the mass of the flask and its contents changes with time.
w
ie
ev
100.2
-R
s
es
100
200
300 400 500
Time / seconds
600
323
700
rs
C
Pr
100.0
ity
op
y
-C
100.1
0
w
[1]
C
op
ni
U
id
100.3
ge
100.4
am
R
b
balance
br
ev
ie
a
Mass of flask and contents/grams
w
100.4
i
ii
op
C
ie
ev
-R
s
es
Pr
expansion
large
rapid
slow
small
In flour mills, there is oten the risk of an explosion due to the rapid
very
particles which have a very
to react.
of the
op
y
surface area
C
U
w
e
ev
ie
id
g
es
s
-R
br
am
-C
[2]
[1]
[1]
ni
ve
rs
combustion
ity
op
y
-C
How does the speed (rate) of this reaction change when:
i the temperature is increased
ii smaller pieces of calcium carbonate are used?
Copy and complete the following sentence using words from the list.
R
ev
ie
w
C
d
[1]
w
ge
id
br
am
On a copy of the graph, draw a curve to show how the mass of the flask and contents
changes with time when hydrochloric acid of half the concentration was used.
c
[1]
y
ve
ni
U
R
ev
ie
At what time was the reaction just complete?
On a copy of the graph, mark with an X the point where the speed (rate) of
reaction was fastest.
iii The student repeated the experiment but altered the concentration of the hydrochloric
acid so that it was half the original value. In both experiments, calcium carbonate was in
excess and all other conditions were kept the same.
Copyright Material - Review Only - Not for Redistribution
[3]
(continued)
ve
rs
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e
w
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C
U
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op
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Cambridge IGCSE Combined and Co-ordinated Sciences
glucose + oxygen
carbon dioxide + water
-R
State the name of this process.
In this process enzymes act as catalysts. What do you understand by the term catalyst?
y
[Cambridge IGCSE Chemistry 0620 Paper 2 Q5 June 2009]
ve
rs
ity
C
w
2H2O2
ev
ie
2H2O + O2
U
R
ni
C
op
y
A student used the apparatus shown below to study how changing the concentration of
hydrogen peroxide afects the speed of this reaction.
-R
am
br
ev
id
ie
w
ge
oxygen collects here
es
s
-C
gas syringe
Pr
y
hydrogen peroxide
op
ni
C
40
C
ie
1
ity
20
40
30
Time / s
60
-R
s
es
-C
am
br
ev
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id
g
w
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C
U
50
y
20
op
w
10
R
ev
ev
-R
s
Pr
2
es
B
60
ni
ve
rs
op
y
3
A
80
0
w
ge
am
br
id
Concentration of
hydrogen peroxide
in g/dm3
-C
Volume of oxygen/cm3
100
C
y
ve
Apart from the volume of hydrogen peroxide, state two things that the student must keep
the same in each experiment.
The student measured the volume of oxygen produced using three diferent concentrations
of hydrogen peroxide. The results are shown on the graph below.
U
ie
ev
b
rs
manganese(IV) oxide
a
R
ity
op
w
C
324
ie
[1]
[1]
Hydrogen peroxide decomposes slowly at room temperature to form water and oxygen.
The reaction is catalysed by manganese(IV) oxide.
op
4
Pr
es
s
-C
i
ii
ev
ie
am
br
id
Cells in plants and animals break down glucose to carbon dioxide and water.
Copyright Material - Review Only - Not for Redistribution
[2]
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C
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op
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C7: How far? How fast?
Pr
es
s
y
130
15
did not produce any oxygen
manganese(IV) oxide
18
ge
magnesium oxide
br
ev
id
C
op
ni
U
lead(IV) oxide
es
s
-C
[Cambridge IGCSE Chemistry 0620 Paper 22 Q3 November 2011]
op
Pr
y
The equation for the reaction between sodium thiosulfate and hydrochloric acid is given below.
325
2NaCl(aq) + S(s) + SO2(g) + H2O(l)
ity
Na2S2O3(aq) + 2HCl(aq)
The speed of this reaction was investigated using the following experiment. A beaker containing
50 cm3 of 0.2 mol/dm3 sodium thiosulfate was placed on a black cross. 5.0 cm3 of 2.0 mol/dm3
hydrochloric acid was added and the clock was started.
y
op
ni
ev
ve
ie
w
rs
C
[1]
-R
am
Put these compounds in order of their efectiveness as catalysts.
worst catalyst
best catalyst
ie
w
ge
id
-R
am
ev
sodium thiosulfate
and hydrochloric acid
br
solution turns
from colourless
to cloudy
C
U
R
look down at
cross on paper
-C
paper
s
es
Pr
view looking down
ity
Initially the cross was clearly visible. When the solution became cloudy and the cross could no
longer be seen, the clock was stopped and the time recorded.
ni
ve
rs
y
-R
s
-C
am
br
ev
ie
id
g
w
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C
U
op
ev
R
(continued)
es
C
op
y
cross on paper
ie
w
[1]
w
ev
ie
copper(II) oxide
R
[1]
Time taken to produce 20cm3 of oxygen / s
Compound
5
[1]
[1]
ie
w
C
op
c
ve
rs
ity
y
-C
-R
am
br
id
ev
ie
i Describe how the speed of the reaction varies with the concentration of hydrogen peroxide.
ii Explain why the final volume of oxygen given of is less for graph B than for graph A.
iii From the graph, determine:
■ the time taken for the reaction to be completed when 3 g/dm3 hydrogen peroxide
(line A) was used.
■ the volume of oxygen produced by 2 g/dm3 hydrogen peroxide (line B) in the first
15 seconds.
The student then tested various compounds to see how well they catalysed the reaction.
He used the same concentration of hydrogen peroxide in each experiment. The table shows
the time taken to produce 20 cm3 of oxygen using each compound as a catalyst.
Copyright Material - Review Only - Not for Redistribution
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rs
ity
w
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C
U
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op
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Cambridge IGCSE Combined and Co-ordinated Sciences
-C
Volume of thiosulfate / cm
op
y
Volume of water / cm
40
25
10
0
10
25
40
5
5
5
55
55
55
60
96
55
ve
rs
ity
48
y
w
5
ie
25
5
s
42
40
Pr
y
es
96
[4]
[Cambridge IGCSE Chemistry 0620 Paper 32 Q3 June 2011]
ity
op
The diagram below shows the apparatus a student used to investigate the efect of changing the acid concentration
on the rate of reaction between excess dilute hydrochloric acid and magnesium. At the start of the experiment the
measuring cylinder contained no gas and was full of water.
y
op
C
ie
ev
id
measuring
cylinder
br
side-arm
test-tube
gas
ge
bung
w
U
R
ni
ev
ve
rs
C
w
25
ev
am
Time / s
ie
25
20
-C
Temperature / °C
25
-R
am
dilute hydrochloric
acid
-C
water
es
s
magnesium
Pr
ity
-R
s
es
-C
am
br
ev
ie
id
g
w
e
C
U
op
ev
R
y
ni
ve
rs
C
op
y
To carry out his investigation the student used the following method.
• He dropped the magnesium into the dilute acid.
• He immediately placed the bung into the side-arm test-tube and started a stopclock.
• He measured the volume of gas in the measuring cylinder every half minute, for eight minutes.
ie
w
[2]
[1]
[3]
-R
br
Volume of water / cm
3
Volume of acid / cm3
6
C
op
ni
U
ge
id
Volume of sodium thiosulfate / cm3
326
4
50
5
3
Time / s
C
w
ev
ie
Total volume / cm
3
3
i Explain why it is necessary to keep the total volume the same in all the experiments.
ii Copy and complete the table.
iii How and why does the speed of the reaction vary from experiment 1 to 4?
The idea of collisions between reacting particles is used to explain changes in the speed of reactions.
Use this idea to explain the following results.
b
R
Volume of acid / cm
3
2
-R
1
3
Pr
es
s
am
br
id
Experiment
ev
ie
The experiment was repeated with 25 cm3 of 0.2 mol/dm3 sodium thiosulfate and 25 cm3 of water.
Typical results for this experiment and a further two experiments are given in the table.
a
Copyright Material - Review Only - Not for Redistribution
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rs
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w
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C
U
ni
op
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C7: How far? How fast?
am
br
id
-R
State two other variables that the student needed to keep the same in experiments A and B.
The following graph shows the results the student obtained for experiments A and B.
-C
experiment B
op
y
40
35
C
[1]
Pr
es
s
45
ve
rs
ity
a
b
ev
ie
He carried out two experiments, A and B, in which the only variable that he changed was the concentration of the
hydrochloric acid.
C
op
ni
25
20
U
w
ev
ie
R
volume of gas
collected / cm3
y
experiment A
30
w
ge
15
ev
0
0
2
3
4
6
7
8
9
time / minutes
Pr
y
In which experiment, A or B, did the student use hydrochloric acid which had the higher concentration?
Explain your answer.
The student was told that he could calculate the average rate of reaction using:
maximum volume of gas collected
average rate of reaction =
minimum time taken to collect maximum value
ity
op
C
i
[1]
ni
op
y
ve
ie
ev
[3]
w
ge
C
U
R
Use the information in the graph to calculate the average rate of reaction for experiment A.
Show your working and state the units.
ie
The balanced symbolic equation for the reaction between hydrochloric acid and magnesium is shown below.
id
ev
-R
s
What is meant by the state symbol (aq) in this equation?
Suggest why the reaction in both experiments A and B above produced the same volume of gas.
es
Pr
ity
-R
s
am
br
ev
ie
id
g
w
e
C
U
op
y
ni
ve
rs
C
w
ie
ev
R
-C
[1]
[2]
[Cambridge IGCSE Combined Science 0653 Paper 31 Q4 June 2012]
op
y
-C
i
ii
MgCl2(aq) + H2(g)
am
br
Mg(s) + 2HCl(aq)
es
c
327
rs
ii
w
5
s
1
es
-C
0
-R
am
br
id
ie
10
Copyright Material - Review Only - Not for Redistribution
op
y
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rs
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C
U
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am
br
id
Pr
es
s
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y
ni
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br
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U
R
C
ve
y
C
-R
-C
IV
V
VI
VII
B
C
N
O
F
Ne
y
op
C
id
g
Ra
-R
s
es
am
br
Fr
The melting points of the alkali metals decrease gradually
as you go down the group. There is a similar trend in the
hardness of the metals. They are all sot, low-density metals.
e
Cs Ba
w
Sr
ie
Rb
ev
Ca
U
Na Mg
The distinctive metals of Group I are called the
alkali metals. The most memorable thing about them is
their spectacular reaction with cold water (Image C8.01).
These metals do not have many uses because they are so
reactive and tarnish easily. They have to be stored under
oil. The one familiar use of sodium is in sodium vapour
lamps. These are the yellow street and motorway lights
seen throughout towns and cities.
s
es
III
Be
K
Pr
II
Li
ni
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I
He
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0
H
-C
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C8.01 the alkali metals
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■
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■
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■
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■
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■
the alkali metals – trends in properties
aluminium and its protective oxide layer
the transition elements – distinctive properties of these metals
the reactivity series
methods of extraction in relation to reactivity
metal displacement reactions.
id
■
br
R
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This chapter covers:
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C8
Patterns and properties of metals
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TIP
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Pr
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C
2NaOH(aq)
+
H2(g)
The reaction gets more vigorous as you move down the
group. The reaction of lithium with water is quite steady:
the metal does not melt and the hydrogen does not
ignite. Sodium reacts more strongly: the metal melts
but, if the sodium is free to move, the hydrogen does not
usually ignite. Restricting the movement of the sodium,
by placing it on a piece of filter paper on the water surface,
results in the hydrogen gas igniting. The flame is coloured
yellow by the sodium. Potassium reacts so strongly
with water that the hydrogen gas ignites spontaneously.
The potassium may even explode dangerously. The flame
is coloured lilac. Rubidium and caesium explode as soon
as they are put into water. The metal hydroxide produced
in each case makes the water become alkaline.
op
C
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TIP
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Remember to read any questions carefully. When asked,
‘What would you observe?’, make sure that you give your
observations carefully – talk about what you see, hear and
smell. If there is a gas given of, then state its colour, for
instance. Make sure you give detail.
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s
-R
br
am
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sodium hydroxide + hydrogen
water
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id
br
am
-C
They are all reactive metals. They have to be stored
under oil to stop them reacting with the oxygen
and water vapour in the air (Table C8.01).
They are sot and can be cut with a knife.
Like all metals, they form positive ions. The metals
of Group I form ions with a single positive charge
(for example, Li+, Na+, K+).
As a result, they form compounds that have similar
formulae; for example, their carbonates are lithium
carbonate (Li2CO3), sodium carbonate (Na2CO3) and
potassium carbonate (K2CO3).
They all react strongly and directly with non-metals
to form salts. These salts are all white, crystalline,
ionic solids that dissolve in water.
op
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329
2Na(s) + 2H2O(l)
ni
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■
metal hydroxide + hydrogen
op
w
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sodium +
The common properties of the alkali metals
■
metal + water
For example:
There are many ways in which the diferent elements of
Group I show similar properties. Some of these common
characteristics are given in the box that follows.
■
All the alkali metals react spontaneously with water
to produce hydrogen gas and the metal hydroxide
(Table C8.01). The reactions are exothermic. The heat
produced is suficient to melt sodium and potassium
as they skid over the surface of the water. Lithium does
not melt as it reacts. This begins to show the gradual
diferences in reactivity between the metals as you go
down the group. Lithium (at the top) is the least reactive
and caesium (at the bottom) is the most reactive. The
reaction with water is the same in each case:
w
ge
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R
Lithium is the hardest, but it can still be cut with a knife.
The metals get easier to cut going down the group.
The density of the metals tends to increase down the group,
though potassium is an exception, being slightly less
dense than sodium.
■
Remember that you can be asked to ‘predict’ properties
of these elements by comparison with others in the group,
so practise that type of question.
The reaction of the alkali metals with water
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Pr
es
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-R
Make sure of the wording of your comments when
discussing these metals. The alkali metals have ‘similar’
properties to each other, they are not the same. There is a
gradual change in properties as you go down the group.
Image C8.01 The reaction of sodium with water. Note that
the hydrogen released burns with the metal’s characteristic
flame colour.
■
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C8: Patterns and properties of metals
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Cambridge IGCSE Combined and Co-ordinated Sciences
Reaction with water
Reaction with air
lithium
reacts steadily
2Li + 2H2O 2LiOH + H2
tarnishes slowly to give a layer
of oxide
reacts strongly
2Na + 2H2O 2NaOH + H2
tarnishes quickly to give a layer
of oxide
reacts violently
2K + 2H2O 2KOH + H2
tarnishes very quickly to give a
layer of oxide
-C
sodium
increasing reactivity
op
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Pr
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potassium
-R
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Element
Na+
potassium
K+
y
ni
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yellow
Magnesium reacts very slowly when placed in cold water.
A much more vigorous reaction is obtained if steam is
passed over heated magnesium. The magnesium glows
brightly to form hydrogen and magnesium oxide:
U
lilac
id
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w
Table C8.02 Flame colours of Group I metals.
br
ev
Flame tests for the alkali metals
II
III
IV
V
VI
VII
Li
Be
B
C
N
O
F
Ne
Na Mg
Al
K
Ca
Ga
Rb
Sr
In
Cs Ba
Tl
-R
s
Pr
op
C
w
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Pr
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ni
ve
rs
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State two physical characteristics of the
alkali metals.
C8.02
Give the colours of lithium, sodium and potassium
if their salts are tested in the flame test.
C8.03
What gas is given of when the alkali metals are
reacted with water?
C8.04
Name the product, other than hydrogen,
when potassium is reacted with water.
C8.05
Write a word equation for the reaction of sodium
with water.
C8.06
Write a balanced chemical equation for the
reaction of potassium with water.
y
id
g
2MgO(s)
Which of the alkali metals does not melt when a
piece of it is placed on the surface of water?
es
s
-R
br
ev
C8.07
As in Group I, the reactivity of the alkaline earth metals
increases going down the group. Beryllium (at the top) is
am
C8.01
U
magnesium oxide
Trends in reactivity
-C
H2(g)
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It burns even brighter in pure oxygen, producing a
white ash, magnesium oxide:
+ O2(g)
+
QuEStiONS
Magnesium metal burns fiercely with a brilliant (very
bright) white light. For this reason it is used in distress
flares, in flashbulbs and in fireworks that give a white light.
2Mg(s)
Ca(OH)2(aq)
y
ve
ni
U
-C
am
The Group II metals are called the alkaline earth metals.
Group II shows similar trends in reactivity to Group I.
They are less reactive than the metals in Group I, but still
take part in a wide range of reactions.
magnesium + oxygen
H2(g)
Calcium hydroxide is more soluble than magnesium
hydroxide, so an alkaline solution is produced (limewater).
As the reaction proceeds, a white suspension is obtained
because not all the calcium hydroxide dissolves.
ge
Ra
+
calcium hydroxide + hydrogen
+ 2H2O(l)
Ca(s)
br
Fr
calcium + water
rs
I
He
MgO(s)
Calcium, however, reacts strongly with cold water, giving of
hydrogen rapidly:
id
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0
H
+ H2O(g)
Mg(s)
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Group II metals
330
magnesium oxide + hydrogen
magnesium + steam
es
-C
am
Compounds of the alkali metals can be detected by a
flame test. All alkali-metal ions give characteristic colours
in a Bunsen flame. Table C8.02 lists the colours obtained.
op
sodium
red
C
Li+
w
lithium
the least reactive and barium (at the bottom) is the most
reactive. Again the change in reactivity is best shown by
using their reactions with water.
Flame colour
ie
Formula
ve
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Metal ion
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C
Table C8.01 Reactions of lithium, sodium and potassium with air and water.
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C8: Patterns and properties of metals
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IV
V
VI
VII
B
C
N
O
F
Ne
Mg
Al
Si
Ca
Ga Ge
Sr
In
Sn
Ba
Tl
Pb
ve
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III
Be
Pr
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s
II
Li
-C
I
He
Ra
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0
H
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C8.02 Aluminium
U
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Aluminium was, for a long time, an expensive and littleused metal. In France, around the 1860s, at the Court
of Napoleon III (the nephew of Napoleon Bonaparte),
honoured guests used cutlery made of aluminium rather
than gold. At that time the metal was expensively extracted
from aluminium chloride using sodium or potassium:
3NaCl(s) + Al(s)
s
AlCl3(s) + 3Na(s)
es
-C
am
aluminium chloride + sodium
sodium chloride + aluminium
-R
br
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Image C8.02 The supersonic passenger jet Concorde was
built out of an aluminium alloy.
transport (aircrat,
etc.) 17%
building (windows,
etc.) 22%
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op
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packaging
(foil, etc.) 16%
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consumer goods 9%
Figure C8.01 The widespread and increasing uses of the
aluminium produced in the USA.
es
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-R
br
am
-C
ity
Pr
op
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Aluminium is particularly useful because it is protected
from corrosion by the stable layer of aluminium oxide
that forms on its surface. This protective layer stops the
aluminium (a reactive metal) from reacting. This makes
aluminium foil containers ideal for food packaging
because they resist corrosion by natural acids. Aluminium
is also used for external structures such as window frames
because they resist weathering. Figure C8.01 shows the
uses made of aluminium produced in the USA.
ni
ve
rs
y
C8.09
Why does aluminium have to be extracted
by electrolysis?
op
Give two characteristic properties of aluminium
that make it very useful for construction.
ie
id
g
Why does aluminium not corrode like iron?
ev
C8.10
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br
am
-C
C8.08
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QuEStiONS
C
C
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machinery 8%
Aluminium is a light, strong metal and has good electrical
conductivity. Increasingly it is being used for construction
purposes. The Lunar Rover ‘moon-buggy’ was built out
of aluminium, and so too are some modern cars. For
use in aeroplanes, it is usually alloyed with other metals
such as copper (Image C8.02). Its low density and good
conductivity have led to its use in overhead power lines.
ev
C
export 7%
U
R
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others 7%
Aluminium is the most common metal in the Earth’s crust.
The one major ore of aluminium is bauxite, and aluminium
oxide is purified from this. Electrolysis of molten
aluminium oxide produces aluminium at the cathode.
Aluminium’s usefulness
R
electrical
cables 14%
Pr
y
The breakthrough came in 1886 when Charles Hall and
Paul Héroult independently found a way to obtain the
metal by electrolysis.
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Cambridge IGCSE Combined and Co-ordinated Sciences
-C
Sr
Zr Nb Mo Tc Ru Rh Pd Ag Cd
In
Hf
Tl
Ta
Cr Mn Fe Co Ni Cu Zn Ga
W
Re Os
Ir
Au Hg
w
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ni
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Ra
w
ev
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The famous bridge at Ironbridge in Shropshire,
England (Image C8.03), marks a historic industrial
revolution in Europe. Made from cast iron and opened
in 1781, it was the first iron bridge in the world.
The metal iron is a transition element (or transition
metal). We use about nine times more iron than all the
other metals put together. Modern bridges (such as the
first Forth Road Bridge and the new Queensferry Crossing,
in Scotland) are now made of steel, where iron is alloyed
with other transition elements and carbon to make
it stronger.
-R
s
rs
These general properties mean that the transition
elements are useful in a number of diferent ways.
In addition there are particular properties that
make these metals distinctive and useful for more
specific purposes.
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Image C8.03 The bridge at Ironbridge was the first ever
built of iron.
The general features of transition elements make them
the most useful metallic elements available to us.
They are much less reactive than the metals in Groups I
and II. Many have excellent corrosion resistance, for
example chromium. The very high melting point of
tungsten (3410 °C) has led to its use in the filaments of
light bulbs.
C
332
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R
Pt
ve
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Ti
Ba
C
Al
Ca Sc
Y
V
B
-R
Be
Mg
III
Pr
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II
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C8.03 the transition elements
-R
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The distinctive properties of the
transition elements:
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Many of their compounds are coloured.
■ These metals oten show more than one valency –
they form more than one type of ion.
■ The metals or their compounds oten make
useful catalysts.
■ A few of the metals are strongly magnetic (iron,
cobalt and nickel).
■
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are hard and strong
have high melting points
have high densities
are good conductors of heat and electricity
are malleable and ductile.
ni
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■
Pr
op
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-C
The transition elements have all the major
properties we think of as being characteristic of
metals. They:
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rs
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copper(II)
Cu2+
blue
Fe
Fe3+
green
cobalt(II)
Co2+
pink
MnO4–
purple
2–
4
yellow
y
op
CrO
dichromate(vı)
Cr2O72–
ve
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chromate(vı)
orange
C
op
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U
R
Table C8.03 The colours of some transition-element ions
in solution.
w
Catalytic properties
-R
iron
Iron is only a moderately reactive metal, but it will still
react with steam or acids to displace hydrogen gas.
For example:
rs
C
w
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The reactions of certain transition elements
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op
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The transition elements are one of the major
contributors to colour in our lives. The impressive colours
of stained glass windows are produced by the presence
of these metal ions in the glass (Image C8.04).
w
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C
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R
Catalysts are substances that speed up a
chemical reaction without themselves being used
up or changed at the end of the reaction. Many of the
important industrial catalysts are either transition
elements or their compounds, for example iron in the
Haber process.
333
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Pr
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s
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am
br
The salts of the metals in Groups I, II and III are generally
white solids. They give colourless solutions if they dissolve
in water. In contrast, the salts of the transition elements
are oten coloured and produce coloured solutions when
dissolved. For example, vanadium compounds in solution
can be yellow, blue, green or purple. Some other examples
of the colours produced by transition-element ions are
given in Table C8.03. The presence of such metals in
negative ions also gives rise to colour.
ev
id
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Coloured compounds
id
ie
iron + hydrochloric acid
es
s
-C
ity
Pr
op
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+
H2(g)
Copper
Copper has a distinctive colour. It is one of the
least reactive metals in common use. It does not react
with dilute acids to produce hydrogen. If the metal is
heated in air, a black layer of copper(II) oxide is formed on
the metal:
ni
ve
rs
C
y
C
U
op
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heat
copper(II) oxide
2CuO(s)
Copper statues and roofs become coated in a green layer
of basic copper(II) carbonate (Image C8.05) when exposed
to the atmosphere for a long time.
w
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id
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es
s
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br
am
FeCl2(aq)
2HCl(aq)
Copper + oxygen
2Cu(s) + O2(g)
Image C8.04 Meherangarah Fort stained glass
windows, Jodhpur, India. The colours of the stained
glass are due to the presence of transition metal ions in
the glass.
-C
iron(II) chloride + hydrogen
-R
am
br
ev
Fe(s) +
ev
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The colours associated with transition elements also
help in chemical analysis. When testing a salt solution by
adding sodium hydroxide, the transition elements give
hydroxide precipitates with a characteristic colour. For
example, iron(II) hydroxide is grey-green whereas iron(III)
hydroxide is red-brown.
y
Including some negative ions that contain these metals.
ni
(a)
Pr
es
s
Cr
-C
chromium(III)
C
w
red-brown
3+
manganate(vıı)
ev
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green
-R
iron(III)
am
br
id
iron(II)
2+
Similar trace amounts (very small amounts) of metals
produce the colours of gemstones such as sapphire
and ruby. These stones are corundum, the naturally
occurring crystalline form of aluminium oxide (A12O3).
Pure corundum is colourless but trace amounts of
titanium and iron ions together produce the blue colour
of sapphires, while chromium ions (Cr3+) produce the red
colour of rubies.
w
Colour
ev
ie
Formula
ge
Metal ion in solution(a)
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C8: Patterns and properties of metals
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Cambridge IGCSE Combined and Co-ordinated Sciences
am
br
id
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This is simply a physical change that occurs on heating; it is
not a chemical reaction.
Give three distinctive properties of the
transition metals.
C8.12
What are the two oxidation states (valencies) of
iron in its compounds?
C8.13
What colour do you associate with copper
compounds?
C8.14
Iron corrodes to form rust. What is the chemical
name and formula for ‘rust’?
C8.15
Name an important industrial process for which
iron is the catalyst.
ev
s
es
Pr
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op
rs
C
w
y
CO2(g)
op
ve
black
ni
CuO(s)
green
C
U
ie
copper(II) oxide + carbon dioxide
CuCO3(s)
R
ev
copper(II) carbonate
zinc oxide + hydrogen
ZnO
es
+
H2(g)
C
ity
op
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H2(g)
ZnCl2(aq)
2HCl(aq)
zinc oxide + carbon dioxide
ZnO(s) +
CO2(g)
white
w
e
white
op
heat
C
ZnCO3(s)
heat
U
zinc carbonate
y
ni
ve
rs
Zinc carbonate decomposes on heating to give of
carbon dioxide:
w
ie
id
g
Image C8.06 This Viking sword has a handle made from
gold and silver and an iron blade. The blade has corroded
but the handle is untouched.
-R
s
es
am
br
ev
Interestingly, when hot, the zinc oxide produced is yellow.
However, when it cools down it turns white again.
-C
ie
+
zinc chloride + hydrogen
zinc + hydrochloric acid
ev
-R
Zn(s) + H2O(g)
heat
s
heat
-C
zinc + steam
Pr
am
br
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Zinc
Zinc is a moderately reactive metal that will displace
hydrogen from steam or dilute acids:
R
Most of the elements in the Periodic Table are metals.
Many of them are useful for a wide variety of purposes;
some, such as iron, have an enormous number of uses. The
early history of human life is marked by the metals used in
making jewellery, ornaments and tools. Early civilisations
used metals that could be found ‘native’ (for example,
gold) for decorative items, and then alloys such as bronze.
Later, iron was used for tools. Even ater the Bronze and
Iron Ages, only a few metals continued to be used widely.
Other more reactive metals could not be obtained until the
19th century. Even among the metals that were available,
there were obvious diferences in resistance to corrosion.
The Viking sword in Image C8.06 emphasises the diferent
reactivities of the gold and silver of the hilt and the iron of
the blade.
-R
br
am
-C
y
Copper(II) carbonate is also found in the Earth’s crust
as the mineral malachite. Like most other carbonates,
copper(II) carbonate will decompose on heating to release
carbon dioxide:
Zn(s) +
C
op
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id
C8.04 the reactivity of metals
Image C8.05 The copper sheets of this roof have become
coated in a layer of green copper(II) carbonate.
334
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C8.11
U
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Pr
es
s
-C
-R
QuEStiONS
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C8: Patterns and properties of metals
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ev
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-R
am
br
id
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We have seen how reactivity changes in a particular group.
But the more important metals we use come from more
than one group. Is there a broader picture in which we can
compare these?
Pr
es
s
-C
An overview of reactivity
op
y
We can get information on reactivity by investigating the
following aspects of metal chemistry:
• reactions with water
Image C8.07 A piece of copper found ‘native’.
ni
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• reactions with dilute acids
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• reactions with air or oxygen
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• ease of extraction
• metal displacement reactions
w
ge
copper (Image C8.07), gold and silver. The metals that
occur native form the first broad group of metals. They are
found as metals in the Earth’s crust.
C
U
Pb
(hydrogen
H)
copper
Cu
silver
Ag
ie
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-R
s
lead
es
Sn
Pr
tin
w
r e a c t i v i t y
Fe
Metal
copper pyrites
copper iron sulfide, CuFeS2
iron
hematite
iron(III) oxide, Fe2O3
sodium
rock salt
sodium chloride, NaCl
Au
tin
cassiterite
tin(IV) oxide, SnO2
zinc-blende
zinc sulfide, ZnS
galena
lead(II) sulfide, PbS
op
C
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s
Table C8.04 Some metals and their ores.
es
am
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lead
ie
zinc
Pt
y
copper
U
aluminium oxide, Al2O3
Figure C8.02 The reactivity series of metals.
-C
Compound present
aluminium bauxite
br
platinum
id
g
gold
Name of ore
ni
ve
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iron
ity
Zn
i n c r e a s i n g
C)
-C
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y
C
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Al
(carbon
zinc
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Mg
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br
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aluminium
ge
magnesium
R
y
Ca
Extracting metals with charcoal
Skills:
AO3.1 Demonstrate knowledge of how to safely use
techniques, apparatus and materials (including
following a sequence of instructions where
appropriate)
AO3.3 Make and record observations, measurements
and estimates
In this activity, copper is extracted from its oxide using
powdered charcoal.
A worksheet is included on the CD-ROM.
op
calcium
ity
Na
rs
sodium
ACtivity C8.01
ve
K
335
ni
potassium
R
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Pr
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A few metals are so unreactive that they occur in an
uncombined state. These unreactive metals include
es
s
-C
The extraction of metals
However, most metals are too reactive to exist on their own
in the ground. They exist combined with other elements
as compounds called ores (Table C8.04). These are the raw
materials for making metals. The metals that must be mined
as ores can be subdivided into two other broad groups.
-R
am
br
The overall picture that emerges is summarised in
Figure C8.02. This is known as the reactivity series of metals.
ev
id
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• heat stability of metal compounds.
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2ZnO(s)
+
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lead
copper
silver
ni
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gold
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Table C8.05 Methods of extraction in relation to the
reactivity series.
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s
-R
br
am
-C
Pr
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ACtivity C8.02
Reacting iron wool with steam
Skill:
AO3.3 Make and record observations, measurements
and estimates
In this demonstration, steam is passed over red-hot
iron wool. The gas produced in the reaction is collected
and tested with a lighted splint.
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occur native
(uncombined) in
the ground
C
op
C
However, some metals are too reactive to be extracted by
this method. The very reactive metals such as aluminium,
magnesium and sodium have to be extracted by
electrolysis of their molten ores. The three broad groups
are summarised in Table C8.05.
reduction of oxides
with carbon (sulfide ores
heated to give oxide)
iron
tin
So this group of moderately reactive metals can be
extracted by reduction with carbon using essentially the
blast furnace method (Section C9.01).
w
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Image C8.08 The major ores or iron: limonite, hematite and
magnetite.
id
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iron wool
safety tube
water
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am
br
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hydrogen
heat
y
heat
A worksheet is included on the CD-ROM.
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Pr
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steam
generator
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Questions
id
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A1 What colour is the surface of the iron ater the reaction?
ev
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A2 The form of iron oxide produced in this reaction has
the formula Fe3O4. Write a balanced symbol equation
for the reaction taking place.
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s
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br
am
Image C8.09 ‘Fool’s gold’ – a notorious ore of iron called
iron pyrites (FeS2).
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336
zinc
decreasing
reactivity
2SO2(g)
electrolysis of
molten ores
aluminium
The oxide must then be reduced to give the metal. Carbon,
in the form of coke, is used for this. Coke can be made
cheaply from coal. At high temperatures, carbon has a
strong tendency to react with oxygen. It is a good reducing
agent and will remove oxygen from these metal ores.
w
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magnesium
y
+ 3O2(g)
potassium
C
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2ZnS(s)
Method of extraction
calcium
zinc oxide + sulfur dioxide
zinc sulfide + oxygen
Metal
sodium
Pr
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am
br
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The moderately reactive metals such as iron, occur as
oxide or as sulfide ores (Image C8.08). One sulfide ore that
is quite noteworthy is iron pyrites which, because of its
colour, became known as ‘fool’s gold’ (Image C8.09). The
sulfide ores can easily be converted to the oxide by heating
in air. For example:
C
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Copyright Material - Review Only - Not for Redistribution
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C
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C8: Patterns and properties of metals
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Reactions of metals with air, water and
dilute acids
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Metal displacement reactions
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ZnSO4(aq) +
CuSO4(aq)
colourless
Pr
rs
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The observed efect of the reaction is that the zinc metal
becomes coated with a red-brown layer of copper.
The blue colour of the solution fades. The solution will
eventually become colourless zinc sulfate (Image C8.10).
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Cu(s)
red-brown
es
blue
y
grey
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Zn(s) +
-C
zinc sulfate + copper
s
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In this type of reaction, the two metals are in direct
competition. If a piece of zinc is let to stand in a solution of
copper(II) sulfate, a reaction occurs:
zinc + copper(II) sulfate
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Zinc displaces copper from solution, so zinc is more
reactive than copper.
b
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a
Displacement reactions of metals
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
AO3.5 Evaluate methods and suggest
possible improvements
Wear
eye protection.
!
In this experiment, you will investigate the reactions
between metals and solutions of their salts.
1 Using a measuring cylinder, pour 10 cm3 of zinc sulfate
solution into a boiling tube.
2 Place the tube in a rack and, using a stirring
thermometer, record the temperature of the solution.
3 Add one spatula measure of magnesium powder to the
tube, start a stopclock and stir.
4 Record the temperature every 30 seconds for
5 minutes, stirring between each reading.
5 Using a fresh tube, repeat the above experiment
using copper sulfate solution and zinc powder.
6 Again, record the temperature change over
5 minutes.
7 Repeat the experiment again, this time using
copper sulfate solution and iron powder.
8 Plot three graphs on the same grid showing the
temperature change over time for each metal.
A worksheet is included on the CD-ROM.
The Notes on activities for teachers/technicians
contain details of how this experiment can be used
as an assessment of skill AO3.4. This activity could be
used as a pilot for Activity C7.03.
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A displacement reaction can help us to place particular
metals more precisely in the reactivity series. We can
use it to compare directly the reactivity of two metals.
In a displacement reaction, a more reactive metal
displaces a less reactive metal from solutions of salts of
the less reactive metal.
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Pr
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Considering the methods of extraction of the metals gives
a broad pattern of reactivity. More detail can be found by
looking at certain basic reactions of metals. The results are
summarised in Table C8.06.
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ACtivity C8.03
Questions
op
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A2 How could you improve the accuracy of your
experiment?
e
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A1 What would you expect to happen if the experiment
was carried out using iron(II) sulfate solution and zinc
powder? Explain your answer.
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The reverse reaction does not happen. A piece of copper
does not react with zinc sulfate solution.
-R
It is possible to confirm the reactivity series using
displacement reactions of this type. For example,
s
es
-C
am
br
Image C8.10 Zinc is more reactive than copper and
displaces copper from copper(II) sulfate solution. Note the
brown deposit of copper, and the fact that the blue colour
of the solution has faded.
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337
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aluminium(a)
do not react
do not react
copper
react slowly to form oxide
layer when heated
silver
do not react
do not react
do not react
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gold
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lead
y
react less strongly to
give hydrogen
y
burn less strongly in air to
form oxide
iron
ev
ie
react very strongly to
give hydrogen
react with steam, when
heated, to give hydrogen
zinc
R
w
react with cold water to
give hydrogen
ev
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burn very strongly in air to
form oxide
Dilute HCl
Pr
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magnesium
Water
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calcium
Air
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sodium
Reaction with …
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Reactivity series
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Cambridge IGCSE Combined and Co-ordinated Sciences
(a)
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These reactions only occur if the protective oxide layer is removed from the aluminium.
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Table C8.06 The reaction of metals with air, water and dilute hydrochloric acid.
ev
REDUCING AGENT
• Zn loses electrons
• Zn is oxidised
• Oxidation number
increases
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Other redox competition reactions
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zinc + copper(II) ions
y
Cu(s)
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This shows that the reaction is a redox reaction
involving the transfer of two electrons from zinc atoms
to copper(II) ions. Zinc atoms are oxidised to zinc ions,
while copper(II) ions are reduced (Figure C8.03). In general,
the atoms of the more reactive metal lose electrons to
become positive ions.
es
s
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br
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A worksheet is included on the CD-ROM.
am
zinc ions + copper
Zn2+(aq) +
Cu2+(aq)
Zn(s) +
This activity uses microscale Comboplates® or white
spotting tiles to investigate the reactions between
powdered metals and their solutions. This observational
exercise allows the metals to be placed in series
depending on their reactivity.
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C
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
Reactive metals are good reducing agents. The nature of
the reaction taking place between zinc and copper sulfate
can be explored in more detail by looking at the ionic
equation:
s
am
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AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
R
Cu
Figure C8.03 The displacement reaction between zinc
and copper(II) sulfate is a redox reaction. A summary of the
redox change in terms of oxidation number and electron
exchange is shown.
the reactivity series
Skills:
Cu2+
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ACtivity C8.04
2e–
OXIDISING AGENT
• Cu2+ gains electrons
• Cu2+ is reduced
• Oxidation number
decreases
Copper displaces silver from solution, so copper is
more reactive than silver.
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338
es
Cu(NO3)2(aq) + 2Ag(s)
op
y
2AgNO3(aq) + Cu(s)
Zn2+
Zn
Pr
-C
am
br
if copper metal is put into colourless silver nitrate
solution, the copper will become coated with silver, and
the solution becomes blue because of the formation of
copper nitrate solution:
Copyright Material - Review Only - Not for Redistribution
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C8.17
Select from this list a metal that will not react
with hydrochloric acid to produce hydrogen:
magnesium, iron, copper.
Pr
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that the transition metals are less reactive than
the metals in Groups I and II and have certain
distinctive properties
how metals can be arranged into a series based on
their reactivity, with the most reactive metals lying to
the let of the Periodic Table
how a more reactive metal will displace a less reactive
metal from its oxide
how a more reactive metal can displace a less reactive
metal from a solution of one of its salts
that these displacement reactions are redox reactions
involving the transfer of electrons.
ev
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■
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that the alkali metals (Group I) are sot metals with low
densities – they are the most reactive group of metals,
displacing hydrogen from cold water and having to be
stored under oil
how reactivity increases as you move down a group
and that this is true for both Group I and Group II
(the alkaline earth metals)
that aluminium is a useful construction metal because
it is strong but has a low density
that aluminium is resistant to corrosion because of its
protective oxide coating
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■
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id
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Which properties of metals and their alloys are important when selecting the right metal for
a particular job?
Brass conducts electricity less well than copper. Explain why it is used in plugs and switches.
Reaction with cold water
calcium
reacts rapidly
copper
no reaction
magnesium
reacts very slowly
reacts rapidly
zinc
no reaction
reacts
y
no reaction
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reacts very rapidly
Put these metals in order of their reactivity.
least reactive
most reactive
Iron is a metal between zinc and copper in the reactivity series. Predict the reactivity of iron
with cold water and with steam.
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Reaction with steam
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[1]
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Pr
Metal
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A student observed the reaction of various metals with both cold water and steam. Her results are
shown below.
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End-of-chapter questions
1
R
Write a balanced chemical equation and
an ionic equation for the reaction between
magnesium and copper(II) sulfate solution.
ni
You should know:
R
C8.20
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Summary
■
State two observations you would see
when a piece of magnesium ribbon is placed in
copper(II) sulfate solution.
Write a word equation for the reaction between
magnesium and copper(II) sulfate solution.
C
op
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C8.18
C8.19
-R
Write a word equation for the reaction of zinc and
dilute hydrochloric acid.
-C
C8.16
■
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QuEStiONS
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C8: Patterns and properties of metals
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[2]
(continued)
339
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Zn + H2O
ZnO + H2
-R
Pr
es
s
Melting point / °C
Hardness
lithium
Density / g/cm3
fairly hard
63
rubidium
39
caesium
29
sot
1.53
extremely sot
1.88
U
very sot
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es
Pr
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–
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E
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+
C
A
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the electrolyte?
s
-C
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br
the anode?
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Which letter in the diagram above represents:
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–
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[5]
s
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D
B
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[1]
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[2]
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i Write a word equation for this reaction.
ii Sodium reacts with water in a similar way to lithium.
Write a symbol equation for the reaction of sodium with water.
Describe the reactions of lithium, sodium and potassium with water. In your description,
write about:
i the diference in the reactivity of the metals
ii the observations you would make when these metals react with water.
The diagram below shows an electrolysis cell used to manufacture sodium from molten
sodium chloride.
ve
b
Pr
2LiOH + H2
y
op
C
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[Cambridge IGCSE Chemistry 0620 Paper 21 Q6 June 2011]
The equation for the reaction of lithium with water is
2Li + 2H2O
ie
[1]
[1]
[1]
Lithium, sodium and potassium are in Group I of the Periodic Table.
a
340
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potassium
fairly sot
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98
ni
sodium
0.53
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Metal
i Estimate the melting point of lithium.
ii How does the hardness of these metals change down the group?
iii Estimate the density of potassium.
3
[1]
[3]
Write a word equation for this reaction.
State three physical properties that are characteristic of most metals.
Some properties of the Group I metals are shown in the table.
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d
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The equation for the reaction of zinc with steam is:
am
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b
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Cambridge IGCSE Combined and Co-ordinated Sciences
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[2]
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State the name of the product formed:
am
br
id
ii
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C8: Patterns and properties of metals
at the positive electrode
[2]
-R
at the negative electrode
iodine
sodium
[1]
[Cambridge ICCSE Chemistry 0620 Paper 21 Q6 June 2012]
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am
es
s
-C
Pr
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341
ity
C
In two of the test-tubes, bubbles of hydrogen gas are produced.
i
y
op
[1]
ie
magnesium chloride
solution
y
test-tube B
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test-tube A
w
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C
U
Describe how the appearance of the contents of test-tube A would change ater one hour.
Explain why you would not expect a chemical change in the contents of test-tube B.
-R
s
es
-C
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br
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i
ii
ity
C
Pr
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am
metal X
copper chloride
solution
w
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[2]
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Complete the balanced symbol equation for the reaction between magnesium and
hydrochloric acid.
+
MgCl2 +
ii List the three metals X, copper and magnesium, in order of reactivity, from the most
reactive to the least reactive.
The diagram below shows an experiment in which the metal X is placed in solutions of
copper chloride and magnesium chloride.
metal X
ev
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magnesium
copper
ge
R
X
b
R
[2]
The diagram below shows an experiment to compare how three metals react with dilute hydrochloric acid.
ni
a
magnesium
Lithium, sodium and potassium are metals with a low density. State two other physical
properties of these metals.
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4
Pr
es
s
graphite
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d
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iii Which one of the following substances is most likely to be used for the anode?
Copyright Material - Review Only - Not for Redistribution
[2]
[1]
(continued)
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[2]
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[Cambridge IGCSE Combined Science 0653 Paper 33 Q1 June 2014]
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es
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br
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C
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C
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Pr
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342
[1]
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ii
Copper can be extracted from copper oxide by heating it with carbon. The process involves the
reduction of copper oxide.
State what is meant by the term reduction.
Aluminium is extracted by the process of electrolysis of molten aluminium oxide.
Aluminium metal is deposited at the cathode of the electrolytic cell. Explain why metals are
always deposited at the cathode, rather than the anode, during electrolysis.
am
br
id
c
C
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op
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Cambridge IGCSE Combined and Co-ordinated Sciences
Copyright Material - Review Only - Not for Redistribution
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Pr
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C
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br
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Pr
ity
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■
ni
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■
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■
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■
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■
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■
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■
the production of iron in the blast furnace
steel making
rusting of iron and barrier methods for its prevention
sacrificial protection and galvanization as rust prevention methods
the extraction of aluminium
the Haber–Bosch process for the manufacture of ammonia
the manufacture and use of fertilisers
the manufacture of sulfuric acid
the commercial electrolysis of brine
limestone and its uses
the production of lime and its uses
recycling.
am
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■
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This chapter covers:
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C9
Industrial inorganic chemistry
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343
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Cambridge IGCSE Combined and Co-ordinated Sciences
C
sealing valves
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Figure C9.01 The blast furnace reduction of iron ore
to iron.
s
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Pr
y
A series of chemical reactions takes place to produce
molten iron (Figure C9.02). The most important
reaction that occurs is the reduction of the ore by
carbon monoxide:
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C
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ni
2Fe(s) + 3CO2(g)
C
U
Fe2O3(s) + 3CO(g)
One of the major impurities in iron ore is sand (silica, SiO2).
The limestone added to the furnace helps to remove
this impurity. The limestone decomposes to lime in the
furnace. This then reacts with the silica:
CaCO3(s)
heat
lime
op
+ silica
C
w
e
id
g
+ carbon dioxide
CaO(s) +
y
heat
CO2(g)
calcium silicate
CaSiO3(l)
ev
ie
The calcium silicate formed is also molten. It flows
down the furnace and forms a molten layer of slag on
top of the iron. It does not mix with the iron, as it is
less dense. The molten slag is ‘tapped of’ separately.
es
s
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br
am
limestone
CaO(s) + SiO2(s)
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C
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The iron produced flows to the bottom of the furnace
where it can be ‘tapped of’ because the temperature at
the bottom of the furnace is higher than the melting point
of iron.
lime
Image C9.01 A worker in protective clothing takes a sample
from a blast furnace in a steel works.
-C
molten iron
-R
br
am
-C
The main ore of iron is hematite (Fe2O3). The iron is
obtained by reduction with carbon in a blast furnace
(Image C9.01 and Figure C9.01). The furnace is a steel
tower about 30 metres high. It is lined with refractory
(heat-resistant) bricks of magnesium oxide which are
cooled by water. The furnace is loaded with the ‘charge’,
which consists of iron ore, coke (a form of carbon made
from coal) and limestone (calcium carbonate). The charge
is sintered (the ore is heated with coke and limestone) to
make sure the solids mix well, and it is mixed with more
coke. Blasts of hot air are sent in through holes near the
bottom of the furnace. The carbon burns in the air blast
and the furnace gets very hot.
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hot air
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hot air
molten slag
The production of iron in the blast furnace
344
walls of heatresistant magnesium
oxide bricks, cooled
by water
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Steel is mainly iron with between 0.2 and 1.5% carbon.
The carbon makes the iron harder and stronger. Small
quantities of other transition metals can also be added
to make special steels. Steels are alloys in which the main
metal is iron. The magnetic properties of iron make it easy
to separate steel products from other waste, so the metal
can be easily recycled.
waste gas to heat
exchanger, to heat
incoming air
limestone,
coke, iron ore
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Pr
es
s
-C
In our modern world, we have invented and shaped many
machines and clever devices. These are oten made of steel.
It is the most widely used of all metals. The durability, tensile
strength and low cost of steel make it the basis of countless
industries, from ship-building to watch-making. Iron and
steel making are at the centre of our heavy industries.
-R
Iron and steel
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C9.01 the extraction of metals
by carbon reduction
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C9: Industrial inorganic chemistry
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Steel-making
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The hot waste gases escape from the top of the furnace.
They are used in heat exchangers to heat the incoming air.
This helps to reduce the energy costs of the process. The
extraction of iron is a continuous process.
C
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molten iron and
scrap steel, lime
molten iron
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scrap steel
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slag
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slag
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TIP
molten steel
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w
Figure C9.03 The diferent stages of the steel-making
process (the basic oxygen process). a The furnace is
charged with scrap steel and molten iron. b Oxygen is
blown in through an ‘oxygen lance’. c The molten steel, and
then the slag, are poured from the furnace by tilting it in
diferent directions.
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For the blast furnace it is important that you are aware of
the diferent aspects of how it works. You should be able
to label a diagram of it and know what is fed into it.
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Importantly, you should also know the key reactions of
the furnace, including the formation of slag.
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water-cooled
oxygen lance
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uses iron ore, coke, limestone and hot air
■ involves the reduction of iron(III) oxide by carbon
monoxide
■ uses limestone to remove the main impurity (sand)
as slag (calcium silicate).
fumecollecting
hood
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The blast furnace extraction of iron:
■
oxygen
b
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345
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Figure C9.02 Iron is produced in the blast furnace by a
series of reactions. Carbon monoxide is thought to be the
main reducing agent.
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Carbon burns strongly at the base of the
furnace (temperatures reach 1900 ºC).
carbon + oxygen
carbon dioxide
C + O2
CO2
ie
Most of the pig iron produced is taken to make steel.
The carbon content is reduced by burning it of as
carbon dioxide. This basic oxygen process is carried
out in a tilting furnace (Figure C9.03). Scrap steel is
added to the molten pig iron for recycling.
A high-speed jet of oxygen is blown into the vessel
through a water-cooled lance. Some impurities,
for example silicon and phosphorus, do not produce
gaseous oxides, so lime (CaO) is added to the furnace.
The impurities form a ‘slag’, which floats on top of the
molten iron. The molten iron is poured of by tilting
the furnace. Controlled amounts of other elements
such as chromium, manganese, tungsten or other
transition metals are added to make diferent types
of steel (see Tables C9.01 and C9.02).
C
op
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Hot gases rise up the furnace.
C
w
R
Molten iron flows down the furnace.
Carbon dioxide is reduced as it rises through
the furnace – carbon monoxide is produced
(temperature about 1000 ºC).
carbon
carbon
+ carbon
monoxide
dioxide
2CO
CO2 + C
-R
carbon
iron +
dioxide
2Fe + 3CO2
iron(III)
carbon
+
oxide monoxide
Fe2O3 + 3CO
ev
ie
The iron produced by the blast furnace is known as
‘pig iron’ or ‘cast iron’ and is not pure. It contains about
4% carbon, and other impurities. This amount of carbon
makes the iron brittle.
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br
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The iron ore is reduced by carbon monoxide
(temperature about 600 ºC).
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< 0.25
easily worked; not brittle
car bodies, chains, pylons
0.25–0.45
tougher than mild steel
car springs, axles, bridges
0.45–1.5
hard and brittle
iron
74%
chromium
18%
5%
87%
drill bits, springs
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Rust is a red-brown powder consisting mainly of
hydrated iron(III) oxide (Fe2O3 · xH2O). Water and oxygen
are essential for iron to rust (Figure C9.04).
rs
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tube 1 (control
experiment)
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very rusty
iron nails
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distilled
water
distilled
water
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anhydrous
calcium chloride
(drying agent)
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op
C
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When a metal is attacked by air, water or other surrounding
substances, it is said to corrode. In the case of iron and
steel, the corrosion process is also known as rusting.
Rusting is a serious economic problem. Large sums of
money are spent each year replacing damaged iron and
steel structures, or protecting structures from such damage.
boiled distilled
water (boiled
to remove any
dissolved air)
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g
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Figure C9.04 The results of an experiment to investigate
the factors that are involved in rusting. In tube 2, the air is
dry, so the nails do not rust. In tube 3, there is no oxygen in
the water, so the nails do not rust. In tube 4, pure oxygen
and water are present, so the nails are very rusty.
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s
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br
am
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rusty iron
nails
The rusting of iron and its prevention
pure
oxygen
layer of
olive oil
(prevents air
dissolving
in the water)
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Pr
op
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The uses of other substances are also explicitly stated in
the syllabus – so go through and make a list of these and
specifically learn them.
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air
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The syllabus very clearly states some examples of the
major uses of mild and stainless steel. Make sure that you
are aware of these.
tube 4
dry air
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air
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TIP
tube 3
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But carbon steels tend to rust unless protected. So other
metals, for example chromium, are added to prevent
corrosion and to make the steel harder. Some of these
alloy steels are listed in Table C9.02.
tube 2
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C
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Pr
y
There is a wide variety of steels to suit particular
applications. Some steels are alloys of iron and carbon
only. The amount of carbon in steels can vary between
0.2% and 1.5%. These carbon steels, which include the
mild steel used for car bodies, are listed in Table C9.01.
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tough; springy
br
am
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Carbon steels and alloy steels
R
edges of high-speed cutting tools
All these alloys have a low content of carbon (< 0.45%).
Table C9.02 Some typical alloy steels.
346
tough; hard, even at
high temperatures
13%
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manganese
(a)
cutlery, surgical instruments,
kitchen sinks, chemical plant
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95%
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tungsten
manganese steel iron
tough; does not
corrode
C
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iron
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tungsten steel
Uses
8%
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nickel
Properties
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stainless steel
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C
Typical composition
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Table C9.01 Cast iron and carbon steels.
Steele(a)
chisels, cutting tools, razor blades
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high-carbon steel
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gear boxes, engine blocks, brake discs
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medium steel
cheaper than steel; easily moulded
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mild steel
Uses
2.5–4.5
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cast iron
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Carbon content / % Properties
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Metal
C
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C9: Industrial inorganic chemistry
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• Galvanising: An object may be coated with a layer of
the more reactive metal, zinc. This is called galvanising.
It has the advantage over other plating methods in that
the protection still works even if the zinc layer is badly
scratched.
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op
Pr
y
Aluminium is more reactive than iron, but it does not
corrode in the damaging way that iron does. Both metals
react with air. In the case of aluminium, a very thin single
layer of aluminium oxide forms, which sticks strongly to
the surface of the metal. This micro-layer seals the metal
surface and protects it from further attack.
• Sacrificial protection: This is a method of rust
prevention in which blocks of a reactive metal are
attached to the iron surface. Zinc or magnesium
blocks are attached to oil rigs and to the hulls of ships
(Figure C9.05). These metals are more reactive than iron
and will be corroded in preference to it. Underground
gas and water pipes are connected by wire to blocks of
magnesium to obtain the same protection. In all cases,
an electrochemical cell is set up. The metal blocks lose
electrons in preference to the iron and so prevent the
iron forming iron(III) oxide.
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The problem is made worse by the presence of salt;
seawater increases the rate of corrosion. Pictures from
the seabed of the wreck of the Titanic show that it has a
huge amount of rust (see Image C9.02). Acid rain also
increases the rate at which iron objects rust.
ev
• Electroplating: An iron or steel object can be
electroplated with a layer of chromium or tin to protect
against rusting. A ‘tin can’ is made of steel coated on
both sides with a fine layer of tin. Tin is used because it
is unreactive and non-toxic. However, this does raise a
problem. With both these metals, if the protective layer
is broken, then the steel beneath will begin to rust.
y
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Image C9.02 Photograph of the highly rusted bow of the
Titanic taken from a submersible.
In contrast, when iron corrodes, the rust forms in flakes.
It does not form a single layer. The attack on the metal
can continue over time as the rust flakes come of.
Indeed, a sheet of iron can be eaten right through by the
rusting process.
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• Plastic coatings: These are used to form a protective
layer on items such as refrigerators and garden chairs.
The plastic poly(vinyl chloride), PVC, is oten used for
this purpose.
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film, preventing rusting. Again, the treatment must be
repeated to continue the protection.
zinc bar
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C
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• Painting: This method is widespread, and is used for
objects ranging in size from ships and bridges to garden
gates. Painting only protects the metal as long as the
paint layer is unscratched. Regular re-painting is oten
necessary to keep this protection intact.
C
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2e–
• Oiling and greasing: The oiling and/or greasing of
the moving parts of machinery forms a protective
zinc (Zn)
Zn
2+
iron (Fe)
hull
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Figure C9.05 Blocks of zinc (or magnesium) are used for the
sacrificial protection of the hulls of ships.
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s
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am
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water
Pr
op
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es
The need to protect iron and steel from rusting has
led to many methods being devised. Some of these are
outlined here.
s
-C
Rust prevention
ship’s hull
made of steel
(mainly iron)
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347
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Cambridge IGCSE Combined and Co-ordinated Sciences
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C9.02
Write an equation for the reduction of
iron(III) oxide.
C9.03
Which element is used to remove the carbon from
cast iron?
C9.04
Why is chromium sometimes added to steel?
C9.05
Which two substances are essential for the
rusting of iron?
C9.06
Give two ways in which zinc can be used to stop
the rusting of iron.
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C9.02 the extraction of metals
by electrolysis
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am
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Pr
The copper produced from this ore is suitable for piping,
boilers and cooking utensils. When it is to be used for
y
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• mining the ore
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op
• purification of the ore
U
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Copper is less reactive than the other metals we have
considered so far. It can be found native in the USA, but
most copper is extracted from copper pyrites, CuFeS2.
C
• electrolysis of the molten ore.
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the extraction of copper and the
reactivity series
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
This activity explores the reactivities of copper, hydrogen
and carbon using microscale apparatus. The aim is to
see whether copper(II) oxide can be reduced to copper by
either hydrogen or carbon.
A worksheet is included on the CD-ROM. Details of a
scaled-up version of this experiment are given in the
Notes on activities for teachers/ technicians.
The extraction of aluminium
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Bauxite, the major ore of aluminium, takes its name
from the mediaeval village of Les Baux in France, where
it was first mined. Napoleon III saw its possibilities for
military purposes and ordered studies on its commercial
production. A method of extraction using sodium to
displace aluminium from aluminium chloride existed at
that time. However, in 1886, the Hall–Héroult electrolytic
method for extracting aluminium was invented by
Hall (an American) and Héroult (a Frenchman).
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s
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am
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The extraction of a metal by electrolysis is expensive.
Energy costs to keep the ore molten and to separate the
ions can be very high. Because of this, many of these
metals are extracted in regions where hydroelectric
power is available. Aluminium plants are the most
important examples. They produce suficient aluminium
to make it the second most widely used metal
ater iron.
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ACtivity C9.02
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Reduction with carbon does not work for more reactive
metals. The metals are held in their compounds
(oxides or chlorides) by stronger bonds which need
a lot of energy to break them. This energy is best
supplied by electricity. Extracting metals in this way is a
three-stage process:
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The extraction of copper
348
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Why is limestone added to the blast furnace?
U
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C9.01
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QuEStiONS
Pr
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Preventing rusting
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.2 Plan experiments and investigations
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
In this activity, iron nails are protected from rusting using
a variety of methods, including painting, greasing and
sacrificial protection. By using corrosion indicator solution,
the efectiveness of the diferent types of protection can
be assessed.
A worksheet, with a self-assessment checklist, is
included on the CD-ROM.
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electrical wiring, it must be refined (purified) by electrolysis
(see Section C4.05).
am
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ACtivity C9.01
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carbon + oxygen
Pr
C(s)
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When electrolytic cells are set up with appropriate metal
electrodes, metal can be efectively transferred from the
anode to the cathode. Such methods can be used to plate
objects with metals such as chromium or tin, or to refine
copper to a very high degree of purity.
Pr
ity
C9.08
Why is cryolite added to the cell as well
as alumina?
C9.09
Why do the anodes need replacing regularly?
y
Why is aluminium expensive to extract?
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C9.07
w
Write an equation for the reaction at the cathode.
ie
C9.10
C
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Al
C9.11
Aluminium is a reactive metal. Why, then, is it
useful for window frames and aircrat?
es
s
Oxide ions are attracted to the anode where they are
discharged to form oxygen gas. At the high temperature of
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g
CO2(g)
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id
Aluminium ions are attracted to the cathode where they
are discharged to form liquid aluminium metal:
am
O2(g)
carbon dioxide
heat
QuEStiONS
3 The molten mixture of aluminium oxide and cryolite
is electrolysed in a cell fitted with graphite electrodes
(Figure C9.06).
-C
+
heat
Electroplating and copper refining
br
am
-C
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y
C
–
The Hall–Héroult process uses a great deal of energy.
It is also costly to replace the anodes, which are burnt
away during the process. It is much cheaper to recycle
the metal than to manufacture it. The energy requirement
for recycling is about 5% of that needed to manufacture
the same amount of ‘new’ metal.
1 The bauxite is treated with sodium hydroxide to obtain
pure aluminium oxide (alumina). The alumina produced
is shipped to the electrolysis plant.
w
carbon cathode
brick insulation
The anodes burn away and have to be replaced regularly.
2 The purified aluminium oxide (Al2O3) is dissolved in
molten cryolite (sodium aluminium fluoride, Na3AlF6).
Cryolite is a mineral found naturally in Greenland. It
is no longer mined commercially there, and all the
cryolite now used is made synthetically. Cryolite is used
to lower the working temperature of the electrolytic
cell. The melting point of aluminium oxide is 2030 °C.
This is reduced to 900–1000 °C by dissolving it in
cryolite. The cryolite thus provides a considerable
saving in energy costs.
ie
anode +
the cell this reacts with the carbon of the anode to form
carbon dioxide:
The Hall–Héroult process involves the following stages.
Al3+ + 3e–
frozen crust
Figure C9.06 A cross-section of the electrolytic cell for
extracting aluminium. At the cathode: Al3+ + 3e–
Al.
At the anode: 2O2–
O2 + 4e–.
am
-C
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molten
electrolyte
containing
alumina
steel shell
carbon block
lining
liquid aluminium
–
Bauxite (Image C9.03) is an impure form of aluminium
oxide. Up to 25% of bauxite consists of the impurities
iron(III) oxide and sand. The iron(III) oxide gives it a
red-brown colour.
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hood
Pr
es
s
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The Hall–Héroult process
ev
+
alumina
hopper
anode +
Image C9.03 The major ore of aluminium is bauxite.
It is usually mixed with iron(III) oxide, which gives the ore
its brown colour.
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+
-R
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C9: Industrial inorganic chemistry
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349
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Cambridge IGCSE Combined and Co-ordinated Sciences
is important for agriculture. Most plants cannot directly
use (or fix) nitrogen from the air. The main purpose of
industrial manufacture of manufacture of ammonia is to
make agricultural fertilisers.
am
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C9.03 Ammonia and fertilisers
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Ammonia has the following general properties as
a gas:
In the Haber process (Figure C9.07), nitrogen and hydrogen
are directly combined to form ammonia:
N2(g)
+
3H2(g)
ammonia
2NH3(g)
ni
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As a raw material for both fertilisers and explosives,
ammonia played a large part in human history. It helped to
feed a growing population in peacetime, and it was used
to manufacture explosives in wartime.
ge
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am
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Nitrogen is an unreactive gas, and changing it into
compounds useful for plant growth (nitrogen fixation)
s
pump
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N2, H2
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C
350
nitrogen + hydrogen
Nitrogen is obtained from air, and hydrogen from natural
gas by reaction with steam. The two gases are mixed
in a 1 : 3 ratio and compressed to 200 atmospheres.
They are then passed over a series of catalyst beds
containing finely divided iron. The temperature of the
converter is about 450 °C. The reaction is reversible and
does not go to completion. A mixture of nitrogen, hydrogen
and ammonia leaves the converter. The proportion of
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colourless
■ distinctive smell
■ less dense than air
■ very soluble in water to give an alkaline solution.
■
beds of
catalyst
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pump
converter
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compressor
Pr
N2, H2, NH3
N2 , H 2
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gases mixed
and scrubbed
cooler
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H2
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liquid ammonia
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storage tanks
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N2
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pump
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am
Figure C9.07 A schematic drawing of the diferent stages of the Haber process. Nitrogen and hydrogen are mixed in a
ratio of 1 : 3 at the start of the process.
Copyright Material - Review Only - Not for Redistribution
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The ammonium nitrate can be crystallised into pellet form
suitable for spreading on the land.
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ev
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For the Haber process, and the Contact process for
making sulfuric acid, it is important that you know the
conditions used and how these are chosen. Remember
these are both reversible reactions that reach an
equilibrium under the conditions used (see Section C7.05).
C
op
y
Ammonium salts tend to make the soil slightly acidic.
To overcome this, they can be mixed with chalk (calcium
carbonate), which will neutralise this efect. ‘Nitro-chalk’ is
an example of a compound fertiliser.
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U
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Ammonium nitrate is soluble in water, as are all other
ammonium salts, for example ammonium sulfate,
(NH4)2SO4. This solubility is important because plants
need soluble nitrogen compounds that they can take
up through their roots. There are two types of nitrogen
compounds that plants can use – ammonium compounds
(which contain the NH4+ ion) and nitrates (which contain
the NO3– ion). Ammonium nitrate provides these ions.
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TIP
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ammonia in the mixture is about 15%. This is separated
from the other gases by cooling the mixture. Ammonia has
a much higher boiling point than nitrogen or hydrogen, so
it condenses easily. The unchanged nitrogen and hydrogen
gases are re-circulated over the catalyst. By re-circulating
in this way, an eventual yield of 98% can be achieved. The
ammonia produced is stored as a liquid under pressure.
C
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C9: Industrial inorganic chemistry
ev
A modern fertiliser factory will produce two main
types of product:
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Ammonium nitrate (‘Nitram’) is the most important of
the nitrogenous fertilisers. It contains 35% by mass of
nitrogen. It is produced when ammonia solution reacts
with nitric acid:
y
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es
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s
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am
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NH4NO3(aq)
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NH3(aq) + HNO3(aq)
id
ammonium nitrate
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Ammonium nitrate and other fertilisers
ammonia + nitric acid
straight N fertilisers are solid nitrogen-containing
fertilisers sold in pellet form, for example
ammonium nitrate (NH4NO3), ammonium sulfate
((NH4)2SO4) and urea (CO(NH2)2)
■ NPK compound fertilisers (Image C9.04) are
mixtures that supply the three most essential
elements lost from the soil by extensive use, namely
nitrogen (N), phosphorus (P) and potassium (K).
They are usually a mixture of ammonium nitrate,
ammonium phosphate and potassium chloride, in
diferent proportions to suit diferent conditions.
■
es
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am
br
Most of the ammonia produced is used to manufacture
fertilisers. Liquid ammonia itself can in fact be used
directly as a fertiliser, but it is an unpleasant liquid to
handle and to transport. The majority is converted
into a variety of solid fertilisers. A substantial amount
of ammonia is converted into nitric acid by oxidation
(Figure C9.08).
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op
others
10%
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nylon
5%
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nitric
acid
10%
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fertilisers 75%
s
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Image C9.04 Some fertiliser products; note the three
key numbers (N : P : K) on the fertiliser bags.
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Figure C9.08 The uses of ammonia produced by the
Haber process.
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351
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C9.13
What conditions are needed to ensure the
Haber process works eficiently?
C9.14
Why are the unreacted gases re-circulated?
C9.15
Why do many fertilisers contain N, P and K?
C9.16
How can fertilisers cause pollution?
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C
op
Sulfur is burnt in air to form sulfur dioxide. The main
reaction in the Contact process (Figure C9.09a) is the
one in which sulfur dioxide and oxygen combine to form
sulfur trioxide. This reaction is reversible. The conditions
needed to give the best equilibrium position are carefully
considered. A temperature of 450 °C and 1–2 atmospheres
pressure are used. The gases are passed over a catalyst of
vanadium(V) oxide. A yield of 98% sulfur trioxide is achieved.
ie
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sulfur
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sulfur burnt to form sulfur dioxide
S(s) + O2(g) SO2(g)
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How is hydrogen obtained for use in the
Haber process?
Sulfuric acid is a major product of the chemical industry.
It is made from sulfur by the Contact process.
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C9.12
C9.04 Sulfur and sulfuric acid
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Making a fertiliser
Skills:
AO3.1 Demonstrate knowledge of how to safely
use techniques, apparatus and materials
(including following a sequence of instructions
where appropriate)
AO3.3 Make and record observations, measurements
and estimates
AO3.4 Interpret and evaluate experimental
observations and data
The introduction of the Haber process revolutionised
agriculture by making it possible to manufacture artificial
fertilisers. An example is ammonium sulfate and it is
made in this activity by neutralising sulfuric acid with
ammonia solution:
(NH4)2SO4(aq)
H2SO4(aq) + 2NH3(aq)
The ammonium sulfate solution can be concentrated by
heating. It is then cooled to allow crystals to form.
A worksheet is included on the accompanying
CD-ROM.
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ACtivity C9.03
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Cambridge IGCSE Combined and Co-ordinated Sciences
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mixture of gases reacted
2SO2(g) + O2(g) 2SO3(g)
conditions: 450ºC, 1–2 atmospheres,
vanadium(V) oxide catalyst
yield: 98% SO3
unreacted
gases recycled
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SO3 dissolved in 98% H2SO4
SO3 + H2SO4 H2S2O7
(or SO3 + H2O H2SO4)
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gases mixed and cleaned by
electrostatic precipitation
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concentrated sulfuric acid
diluted when needed
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Figure C9.09 a The Contact process plant at Billingham, Teesside, in the UK. b A flow chart for making sulfuric acid by
this process.
Copyright Material - Review Only - Not for Redistribution
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C9: Industrial inorganic chemistry
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We have seen earlier in Chapter C1 that chlorine is
used on a large scale in producing a clean domestic
water supply. You will probably also be familiar
with its similar use in killing microbial organisms in
swimming pools. Chlorine has a wide range of other
uses in industry.
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Sodium hydroxide solution
alkaline and corrosive
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Hydrogen
a colourless, flammable gas
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used for making
margarine
nylon
hydrogen chloride and hydrochloric acid
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also used for
killing bacteria in the water supply
killing bacteria in swimming pools
Figure C9.11 The chlor–alkali industry.
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used for making
PVC (poly(chloroethene))
solvents for dry-cleaning (e.g. trichloroethane)
pain