Tips & Tricks to ace IGCSE
CHEMISTRY-0620 exam
Aminath Ulfa Adil
Chemistry Teacher
Bachelor of Teaching- Secondary (majored in Chemistry & Maths), MEd
Details of the assessment
MCQ
40 marks
40 Questions
Duration- 45mins
30%
THEORY
80marks
Usually 7 questions
Duration 1 hr 15mins
50%
ATP
40 Marks
4 questions
Duration- 1hr
20%
Electrolysis
Decomposition of an ionic compound in molten or aqueous
solution by passing electricity. (2marks)
•
Charge carried by:
Electrons - in the external circuit and move from anode to
cathode
– + ve ions & - ve Ions- in electrolyte
–
ANOX
I
L
REDCAT
I
G
Aqueous - dissolved in water’
Dilute
Concentrated
Cathode- The ion which is
lower in Electrochemical
Series
Cathode- The ion which
is lower in
Electrochemical Series
Anode- OH- discharged
Anode- If halide (Cl-/Br/I-) present then it gets
discharged
-Metal ions discharged to form metal
-H+ discharged to produce hydrogen gas
-O 2- discharged to produce oxygen gas
-Halides produce Halogens
-OH- produce Oxygen gas
“in one breath drink 2 glasses of
water’’
Purification
Electrolysis in
Anode- impure metal
Obs: ↓ in size and mass/ anode
dissolves away
Cathode- made from pure metal
Obs: ↑ in size and mass
Electrolyte- solution of metal used
electroplating
Anode- pure metal
Obs: ↓ in size and mass/ anode dissolves
away
Cathode- objeCT
Obs: (metal) coating formed on object
(REMEMBER TO WRITE THE
APPEARANCE OF THE METAL)
Electrolyte- solution of metal used
Observations at Electrodes
CATHODE
ANODE
Metal- silvery deposit
Cl2- Pale yellow-green gas
Copper- pink/brown solid
deposited
Br2- Red-brown vapour
Lead- Grey metal coating
I2- Purple vapours
If a colourless gas is produced always write
bubbles formed/fizzing/effervescences
DYNAMIC EQUILIBRIUM
A reversible chemical reaction in a closed system where
forward and backward reaction are taking place simultaneously
at equal rates without any further change in concentration of
reactants or products
FACTORS AFFECTING POSITION of EQUILIBRIUM
Concentration
Temperature
Pressure
Temperature
Pressure
ITEN
DTEX
IPL
DPM
Increasing temperature: favours
Increasing pressure: reaction which
ENDOTHERMIC reaction
produces Less no. of moles of gas
Decreasing temperature: favours
Decreasing pressure: reaction which
EXOTHERMIC reaction
produces More no. of moles of gas
Concentration
✔ Whenever the concentration of a substance is INCREASED, system try to
DECREASE it by favouring the reaction which uses up the particular
substance
✔ Whenever the concentration of a substance is DECREASED, system try to
INCREASE it by favouring the reaction which produces the particular
substance
C2H4 (g) + H2O(g) ⇌
C2H5OH (g)
ΔH= -47.8kJ/mol
State & Explain what happens to the position of equilibrium when:
(a)Temperature increases ITEN
(b)Temperature decrease DTEX
(c)Pressure increases
IPL
2 ⇌ 1
(d)Pressure decrease DPM
(e)Concentration of ethene increases
(f) Concentration of ethanol increases
(g)Adding a catalyst
QUESTION 1
4 questions in
ATP
Experimental
Analysis
Question 2
Titration / Graph
Question 3
Salt Analysis
Question 4
Planning an
investigation/
Experimental
design
Solubility RULES
Group 1 & NH4+
Chlorides
All soluble
All soluble
All soluble except Pb &
Ag
Sulfates
Hydroxides
Nitrates
All soluble except CaBaPb
All insoluble except Gr. 1 & NH4+,Sr &
Ba
Carbonates
All insoluble except Gr. 1 & NH4+
- At a higher temperature
a given volume of water
can dissolve more salt
than at a lower
temperature.
Why do crystals form as the solution cools down?
- Because solubility decreases when temperature decreases
Salt Preparation
INSOLUBLE
SOLUBLE
PRECIPITATION
S1
ACID + EXCESS METAL
MASH for MAZIT Salt
S2
ACID + EXCESS METAL CARBONATE
CAWCS
S3
ACID + EXCESS METAL OXIDE
BAWS
S4
ACID + ALKALI (Titration method)
Preparation of Highly reactive
metal(grp 1) & NH4+ salts
Salt Preparation
Insoluble
• Mix two aqueous salt solutions
• Filter off the PPT
• Rinse with cold distilled water
• Dry PPT with filter paper
common exam QUestions
• How is the precipitate removed?
• By filtration
• Why do we need to wash the residue/ppt with
distilled water?
• To remove soluble impurities
Salt Preparation
Soluble
• Add Excess metal/metal carbonate/metal
oxide into dilute acid until it stops dissolving.
• Filter off the excess solid.
• Heat the filtrate upto crystallization point
• Allow the solution to cool and form crystals
• Filter off the crystals, rinse with cold
distilled water
• Dry the crystals between filter papers
• For reactions of acids with metal or metal
carbonate excess solid is added until
bubbles stop evolving
• For acid+ MO heat is needed
• For acid + carbonate no heat is needed
Salt Preparation- common exam Questions- soluble salts
What is meant by the
term excess?
• More than enough to react
How is the excess
solid removed?
• filtration
Why is the solid added in
excess?
• To ensure all acid has used up
How did the student know that all of
the dilute acid had reacted?
(OR)
State two observations that would
indicate that the metal/metal
carbonate is in excess
• no more bubbling /
fizzing /
effervescence
• solid or powder
stops dissolving
Salt Preparation- common exam Questions- soluble salts
How can we identify
crystallization point?
(or) What evidence would
show that the solution was
saturated?
• when crystals form on a glass rod withdrawn
from solution / on a sample of solution placed
on microscope slide etc.
Why you shouldn’t
overheat crystals?
(or) Why should the
crystals not be heated to
dryness
• (heating to dryness) would remove water of
crystallisation
Why is actual yield always
less than the expected
yield?
• some salt remains in solution / some salt may
not form crystal
• some of the crystals decomposed
• some crystals lost in transfer
Question 2- Titration
Titration is usually done to find the concentration of a
solution by using a standard solution ( a solution of
known concentration)
Used to prepare salts of group 1 and Ammonium Salts
In titration colour
change at end-point
is from
FLASK to NEUTRAL
Question 2- Titration
▪
▪
▪
▪
Transfer 25cm3 of alkali into a conical flask using a pipette
Add few drops of indicator into the flask
Fill the burette with acid.
Slowly add acid from the burette until a colour change
occurs.
▪ Find the volume of acid needed for the neutralization.
Volumes of Liquids
1 dm3 = 1 000 cm3
Apparatus for measuring liquids depends on:
▪ The volume being measured
▪ How accurate the measurement needs to be
Beakers are more difficult when it comes to
swirling / mixing / shaking; compared to a conical
flask.
Measuring cylinder has accuracy to 1 cm3.
If reading is 23 cm3, should not write 23.0 cm3 as the ‘0’
means accurate to 0.1 cm3.
Volumes of Liquids
✔ Burette has long scale of 0 – 50 cm3,
accurate to 0.1 cm3.
✔ Liquid level to be measured before and
after tap opening. The difference of volume
gives the liquid volume poured off.
✔ Bulb pipette measures exact volumes such
as 20.0, 25.0 or 50.0 cm3, not odd
volumes such as 31.0 cm3.
ACCURACY OF APPARATUS
Burette and pipeTte
0.1 cm3 - Values must be given correct to 1dp
Measuring cylinder
1cm3 - Values must be given correct to whole number
thermometer
0.1 0 C - Values must be given correct to 1dp
apparatus
Rinsed with
reason
Burette
distilled water and solution added to ensure that the
In Titration
Pipette
Conical flask
concentration is same
distilled water and solution added to ensure that the
concentration is same
Tap water and distilled water
to remove impurities /
chemicals / residue /
solution
If the same conical flask is used for all experiments, the conical flask must be
rinsed with distilled water to remove any residue/ chemicals left from the
previous experiment.
Determining the more concentrated solution
If the solutions with unknown concentration(analyte) is
in the conical flask:
Sodium hydroxide
(known
concentration)
HCl solution D
HCl solution C
The more concentrated
HCl solution would
require a greater
volume of NaOH for
neutralisation.
Burette volume
(difference) would be
larger
Determining the more concentrated solution
If the solutions with unknown concentration (analyte) is
in the burette:
HCl solution D
HCl solution C
Sodium hydroxide
(known
concentration)
Less volume of HCl is
needed if it has a
greater concentration
Burette volume
(difference) would be
smaller.
Determining the more concentrated solution
If the solutions with unknown concentration (analyte) is
in the conical flask:
Sodium hydroxide
required = 12.5cm3
Sodium hydroxide
required = 25.0cm3
HCl solution D
HCl solution C
Solution D is more
concentrated than
solution C as it
required a greater
volume of NaOH to
neutralise
(25/12.5) =So solution
D is twice more
concentrated than C
Determining the more concentrated solution
If the solutions with unknown concentration (analyte) is
in the conical flask:
Sodium hydroxide
required = 21.3cm3
Sodium hydroxide
required = 14.2cm3
HCl solution D
HCl solution C
Solution C is more
concentrated than
solution D as it
required a greater
volume of NaOH to
neutralise
(21.3/14.2) =1.5
So solution C is 1.5
times more
concentrated than D
Determining the more concentrated solution
If the solutions with unknown concentration (analyte) is
in the burette:
HCl solution D=
15.0 cm3
HCl solution C
Added= 30.0cm3
Sodium hydroxide
(known
concentration)
Solution D is more
concentrated than solution
C as less volume was
needed for neutralization
30.0/15.0 =2
So solution D is 2 times
more concentrated than C
Titration- Common Questions
Why can’t we use universal
indicator in titration?
• It gives a range of colours so it is
difficult to identify end-poit
why it is important to run some
solution out of the burette after
it has been filled for the first
time in an experiment.
• make sure the level of the solution is on the
scale.
• to fill tap / to fill part of burette below tap /
to fill all of the burette / to fill the jet
Why do we use a white tile
under the flask?
• colour change can be seen clearly /
easily
In experiments where the solution
in burette requires more than
50cm3 to react or neutralize:
• Refill the burette
• Use more than one burette
Remember to take the burette reading correct to 1 d.p
Sources of error in Titration
✔using a measuring cylinder to measure solution /
solution
✔only carrying out the experiments once
✔going past the end-point (overshooting)
✔use a pipette / burette
✔repeat the experiment and take average
✔Add the solution from burette more slowly,
swirl and observe closely
Titration
• unchanged because moles of acid is the same in each titration
• the water dilutes solution B / makes solution B less concentrated
• Note: heating/ warming the solutions has no effect on a volumes
required in titration because no.of.moles of the reactant do not
change.
Q2- Rate of Reaction- common exam Questions
-Why does mass decrease by time?
- because gas escapes
-What is the purpose of cotton wool?
- to allow gas to escape AND to prevent loss of acid
- Why is cotton wool used to plug the flask instead of a
rubber bung?
- Rubber bung would not allow gas to escape
- Why do we use powdered
Calcium carbonate instead of lumps?
- More surface area
-rate of reaction increase
Rate of Reaction- common exam Questions
CaCO3 starts to react with the acid as soon as it is added. When using
gas syringe method:
-Why does this decrease the accuracy?
-Describe one improvement that you could make to overcome this
problem.
-Use a divided flask/ glass divider
-Use cotton thread to hold a test-tube
(containing the acid/solid) in the flask
-Loosen / cut the string to start reaction
-tip/shake the flask to start the
reaction
SOURCES OF ERROR
Measuring temperature change
Use a polystyrene cup instead of a beaker- to minimize heat loss to
surrounding
In titration
Use burette/pipette for more accurate results
For reliable results
Repeat the experiment and take average
Why is the volume/ temperature measured every 10seconds
instead of every 20seconds?
To obtain more data/ plotting so a smoother/better graph can be
drawn
Tips for Experimental chemistry
Exam Tips
Take readings correctly by referring to the scale
10 divisions
Tips
for
Graph
Exam Tips
3
6
9
12 15 18 21 24 27
30
0
Before drawing the curve read the instructions carefully
Points on the graph should be clearly marked as crosses (×) or
encircled dots (ʘ) of appropriate size.
A best-fit line (trend line) should be a single, thin, smooth
straight-line or curve.
Do not over draw
Tips for Graph
• If scale is not given, take the scale in such a way that more than
half of the grid is covered with the drawn graph.
• Remember to label the graph where necessary.
• Extrapolation : the extension of a graph, curve, or range of values
by inferring unknown values from trends in the known data.
• When asked to show how you obtain a value from the graph, show
it using dotted lines.
• Remember to write units in your answer.
Drawing line graphs
SMOOTH LINE – CURVE WITH
FREE HAND.
DON’T INCLUDE ANOMALOUS
POINT IN THE CURVE/LINE .
Gas syringe
Collecting & Drying Gases
Collect and measure
volume of gas
Upward delivery/ downward
displacement of air
Over water/
Gases that are insoluble in water or
slightly soluble in water. (eg: ethane,
O2)
DOWNWARD delivery/ upward
displacement of air
Gases which are
dense than air. (eg:
eg: Cl2,CO2, HCl)
Gases which are less dense than air. (eg:
H2,NH3)
Suck-back effect/ Back-flow
- Occurs when we are strongly heating a test tube and collecting a gas over
water.
- When heating a liquid, if you take away /turn off heat and leave the delivery
tube in the water bath, it will suck the cold water back to the delivery tube.
- When this reaches the hot part of the tube, it could shatter.
CAUTION- before you turn off heat ALWAYS remove the delivery tube from the
water bath immediately!!
Gas Collection (drying)
Which method will work?
drying agent
Concentrated sulfuric acid
REMEMBER…
Wet gas must be bubbled into
drying agent
Gas Collection (drying)
A student is asked to produce a dry sample of hydrogen by passing it
through a drying agent. The direction of flow of the gas through the
apparatus is shown by the arrows.
Which apparatus, X, Y, or Z, should be used? Explain
your answer.
the gas / hydrogen chloride is colourless or cannot be seen (to read the
volume)
Question 3- Tips for Salt Analysis
• If no results are obtained with aq. NaOH or aq. NH3 :
The compound may contain group one metal ions
• While testing for Nitrate ion it is important to warm
gently: NaOH is an irritant and boiling may cause NaOH
solution to spit out of the test tube.
• If no results are obtained with a test for particular ion: “does not
contain that ion” can be a conclusion
If coloured solution is formed when dissolved: It must contain
transition metal ions.
Question 3- Tips for Salt Analysis
If acidified KMnO4 is added to a solution containing Fe2+ ions and
heated, it will get oxidized to Fe3+ and so will give a brown ppt
when NaOH is added.
If heating a solid produces condensation (liquid) : the compound
must be hydrated.
Acid is added in anion test- to remove any carbonate ions that might
` ` ` be present
In nitrate test- Aluminium acts as a reducing agents, reducing
nitrates to ammonia
Appearance
All metals are silvery/grey
Copper is pink/brown
Copper oxide is Black
If compound doesn’t contain a transition metal ion- white solid
If the compound contains a transition metal ion- coloured solid
Copper Salts are blue except copper carbonate which is
green
Fe2+ salt are green and Fe3+ salts are red-brown
Acids and alkalis are colourless
Hydrocarbons burn with a smoky flame
Nitrogen dioxide is a brown gas
Flame test
The flame test is used to identify the metal cations by the colour of the flame
they produce.
1. Clean a Platinum or nickel- chromium wire loops by dipping in
hydrochloric or nitric acid, followed by rinsing with distilled or
deionized water.
2. The clean loop is dipped in either a powder or solution of an ionic
(metal) salt.
3. The loop with sample is placed in the clear or blue part of the flame
(non-luminous flame) and the resulting color is observed.
-In flame test we use non-luminous/blue
flame because it does not interfere with
flame test colours.
- In a Bunsen burner, non-luminous
flames are formed when the air-hole is
opened completely.
lithium, Li+ →
red
calcium, Ca2+ → orange-red
sodium, Na+ → yellow
barium, Ba2+ → light green
potassium, K+ → lilac
copper(II), Cu2+ → blue-green
Investigation Basics
•
•
•
•
It’s always a good idea to write down
some preliminary notes before answering this part of the exam.
Use bullet points and include the main aspects of your investigation
such as:
Steps, Apparatus and conditions needed
Constant variables (eg; mass of solid, volume and concentration of
acid, and temperature)
•
Safety issues
•
Amounts of substances, how to measure them, accuracy
•
Priniciple used to measure the result
•
Relevant practical procedures, calculations, equations,
•
Conclusion
Planning Questions
The label on a bottle of orange drink stated ‘contains no artificial colours’.
A scientist thought that the orange colour in the drink was a mixture of two
artificial colours:
• Sunset Yellow E110
• Allura Red E129.
Plan an investigation to show that the orange colour in the drink did not
contain these two artificial colours. You are provided with samples of E110,
E129 and the orange colouring from the drink. You are also provided with
common laboratory apparatus. You may draw a diagram to help answer the
question.
Planning Questions
Sample Answer:
Chromatography method can be used to separate the components. Draw the base
line using a pencil towards the bottom end of the chromatography paper. Place
samples of orange colouring, E110 and E129 on the baseline using a dropper. Dip
the paper in to solvent, ensuring that the solvent level is below the base line.
Allow the separation to complete and check the heights of spots of E110 and E129
against orange colouring. If none of the spots in E110 and E129 are at the same
height as the spots from orange colour, then that means that the orange colour in
the drink did not contain these two artificial colours
Planning Questions
When solid barium hydroxide is added to solid ammonium chloride a
reaction takes place.
Describe an experiment to show that this reaction is endothermic.
• M1 measure initial temperature of (solid) ammonium chloride
/ barium hydroxide
• M2 add barium hydroxide / ammonium chloride / other solid
AND mix / stir
• M3 use a thermometer
• M4 measure the temperature of the mixture / final
temperature
• M5 temperature decreases / test-tube feels cold
CHromatography
State 2 errors
CHromatography
•
If two or more substances are the same, they will produce identical
chromatograms
•
An impure substance will show up with more than one spot, a pure
substance should only show up with one spot.
•
If the substance has not separated: the substance must be insoluble
in the solvent used and so must try with a different solvent.
•
Locating agent: to make the colourless components visible.
CHromatography
•
It is important to hold the paper in place with paper clips to
prevent the sample from getting submerged in the solvent and
dissolving in it.
•
The container must be covered with a lid to prevent the solvent
from evaporating which would effect the movement of the solute
and the solvent system.
Safety Precautions
Prevents inhalation of toxic fumes
NOTE: We can use an electric heater instead of direct
flame in heating alcohols (flammable mixtures)
heating experiments
Suggest a reason, other than student error, to explain why the value
obtained is lower
✔ heat loss to surroundings / incomplete combustion of the fuel / some
heat transferred to the can or tripod / some hexane or
✔ octane evaporates
heating experiments
• prevent solid / MCO3 / MO escaping
allow gas or carbon dioxide to escape / to prevent
pressure building up
heating experiments
A Test states that the solid should be
heated gently then strongly.
In terms of safety, explain why it is
necessary to heat gently at first.
To minimize errors
Suggest an advantage of
taking the temperature
readings every 15 seconds.
solid spits out of the tube / the
tube might crack
• insulation / use a lid; to reduce heat losses; OR
• repeats; average results;OR
• measure water or sulphuric acid or methyl orange
using a burette / use a 2 d.p. stopwatch / digital
thermometer; reference to accuracy;
more readings / points / data
• smoother curve / better or more accurate graphmore
readings / points / data
• smoother curve / better or more accurate graph
heating experiments
Explain why a
polystyrene cup is
used in the
experiments
-polystyrene is an insulator / copper is a (good)
conductor
reduced heat losses and not a copper (metal) can.
- therefore, the temperature is more accurate
Experiments are often repeated and the results compared to check that they are
reliable. Suggest why this is difficult to do for heating experiments
Because it is difficult to get the temperature (exactly)the same for every experiment
Suggest why one of the reagents is warmed before adding to the other reagent
rather than after it has been added.
the temperature is changing while it reacts
Suggest why it is not a good idea to put the reagent in a polystyrene cup before it is
warmed.
the polystyrene would melt when heated.
To improve accuracy
✔ use a burette / graduated pipette / gas syringe
✔ use cotton thread to hold a test-tube (containing the
acid) in the flask (no air is collected)
✔ repeat the experiment, take average / more frequent
readings
THANK YOU!