Lower Secondary 7 Science Checkpoint
Essential Question:
How are elements, compounds,
and mixtures related?
Matter is anything that has mass and
takes up space (volume)
There are different types of Matter:
Pure Substances (elements and
compounds) and Mixtures
The composition (structure) of a
substance determines its Matter type.
Characteristics of
Pure Substances
• Fixed composition
• Distinct properties
• Cannot be separated into simpler substances
by physical methods
• Can only be changed in identity and
properties by chemical methods
• Properties do not vary one sample to another
sample
How many different substances are there?
How many
different substances
can you think of?
There are millions of different substances!
What are they all made of?
All substances are made of atoms
All substances are made of very tiny particles called atoms.
Many substances are made up of different types of atoms.
hydrogen and
oxygen atoms
carbon and
hydrogen atoms
iron, aluminium,
silicon, oxygen
and boron atoms
carbon, nitrogen,
hydrogen, oxygen
and sulphur atoms
Types of
Pure Substances:
Elements
And
Compounds
What is an element?
All substances are made of very tiny particles called atoms.
There are about one hundred substances that are made up
of just one type of atom. These are the elements.
carbon
helium
copper
The elements are the simplest substances in the universe.
The elements are the building blocks of all other substances.
Elements
• Made up of one type of atom
• Cannot be broken down by
physical and chemical methods
• Examples: Oxygen, Nitrogen, Carbon
Sample of the
Element Lead
Sample of the
Element Chlorine
Atoms in elements
An element is a substance made up of only one type of atom.
Copper is an
element made up of
copper atoms only.
Carbon is an
element made up of
carbon atoms only.
Helium is an
element made up of
helium atoms only.
Atoms and molecules of elements
An element is a substance made up of only one type of atom.
In some elements, the atoms are joined in groups of two or
more. A particle containing atoms grouped in this way is
called a molecule.
Oxygen is an
element made up of
oxygen atoms only.
How many atoms
are there in an
oxygen molecule?
Other elements, that contain atoms joined in molecules are
hydrogen, nitrogen, chlorine and bromine.
Introducing chemical symbols
A standard set of symbols is used to represent elements:
 The symbol for many of the more common elements uses
just the first letter of the name.
H = hydrogen
C = carbon
F = fluorine
 Others elements have
the first two letters.
Ca = calcium
Al = aluminium
He = helium
O = oxygen
N = nitrogen
I = iodine
 Some of the symbols are not
always as you might expect.
Mg = magnesium
Au = gold
Ag = silver
How to write symbols for elements
Two important rules should be followed when writing the
symbols of elements so that there is no confusion.
1. The first letter of an element’s symbol is always
a capital letter.
e.g.
N (not n) for nitrogen
2. If there are two letters in the element’s symbol,
the second letter is always a small letter.
e.g.
Co (not CO) for cobalt
No, Watson! It was
carbon monoxide
poisoning – not cobalt.
The Periodic Table
•
•
•
•
arranging the elements
It is organized in rows and columns
The rows are called periods
The columns are called groups
Group
Period
METALS
• are very useful materials
• are the substances from which objects are made
Properties of Metals
• are usually shiny when polished or freshly cut
• ring like a bell when you hit them
• strong and tough, they do not shatter when
dropped and they do not cracks easily
• can be shaped by bending them. They are
malleable, which means that they can be
hammered into shape. They are ductile, which
means that they can be drawn out into wires.
Properties of Metals
• don’t melt easily
• Mercury is the only metal that is liquid at room
temperature
• Are magnetic: like iron, steel, nickel, and cobalt
• Are good conductors of heat. When you touch
them they conduct heat energy away from the
hand so they feel cold.
Non-metals
Comparing metals and non-metals
Classification of Elements as
Metals and Non- Metals
No.
METALS
NON- METALS
1
Good conductors of
electricity & heat
Bad conductors of heat &
electricity
2
Shiny in appearance
Usually dull in appearance
3
Strong & hard
Usually weak & soft
4
Malleable & ductile
Brittle
ELEMENTS & SYMBOLS
NON-METALS
METALS
No
1
2
3
4
5
6
7
8
9
10
NAME
SYMBOL
PHYSICAL
STATE
NAME
Aluminium
Al
Solid
Argon
Calcium
Ca
Solid
Bromine
Copper
Cu
Solid
Carbon
Iron
Fe
Solid
Chlorine
Magnesium
Mg
Solid
Silicon
Mercury
Hg
Liquid
Sulphur
Potassium
K
Solid
Hydrogen
Sodium
Na
Solid
Iodine
Zinc
Zn
Solid
Nitrogen
Solid
Gold
Au
oxygen
* Physical states are given at room temperature.
SYMBOL
PHYSICAL
STATE
Ar
Br
C
Cl
Si
S
H
I
N
O
Gas
Liquid
Solid
Gas
Solid
Solid
Gas
Solid
Gas
Gas
Alloys
Stainless steel is a mixture
of iron and chromium.
Brass is an alloy of copper
and zinc.
Metal or non-metal?
Is this element a metal or a non-metal ?
Hard to
say!
H
He
Li Be
B C N O F Ne
iron
(Fe)
antimony
(Sb)
iodine
(I)
copper
(Cu)
phosphorus
(P)
sulfur (S)(Mg)
magnesium
Na Mg
Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac Rf Db Sg Bh Hs Mt ? ? ?
Metals and non-metals in the periodic table
The periodic table is a list of all the known elements which
are arranged according to the similarities in their properties.
Non-metals are
Metals are on the
mostly on the right.
left and in the centre.
Xe
What type of elements are between metals and non-metals?
Metal, non-metal or metalloid?
francium
krypton
(Kr)
(Fr)
silicon(Co)
(Si)
scandium
(Sc)
cobalt
Which
side
What are
areare
thethe
nonmetalloids?
metals on?
H
Li Be
Na Mg
K Ca Sc
Rb Sr Y
Cs Ba La
Fr Ra Ac
Ti
Zr
Hf
Rf
metalloid
non-metal
metal
He
B C N O F Ne
Al Si P S Cl Ar
V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Db Sg Bh Hs Mt ? ? ?
Metals are on
Metalloids sometimes
Non-metals
the left and in
behave like metals and
are mostly
the centre. sometimes like non-metals. on the right.
Everyday materials and their properties
Glass
• Transparent
• Waterproof
• It can be recycled
Plastic
• Transparent or opaque
• Used for containers for food and drink. It does
not react with the food.
• It is light in weight.
• It can be recycled.
What is a compound?
A compound is the substance produced when two or more
elements combine in a chemical reaction.
Two elements, hydrogen (H)
and oxygen (O), combine to
make the compound, water.
Which two elements combine
to make the compound
carbon dioxide?
A compound is always made up of two or more
different types of atom.
Compounds
• Form when two or more
different elements join (bond)
together chemically
• Composition is identical in each sample
• Can be separated only by chemical methods
• Properties of a compound are totally
different than the properties of the elements
that form them
• Examples: Water, Carbon dioxide, Sugar
Animated images and notes from http://www.chem.purdue.edu/gchelp/atoms/elements.html
Compounds
Atoms in elements and compounds
Elements are materials made up of one type of atom only.
The element, hydrogen, exists
as molecules.
Each hydrogen molecule is
made up of two hydrogen atoms
joined together.
H H
Compounds contain two or more different types of atom.
The compound, water, exists
as molecules.
Each water molecule consists
of two hydrogen atoms joined
to one oxygen atom.
Making a compound – water
A compound is made when atoms of different elements
react and join together.
For example, water is produced from the chemical reaction
between hydrogen and oxygen.
hydrogen
+
oxygen
water
H H
+ O
O
H H
Two hydrogen molecules react with one oxygen molecule
to produce two molecules of water.
Why does water have different properties to its elements?
Making a compound – carbon dioxide
A compound has very different properties to the elements
from which it is made.
elements
carbon
oxygen
combines
with
A black solid
which can be
used as a fuel.
C
compound
carbon dioxide
to make
A colourless gas
which is essential
for life.
+
O O
A colourless gas
which is used
to put out fires.
Making a compound – water
What are the elements which make up water?
In what ways are the elements different to their compound?
elements
compound
oxygen
hydrogen
combines
with
A colourless gas
which is used in
hot air balloons.
water
to make
A colourless gas
which is essential
for life.
A liquid which is
essential to our
lives and has many
different uses.
A compound is not a mixture
The atoms in a mixture of
hydrogen gas and oxygen
gas, which have not reacted
with each other, look like this…
The atoms in water, the
compound made when
hydrogen and oxygen react
and their atoms become
chemically joined to each
other, look like this…
What is a mixture?
A mixture contains two or more
substances that are mixed together
but have not reacted with each other.
Sea water is a mixture of salts, water
and other substances.
A mixture is not the same as a compound:
1. The proportions of the substances in a mixture are not fixed.
2. The properties of a mixture are often an “average” of the
properties of its ingredients (e.g. a mixture of a black and
white powder is grey).
3. The substances in a mixture are just mixed, not chemically
joined, and so it is usually quite easy to separate the
ingredients (e.g. it is easy to get salt from sea water).
Mixtures
• Form when elements and/or compounds
are combined physically
• Properties of a mixture are related to its
components
• Composition varies from sample to sample
• Can be separated by physical methods
• Examples of Mixtures: Tea, Perfume, Air,
Salad, Beach sand, oil and vinegar salad
dressing, etc.
Mixtures
Mixture of
Different Elements
Mixture of Different
Compounds
Mixtures
• A mixture of lead atoms and
chlorine atoms. They exist in
no particular ratio and are not
chemically combined with
each other. They can be
separated by physical means.
• A mixture of PbCl2 and PbCl4
formula units. Again, they are
in no particular ratio to each
other and can be separated
without chemical change.
Mixtures
Mixtures are often referred to
as homogeneous or
heterogeneous.
Mixtures
• Homogeneous mixtures (Solutions) have
a uniform distribution.
• For example: Tea, Perfume, Air
Mixtures
• Heterogeneous mixtures
do not have a uniform
distribution.
• Parts are often visible
• For example:
Salad, Beach Sand,
Oil and Vinegar dressing
Types of Mixtures
There are THREE types of mixtures:
 Solutions
 Colloids
 Suspensions
Solutions
• A solution is a mixture that appears to be a single
substance, but it is actually composed of 2 or
more substances that are distributed evenly
amongst each other.
• A solution always has solute (a substance that is
dissolved) and solvent (a substance that does the
dissolving)
▫ SOLUTIONS ARE HOMOGENEOUS
carbon dioxide is
dissolved in water
salt is dissolved
in water
sugar is dissolved in water
Types of solutions
Solute Solvent
Example
Gas
Gas
Gas
Liquid
Solid
Liquid
Air (oxygen in nitrogen)
Soda water (carbon dioxide in
water)
Ocean water (salt in water)
Solid
Solid
Gold jewelry (copper in gold)
Metals dissolved in metals are called alloys.
Colloids
• A colloid is a mixture in which the particles are
mixed together but not dissolved
▫ COLLOIDS ARE HOMOGENEOUS
Colloids have
properties of both
solutions and
.
suspensions
cloud
mayonnaise
milk
Suspensions
• A suspension is a mixture in which particles of a
material are dispersed throughout a liquid or gas,
but are large enough that they settle out.
▫ SUSPENSIONS ARE HETEROGENEOUS
muddy water
Italian dressing
Distinguishing between Elements,
Compounds, and Mixtures
Distributed Summarizing
The diagram below shows how two elements can be mixed
together…Which is a Compound? A Mixture?
C.
Compound
A.
B.
D.
Mixture
Match the Picture to the Description
Compound of 2
Elements
Mixture of Molecules
Element Molecule
Compound of 3
Elements
Element/ Atoms
Mixture of Atoms
Elements, Compounds, and Mixtures Constructed Response
Use the list of substances below to answer the following question.
Summarizing Strategy:
Elements, Compounds,
& Mixtures
Constructed Response







Water
Table salt (NaCl)
Pure gold ring
Pizza
Air
Carbon dioxide (dry ice)
Nitrogen
a. Categorize each substance as an element, a compound, or a mixture.
b. Explain how each substance belongs in the category you chose.
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