PURE SUBSTANCES AND MIXTURES

Matter can be classified into two main groups: pure substances and mixtures.

Figure 1: Classification of matter

Pure Substances
A pure substance consists of only one type of material.
They can be either elements or compounds.
Their component parts cannot be separated by physical means.
They have fixed, constant compositions and properties:
A sharply defined, constant melting point or freezing point
A sharply defined, constant boiling point
A constant density
Impurities lower the melting point and raise the boiling point of a pure substance.
To find out if a substance is pure, its melting point or boiling point is determined.

Elements
An element is the simplest form of matter.
It is a pure substance which cannot be spilt into simpler substances by any physical means or
chemical processes (excluding nuclear reactions).
An atom is the smallest particle in an element that has the same properties as the element.
Each element is composed of only one kind of atom.
Examples of elements are iron (Fe), copper (Cu), oxygen (O2), hydrogen (H2), etc.
Elements in the periodic table are divided into metals and non-metals.
Table 1: Properties of metals and non-metals
Property Metals Non-metals
State at room temperature Solid (except mercury) Solid, liquid, gas
Appearance of solid Shiny Dull
‘Bendability’ of solid Malleable and ductile Brittle
Density Generally high Generally low
Melting and boiling points Generally high Generally low
Electrical and thermal conductivity Good Poor (except graphite)

Malleable means that it can be bent and hammered into different shapes.
Ductile means that it can be drawn out into wires.

Compound
A compound is a pure substance containing two or more different types of elements bonded
together chemically in fixed proportions and in such a way that their properties are changed.
Compounds cannot be separated by physical means.
E.g. magnesium + Oxygen → magnesium oxide
(element) (element) (compound)
Examples of compounds are water (H2O), sodium chloride (NaCl), methane (CH4), etc.

Mixture
A mixture is an impure substance consisting of two or more substances (elements and/or
compounds) combined together in varying proportions. Each component retains its independent
properties and has undergone no chemical changes.
A physical change is a change which occurs when one or more of the physical properties of a
substance is changed.
A chemical change is a change which alters the chemical properties of a substance to form a
new substance.
Physical properties are characteristics that can be observed or measured without changing the
chemical composition of a substance, e.g. shape, volume, density, compressibility, solubility,
melting and boiling point, etc.
Chemical properties are characteristics that can be observed when a substance undergoes a
chemical change or reaction, e.g. oxidation state, acidity or basicity, types of chemical bonds
formed, etc.
Table 2: Properties of mixtures and compounds
Property Mixture Compound
Ease of separation Easy Difficult
Properties compared to the properties Same Different
of the constituent elements
Change in energy Energy is not given out or Energy change occurs
taken in when made when made
Composition Variable Fixed

Homogeneous mixture
A homogeneous mixture is one in which the properties and composition are uniform throughout
the mixture.
The component parts cannot be distinguished from each other.
A solution is a homogeneous mixture.
Examples of homogeneous mixtures: air, salt dissolved in water, metal alloys (e.g. brass), etc.

Mechanical mixture
These are mixtures containing solids that can be easily separated by physical means – based on
the colours, magnetic properties, densities, particle sizes or boiling points.
Examples of mechanical mixtures: soil, iron and sand, green and yellow balloons, etc.

Heterogeneous mixture
A heterogeneous mixture is a non-uniform mixture, for example, a mixture in which the
component parts are in different states.
The component parts can be distinguished from each other, although not always with the naked
eye.
Suspensions and colloids are heterogeneous mixtures.
Examples of heterogeneous mixtures: salt and sand, mayonnaise, muddy water, etc.

Elements, compounds and mixtures can be distinguished by looking at the particles that make up
the substance:
 If there is only one kind of atom, then it is an element.
 If there are two or more kinds of atoms joined together in the same ratio, then it is a
compound.
 If there is a combination of two or more elements and/or compounds, then it is a mixture
Figure 2: Elements, compounds and mixtures
Elements: D, E, H
Compounds: B, C, G
Mixtures: A, F, I (I is a mixture and not a compound because the blue atoms are not arranged
regularly among the green atoms).

Solutions
A solution is a homogeneous mixture of two or more substances; one substance is usually a
liquid.
The particle sizes of a solution are less than 1 nm and are too small to be visible to the naked eye.
Solutions do not settle on standing.
A solution is composed of:
 The solvent, which is the substance that does the dissolving. The solvent is present in the
higher concentration.
 The solute, which is the substance that dissolves. The solute is present in the lower
concentration. A solution may contain more than one solute.
Solutions in which the solvent is water are known as aqueous solution.
A saturated solution is a solution in which the solvent cannot dissolve any more solute at a
particular temperature, in the presence of undissolved solute.
Table 3: Different types of solutions
State of State of Example Components
solute solvent
Solid Liquid Sea water Sodium chloride dissolved in water
Liquid Liquid White vinegar Ethanoic acid dissolved in water
Gas Liquid Soda water Carbon dioxide dissolved in water
Solid Solid Bronze (metal Tin dissolved in copper
alloy)
Gas Gas Air Oxygen, carbon dioxide, noble gases and water
vapour dissolved in nitrogen.

Suspensions
A suspension is a heterogeneous mixture in which very small, visible particles of one substance
are dispersed in another substance, which is usually a liquid.
Suspension particles are greater than 1000 nm in diameter and are opaque and scatter light.
The components settle when left to stand (they undergo sedimentation).
Examples:
 Mud in water and powdered chalk in water. These are suspensions of solid particles in
a liquid.
 Oil shaken in water. This is a suspension of liquid droplets in a liquid.
 Dust in the air. This is a suspension of solid particles in a gas.

Colloids
A colloid is a heterogeneous mixture in which very small particles of one substance are dispersed
in another substance, which is usually a liquid. The dispersed particles are larger than those of a
solution, but smaller than those of a suspension.
The particles of a colloid:
 are intermediate in size and properties are between those of a solution and those of a
suspension.
 are between 1 and 1000 nm in diameter and cannot be seen even with a microscope.
 do not settle on standing
Colloids are opaque and scatter light.
Table 4: Different types of colloids
Type of colloid Composition Examples
Aerosol Solid dispersed in a gas Smoke particles in air
Foam Gas dispersed in a liquid Whipped cream
Emulsion Liquid dispersed in a liquid Milk (fat in water)
Sol Solid dispersed in a liquid Paint
Gel Liquid trapped in a solid Jelly

Ink contains a colloid of different coloured dyes dissolved in a solvent. Colloids with inks, dyes
and chlorophyll pigments are sols.