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2 Pure substances, mixtures and separations
Matter can be classified into pure substances and mixtures. These can be further classified as
Figure 2.1 Classification of matter
Pure substances
A pure substance is composed of
Any pure substance possesses certain general characteristics:
Its composition is fixed and constant.
Its properties are fixed and constant, for example, its melting point, boiling point and density.
The component parts cannot be separated by any physical process.
To find out if a substance is pure, its melting point or boiling point can be measured. If any impurities are present they will usually lower its melting point and raise its boiling point.
Elements
Elements are the simplest form of matter.
An element is a pure substance that cannot be broken down into simoter substance by using any ordinary physical or chemical process.
An atom is the smallest particle in any element. Each element is composed of atoms of one kind only. Most elements are made up of individual atoms, e.g. silver (Ag) is made up of individual silver atoms.
A few elements are made up of molecules, e.g. nitrogen (N.) is made up of nitrogen molecules, each molecule being composed of two nitrogen atoms.
There are 118 known elements and they can be classified as metals or non-metals.
2 Pure substances, mixtures and separations
Compounds
A compound is a pure substance that is formed from of more different types of elements which are chemically bonded together in fixed proportions and in a way that their properties have changed.
Example
sodium chlorine (element) (element)
sodium chloride
(compound)
The proportions, by mass, of sodium and chlorine in any pure sample of sodium chloride the same and the elements cannot be separated by bonded together. The properties chlorine. are always always y physical means because they are chemically of sodium chloride are different from those of both sodium and
Compounds can be represented by chemical formulae, e.g. the chemical formula for sodium chloride is NaCl and for water it is H,O.
Mixtures
A mixture consists of two or more substances telements and/or compounds) which are physically combined together in variati variable proportions. Each component retains its own individual properties and is not chemically bo bonded to any other component of the mixture.
Any mixture possesses certain general characteristics:
Its composition can vary.
Its properties are variable because its component parts keep their individual properties.
Its component parts can be separated by physical means (see p. 12).
Homogeneous mixtures
A homogeneous mixture is a uniform mixture; it has the same composition and properties throughout the mixture. It is not possible to distinguish the component parts from each other. All solutions are homogeneous mixtures.
Heterogeneous mixtures
A heterogeneous mixture is a non-uniform mixture; it is possible to distinguish the component parts from each other, though not always with the naked eye. Heterogeneous mixtures include suspensions and colloids.
Solutions, suspensions and colloids
Solutions
A solution is a homogeneous mixture of two or more substances, mesis usually a liquid.
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 solutions.
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.
2 Pure substances, mixtures and separations
Table 2.4 Different types of solutions
Components
State of solute State of solvent
Example
Solid
Liquid
Sea water
Liquid
Liquid
White vinegar
Gas
Liquid
Soda water
Solid
Solid
Gas
Gas
Air
Sodium chloride dissolved in water
Table 2.6 Com
Ethanoic acid dissolved in water Carbon dioxide dissolved in water
Property
Size of dispe particles
Bronze (a metal alloy)
Tin dissolved in copper
water vapour dissolved in nitrogen Oxygen, carbon dioxide, noble gases and
Visibility of dispersed p
Sedimentat
Passage of
Appearan
Suspensions
A suspension is a heterogeneous mixture in which minute, visible particles of one substance are dispersed in another substance, which is usually a liquid.
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 minute particles of one substane in another substance, which is usually a liquid. The dispersed particles are far solution, but smaller than those of a suspension. dispersed those of a
Colloids are intermediate between a solution and a suspension.
Table 2.5 Different types of colloids
Type of colloid
Composition
Gel
Emulsion
Solid particles dispersed in a liquid
Liquid droplets dispersed in a liquid
Examples
Gelatin, jelly
Mayonnaise, milk, hand cream
Foam
Gas bubbles dispersed in a liquid
Solid aerosol
Solid particles dispersed in a gas
Whipped cream, shaving cream
Smoke
Liquid aerosol
Liquid droplets dispersed in a gas
Fog, aerosol sprays, clouds
solution
particle size increases
colloid
suspension
Figure 2.2 Comparing the particle sizes in a solution, a colloid and a suspension