Pure substances and mixtures

The meaning of pure

The word 'pure' is used in chemistry in a different way from its everyday meaning. For example, shops sell cartons labelled as 'pure' orange juice. The label means that the contents are just orange juice, with no other substances added. However, the juice is not pure in the chemical sense because it contains different substances mixed together. In chemistry:

  • a pure substance consists only of one element or one compound
  • a mixture consists of two or more different substances, not chemically joined together
A mixed pile of sweets is separated into four piles of different colours - red, green, yellow and purple.The components of a mixture can be separated without chemical reactions

Mixtures and compounds

Hydrogen and oxygen are both gases. Together, as a mixture, hydrogen and oxygen can react and form water. Water is a compound of hydrogen and oxygen. There are important differences between the properties of a mixture and a compound.

In this table, the column 'Mixture' refers to the gasses hydrogen and oxygen, and the column named 'Compound' refers to water.

MixtureCompound
CompositionVariable composition - the relative amounts of the two gases can be changed.Constant composition - water always contains the same ratio of hydrogen to oxygen. This ratio is shown in the chemical formula of the compound - H2O.
Joined or not?The hydrogen and oxygen are not joined together.The hydrogen and oxygen have joined together to form the new substance water.
PropertiesKeeps the properties of the substances involved. This mixture is in the gas state.Properties are different from those of the elements it contains. This compound is a liquid.
SeparationThe substances in the mixture can be separated.Cannot be separated but can be obtained by using chemical reactions.

Distinguishing between pure substances and mixtures

Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a hot liquid is measured as it cools and freezes. The graph shows the cooling curve for a sample of a compound called salol.

A cooling curve for salolThe temperature stays the same while a pure substance changes state

The horizontal part of the graph shows that the salol has a sharp melting point, so it is pure. Impure salol (a mixture of salol and other substances) would produce a gradual decrease over a range of temperatures as it freezes.