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

The substances in a mixture can be elements, or compounds, or both. Being part of a mixture does not change the chemical properties of the substances that are in it.

Separating mixtures

Mixtures can be separated by physical processes. These processes do not involve chemical reactions, and no new substances are made.

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