Purity and separating mixtures
There are different ways to separate mixtures, for example by filtration, crystallisation, distillation, or chromatography. The method chosen depends upon the type of mixture.
Pure substances and mixtures
Different types of chemical substance
Substances exist as elements, compounds or mixtures:
- An element contains atoms with the same atomic number.
- A compound contains two or more elements chemically joined together.
- A mixture contains two or more different substances that are not chemically joined together. The different substances in a mixture can be elements and/or compounds.
The table shows some examples:
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
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 liquid is measured as it cools and freezes. The graph shows the cooling curve for a sample of a compound called salol.
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 fall in temperature as it freezes.