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 A substance made of one type of atom only. or one A substance formed by the chemical union of two or more elements.
- a Two or more substances that are not joined together. The substances can be elements, compounds, or both. consists of two or more different substances, not chemically joined together
Different types of chemical substance
- an element contains just one type of The smallest part of an element that can exist.
- a compound contains two or more types of atom joined together
- a mixture contains two or more different substances that are not joined together
- the different substances in a mixture can be elements or compounds
The table shows some examples:
Notice that the different substances in a mixture can be single atoms, molecules of elements or molecules of compounds.
Distinguishing between pure substances and mixtures
Pure substances have a sharp The temperature at which a solid changes into a liquid as it is heated. but mixtures The process that occurs when a solid turns into a liquid when it is heated. over a range of temperatures. This difference is most easily seen when the temperature of a hot liquid is measured as it cools and A change of state in which liquid becomes solid by cooling.. 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 decrease over a range of temperatures as it freezes.