1. A measure of the ability of water to form a carbonate scale when boiled, or to prevent the sudsing of soap. Permanent hardness is due mainly to dissolved calcium and magnesium sulphate or chloride, the bicarbonate ion causes temporary hardness. Dissolved carbon dioxide and the weathering of carbonate rocks are the main sources of hardness in water.

2. A physical property of minerals and one of the most useful tests for mineral identification. Mohs’s scale of hardness (H), which ranks minerals by their hardness and thus makes possible a diagnostic test in which one mineral is used to scratch another, was introduced in 1822 and is still the standard used today. Useful tools for determining hardness are the finger nail (H about 2.5) and a penknife (H about 5.5). With a little practice, the hardness of a mineral may be determined by means of a scratch to within one or two points on Mohs’s scale. See also vickers hardness number.