HARDNESS OF SOLIDS

Some solids are literally “more solid” than others. A quantitative means of expressing hardness, known as the Mohs scale, classifies solids from 1 to 10. The lower numbers represent softer solids, and the higher numbers represent harder ones. The standard substances used in the Mohs scale, along with their hardness numbers, are shown in Table 1. The test of hardness is simple and twofold: (1) a substance always scratches something less hard than itself, and (2) a substance never scratches anything harder than itself.
An example of a soft solid is talc, which can be crumbled in the hand. Chalk is another soft solid. Wood is somewhat harder than either of these. Limestone is harder still. Then, in increasing order of hardness, there are glass, quartz, and diamond. The hardness of a solid always can be determined according to which samples scratch other samples.
Many substances have hardness numbers that change with temperature. In general, colder temperatures harden these materials. Ice is a good example. It is a fairly soft solid on a skating rink, but on the surface of Charon, the bitterly cold moon of the planet Pluto, water ice is as hard as granite.

Table 1 The Mohs Scale of Hardness (Higher numbers represent harder substances. Relative hardness is determined by attempting to scratch one substance with another.)

Hardness is measured by maintaining laboratory samples of each of the 10 substances noted in Table 1. A scratch must be a permanent mark, not just a set of particles transferred from one substance to the other. Substances commonly have hardness values that fall between two whole numbers on the scale. The Mohs hardness scale is not especially precise, and many scientist prefer more elaborate methods of defining and measuring hardness. DENSITY