hardness

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hardness property of matter commonly described as the resistance of a substance to being scratched by another substance. The degree of hardness is relative, different substances being compared with one another. Mohs's scale of hardness (named for Friedrich Mohs), used commonly in mineralogy, lists certain minerals in order of hardness: talc, 1; gypsum, 2; calcite, 3; fluorite, 4; apatite, 5; orthoclase, 6; quartz, 7; topaz, 8; corundum, 9; diamond, 10. The listing indicates merely that gypsum (hardness=2) is harder than—i.e., capable of scratching—talc (hardness=1). The listing does not indicate that gypsum (2) is twice as hard as talc (1). The hardness of many minerals falls between those included in the list. For example, the hardness of barite is 3.3. Hardness may differ with the direction of the scratch made on the substance. Thus the mineral kyanite has a hardness of 5 parallel to the length of its crystals and of 7 at right angles to this direction. There are several other methods based on the resistance to indentation for testing engineering materials. The solid elements have been thus classified: diamond (an allotropic form of carbon) is hardest and listed as 10, with cesium the softest, rated as 0.2, the same degree of hardness as wax (hardness=0.2 at 0°C). The hardness numeral of the Brinell scale is based upon the indentation produced when pressure is exerted by a sphere on the substance. The value thus obtained has a direct relation to the tensile strength of the substance. The hardness of a material may be modified by the presence of small quantities of another substance, as in metallic alloys, or by impurities in minerals.

hardness
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 by the German-Austrian mineralogist Friedrich Mohs (1773–1839) and is still the standard used today. Useful tools for determining hardness are the fingernail (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.

hardness
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.

hardness Resistance of a material to abrasion, cutting, or indentation. The Mohs' scale is a means of expressing the comparative hardness of materials, particularly minerals, by testing them against ten standard materials. These range from (1) talc to (10) diamond (the hardest). A mineral hard enough to scratch material 3, but soft enough to be scratched by material 5, would be rated as having hardness 4 on the Mohs' scale.