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出典:『Wikipedia』 (2011/06/14 10:46 UTC 版)

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In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are diamond, gold, salt (sodium chloride) and sugar (sucrose). Most chemical substances occur as mixtures with other chemical substances. For example, drinking water is a mixture of water, sodium chloride and many other chemical substances. Generally, chemical substances exist as a solid, liquid, gas, or plasma and may change between these phases of matter with changes in temperature or pressure. Chemical reactions convert one chemical substance into another.