In chemistry, a 'chemical substance' is a material with a specific chemical
composition. 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. Some typical chemical substances are diamond,
gold, salt (sodium chloride) and sugar (sucrose). 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. Forms of energy, such as light and heat, are not considered to be matter, and
thus they are not "substances" in this regard.
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