Hydrates are compounds formed by the union of water with some other substance, generally forming a neutral body, as certain crystallized salts. (If the water is heavy water, where the hydrogen consists of the isotope deuterium, then the term deuterate may be used in place of hydrate.)

These substances do not contain water as such, but have their constituents (hydrogen, oxygen, hydroxyl) so arranged that water may be eliminated. Hence, hydrates are derivatives of, or compounds with, hydroxyl.

Such a substance is considered to be anhydrous when it is not in such a union with water.

The notation of anhydrous compound·nH₂O, where n is the number of water molecules per molecule of anhydrous salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to exist.

Examples include: borax, hydroxide, ethyl hydrate (ethanol), clathrate hydrate, calcium hydroxide and chalcanthite.

See also: Anhydrate