Hydrate
Hydrate, any chemical compound formed by the loose combination of water with some other substance. The water retains its molecular structure; that, is, its hydrogen and oxygen atoms remain together and do not unite separately with the atoms of the other substance. When certain salts crystallize from a water solution they carry with them a definite number of water molecules and form hydrates. The hydrate, as a rule, is a solid substance and not wet. Several minerals are hydrates, including gypsum and borax.
The water in a hydrate is called the water of crystallization. This water can usually be driven off without decomposing the rest of the compound by heating the hydrate to 212 F. (100 C.). In some hydrates the water of crystallization is held so loosely that it escapes when exposed to moderately dry air at ordinary temperature. These hydrates are said to be efflorescent.