The salts which contain water molecules are known as hydrated salts.A hydrated salt is a crystalline salt molecule that is loosely attached to a certain number of water molecules.A salt molecule that is not bound to any water molecules is an anhydrate, and a salt molecule that is bound to water molecules is a hydrated salt. For example the crystals of copper sulphate contain five molecules of water called water of crystallization. So the formula of hydrated salt of copper sulphate is CuSO4.5H2O.