In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs inside crystals. Water is often necessary for the formation of crystals. In some contexts, water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation