Crystalline salts frequently contain water in two different states of combination, which are distinguished by the terms water of constitution (or sometimes basic water, or water of hydration), and water of crystallization. The water of crystallization is much less intimately combined with the salt than the water of the constitution and is, therefore, more easily expelled.
The reason for applying the term water of crystallization is the portion of combined water which is most easily expelled, is found in the influence which exerts upon the crystallization of the salt. Some substances are capable of forming crystals with different proportions of water of crystallization. For example, when anhydrous copper sulfate a white powder is dissolved in water and the solution is set aside to crystallize the product consists of blue prisms of copper sulfate pentahydrate.