Hydrate, HY drayt, is a chemical compound made of a definite amount of water and another substance. For example, the hydrate blue vitriol (copper sulfate pentahydrate) is formed by the combination of a copper atom, a sulfur atom, and four oxygen atoms with five water molecules. When the water in a solution of copper sulfate evaporates, the blue vitriol crystals that form are dry. But they retain a certain amount of water. This water is called water of crystallization or water of hydration.