hydrate (hĪˈdrāt) [key], chemical compound that contains water. A common hydrate is the familiar blue vitriol, a crystalline form of cupric sulfate. Chemically, it is cupric sulfate pentahydrate, CuSO4·5H2O. When a crystal of the substance is formed, five molecules of water (H2O) are combined in the crystal with each molecule of cupric sulfate (CuSO4). This water is called water of crystallization. When cupric sulfate pentahydrate is heated above 150°C the water of crystallization is driven off and anhydrous cupric sulfate is formed. It has several properties different from the pentahydrate, e.g., color, density, and crystal structure. Glauber's salt is sodium sulfate decahydrate, Na2SO4·10H2O. Crystals of it readily give up their water of crystallization at ordinary temperatures, forming a powdery coating of the anhydrous salt; this phenomenon (efflorescence) is exhibited by many hydrates. The number of molecules of water present in a given hydrate is fixed. However, some substances form several different hydrates. There are four different hydrates of ferrous sulfate, each with its own unique physical properties. In organic chemistry a compound formed by addition of water to a carbon-carbon double bond is sometimes called a hydrate; it contains a hydroxyl functional group and usually cannot be dehydrated. In commerce a metal hydroxide is sometimes called a hydrate; e.g., calcium hydrate is calcium hydroxide.