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.
In many cases, the water of hydration is driven off by heating the hydrate. The compound then becomes dehydrated. When all the water is gone, the compound becomes anhydrous. Anhydrous salts often differ from their hydrates in color and crystal form. The blue crystals of blue vitriol change into a white powder when the compound becomes anhydrous.
Some hydrates, such as washing soda (sodium carbonate decahydrate), spontaneously lose their water of hydration when exposed to the air at room temperature. These compounds are called efflorescent. Their crystals slowly turn into a powder as they lose water. Efflorescence takes place faster when the humidity is low.
