DEHYDRATION, in chemistry, the removal of molecules of water from a chemical compound. The water may be present in the form of water of crystallization (see HYDRATE) in which case it is removed by heating the crystals, or by placing them in an evacuated desiccator in the presence of a drying agent such as phosphorus pentoxide. Alternatively, water may form part of a molecule of an organic compound, in which case it is removed by heating the compound in the presence of a metallic oxide, which acts as catalyst (see CATALYSIS) ; e.g., the alcohols (excepting methyl alcohol) are converted into the corresponding olefines. (See ALCOHOLS and OLEFINES.)