HYDRATE  , in

chemistry, a compound containing the elements of water in combination; more specifically, a compound containing the monovalent hydroxyl or OH group . The first and more general definition includes substances containing water of crystallization; such salts are said to be hydrated, and when deprived of their water to be dehydrated or anhydrous . Compounds embraced by the second definition are more usually termed hydroxides, since at one time they were regarded as combinations of an oxide with water, for example, calcium oxide or lime when slaked with water yielded calcium hydroxide, written formerly as CaO•H2O . The general formulae of hydroxides are: Mi OH, Mii(OH)2, Mili(OH)3, M1v(OH)4, &c., corresponding to the oxides M21O, M110, M2iiiO3, MivO2, &c., the Roman index denoting the valency of the element . There is an important difference between non-metallic and metallic hydroxides; the former are invariably acids (oxyacids), the latter are more usually basic, although acidic metallic oxides yield acidic hydroxides . Elements exhibiting strong basigenic or oxygenic characters yield the most_ stable hydroxides; in other words, stable hydroxides are associated with elements belonging to the extreme groups of the periodic system, and unstable hydroxides with the central members . The most stable basic hydroxides are those of the alkali metals, viz. lithium, sodium, potassium, rubidium and caesium, and of the alkaline earth metals, viz. calcium, barium and strontium; the most stable acidic hydroxides are those of the elements placed in groups VB, VIB and VIIB of the periodic table .