HYDRATE, in chemistry, a liquid or solid compound con taining more or less combined water. Frequently the crystalline character of a solid hydrate depends on this water which is present as water of crystallization. Common examples of hydrates are supplied by the well known sodium compounds :- Glauber's salt (q.v.), which is decahydrated sodium sulphate, I ; washing soda, which is decahydrated sodium car bonate, I ; and borax, which is sodium pyroborate, with five or ten molecules of water. Blue vitriol, CuSO,,5H2O, the pentahydrated copper sulphate, is another com mon example. The alums owe their characteristic crystalline form partly to the presence of combined water; for example, potash alum, KA1(SO1) 2, I 2H20, crystallizes in well defined octahedra, which crumble into powder on removing the water. Acids and bases also form hydrates. hydrated barium hydroxide is a well crystallized substance. Sulphuric acid and hy drochloric acid combine with water to form hydrates, and since certain of these are stable only at comparatively low tempera tures they are termed cryohydrates. A remarkably stable solid hydrate is chloral hydrate, produced by adding water to liquid chloral (q.v.). In this substance the added water has probably become hydroxide of constitution, so that this hydrate may be regarded as a connecting link between the loosely knit hydrates and the true hydroxides (q.v.).
(G. T. M.)