What are hydrates?

   Hydrate, in chemistry, is a compound of a class which may be regarded as formed upon the same type as water, or by the substitution of a metallic atom, or a basic radical, for one of the atoms of hydrogen in water, as potassium hydrate, ammonium hydrate. A substance containing water in solution is sometimes called a hydrate, and its crystals are called hydrated crystals, especially when it is desired to distinguish them from the anhydrous crystals of the same substance. Many chemists use the term hydrate as synonymous with hydroxide, but others reserve the term hydroxide for basic compounds. Thus the term sodium hydrate is sometimes applied to caustic soda, which is extensively manufactured, especially for soap-boilers' use, and is more commonly called sodium hydroxide. Potassium hydrate and calcium hydrate are also often called by chemists potassium hydroxide and calcium hydroxide. The hydrates of metals are true chemical compounds, but other so-called hydrates are merely aqueous solutions of various substances.