Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood.

Chemical nature of hydrates

In organic chemistry, a hydrate is a compound formed by the addition of water or its elements to a host molecule. For example, ethanol, CH₃—CH₂—OH, can be considered as a hydrate of ethylene, CH₂=CH₂, formed by the addition of H to one C and OH to the other C. A molecule of water may be eliminated, for example by the action of sulfuric acid. Another example is chloral hydrate, CCl₃—CH(OH)₂, which can be formed by reaction of water with chloral, CCl₃—CH=O.

Molecules have been labeled as hydrates for historical reasons. Glucose, C₆H₁₂O₆, was originally thought of as C₆(H₂O)₆ and described as a carbohydrate, but this is a very poor description of its structure as known today. And methanol is often sold as “methyl hydrate”, implying an incorrect formula CH₃OH₂, although the correct formula is CH₃—OH.

In inorganic chemistry, hydrates contain water molecules that are either bound to a metal center or crystallized with the metal complex. Such hydrates are also said to contain "water of crystallization" or "water of hydration". If the water is heavy water, where the hydrogen involved is the isotope deuterium, then the term deuterate may be used in place of hydrate.

The notation of hydrous compound , where n is the number of water molecules per molecule of salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to exist. In a monohydrate n is one, in a hexahydrate n is 6 etc. Such water is also referred to as water of crystallization. Examples include borax decahydrate and chalcanthite.

Gas hydrates are clathrate hydrates (a class of solid hydrates of gases): water ice with gas molecules trapped within. When the gas is methane it is called a methane hydrate.

A hydrate which has lost water is referred to as an anhydride, and can normally lose further water only upon strong heating, if at all. A substance that does not contain any water is referred to as anhydrous.

Construction

Generally, in construction and refractories, inorganic binders are often deprived of water during manufacture. For instance, both in cement and gypsum products, heat is applied to the raw materials. Once water is added on a construction site, the powder is re-hydrated and able to form bonds with other substances that are present. Thus, one goes from powder, to slurry, or paste and then forms "cement stone". Water that is not chemically bound, or converted into hydrates, can come off again as steam, especially due to the heat of hydration, with cement products in particular, which undergo an exothermic chemical reaction with water.