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.
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 sulphuric acid. Another example is chloral hydrate, CCl3—CH(OH)2, which can be formed by reaction of water with chloral, CCl3—CH=O.
Other 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 consists of the isotope deuterium, then the term deuterate may be used in place of hydrate.
A colorful example is cobalt(II) chloride, which turns from blue to magenta (red) upon hydration, and can therefore be used as a water indicator.
The notation of hydrous compoundTemplate:Hydrate, 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, clathrate hydrates (a class of solid hydrates of gases), and chalcanthite. Gas hydrates are clathrate hydrates: water ice with gas molecules trapped within. When the gas is methane it is called a methane hydrate.
A substance 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 which contains no water is referred to as anhydrous.
The presence of hydrates is quite purposeful in the three fields of endeavour. 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.
Generally, the longer one can keep cementitious products wet immediately after placement, the better. The wetter cementitious products are kept, the more water will be converted into hydrates, instead of evaporating off due to the heat of hydration and other environmental influences. Premature drying is a cause for severe concrete problems, such as cracking and shrinking.
Passive fire protection
Avoiding premature drying is important to all other cementitious building products, such as spray fireproofing and firestop mortars in particular, where the slightest cracking can lead to rejections. The chemically bound water, is the source for endothermic reactions when exposed to fire. Fire temperatures in a building can reach 1100°C, depending on the fuel present and the availability of oxygen. The presence of hydrates keep the item exposed to the heat at or below 100 °C, until all the water is spent. Therefore, the more hydrates, the longer the fire-resistance duration. This is what lends fire-resistive characteristics to basic, or "old" building materials, like gypsum, concrete or plaster.