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, 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 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.
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.
Avoiding premature drying is important to all other cementitious building products, particularly spray fireproofing and firestop mortars, where the slightest cracking can lead to rejection. The chemically bound water is used up by endothermic reactions when exposed to the heat of a fire. Fire temperatures in a building can reach 1100°C, depending on the fuel present and the availability of oxygen, but the hydrates keep the temperature of the item 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.