"Hydrate" is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The name of a hydrate follows a set pattern: the name of the ionic compound followed by a numerical prefix and the suffix "-hydrate." For example, CuSO4 · 5H2O is "copper(II) sulfate pentahydrate." The notation of hydrous compound · nH2O, where n is the number of water molecules per formula unit of the 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. (Typical prefixes are mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and deca-, as shown in Figure 1.)
A hydrate that has lost water is referred to as an anhydride. An anhydride can normally lose more water only with significant heating, if at all. A substance that does not contain any water is referred to as anhydrous.
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.
In organic chemistry, a hydrate is a compound formed by the addition of water or its elements to another molecule. For example, ethanol, CH3–CH2–OH, can be considered a hydrate of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C. Another example is chloral hydrate, CCl3–CH(OH)2, which can be formed by the 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 was described as a carbohydrate, but this is a very poor description of its structure given what we know about it today. Methanol is often sold as “methyl hydrate,” implying the incorrect formula CH3OH2; the correct formula is CH3–OH.