"Hydrate" is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains loosely bonded 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 · 5 H2O 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.
A hydrate that has lost water is referred to as an anhydride. An anhydride can normally lose water only with significant heating. A substance that no longer contains any water is referred to as anhydrous.
In organic chemistry, hydrates tend to be rarer. An organic 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 is known about it today. Methanol is often sold as "methyl hydrate," implying the incorrect formula CH3OH2. The correct formula is CH3–OH.