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 the incorrect formula CH3OH2, while the correct formula is CH3–OH.
Many organic molecules, as with inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule (water of crystallization). The sugar trehalose, for example, exists in both an anhydrous form (melting point 203°C) and as a dihydrate (melting point 97°C). Protein crystals commonly have as much as 50% water content.
{|style="float:right; margin: 0 0 1em 1em; border:1px solid black;" | || |- align="center" | Anhydrous cobalt(II) chlorideCoCl2|| Cobalt(II) chloride hexahydrateCoCl2.6H2O |} 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 compound, 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. Such water is also referred to as water of crystallization. Examples include borax decahydrate and chalcanthite.
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. Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or desiccants.
Nonpolar molecules such as methane can form clathrate hydrates with water, especially under high pressure. Although there is no hydrogen bonding of water molecules when methane is the guest molecule of the clathrate, guest-host hydrogen bonding often forms with guest molecules in clathrates of many larger organic molecules, such as tetrahydrofuran. In such cases the guest-host hydrogen bonds result in the formation of L-type Bjerrum defects in the clathrate lattice.
Category:Chemical compounds Category:Oxygen compounds Category:Hydrogen compounds
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