Hydrate

In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.

Chemical nature of hydrates

Organic chemistry

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, is the product of the hydration reaction of ethene, CH₂=CH₂, formed by the addition of H to one C and OH to the other C, and so can be considered as the hydrate of ethene. 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.

Many organic molecules, as well as 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.