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Substance containing water or its constituent elements
chemical compounds
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.

In organic chemistry, a hydrate is a compound formed by the hydration, i. e. "Addition of water or of the elements of water (i. e. H and OH) to a molecular entity". For example: ethanol, |CH3\sCH2\sOH, is the product of the hydration reaction of ethene, |CH2\dCH2, 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\sCH(OH)2, which can be formed by reaction of water with chloral, |CCl3\sCH\dO. 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. Molecules are also labeled as hydrates for historical reasons not covered above. Glucose, |C6H12O6, was originally thought of as |C6(H2O)6 and described as a carbohydrate. Hydrate formation is common for active ingredients. Many manufacturing processes provide an opportunity for hydrates to form and the state of hydration can be changed with environmental humidity and time. The state of hydration of an active pharmaceutical ingredient can significantly affect the solubility and dissolution rate and therefore its bioavailability.

Clathrate hydrates (also known as gas hydrates, gas clathrates, etc.) are water ice with gas molecules trapped within; they are a form of clathrate. An important example is methane hydrate (also known as gas hydrate, methane clathrate, etc.). Nonpolar molecules such as methane can form clathrate hydrates with water, especially under high pressure. Although there is no hydrogen bonding between water and guest molecules when methane is the guest molecule of the clathrate, guest–host hydrogen bonding often forms when the guest is a larger organic molecule such as tetrahydrofuran. In such cases the guest–host hydrogen bonds result in the formation of L type Bjerrum defects in the clathrate lattice.

The stability of hydrates is generally determined by the nature of the compounds, their temperature, and the relative humidity (if they are exposed to air).