Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood.
Chemical nature of hydrates
In organic chemistry, 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 as a hydrate of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C. 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.
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.
Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal" that are either bound to a metal center or that have crystallized with the metal complex. Such hydrates are also said to contain water of crystallization or water of hydration. If the water is heavy water, where the hydrogen involved is the isotope deuterium, then the term deuterate may be used in place of hydrate.
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.
Full article ▸