In the loosest sense, the term hydrate refers to the presence of water in compounds, where the water molecules may be bonded to positive or negative ions. Cases exist, however, in which water molecules occupy definite positions inside crystals without being coordinated with either positive or negative ions. And in noncrystalline materials such as colloidal gels, water may be present even in the absence of any chemical bonding. To add to the confusion, organic chemists sometimes refer to a compound formed by adding water to a carbon-carbon double bond as a hydrate; such compounds contain a hydroxyl functional group and usually cannot be dehydrated. And metallurgists frequently refer to a metal hydroxide as a hydrate; e.g., calcium hydrate is the same compound as calcium hydroxide.

By a more rigorous convention, however, a hydrate is defined as a chemical compound that contains water in a definite proportions, and...