Hydrate is a chemical term to indicate that a substance contains water. The chemical state of water in the molecule varies depending on how they are attached and function.
For organic chemistry, a hydrate is formed when an organic molecule is combined with water and does some reaction. For example, ethanol is considered a hydrate of ethene.
`CH_(2)=CH_(2) + H_2O -> CH_(3)-CH_(2)OH`
For inorganic chemistry, salts can acquire water molecules from the surroundings that will become part of the crystal structure. That crystal structure is termed to have "water of crystallization". For example, cobalt (II) chloride is used as a desiccant that absorbs water to form the hydrated molecule.
`CoCl_(2) + 6 H_2O -> CoCl_2*6H_2O`
The general formula for inorganic hydrates can be written as:
n = number of water molecules per formula unit of the salt