Water of Hydration Definition

Blue crystals of copper sulfate pentahydrate or copper sulphate.
These are blue crystals of copper sulfate pentahydrate, which is known as copper sulphate pentahydrate in the UK. Anne Helmenstine

Water of hydration is water that is stoichiometrically bound into a crystal. While the water is found within a crystal, it is not directly bonded to a metal cation. Crystal salts containing water of hydration are called hydrates.

Also Known As: water of crystallization, crystallization water

How Water of Hydration Forms

Many crystals form from particles dissolved in water or aqueous solutions. It's common for some of that water to be incorporated into the framework of a crystal.

Usually, heating a crystal drives off water of hydration, but the structure of the crystal is lost.

Water of Hydration Example

Commercial root killers often contain copper sulfate pentahydrate (CuSO4·5H2O) cyrstals. The five water molecules are called water of hydration.

Proteins typically contain around 50 percent water within their crystal lattice.


Water of hydration may be indicated a few different ways:

(1) For a compound consisting of a hydrate with coordinated water, the formula is written:

hydrated compound(H2O)n

Example: ZnCl2(H2O)4

(2) When a compound contains lattice water, but its stoichiometric ratio changes or is unknown, the formula may be written simply: hydrated compound·nH2O

Example: CaCl2·2H2O

(3) The notations may be combined:

Example: [Cu(H2O)4)SO4·H2O