Water of Crystallisation

Summary: Water molecules chemically bonded within a crystal structure. Represented by ·xH₂O in formulae. Tags: igcse chemistry definition practical-skills Created: 2026-07-14 Last Updated: 2026-07-14

Water of crystallisation refers to water molecules that are chemically bound within the crystal lattice of a salt in a definite, fixed stoichiometric ratio, and these water molecules are represented in the chemical formula with a centred dot followed by the number of water molecules per formula unit, such as CuSO₄·5H₂O for hydrated copper(II) sulfate pentahydrate. These water molecules are not merely adsorbed on the surface or physically trapped — they occupy specific coordination sites in the crystal structure, often forming dative covalent bonds with the metal cation through the lone pairs on the oxygen atom of water, and their presence is essential to the crystal’s geometry, stability, and frequently its colour. When a hydrated salt is heated gently, the water of crystallisation is driven off as water vapour, leaving behind the anhydrous salt, which often has a markedly different appearance — for example, blue hydrated copper(II) sulfate pentahydrate (CuSO₄·5H₂O) becomes white anhydrous copper(II) sulfate (CuSO₄), and pink hydrated cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) turns deep blue when dehydrated. This colour change upon dehydration and the reverse colour change upon adding water is the basis for using anhydrous copper(II) sulfate (white to blue) and anhydrous cobalt(II) chloride (blue to pink) as chemical tests for the presence of water. In IGCSE, students must be able to calculate the number of moles of water of crystallisation per mole of salt from experimental data — by measuring the mass loss upon heating and using the formula masses of the anhydrous salt and water, they can determine the value of x in the general formula salt·xH₂O.

Key term — IGCSE Chemistry (0620/0971) glossary.