Hydrated

Summary: A hydrated substance contains water molecules chemically bonded within its crystal structure (water of crystallisation). Example: CuSO₄·5H₂O (hydrated copper(II) sulfate — blue). Contrast with anhydrous (no water). Tags: igcse chemistry water crystals keyword Created: 2026-07-14 Last Updated: 2026-07-14

A hydrated salt is an ionic compound that contains water of crystallisation — water molecules chemically incorporated into its crystal lattice in a fixed, stoichiometric ratio, such as copper(II) sulfate pentahydrate (CuSO₄·5H₂O) where exactly five water molecules are associated with each formula unit. Hydrated salts form when ionic compounds crystallise from aqueous solution, with water molecules becoming trapped as the crystal lattice grows; the water is not merely wetting the surface but is an integral part of the crystal structure. Heating a hydrated salt drives off the water of crystallisation, converting it to the anhydrous form — CuSO₄·5H₂O (blue) becomes CuSO₄ (white) upon heating, and this colour change is exploited in the classic IGCSE test for water using anhydrous copper(II) sulfate, which turns from white to blue in the presence of water. The reverse reaction — adding water to the anhydrous salt — restores the hydrated form and original colour, confirming that the process is reversible. IGCSE candidates must be able to calculate the number of moles of water of crystallisation per formula unit from experimental mass data (by finding the mass loss on heating and using moles = mass / M_r), and a common exam error is forgetting to subtract the mass of the anhydrous salt from the hydrated mass to find the mass of water driven off.

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