Dehydration
Summary: Dehydration is the removal of water of crystallisation from a hydrated salt by heating. The hydrated salt changes colour and forms the anhydrous salt. Example: CuSO₄·5H₂O (blue) → CuSO₄ (white) + 5H₂O. Tags: igcse chemistry water crystals keyword Created: 2026-07-14 Last Updated: 2026-07-14
Definition
Dehydration (in IGCSE Chemistry context) refers to the removal of water of crystallisation from a hydrated salt, usually by heating. The hydrated salt loses its water molecules and becomes anhydrous, often with a visible colour change.
Examples
| Hydrated Salt | → | Anhydrous Salt | Colour Change |
|---|---|---|---|
| CuSO₄·5H₂O | → | CuSO₄ + 5H₂O | Blue → White |
| CoCl₂·6H₂O | → | CoCl₂ + 6H₂O | Pink → Blue |
| Na₂CO₃·10H₂O | → | Na₂CO₃ + 10H₂O | Colourless → White |
| FeSO₄·7H₂O | → | FeSO₄ + 7H₂O | Green → Yellow-white |
Reversibility
Dehydration of some salts is reversible — adding water restores the hydrated form and original colour. This is the basis for using anhydrous CuSO₄ (white → blue) and CoCl₂ paper (blue → pink) as tests for water.
Practical Note
“Heat to constant mass” — heat, cool, weigh, repeat until two consecutive weighings are equal — ensures ALL water of crystallisation has been driven off.
See Also
- Water of Crystallisation — Full topic page
- Hydrated — Salts containing water of crystallisation
- Anhydrous — Salts without water; used as water tests
- Chemical Tests — Testing for water with anhydrous CuSO₄
- IGCSE-Chem-Index