Crystals that carry water
Show on a big screenBlue crystals in the lab jar look dry. Yet water is inside them.
- Blue crystals hold water in their own structure.
- Heat drives the water out as vapour.
- A white powder is left: a different substance.
- Add water back, and the blue crystals return.
Heat the crystals, then add water. Watch the colour.
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The rule
Heat drives the crystal's held water out and leaves a different substance. Adding water brings the first substance back.
Water of crystallisation is water built into a salt's crystal. A formula unit is the group in a formula, such as CuSO₄. Each formula unit holds a fixed number of water molecules. Copper sulphate holds five per unit.
CuSO₄·5H₂O (blue) → heat → CuSO₄ (white) + water
| Salt | Formula | Water per formula unit |
|---|---|---|
| Copper sulphate | CuSO₄·5H₂O | 5 |
| Washing soda | Na₂CO₃·10H₂O | 10 |
| Gypsum | CaSO₄·2H₂O | 2 |
| Plaster of Paris | CaSO₄·½H₂O | half |
Heat gypsum to 373 K, which is 100 °C. It loses water. The white powder left is Plaster of Paris. Doctors use it to hold a broken bone in place.
Mix Plaster of Paris with water and it sets hard. It becomes gypsum again. So it is kept in a moisture-proof container.
Watch out!
Water of crystallisation does not make a crystal wet. Washing soda, Na₂CO₃·10H₂O, is a dry solid. The water is part of its structure.
Try the lab to finish this lesson.