Hydrated salts and water of crystallisation
Some salt crystals hold a fixed number of water molecules, called water of crystallisation. Heating blue vitriol drives it out and leaves white anhydrous copper sulphate.
What water of crystallisation is
When some salts crystallise from water, a fixed number of water molecules joins each unit of the crystal. This water is called water of crystallisation. The salt is then a hydrated salt. The water gives the crystal its shape and often its colour. If the water is removed by heating, the salt becomes anhydrous (without water) and usually loses its colour and shape. The crystals still feel dry because the water is chemically bound.
Common hydrated salts
| Common name | Chemical name | Formula | Water molecules |
|---|---|---|---|
| Blue vitriol | Copper sulphate pentahydrate | CuSO₄·5H₂O | 5 |
| Green vitriol | Ferrous sulphate heptahydrate | FeSO₄·7H₂O | 7 |
| White vitriol | Zinc sulphate heptahydrate | ZnSO₄·7H₂O | 7 |
| Epsom salt | Magnesium sulphate heptahydrate | MgSO₄·7H₂O | 7 |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O | 2 |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O | 10 |
Activity: heating blue vitriol
- 1Put a few blue crystals of copper sulphate into a dry test tube. Note the blue colour.
- 2Hold the tube with a holder and heat it gently over a burner.
- 3Observe drops of water on the cooler upper part of the tube.
- 4★The crystals turn white and powdery: CuSO₄·5H₂O → CuSO₄ + 5H₂O
- 5Cool the tube and add a few drops of water. The blue colour returns and the tube feels warm: CuSO₄ + 5H₂O → CuSO₄·5H₂O
- 6Conclusion: the crystals contained water of crystallisation, and losing and regaining it is reversible.
★ key step
Worked example: percentage of water in blue vitriol
Find the percentage of water of crystallisation in CuSO₄·5H₂O (Cu = 63.5, S = 32, O = 16, H = 1).
- Mass of CuSO₄ = 63.5 + 32 + 4(16) = 159.5
- Mass of 5H₂O = 5 × 18 = 90
- Molar mass of CuSO₄·5H₂O = 159.5 + 90 = 249.5
- Percentage of water = (90 ÷ 249.5) × 100 = 36.07% (rounded to two decimal places)
Blue vitriol is about 36.07% water by mass.
Ferrous sulphate, the green vitriol salt, is the iron in many iron tablets given to prevent anaemia. Washing soda crystals (Na₂CO₃·10H₂O) are used at home for washing clothes, and gypsum is used to make plaster.
1 marks: “The number of water molecules in one formula unit of blue vitriol is ______.” Five (CuSO₄·5H₂O).
2 marks: “What is water of crystallisation? Give one example.” A fixed number of water molecules chemically combined in a crystal of a salt, e.g. CuSO₄·5H₂O has five.
4 marks: “What happens when blue vitriol is heated? Write the equation. What happens when water is added to the product?” Water drops form and the blue crystals turn white anhydrous CuSO₄: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Adding water makes it blue again and warm.
Blue 5, Green 7, Epsom 7, Gypsum 2, Washing soda 10.