What is water of crystallisation? Describe an activity to show that copper sulphate crystals contain water of crystallisation.

1. 1 · Define in one line. Water of crystallisation is a fixed number of water molecules chemically combined in each crystal unit of a salt.
2. 2 · Give examples. CuSO₄·5H₂O (blue vitriol), FeSO₄·7H₂O (green vitriol), MgSO₄·7H₂O (Epsom salt).
3. 3 · Describe the activity. Heat a few blue copper sulphate crystals in a dry test tube. Water drops collect near the mouth and the crystals turn white.
4. 4 · Draw the set-up. [Test tube held over a burner, crystals at the bottom, water drops near the mouth; label 'blue crystals', 'water droplets', 'burner']
5. 5 · Write the equations.
   • CuSO₄·5H₂O → CuSO₄ + 5H₂O (on heating: blue to white)
   • CuSO₄ + 5H₂O → CuSO₄·5H₂O (on adding water: white to blue, heat given out)
6. 6 · Real-life link. White anhydrous copper sulphate is used to test for water, and ferrous sulphate crystals are used in iron tablets.
7. 7 · Close in one line. Thus the colour change shows that the crystals hold water of crystallisation, and the change is reversible.

Scoring tip: Mention both colour changes, blue to white and white to blue, and write both equations.