Question

What is meant by ‘water of crystallisation’ of a substance ? Describe an activity to show that blue copper sulphate crystals contain water of crystallisation.

Answer 1

The water molecules associated with a crystalline substance is called ‘water of crystallisation’. To show crystalline salts contain water of crystallisation.

Materials Required: CuSO₄.5H₂O (Blue vitriol), boiling tube, burner, cork, delivery tube, test tube, clamp stand. 

Procedure: 

1. Take 2g of CuSO₄.5H₂O in a boiling tube fitted in a clamp stand. 

2. Observe its colour. Fit it with cork and delivery tube bent at two right angles which dips into a test tube

3. Heat crystals in boiling tube. 

4. Observe vapours being condensed in test tube. 

5. Cool the crystals and add few drops of water into it. 

Observation: Water vapours get condensed in a test tube and colour of blue crystals changes into white. On adding water to anhydrous copper sulphate, it changes into blue again. 

Chemical Reaction:

Conclusion: Crystalline substances have water of crystallisation which are lost on heating.

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