Water of Crystallisation

What is Water of Crystallisation?

The water of crystallisation refers to the water molecules that are chemically bound to the ions or molecules of a crystalline substance. It is also known as water of hydration or hydrate water. These water molecules are an integral part of the crystal lattice structure and are not merely physically adsorbed or trapped within the crystal.

Copper sulphate is a pentahydrate and is blue in colour as it contains five water molecules of crystallisation. On heating, it loses its water of crystallisation due to the evaporation of water molecules. First, it loses four water molecules, which is observed by the change in the compound’s colour to bluish-white; this usually occurs at a temperature of about 100 ?. On further heating up to around 250 ?, it gets converted to anhydrous copper sulphate that is white in colour. 

                           100 ⁰C                           250 ⁰C 

CuSO₄.5H₂O(s)     ?      CuSO₄.H₂O(s)    ?        CuSO₄ (s)

On cooling, copper sulphate absorbs moisture from the atmosphere and regains its water of crystallisation.

CuSO₄ +     5H₂O   ?  CuSO₄.5H₂O

(White)                            (Blue)

To perform this experiment, apparatus and chemicals are required: Burner, tripod stands, wire gauze, china dish, and copper sulphate crystals.

Procedure for Experiment

To show that crystals of copper sulphate contain water of crystallisation, we have to follow the given procedure.

1. Take a few grams of copper sulphate crystals (blue) in a china dish and heat it over a burner for some time. 
2. Notice that the colour of the copper sulphate crystals changes to bluish-white first and then to white.
3. Switch off the burner and allow the china dish to cool down to room temperature (25 ⁰C). 
4. Observe the change in the colour of anhydrous copper sulphate from white to blue again.

In this experiment, we learnt that: 

• The crystals of copper sulphate contain water of crystallisation.
• Hydrated copper sulphate crystals that are blue in colour lose their water of crystallisation on heating.
• At 250 ?, the copper sulphate loses all its water molecules to form white anhydrous copper sulphate. 
• The anhydrous copper sulphate, on cooling, regains the water molecules to form blue hydrous copper sulphate again.

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