Activity 2.15 Class 10 Science

This activity explains the effect of heat on copper sulphate crystals. This activity also demonstrates the effect of water of crystallisation on its colour.

This activity is based on Chapter 2, Acids, Bases and Salts, Class 10, NCERT Science Book.

Heating of Copper Sulphate Crystals

Aim of the activity

To observe the effect of heating on hydrated copper sulphate crystals and understand the concept of water of crystallisation.

 

Required Materials

   1. Copper sulphate crystals

   2. Dry the boiling tube

   3. Test tube holder

   4. Bunsen burner

   5. Dropper

   6. Water  

Procedure

We take a few crystals of copper sulphate in a dry boiling tube and heat the tube over a Bunsen burner by holding  the tube using a test tube holder. Now we observe the colour change in the crystals.

Now we add 2-3 drops of water on the heated copper sulphate crystals and observe the colour change.

Observation

 

Steps	Observation
Before heating	Blue crystals of CuSO4.5H2O
During heating	Crystals turn white
After heating	Water droplets appear
After adding water	Crystals turn blue again

 

Explanation

Copper sulphate crystals (CuSO₄.5H₂O) exist in blue colour. When these crystals are heated, the water of crystallisation evaporates and the crystals turn white.

The water of crystallisation can be seen as droplets on the cooler part of the tube.

When water is added to these crystals, the blue colour is restored because the crystals absorb water and become hydrated copper sulphate.

(i) On heating

CuSO₄.5H₂O → CuSO₄   + 5H₂O

(ii) On adding water

CuSO₄   + 5H₂O → CuSO₄.5H₂O

Conclusion

We can conclude that –

   1.  The blue colour of copper sulphate is due to the presence of water of crystallisation.

   2.  On heating, the water of crystallisation evaporates and the blue colour disappears.

   3.  The blue colour appears again when water is added to these crystals.

 

Frequently Asked Questions

 

   1. What is water of crystallisation?

Answer – The fixed amount of water molecules chemically bonded with salt is called water of crystallisation.

   2. Why does copper sulphate lose its colour when heated?

Answer – The water of crystallisation evaporates when copper sulphate crystals are heated, and these crystals become anhydrous copper sulphate, white.

   3.  What is the chemical formula of copper sulphate crystals?

Answer - CuSO₄.5H₂O