Crystallisation of Copper Sulphate (CuSO₄·5H₂O) – Class 11 CBSE

Objective

To prepare crystals of pure copper sulphate from an impure sample by the process of crystallisation.

Materials Required

• Impure sample of copper sulphate
• Distilled water
• Dilute sulphuric acid (H₂SO₄)
• China dish / evaporating dish
• Beaker, filter funnel, filter paper
• Glass rod (policeman)
• Watch glass
• Bunsen burner or sand bath
• Crystallising dish
• Ethyl alcohol (for washing crystals)

Procedure

1. Preparation of Solution: Take about 25-30 ml of distilled water in a beaker. Add small quantities of powdered impure copper sulphate and stir until no more dissolves (saturated solution).
2. Add 2-3 ml dilute sulphuric acid to prevent hydrolysis of copper sulphate during heating and to keep the solution clear.
3. Filtration: Filter the hot solution through a filter paper into a clean china dish to remove insoluble impurities.
4. Concentration: Gently heat the filtrate on a sand bath/china dish, stirring with a glass rod to avoid crust formation, until the solution is reduced to about one-third of the original volume or until it reaches the crystallisation point. To test, dip a glass rod, blow on the drop – if crust or crystals form on cooling, crystallisation point is reached.
5. Cooling: Transfer the hot saturated solution to a crystallising dish and allow it to cool slowly undisturbed, placed in a beaker of water for controlled cooling. Crystals of copper sulphate begin to form as the solution cools.
6. Separation and Drying: Decant the mother liquor carefully. Wash crystals with a little cold ethyl alcohol mixed with water to remove impurities. Dry crystals by placing between folds of filter paper or on a porous plate.

Observations

• Deep blue, transparent crystals of copper sulphate (CuSO₄·5H₂O) separate on cooling.
• Solution becomes clear after filtration.
• Slow evaporation leads to large, well-formed crystals.

Result

Pure, deep blue crystals of copper sulphate pentahydrate (CuSO₄·5H₂O) are prepared from the impure sample.

Precautions

• Heat the solution gently and avoid excessive boiling to prevent decomposition.
• Do not evaporate the solution to dryness; concentrate only to crystallisation point.
• Allow slow cooling without disturbance for proper crystal formation.
• Wash crystals with minimal washing solution several times to remove impurities.
• Use dilute sulphuric acid to prevent hydrolysis during heating.

Chemical Reaction

CuSO₄·5H₂O (impure) → (dissolved in water + H₂SO₄) → saturated solution → crystallisation → CuSO₄·5H₂O (pure crystals)