Activity Project: Preparation of Crystals of Copper Sulphate

Objective

To prepare pure crystals of copper sulphate (CuSO₄·5H₂O) from an impure sample by recrystallization.

Materials Required

• Impure copper sulphate sample
• Distilled water
• Dilute sulfuric acid (H₂SO₄)
• Evaporating dish / china dish
• Beaker, filter funnel, filter paper
• Glass stirrer
• Watch glass
• Heating apparatus (Bunsen burner or hot plate)
• Crystallizing dish
• Cold ethanol (optional, for washing crystals)

Procedure

1. Add distilled water to a beaker and heat it gently.
2. Add the impure copper sulphate sample gradually with constant stirring to prepare a saturated solution.
3. Add 2-3 mL of dilute sulfuric acid to prevent hydrolysis of copper sulphate.
4. Filter the hot solution to remove insoluble impurities.
5. Heat the filtrate to concentrate it by evaporation until crystals just begin to form on cooling (crystallization point).
6. Remove from heat and allow the solution to cool slowly and undisturbed. Crystals of copper sulphate will form.
7. Decant the mother liquor carefully and collect the crystals.
8. Wash the crystals with a small amount of cold distilled water or cold ethanol.
9. Dry the crystals by placing them on filter paper or a porous plate.

Observations

• Blue colored, transparent crystals of copper sulphate are obtained.
• Impurities remain in the mother liquor.

Result

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

Precautions

• Heat the solution gently and avoid excessive boiling to prevent decomposition.
• Do not over-evaporate; concentrate only to crystallization point.
• Allow the solution to cool slowly for good quality crystals.
• Use dilute sulfuric acid to prevent hydrolysis during heating.