The crystallisation method is used to purify solids. For example, the salt we get from sea water can have many impurities in it. To remove these impurities, the process of crystallisation is used. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as –
-Take some (approximately 5 g) impure sample of copper sulphate in a china dish.
-Dissolve it in minimum amount of water.
-Filter the impurities out.
-Evaporate water from the copper sulphate solution so as to get a saturated solution.
-Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.
-You will obtain the crystals of copper sulphate in the china dish.
-This process is called crystallisation.
• some solids decompose or some, like sugar, may get charred on heating to dryness.
• some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
• Purification of salt that we get from sea water.
• Separation of crystals of alum (phitkari) from impure samples.