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The process of obtaining crystals of a soluble substance from its hot saturated solution is called crystallisation.
Crystallisation is only a physical change because no new substances are formed during the process – it involves only a change in physical state.
Crystallisation is used to obtain pure substances from naturally occurring impure substances.
Here’s an experiment to see how crystallisation occurs.
Take 100 ml of water in a beaker and heat it on a Bunsen burner.
When water begins to boil, add about 2 ml of dilute sulphuric acid,
and then a spoonful CuSO4.
Stir the contents with a glass rod.
Stir it continuously and add more impure copper sulphate until no more of it dissolves.
Fasten a thread in the middle of a wooden stick and suspend in a glass.
Stop heating and pour the saturated copper sulphate solution in a glass.
Place the glass aside, and cover the beaker with a cardboard. Leave the setup for 24 hours.
After 24 hours, you will find big blue crystals of pure CuSO4 at the base of the beaker.
Remove the crystal from the glass and dry it in warm air.
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