(i)What is it used for:
It is used to obtain a solid from a solution of the solid in a liquid. It is often used to obtain crystals of salt from solutions in water.
(ii)What happens:
The solution is put into an evaporating basin and heated. The solution boils. The liquid is boiled much of the water had evaporated (1/3 of liquid left).
The hot solution is then left to cool. Crystals form on cooling. The crystals are dried by pressing them between sheets of filter paper.
(iii)How it works:
The water is evaporated to make the hot solution saturated. This means it can no longer dissolve any extra solid.
(iv)Experiment: Obtaining dry copper(II) sulfate crystals from solution
Put aqueous copper(II) sulfate into an evaporating dish and heat till it boils. Boil the liquid till its saturated (about 1/3 of liquid left). Leave the solution to cool. Blue crystals form on cooling. Dry the crystals by pressing them between sheets of filter paper.
Result: Dry Copper(II) sulfate crystals are obtained.
(v)Notes:
-Some water must usually be left when aqueous solutions of salts are being crystallised as the crystals need water for crystallisation.
-Aqueous sodium chloride solution (salt solution) is unusual. The solution must be heated to evaporate all of the water before salt crystals appear.
(vi)Apparatus needed:
Evaporating dish, Bunsen burner
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