What is Crystallisation?
Crystallisation is a process that separates a pure solid in the form of its crystals from a solution.
In this method, a saturated solution of the substance to be purified is prepared. This solution is then heated and then allowed to cool slowly. The slow cooling process results in the formation of crystals, which are the purest form of the substance.
Let us understand this with an example.
Say, we want to remove impurities from a sample of alum.
The first step is to prepare a saturated solution of alum. For this, alum is added to water and stirred. Once the alum dissolves, some more are added and stirred. This is done until no more alum crystals can be dissolved in it. That is, the water molecules have no space to accommodate any more alum molecules.
Preparation of saturated solution
This saturated solution is now heated till it starts boiling and a substantial amount of water has evaporated. It is important that all the water does not evaporate.
Boiling of the saturated solution
The remaining solution is then taken off the flame and filtered. This removes any insoluble solid impurities in the alum.
Filtration
Now, the solution is allowed to cool. As the solution slowly cools, crystals begin to develop in it. And these are pure crystals of alum, which are dried with filter paper and are ready to be used.
Formation of alum crystals
Rapid cooling of the solution will also give crystals, but they will be small in size and irregular in shape. Slow cooling gives crystals of the desired size.
The process of crystallisation is used to obtain salt from seawater and pure forms of chemicals like copper sulphate.