Crystallisation is carried out in four stages (a) preparation of the solution of the substance in a suitable solvent (b) filtration of the hot solution (c) crystallisation by cooling the hot filtrate (d) isolation and drying of the purified substance.
a) Preparation of the solution : The powdered organic substance is taken in a semi-micro round bottom flask and the solvent is added little by little with constant stirring and heating till the amount added is just sufficient to dissolve the solute, when the solution is just boiled. If the solvent is non-inflammable, heating may be done on the wire gauze, while in the case of inflammable solvents, heating should be done on a water bath.
Filtration of the Solution : The hot saturated solution obtained is filtrated through a fluted filter paper placed in a hot water funnel.
c) Crystallisation : When the filtration is over, the beaker containing the solution is allowed to cool when pure crystals separate. Sometimes the crystals do not separate due to super cooling of the solution. Crystallisation can be started in such cases by scratching the sides of the vessel containing the solution with a glass rod or seeding with a tiny crystal of the substances.
d)Isolation and drying of the purified crystals : The purified crystals are separated from the mother liquor by filtration using Buchner funnel and a suction pump.
When the whole of the mother liquor has been drained off, the crystals are washed with small amounts of cold solvent thrice. The crystals are then transfered to a porous plate, pressed by using filter paper and then dried by using infra-red light or by keeping in sunlight. If the crystals are coloured, decolourisation is effected by using animal charcoal.
When the solubility of two substances in any solvent is not much different from one another, then the two compounds can be separated by fractional crystallisation, involving a series of repeated crystallisations. For example, when a solution containing two substances A and B is subjected to crystallisation, the slightly less soluble substances (say A) containing a small amount of the other substance (B) crystallises out. The mother liquor when subjected to crystallisation gives crystals of B containing a small amount of A.
Now, if these crystals are subjected to recrystallisation separately and the process is repeated number of times to get pure A and pure B.