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Explain the various steps involved in crystallization method.

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Most solid organic compounds are purified by crystallization method. This process is carried out by the following steps.

1. Selection of solvent:

organic substances being covalent do not dissolve in water, hence selection of suitable solvent becomes necessary. Hence the powdered organic substance is taken in a test tube and the solvent is added little by little with constant stirring and heating, till the amount added is sufficient to dissolve the organic compound.

If the solid dissolves upon heating and throws out maximum crystals on cooling, then the solvent is suitable. This process is repeated with benzene, ether, acetone and alcohol various solvent. till the most suitable one is sorted out.

2. Preparation of solution:

The organic compound is dissolved in minimum quantity of suitable solvent small amount of animal charcoal can he added to decolonize any coloured substance. The solution may be prepared by heating over a wire gauze or water bath.

3. Filtration of hot solution:

The hot solution so obtained is filtered through a fluted filter paper placed in a funnel.

4. Crystallization:

The hot filtrate is then allowed to cool. Most of the impurities are removed on the filter paper. the pure solid substance separate as crystal. If the rate of crystallization slow, it is induced either by scratching the walls of the beaker with a glass rod or by adding a few crystals of pure compounds to the solution.

5. Isolation and drying of crystals:

The crystals are separated from the mother liquor by filtration is done under reduced pressure using a Buchner funnel. Finally the crystals are washed with small amount of pure cold solvent and dried.

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