Benzoic acid is an organic compound. Its crude sample can be purified by crystallisation from hot water. What characteristic differences in the properties of benzoic acid and the impurity make this process of purification suitable?

Answers (1)

As per the principle of crystallisation, it is known to us that the solute must be more soluble in hot water and less soluble in the cold water or any other similar solvent.

Now, the solubility of Benzoic acid in water is lesser as it is an organic compound. Moreover, it is less polar, whereas, in comparison, the water molecule is highly polar. But when the temperature of the water is increased, we observed that the solubility of benzoic acid is better in hot water in comparison to the cold water. The other reason being that the impurities are not insoluble in the water. Therefore, they can be filtered from the benzoic acid solution (if not soluble) or they would remain dissolved even after cooling the solution(if completely soluble in cold water). They will not be interfering with recrystallisation of the benzoic acid, as the extra solution can be discarded once the process of crystallisation is over.

The properties of benzoic acid and the impurity which makes this process of purification suitable are: -

  • Impurities which are present in benzoic acid are either insoluble in water or soluble in water to such an extent that they remain in solution.
  • The solubility of benzoic acid is slightly higher in hot water as compared to cold water.

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