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?

According to the principle of crystallisation, we know that the solute must be less soluble in the cold water and more soluble in hot water, or any other solvent like water.

Now, Benzoic acid is soluble in water in lesser amount as it is an organic compound and is less polar, whereas the water molecule is very polar. But on increasing the temperature of the water we observed that the benzoic acid is more soluble in hot water as compared to cold water, and the other reason is that the impurities are almost insoluble in water or completely soluble in cold water, so that they either can be filtered from the benzoic acid solution (if not soluble) or they remain dissolved even after cooling the solution(if completely soluble in cold water), and do not interfere with recrystallization of the benzoic acid, as we can discard the extra solution once the crystallisation process is complete and we obtain the purified benzoic acid.

Hence the characteristic properties of Benzoic acids are:

Benzoic acid is relatively more soluble in hot water as compared tothe cold water.
(ii) Impurities present in benzoic acid are insoluble in water or more soluble in water so that they remain in the solution.