Regarding the recrystallization of benzoic acid

Question

1. Why is water a good solvent for the recrystallization of benzoic acid?
2. If charcoal and sugar were present in crude benzoic acid, would recrystallization from water be a good method of purification? Why? Explain.

Answer 1

1). Water (H₂O) would be a good solvent for recrystallizing benzoic acid (C₆H₅COOH) for several reasons. First, both are polar so the benzoic acid would be soluble in water, especially at elevated temperatures, but less soluble in lower temperatures. In addition, water will not react with the benzoic acid, and it is easily evaporated at the end of the process, making it easy to recover the recrystallized product. Finally, water is an inexpensive solvent.

2). If charcoal (C) and sugar (C₁₂H₂₂O₁₁) were present in the crude benzoic acid, water would still be a good solvent to use for recrystallization. The reasons for this are as follows:

The charcoal (non polar) wouldn't dissolve even at elevated temperatures, so could be separated from the dissolved benzoic acid and the dissolved sugar. This could be accomplished by centrifugation or filtration.

The remaining solution would contain benzoic acid and dissolved sugar. Upon lowering the temperature of this solution, the benzoic acid would become insoluble (recrystallizled) while the sugar remained in solution. The sugar could thus be easily separated from the benzoic acid, leaving pure, recrystallized benzoic acid as the only product.