Recrystallization from water as a Method of Purification

Question

If charcoal and sugar were present in crude benzoic acid, would recrystallization from water be a good form of purification, and if so, why?

Answer 1

Recrystallization in water would be a great technique for purifying benzoic acid from charcoal and sugar. The main reason for that is the difference in solubility of benzoic acid at different temperatures in a solvent (in this case, water). Charcoal is insoluble in water, and, interestingly, in the chemical synthesis during the purification of targeted compounds, it's often used to remove color, odor, and other soluble substances from water! Sugars, on the other hand, are soluble in water (even at lower temperatures, which is important for this recrystallization process that we are talking about here). Considering these facts, the procedure for recrystallization of benzoic acid would look something like this:

Use the hot water to pour over the mixture of benzoic acid, sugar, and charcoal. You will observe the heterogeneous mixture because the only substance that is not soluble in the hot water in this mixture is the charcoal. By performing hot filtration (not allowing the solution to cool down), you would separate charcoal from the solution that contains sugar and benzoic acid. At this point, you want to utilize the fact that the difference in solubility of benzoic acid and sugar in cold water will allow you to separate them easily by cooling down the hot solution ( slowly! ). Sugar will stay soluble in the mother liquor while beautiful white crystals will start forming- benzoic acid. Using filtration technique (under atmospheric pressure or reduced pressure) and subsequently rinsing the crystals with cold water, you will be able to identify high purity benzoic acid.

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