Purification of Crystalline Organic Compounds Consider the following statement t

Question

Purification of Crystalline Organic Compounds Consider the following statement to answer questions 1 and 2. Recrystallization of an impure solid must be carried our using a minimum amount of the required solvent(s) at or near its boiling point. Why is it necessary to use only a minimum amount of the required solvent for recrystallization? Why is it necessary to carry out the recrystallization at or near the boiling point of the solvent used? How are insoluble impurities removed during recrystallization? What purpose does the addition of finely divided charcoal serve during the recrystallization of impure solids?

Explanation / Answer

1. It is important to add minimum amount of solvent for recrystallization otherwise the dissolved compound would not recrystallize out of the solution and would remain in solution.

2. In order to used the minimal amount of solvent for recrystallization, one must add solvent and perform recrystalization near its boiling point. The solubility of a compound increases at higher temperature and this when cooled will easily recrystallize out of the solution which is near saturation.

3. Insoluble impurities are removed by boiling the compound in minimal amount of solvent and then filtering out the hot solution through a Whatman filter paper. The impurities stay on top and the disolved compound is solution passes out of paper and collected.

4. Finely divided charcoal is added to remove coloring impurities present in an organic compound. The impurities gets adsorbed on charcoal and retained on it when filtered.

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