Question

A student performed a recrystallization and after the solid dissolved, the solution was immediately placed in an iced bath. Within a few seconds, a large amount of solid formed and was collected by cavuum filtration. The percent recovery was high but the melting point of the dried solid showed that the solid was contaminated with impurities. Could you explain very specifically why it happedn?

Answer 1

Crystallization is actually an equilibrium process. There are attractive and repelling sites on the growing crystal. If the crystallization is done slowly, then any impurities that might have been attracted to the crystal can diffuse away again. However, if done too quickly, those impurities may become embedded in the crystal. That is more likely to happen if the solubility falls too quickly. If a solution is cooled slowly, the solubility can be reduced slowly also.