In recrystallization, what does poor solubility of impurities contribute to?

In recrystallization, the goal is to purify a compound by dissolving it in an appropriate solvent and then allowing it to crystallize out of solution, ideally leaving impurities behind. Poor solubility of impurities in the chosen solvent is critical for effective purification because it allows the desired compound to dissolve while the impurities remain undissolved or only minimally soluble.

When the desired compound is highly soluble and the impurities are poorly soluble, the impurities will remain in the solvent after the crystallization process has occurred. Once the solution cools and crystallization begins, the desired compound forms crystals, while the impurities either stay in the mother liquor or may be removed through filtration. This ability to separate the desired compound from the impurities due to their differing solubilities is fundamental to achieving a high level of purity in the final product.

Thus, the poor solubility of impurities contributes positively to the effectiveness of the recrystallization process, as it ensures that the crystallized product is mostly free from contaminants, leading to effective purification.