Question

When successful, recrystallization yields the crystals of the desired solid only and not of the impurity. Why?

Answer 1

A successful recrystallization depends on the proper choice of solvent. The compound must be soluble in the hot solvent and insoluble in the same solvent when it is cold. For the purpose of recrystallization, consider 3% w/v the dividing line between soluble and insoluble: if 3 g of a compound dissolves in 100 mL of a solvent, it is considered soluble. In choosing a solvent, the bigger the difference between hot solubility and cold solubility, the more product recoverable from recrystallization. OR Recrystallization does not involve a chemical reaction; the crude product is simply dissolved into solution, and then the conditions are changed to allow crystals to re-form. Recrystallization produces a more pure final product. For this reason, experimental procedures that produce a solid product by crystallization normally include a final recrystallization step to give the pure compound. ANS- Simply because compound is more soluble in hot solvent as compared to impurity and impurity is more soluble in cold solvent as compared to compound, that's why we mixture cool down compound form crystal and impurity left behind in a mixture. THANKS HOPE THIS WILL HELP YOU