When selecting a solvent for recrystallization, what is an essential criterion?

In the context of recrystallization, the goal is to purify a solid compound by taking advantage of its differing solubility at different temperatures. For an effective recrystallization process, the solvent should ideally dissolve the compound when heated, enabling the compound to enter solution at high temperatures, and then allow crystallization as the solution cools down.

Choosing a solvent that dissolves the compound at high temperatures ensures that a saturated solution can be formed; as the solution cools, the solubility of the compound decreases, leading to crystallization from the solution. This process effectively allows impurities, which may not dissolve as well, to remain in the solution upon cooling, hence achieving purification.

In contrast, if a solvent were selected that dissolves the compound at low temperatures or is immiscible with water, or is highly volatile, it might either fail to create a saturated solution effectively or lead to loss of the compound or impurities during the recrystallization process. Thus, the requirement for the solvent to dissolve the compound at high temperatures is fundamental for successful recrystallization.