Which characteristic is required of a solvent for recrystallization regarding impurities?

In recrystallization, the primary goal is to purify a solid compound by exploiting differences in solubility. For an effective recrystallization process, it is essential that impurities are soluble at low temperatures; this ensures that when the solution is cooled, the impurities remain dissolved while the desired compound crystallizes out.

If the impurities are soluble at low temperatures, they won't be effectively removed during the crystallization process. Therefore, the ideal scenario is that the compound you want to purify becomes less soluble as the temperature decreases, allowing it to form crystals, while the impurities either stay in the solution or do not crystallize out.

This principle of solubility is integral to the recrystallization technique, making the requirement for impurities to be soluble at low temperatures crucial for successful purification.