Final answer:
Upon cooling a solution in recrystallization, the compound crystallizes out while impurities remain dissolved, thus purifying the compound.
Step-by-step explanation:
When you cool a solution during the process of recrystallization, the compound that was dissolved in the hot solvent becomes less soluble and precipitates, or crystallizes out, leaving the impurities still in solution. This is due to the fact that the solubility of substances generally decreases with a decrease in temperature, particularly for the compound of interest compared to impurities. Recrystallization is a method employed to purify a compound by exploiting the differences in solubility between the compound and its impurities at different temperatures.