Recrystallization is a common technique used to purify compounds. The most typical situation is desired compound A is contaminated by a small amount of impurities B. To perform recrystallization, a small amount of solvent is added to a flask containing impure compounds. Then the mixture is heated until all the compounds dissolve. Next, the solution is cooled down. Pure compounds will precipitates because the solubility decreases. The impurities still remain in the solvent. Then vacuum filtration is applied to collect the precipitates.