The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases. For example, much more table sugar can be dissolved in very hot water (just below the boiling point) than in water at room temperature. What will happen if a concentrated solution of hot water and sugar is allowed to cool to room temperature? As the temperature of the solution decreases, the solubility of the sugar in the water also decreases, and the sugar molecules will begin to crystallize out of the solution. (This is how rock candy is made.) This is the basic process that goes on in the recrystallization of a solid.
Recrystallization is a procedure that is used in the purification of the impure compounds in a given solvent. During the recrystallization process, the compound and the impurities are usually dissolved in an appropriate solvent. When the compound precipitates out, the crystals are usually left behind.