Recrystallization is a process that is used to purify a substance. It can be used in various processes, such as the creation of aspirin. Recrystallization is completed by placing the impure compound in a solvent, heating the solution so the compound dissolves, and filtering the impurities. In some cases, it may be necessary to use carbon to remove colored contaminants from the compound. The mixture is then cooled, allowing pure crystals to form.

The main foundation behind recrystallization is the fact that substances will usually become more soluble when the solvent is hot than when it is cold. Therefore, a compound may dissolve in a warm liquid but be insoluble at room temperature. For example, sugar dissolves better in warm water than in cold water. This is why it is often difficult to dissolve sugar in iced tea even though a person may stir thoroughly. The difference in solubility at varying temperatures allows an impure substance to dissolve at a higher temperature and then crystallize slowly at a lower temperature without re-trapping impurities.

To purify a substance using the recrystallization process, a person must start by choosing the appropriate solvent. Many times, the right solvent will be found by using trial and error. The right solvent will not only dissolve the target compound at a higher temperature while allowing it to crystallize at room temperature, but also should not react with the target compound. The solvent should also not dissolve the impurities at the same temperature as the target compound. Impurities should dissolve at room temperature while the compound is insoluble or should be insoluble at a higher temperature to allow the impurities to be filtered.

During the recrystallization, a person should only use a small amount of solvent to dissolve the target compound. If too much is used, the compound may not recrystallize when the time comes. When the target compound has been completely dissolved, a person can filter out any insoluble impurities. The solution should then be allowed to cool slowly so that crystals can form. If the solution is cooled too quickly, the crystals may trap dissolved impurities and render impure crystals.

Should a white or clear compound be discolored when it is still in the hot solution, there may be colored impurities present. In such a case, activated carbon can be used to remove the color. The carbon will attract the impurities and clear the solution. Then both substances can be filtered. Only a small amount of carbon should be used in this process because too much carbon may start to react with the compound, reducing the final amount of the purified substance.