Students performing recrystallization experiments should demonstrate an understanding of the basic principle on which it is based. Chemical compounds obtained by extraction or synthetic reactions exist in an impure state. These compounds are often contaminated with trace quantities of reactants and other reagents used in the chemical reaction. When the compound is dissolved in a suitable solvent in hot conditions, filtered and then cooled, impurities stay in solution and only the pure compound crystallizes out of solution.
By performing the recrystallization experiment, students familiarize themselves with each aspect of the procedure. They understand how to heat solvents and maintain conditions that are optimal to increase the percentage yield from recrystallization. Students also learn about safety measures when handling inflammable solvents. Recrystallization experiments teach students about associated steps such as vacuum filtration using a Buchner funnel, the use of selective adsorbents such as activated charcoal and the process of seeding to initiate the process of crystal formation.
Recrystallization experiments should focus on helping students understand the factors that determine the choice of the solvent. A good recrystallization solvent does not react with the compound of interest and is capable of easy removal by filtration. The compound to be recrystallized should be very soluble when the solvent is hot and almost insoluble when the solvent is cold. The impurities present must be insoluble in the hot solvent and very soluble in cold conditions.
On completion of the experiment, students must calculate the amount of the compound that they recovered by performing recrystallization. To do this, they must weigh the mass of the compound before and after the procedure. Students should ensure that the weights are obtained using the compound after it has fully dried. The percentage yield is given by multiplying 100 to the ratio of the mass following recrystallization to that before recrystallization.