Recrystallization is one of the most important methods used to purify solids. It involves dissolving an impure sample in a hot solvent and filtering this solution while hot (also c... Recrystallization is one of the most important methods used to purify solids. It involves dissolving an impure sample in a hot solvent and filtering this solution while hot (also called hot filtration). The resulting filtrate is then cooled to form larger and purer crystals, thus the technique’s name. This purification method heavily depends on a solvent’s ability to differentiate the compound of interest (solute) from other sample components (impurities) based on their differing solubilities. Thus, choosing an appropriate solvent is crucial in recrystallization. An ideal solvent should have the following properties: 1. It should readily dissolve the solute at high temperatures but only sparingly at room temperature. 2. It should dissolve the impurities even at room temperature, such that only the solute recrystallizes upon cooling. 3. It should be sufficiently volatile to hasten the drying of the purified crystals. A difficulty often encountered in recrystallization is the failure of the solute to recrystallize. This frequently happens when the concentration of the solute is too high (supersaturated solution) or when an impurity prevents recrystallization. In such cases, techniques such as adding a small crystal of the pure substance (also called seeding) or scratching the inner walls of the container with a glass rod may be used. If the above methods fail to bring about recrystallization, the solution is probably unsaturated. It is, therefore, heated further to remove the excess solvent. In practice, the recrystallization procedure should be repeated until a constant melting point of relatively narrow range is obtained. The melting point is a physical property that can be used to identify a substance and to assess its purity. A narrow temperature range of 0.5 to 1.5 ̊C is observed when a solid compound is pure. On the other hand, when the solid compound is contaminated, the melting point decreases, and the melting point range widens. Alternatively, sublimation can be used for the purification of impure solids. It is a reversible physical process where solid substances are directly converted to the vapor phase when heated without the intermediate appearance of a liquid state. For sublimation to occur, the solid must have an exceptionally high vapor pressure at a temperature below its melting point to avoid passing through the liquid state. Though not all solids can be purified by sublimation and despite its low percent recovery, the method yields crystals of very high purity and does not require a solvent. In this experiment, crude benzoic acid samples will be purified using recrystallization and sublimation. For recrystallization, water will be used as the solvent. Furthermore, the melting point will be determined to evaluate the purity of the benzoic acid obtained from the two techniques. Read more