Recrystallization is a common laboratory technique used to purify solid compounds. It involves dissolving the compound in a solvent, allowing impurities to dissolve as well, and then allowing the solution to cool and solidify so that the compound recrystallizes in a pure form. In this essay, we will discuss the steps involved in performing a recrystallization of benzoic acid, a common laboratory compound with a melting point of 122-123°C.
To begin the recrystallization, a sample of impure benzoic acid is weighed and placed in a clean, dry test tube. The next step is to choose a solvent in which the benzoic acid will dissolve. Common solvents for benzoic acid include water, ethanol, and acetone. It is important to choose a solvent in which the benzoic acid will dissolve, but in which the impurities will not dissolve or will dissolve to a lesser extent.
Once the solvent has been chosen, it is added to the test tube containing the impure benzoic acid. The mixture is then heated until the benzoic acid has completely dissolved. This process is called "hot filtration." The hot solution is then allowed to cool to room temperature, and the recrystallization process begins.
As the solution cools, the benzoic acid begins to recrystallize out of the solution. As the crystals form, they act as nucleation sites, encouraging more benzoic acid molecules to form crystals around them. As the crystals grow, they become visible to the naked eye. Once the crystals have reached a sufficient size, the solution is cooled in an ice bath to further encourage crystallization.
Once the crystals have fully formed, the solution is filtered to separate the pure benzoic acid crystals from the remaining solvent and impurities. The crystals are then washed with a small amount of cold solvent to remove any remaining impurities.
The pure benzoic acid crystals are then dried, either by allowing them to air dry or by drying them in an oven. The dried crystals are then weighed to determine the yield of the recrystallization.
In conclusion, the recrystallization of benzoic acid is a simple but effective method for purifying this compound. By carefully selecting a solvent in which the impurities do not dissolve, and by controlling the cooling rate of the solution, it is possible to obtain pure benzoic acid crystals with a high degree of purity.
