Recrystallization

Learn that recrystallization purifies solids by dissolving them in hot solvent where impurities are liberated from the crystal lattice. The technique exploits differential solubility: the desired compound must be insoluble at room temperature but soluble at high temperatures.

Choose an appropriate solvent with low solubility for the solid at room temperature but high solubility at elevated temperatures. Heat the solvent to boiling or near-boiling temperature.

Add the heated solvent to the impure solid in a beaker and stir while heating until all solid dissolves. Use only the minimum amount of solvent necessary to achieve complete dissolution.

Remove the heat source and allow the beaker to cool to room temperature, then place it in an ice bath to further reduce temperature. Crystals will begin to form as solubility decreases.

If crystallization is slow or does not begin, use a seed crystal or scratch the beaker with a stirring rod to provide nucleation sites for crystal growth.

Transfer the crystallized mixture to filter paper on a Buchner funnel and apply vacuum to separate the pure crystals from the solvent containing dissolved impurities.

Allow the crystals to dry completely on the filter paper. Perform melting point analysis and compare results to tabulated values for the pure compound; a narrow melting point range close to literature values indicates high purity.

Choose the optimal solvent through trial and error, prior experience with different polarities, or by consulting tabulated solubility data. Ideal impurities either remain soluble at room temperature or remain insoluble at elevated temperature.