recrystallisation

Meaning: The impure solid is dissolved in the minimum amount of hot solvent. As the solution cools, the solubility of the compound falls and it crystallizes. A growing crystal lattice favors molecules of the main compound, while impurities, present at lower concentration, mostly remain in the cold solvent (the mother liquor). Crystals are collected by vacuum filtration and rinsed with a small amount of ice-cold solvent. Impurities that do not dissolve in the hot solvent can be removed by hot filtration before cooling.

On the MCAT: Passages describe a purification and ask why a step is done or what happens if it is changed. Predict the direction: Too much solvent: less product crystallizes, so yield drops. Rapid cooling: small crystals form that can trap impurities, so purity drops. Rinsing with warm solvent: some product redissolves, so yield drops. Solvent choice follows like dissolves like, and the compound should be soluble hot but poorly soluble cold. Purity is checked by melting point: a purer sample melts at a higher, sharper range, and an impure one melts lower and over a broader range. Percent recovery is below 100% in practice because some product stays in solution.

Common confusion: Students treat it as a separation by melting point or as the same thing as precipitation from a reaction. It depends on the change in solubility with temperature. It works best when the impurity is a small fraction of the sample. Two compounds present in similar amounts are better separated by chromatography or extraction.