Purification methods (chemistry)
Also known as: separation and purification methods
Purification methods in chemistry are laboratory techniques — extraction, distillation, chromatography, recrystallization, and filtration — that separate a desired compound from a mixture based on differences in physical properties.
Purification methods are the laboratory techniques chemists use to isolate a single compound from a mixture. Each method exploits a difference in physical properties between the target compound and its contaminants — solubility, boiling point, polarity, charge, or size.
Extraction separates compounds by differential solubility between two immiscible liquids, typically water and an organic solvent. Acid-base chemistry supercharges it: converting a compound to its charged (ionized) form pulls it into the aqueous layer, so an acid can be extracted from organics by washing with base. Distillation separates liquids by boiling point — simple distillation works when boiling points differ widely, fractional distillation resolves closer ones, and vacuum distillation protects heat-sensitive compounds by lowering the boiling point.
Chromatography separates by differential affinity between a stationary phase and a mobile phase: thin-layer and column chromatography rely on polarity, ion-exchange on charge, size-exclusion on molecular size, and affinity chromatography on specific binding. Recrystallization purifies solids by dissolving them in a hot solvent and letting pure crystals form on cooling, while filtration and centrifugation separate solids from liquids by size and density. Protein-specific variants — like salting out and electrophoresis — extend the same logic to biochemistry.
The MCAT's chemical and physical foundations section tests separation and purification methods directly. Expect questions matching a scenario to the right technique, predicting which layer a compound enters during an acid-base extraction, and interpreting the order of elution in chromatography.
Key takeaways
- Every purification method exploits a physical property difference: solubility, boiling point, polarity, charge, or size.
- Extraction uses two immiscible solvents; ionizing a compound with acid-base chemistry moves it into the aqueous layer.
- Distillation separates by boiling point — fractional for close boiling points, vacuum for heat-sensitive compounds.
- Chromatography separates by affinity for a stationary phase; recrystallization purifies solids via solubility differences on cooling.