Principle: Most solids are more soluble in hot solvent than cold. Dissolve impure solid in minimum hot solvent → impurities either (a) remain dissolved at lower concentration in cold solution, or (b) are filtered out as insoluble matter from hot solution.
Steps:
- Choose correct solvent (must dissolve compound well hot, poorly cold; must not react with compound)
- Dissolve impure solid in minimum hot solvent (to get saturated solution)
- Filter hot solution (removes insoluble impurities)
- Cool slowly (slow cooling → larger, purer crystals; fast cooling → smaller, less pure)
- Filter crystals by suction filtration (Buchner funnel)
- Wash with small amount of cold solvent
- Dry crystals
Common solvent-solute pairs: Benzoic acid — water. Naphthalene — alcohol. Acetanilide — water. Sugar — water/ethanol mixture.
Decolorizing carbon (activated charcoal): Added to hot solution before filtration to adsorb colored impurities. Use sparingly — excess charcoal adsorbs the desired compound too.