Purification Methods: Crystallisation to Chromatography

A reaction product is rarely pure — it comes mixed with unreacted starting materials, side products or solvents. Purification separates the desired compound from these impurities by exploiting differences in physical properties: solubility for crystallisation, boiling point for distillation, vapour pressure for sublimation, and differential adsorption for chromatography.

These techniques are the backbone of every chemistry laboratory and a staple of Class 11 and JEE/NEET questions. Examiners typically give you a mixture and ask which method separates it — and the answer always follows from one property difference.

Key Concepts

• Crystallisation: dissolve the impure solid in the minimum hot solvent, filter hot, cool slowly — pure crystals form while soluble impurities stay in the mother liquor.
• Distillation: separate liquids by boiling-point difference; simple distillation for large differences, fractional distillation (fractionating column) for close ones.
• Steam distillation: for liquids immiscible with water that decompose at their boiling point (e.g. aniline); the mixture boils below 100 °C.
• Distillation under reduced pressure: for high-boiling liquids that decompose on heating (e.g. glycerol); lowering pressure lowers the boiling point.
• Sublimation: heating converts some solids directly to vapour, separating them from non-sublimable impurities — e.g. camphor, naphthalene, ammonium chloride.
• Chromatography: separation between a stationary phase and a mobile phase; components travel at different speeds by adsorption or partition.

In paper chromatography, the Rf value identifies each component: Rf = (distance travelled by the substance) ÷ (distance travelled by the solvent front), always between 0 and 1. Steam distillation works because the total vapour pressure of two immiscible liquids equals the sum of their individual vapour pressures, so the mixture boils when this sum reaches atmospheric pressure — below the boiling point of either pure liquid.

Formulas

• Rf value: Rf = d(substance) ÷ d(solvent front), both measured from the origin line in the same units (cm). Dimensionless; 0 ≤ Rf ≤ 1.
• Steam distillation condition: P(total) = P(water) + P(organic) = atmospheric pressure at the boiling point of the mixture.
• Assumption: the two liquids are immiscible and chemically unreactive toward each other and toward steam.

Worked Example

In a paper chromatogram, a dye spot travels 3.2 cm from the origin while the solvent front travels 8.0 cm. Calculate the Rf value. Step 1: identify the distances — d(substance) = 3.2 cm, d(solvent) = 8.0 cm. Step 2: Rf = 3.2 ÷ 8.0 = 0.40. Step 3: check the range — 0.40 lies between 0 and 1, so it is valid. A second spot with Rf 0.65 would be a different, less strongly adsorbed compound.

Common Misconceptions

• "Simple distillation can separate any two liquids." No — if boiling points are close, you need fractional distillation with a fractionating column; simple distillation only works for large differences.
• "Sublimation purifies any solid on heating." Wrong — only sublimable solids (camphor, naphthalene, NH₄Cl) sublime; ordinary solids just melt or decompose.
• "Rf depends on how long you run the chromatogram." It does not — Rf is a ratio of distances, so it is independent of run time and solvent volume (for the same system).

Real-World Applications

• Petroleum industry: fractional distillation of crude oil yields petrol, diesel, kerosene and bitumen in one giant column.
• Pharmaceuticals: crystallisation purifies drugs to pharmacopoeia standards; chromatography checks purity and separates enantiomers.
• Forensics and food testing: chromatography detects adulterants, doping agents and pesticide residues.

Every purification method exploits one property difference — solubility, boiling point, vapour pressure or adsorption. Match the mixture to the method, remember Rf = spot distance ÷ solvent distance, and know when to choose fractional, steam or reduced-pressure distillation. That decision tree answers nearly every purification question.