Purification of organic compounds involves: distillation (liquid mixtures), crystallisation (solids from solution), chromatography (mixtures on stationary phase), sublimation (solid to vapour without liquid phase), extraction (using immiscible solvents). Choice depends on: physical state, boiling points, thermal stability, solubility, volatility. Each technique exploits a different physical or chemical property to achieve separation. The purity of a compound is confirmed by sharp melting point (solid) or constant boiling point (liquid), and by spectroscopic methods (IR, NMR, MS).

Used when: components have significantly different boiling points (>25°C difference), or when separating a volatile liquid from a non-volatile solute. Process: mixture heated, more volatile component vaporises first, condenses in condenser, collected as distillate. Less volatile component remains in flask. Examples: CHCl3 (bp 61°C) from aniline (bp 184°C), water from dissolved salts (desalination principle), ethanol-water at low ethanol concentrations. Limitation: cannot separate liquids with similar boiling points. Thermometer placed at side arm of distillation flask to monitor vapour temperature. When used for water purification: called simple distillation or lab-scale desalination.

Used when: mixture contains liquids with close boiling points. Uses a fractionating column (packed with glass beads, Raschig rings, or has distillation plates) between flask and condenser. The column provides many theoretical plates — each plate is an equilibrium stage where vapour and liquid exchange. More volatile component progressively enriches in vapour phase. Industrial applications: petroleum refining (crude oil → LPG, petrol, kerosene, diesel, lubricating oils, bitumen by fractional distillation in tall towers). Ethanol-water separation (azeotrope at 95.6% ethanol prevents 100% separation by simple fractional distillation alone — requires molecular sieves or benzene to break azeotrope). Air separation: fractional distillation of liquid air gives N2 (bp -196°C), O2 (bp -183°C), Ar (bp -186°C).

Used for: high-boiling compounds that decompose at their atmospheric boiling point. Principle: boiling point decreases with decreasing pressure. At reduced pressure, compound boils at much lower temperature — avoiding decomposition. Common examples: glycerol (bp 290°C, decomposes near bp), aniline derivatives, natural products. Equipment: vacuum pump, vacuum adapter, Claisen flask (has two necks for safety). The pressure is monitored by a manometer. Modern alternative: rotary evaporator (rotavap) — reduces pressure while rotating flask to increase evaporation surface area, used for removing solvents from reaction mixtures. Very common in organic synthesis labs.

Used for: compounds that are (a) immiscible with water, (b) steam volatile (have appreciable vapour pressure below 100°C), and (c) cannot be purified by regular distillation (too high bp or thermally unstable). Principle: when two immiscible liquids are heated, they boil when P1 + P2 = Patm (Dalton law of partial pressures). The mixture boils at temperature below the boiling point of either component alone. Aniline (bp 184°C) + water boil at 98.5°C. Useful for heat-sensitive natural products: essential oils from plants (eucalyptus, rose, lavender), aniline, turpentine, coal tar fractions. Mole ratio in distillate: n1/n2 = P1/P2 (directly proportional to vapour pressures at boiling temperature). This allows calculation of composition of distillate.

Used for: purification of solid compounds. Principle: solubility increases with temperature for most solids. Hot saturated solution → cool → pure crystals form (impurities remain in solution if present in smaller amounts). Steps: dissolve in minimum hot solvent → filter hot (remove insoluble impurities) → cool slowly (large pure crystals form) → filter → wash crystals → dry. Choice of solvent: compound must be more soluble hot than cold in chosen solvent (large solubility difference). Common solvents: water, ethanol, acetone, petroleum ether. Mixed solvents: e.g., ethanol-water. Fractional crystallisation: separates two solids with different solubility-temperature profiles. Examples: KNO3 from KCl (large solubility difference), aspirin purification, recrystallisation of NaCl from water.

Separation based on differential distribution of components between stationary and mobile phases. Types: Paper chromatography (PC): stationary phase = paper (adsorbed water), mobile phase = organic solvent. Simple, cheap, used for amino acids, sugars, dyes. Thin Layer Chromatography (TLC): stationary phase = silica gel/alumina on aluminium sheet, mobile = organic solvent. Faster than PC, used to monitor reactions. Rf = distance moved by compound / distance moved by solvent front. Column chromatography: large-scale separation. Silica or alumina column. Fractions eluted with solvents of increasing polarity. Gas-liquid chromatography (GLC/GC): mobile phase = inert gas (N2, He), stationary = liquid on solid support. Separates volatile compounds. Used in forensics, food analysis. HPLC (High Performance Liquid Chromatography): high pressure liquid mobile phase. Most widely used analytical technique in pharmaceutical industry.

Sublimation: solid directly converts to vapour without melting. Used for volatile solids: camphor, naphthalene, iodine, anthracene. Product sublimes onto cold finger or cold surface, leaving non-volatile impurities behind. Extraction (solvent extraction): using immiscible solvent (organic vs water). Distribute compound between two immiscible phases based on partition coefficient. Acidic compounds extracted with NaOH(aq); basic compounds with HCl(aq); neutral with organic solvent. Separating funnel used. Zone refining (for semiconductors): narrow molten zone passed along ingot. Impurities concentrate in molten zone, move to one end. Very high purity silicon/germanium produced this way for semiconductors. Vapour phase chromatography for mixture analysis and separation of very small quantities.