Every method anchored to the one physical property it exploits — with a live diagram for each. Volatility, solubility, adsorption affinity, or travelling with steam.
| Technique | Property exploited | Typical situation |
|---|---|---|
| Sublimation | Solid → vapour directly | Sublimable solid + non-sublimable one |
| Crystallisation | Solubility varies with temperature | Soluble hot, sparingly soluble cold |
| Simple distillation | Large boiling-point gap | Volatile liquid + non-volatile / far-boiling |
| Fractional distillation | Small boiling-point gap | Two miscible, close-boiling liquids |
| Steam distillation | Volatile in steam, water-immiscible | Decomposes near its own b.p. |
| Reduced-pressure distn. | b.p. falls as pressure falls | High-boiling / heat-sensitive liquid |
| Differential extraction | Unequal solubility in two solvents | Compound in water → organic solvent |
| Chromatography | Differential adsorption / partition | Small amounts, very similar components |
Principle. Some solids pass straight from solid to vapour on heating and re-solidify on cooling, without melting. Gentle heat drives the pure compound off as vapour, which re-deposits as a sublimate on a cool surface, leaving the non-sublimable impurity behind.
Applies to camphor, naphthalene, anthracene, benzoic acid, ammonium chloride and iodine — freed from non-sublimable impurities such as NaCl or sand.
Principle. A solid dissolves far more in hot solvent than cold. Dissolve the impure solid in the minimum hot solvent to saturate; on cooling the pure compound — less soluble cold — crystallises out, while soluble impurities stay in the mother liquor and are filtered off.
Two solids of different solubility: repeated, controlled cooling makes the less-soluble one crystallise first. Separate and re-crystallise each crop — the classic route to KClO₃ from KCl.
Principle. Separation by boiling point. Heat the mixture; the more volatile component vaporises, passes to the condenser and is collected as the distillate, while the less-volatile material stays in the flask.
For a large boiling-point gap (≈ 25 K or more), or a volatile liquid from a non-volatile solid — e.g. chloroform (61 °C) from aniline (184 °C), or water from salt water.
For close boiling points. A fractionating column gives many vaporisation–condensation cycles, each enriching the vapour in the more volatile component. Basis of petroleum refining and of separating acetone (56 °C) from methanol (65 °C).
For liquids that are steam-volatile, immiscible with water, and decompose near their b.p. Compound and water each add their own vapour pressure, so the mixture boils when the total reaches atmospheric — below 100 °C.
o-Nitrophenol has intramolecular H-bonding (chelation) → not associated → steam-volatile, distils over.
p-Nitrophenol has intermolecular H-bonding → highly associated → not steam-volatile → stays behind. (Same logic gives ortho the lower boiling point.)
A vacuum pump lowers the pressure above the liquid, which lowers the boiling point. High-boiling or heat-sensitive liquids then distil well below their decomposition temperature — used for glycerol and to concentrate sugarcane juice.
Principle. A compound in water is shaken with an immiscible organic solvent in which it is more soluble. It distributes between the two layers by the distribution (partition) law:
For a fixed total solvent volume, several small portions remove more compound than one large portion — each fresh portion re-establishes the KD equilibrium and pulls across more solute.
Principle. Components separate by their differing affinity for a fixed stationary phase vs a moving mobile phase. Strongly-held components move slowly; those preferring the mobile phase move fast — so the mixture spreads into separate zones. Uniquely powerful for small amounts of very similar substances.
The mixture is loaded on an adsorbent-packed column (silica / alumina) and washed down (eluted) with solvent. The least-adsorbed component moves fastest and emerges first; fractions are collected separately.
The mixture is spotted near the base of an adsorbent-coated plate; solvent rises by capillary action, carrying components to different heights. Each is characterised by its retardation factor:
More strongly adsorbed → travels less → smaller Rf. More soluble in the mobile phase → travels further → larger Rf. Always 0 < Rf < 1, and fixed for a compound under set conditions — so it identifies components.
Paper chromatography works by partition: the stationary phase is water held in the paper, and components partition between it and the moving solvent — the same KD idea as extraction. Widely used for amino acids and sugars.
| Method | Basis | Best example |
|---|---|---|
| Sublimation | Solid → vapour → solid | Camphor, naphthalene, NH₄Cl, I₂ |
| Crystallisation | Solubility hot vs cold | Benzoic acid, sugar, alum |
| Fractional crystln. | Two solids, different solubility | KClO₃ from KCl |
| Simple distn. | Large b.p. gap | Chloroform (61) from aniline (184) |
| Fractional distn. | Small b.p. gap | Crude oil; acetone / methanol |
| Steam distn. | Steam-volatile, water-immiscible | Aniline; o-nitrophenol; oils |
| Reduced-pressure distn. | Lower pressure → lower b.p. | Glycerol; sugarcane juice |
| Differential extraction | Partition (KD) | Compound from water into ether |
| Column / TLC | Differential adsorption | Reaction mixtures; dyes |
| Paper chromatography | Differential partition | Amino acids, sugars |