Separation Techniques

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Separation techniques isolate individual components of a mixture by exploiting differences in particle size, solubility, boiling point, or affinity for a stationary phase, leaving chemical identity unchanged.

Insoluble solid + liquid → filtration (residue on paper, filtrate passes through).
Dissolved solid in solvent → evaporation (recover solute) or crystallisation (recover pure solid).
Miscible liquids with close boiling points → fractional distillation with a fractionating column; single liquid + dissolved solid → simple distillation (Liebig condenser).
Sublimable solid (I₂, NH₄Cl, naphthalene) → sublimation.
Chromatography separates by partition between mobile and stationary phases, reported as Rf = distance moved by solute ÷ distance moved by solvent.

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Choosing the Right Method

The technique selected depends on which physical property differs between the components. Filtration uses filter paper and a funnel to trap an insoluble solid (residue) while the liquid (filtrate) drains through. Decantation pours off a supernatant liquid after settling, but it fails for finely divided suspensions, where centrifugation is more reliable.

Recovering Dissolved Solids

When the solute is dissolved, evaporation to dryness recovers the solid — but it gives an impure product if other dissolved solids are present. Crystallisation purifies by dissolving the solid in hot solvent, filtering out insoluble impurities, then cooling to deposit pure crystals while soluble impurities remain in the mother liquor.

Distillation Family

Simple distillation separates a solvent from a non-volatile solute (e.g. water from salt solution) using a Liebig condenser; the thermometer bulb sits at the side-arm to record vapour temperature. Fractional distillation adds a vertical column packed with glass beads or Raschig rings, providing repeated condensation–evaporation cycles (“theoretical plates”) that resolve miscible liquids whose boiling points differ by less than 25 °C, such as ethanol (78 °C) and water (100 °C).

Chromatography and Sublimation

Paper chromatography partitions solutes between a stationary water phase held in cellulose fibres and a mobile organic solvent; the Rf value = (distance travelled by solute) ÷ (distance travelled by solvent front) and always lies between 0 and 1. Sublimation converts a solid directly to vapour on heating — applicable to I₂, NH₄Cl, and naphthalene — leaving non-sublimable impurities behind.

Technique	Best for	Governing property
Filtration	Insoluble solid + liquid	Particle size
Evaporation	Solute recovery	Volatility of solvent
Crystallisation	Pure solid recovery	Differential solubility
Simple distillation	Solvent + non-volatile solute	Boiling point gap (>25 °C)
Fractional distillation	Miscible liquids, close b.p.	Boiling point gap (small)
Sublimation	Sublimable solid + impurity	Sublimation tendency
Chromatography	Dissolved mixture	Partition coefficient

Exam tip: when a question names two miscible liquids, the answer is fractional, not simple, distillation — even if the wording says “boiling-point difference.”

Common Mistakes Trapped by JAMB

Confusing filtration (insoluble) with evaporation (soluble).
Putting an Rf greater than 1 — impossible by definition.
Reading the thermometer in the liquid bulk of a distillation flask, not at the side-arm.
Using decantation for finely divided precipitates without centrifuging first.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Separation by Solvent Extraction

When a solute distributes itself between two immiscible liquids, the partition coefficient K = C₁/C₂ (concentrations in the two solvents at equilibrium) determines extraction efficiency. Repeated extractions with small volumes of the extracting solvent recover more solute than one large extraction — the principle behind separating organic products from aqueous reaction mixtures using a separating funnel.

Chromatography in Detail

In paper chromatography, the stationary phase is water trapped in cellulose; the mobile phase is a developing solvent (e.g. ethanol/water mixture). Components separate based on relative solubilities. Thin-layer chromatography (TLC) uses a silica-coated plate and offers sharper resolution and faster runs. Column chromatography scales the same partition principle for preparative work — the eluent carrying the most weakly adsorbed component exits first.

Worked Example — Rf Calculation

A dye travels 3.6 cm while the solvent front travels 5.0 cm on a chromatography strip. Rf = 3.6 ÷ 5.0 = 0.72 (dimensionless, always 0 ≤ Rf ≤ 1). If a second dye travels 2.5 cm under identical conditions, Rf = 0.50, so the two dyes are distinct and the first is less polar (more soluble in the mobile phase).

Distillation Efficiency

A fractionating column’s efficiency depends on the number of theoretical plates — each plate represents one condensation–evaporation cycle. Longer columns and fine packing (glass helices, Raschig rings) increase plate count and sharpen separation of liquids whose boiling points differ by as little as 1–2 °C, as in crude-oil refining into petroleum fractions.

Connections to Adjacent Topics

Pure substances vs mixtures — separation is the operational test of purity.
Atomic structure and bonding — intermolecular forces (H-bonding in ethanol/water) explain why fractional, not simple, distillation is needed.
Water treatment — sedimentation, filtration, and chlorination apply these techniques at industrial scale.

Common Mistakes to Avoid

Students often pick simple distillation for “ethanol and water” because both are liquids — the column is what makes fractional distillation work for close-boiling mixtures.

Assuming all solids in solution require evaporation — crystallisation gives purer products.
Believing chromatography separates elements — it separates components of a mixture, not atoms in a compound.
Misreading the meniscus when measuring chromatographic distances, producing Rf errors.

Practice Prompts

A mixture contains iodine (sublimable) and sodium chloride (non-sublimable). State the technique, the apparatus change observed, and the purity of the product.
A student records a solute distance of 4.2 cm and solvent front of 3.5 cm. Identify the error and recalculate the correct Rf using the right denominator.

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Separation Techniques