Separation Techniques

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Separation techniques are physical methods that isolate the components of a mixture without changing their chemical identity. The technique chosen depends on which physical property differs most sharply between the components: particle size, density, boiling point, solubility, magnetic behaviour, or tendency to sublime.

Mnemonic — “Pick the property, pick the process”: insoluble solid in liquid → filtration; two liquids of different densities → separating funnel; soluble salt from solution → crystallisation; miscible liquids with close boiling points → fractional distillation; volatile solid from non-volatile → sublimation; coloured solutes in solution → paper chromatography.

Property exploited	Method
Particle size	Filtration
Density	Decantation, separating funnel
Boiling point	Simple/fractional distillation
Solubility	Crystallisation, solvent extraction
Adsorption (paper)	Chromatography
Magnetism	Magnetic separation
Sublimation behaviour	Sublimation

R_f = (distance moved by solute) ÷ (distance moved by solvent front) — always a value between 0 and 1.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Definitions you must know cold

A mixture contains two or more substances mixed in no fixed ratio; its constituents keep their individual properties and can be separated physically. A pure substance has a fixed composition and sharp, characteristic physical constants. The liquid that passes through filter paper is the filtrate; the solid trapped is the residue. The condensed vapour from distillation is the distillate, while the substance dissolved in a liquid is the solute and the dissolving medium the solvent.

How each technique works

Filtration uses porous filter paper to catch an insoluble solid while the liquid passes through under gravity. Decantation and sedimentation simply let gravity pull a denser solid to the bottom of a beaker, after which the clear liquid is poured off. Centrifugation mimics gravity thousands of times over, packing dense solids at the bottom of a spinning tube in seconds.

Trap: many candidates reverse filtrate and residue. Remember — the filtrate flows through; the residue stays behind.

Evaporation boils off a volatile solvent to recover a dissolved solute. Crystallisation is gentler: a hot saturated solution is cooled, and pure solid crystals grow while soluble impurities stay dissolved in the mother liquor.

Simple distillation (Liebig condenser) separates a volatile solvent from a non-volatile solute — e.g. pure water from sea water. Fractional distillation adds a vertical fractionating column packed with glass beads; vapour repeatedly condenses and re-evaporates on the surfaces, giving many “theoretical plates” of separation that simple distillation cannot. This is how crude oil is split into petrol, kerosene and diesel, and how ethanol is concentrated above its ~95.6% azeotrope with water.

Trap: the all-important piece in fractional distillation is the fractionating column, not the Liebig condenser (which both share).

A separating funnel drains immiscible liquids in order of density — the denser liquid is run out through the bottom tap first, then the lighter one is poured from the top. It will never separate miscible liquids such as ethanol and water.

Sublimation converts a solid straight to vapour on heating (iodine, ammonium chloride, naphthalene); vapour then deposits as crystals on a cold finger or watch-glass. Magnetic separation pulls out iron filings with a bar magnet. Solvent extraction shakes a solute between two immiscible liquids and collects it in whichever it dissolves better.

Paper chromatography uses capillary flow of a mobile phase (solvent) up a paper stationary phase. Different solutes travel different distances; their R_f values identify them under fixed conditions.

Typical WASSCE question patterns

Matching a named mixture to its correct method (salt–sand–water, ethanol–water, iodine–sand, oil–water).
Labelled diagrams of simple and fractional distillation apparatus (1 mark per correct label).
Calculating R_f from given distances (typically a 2-mark numerical).
Explaining why a separating funnel cannot separate miscible liquids.

The highest-yield revision activity is memorising the property-to-method table and practising R_f calculations.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge cases and the physics behind the choice

The textbook says “choose by property”, but WASSCE candidates are tested on the exceptions. Three land in almost every paper:

Ethanol–water cannot be separated by simple distillation because they form an azeotrope at ~95.6 % ethanol / 4.4 % water, boiling at 78.2 °C — below either pure boiling point. Only fractional distillation (with a side-draw or a drying agent such as anhydrous CaO) pushes ethanol past 95.6 %.
Heat-sensitive solutes (sugar, benzoic acid) cannot be recovered by evaporation to dryness — they char. Use crystallisation by cooling instead, then filter and dry between filter papers.
Miscible liquids (ethanol + water, propanone + water) cannot be separated by a separating funnel. The funnel only works because the two liquids form a visible meniscus; miscible liquids form a single phase.

Worked micro-example — R_f calculation

A solute spot moves 4.2 cm up a chromatography strip, while the solvent front moves 6.0 cm.
R_f = 4.2 ÷ 6.0 = 0.70 (dimensionless, between 0 and 1). Comparing with standards run on the same paper at the same temperature identifies the spot.

Worked micro-example — % yield of crystals

5.0 g of copper(II) sulphate is dissolved in 20 cm³ of hot water, then cooled. The dried crystals recovered weigh 4.2 g.
% yield = (4.2 ÷ 5.0) × 100 = 84 %.

Exam strategy (3% of total marks ≈ 3 MCQs + part of an essay)

The weightage is small, so do not over-invest time. Master the matching table and you already answer 80 % of questions. In objective items, watch for distractors (e.g. “Liebig condenser” as the defining apparatus of fractional distillation) — examiners love that one. For essays, always state the property being exploited first, then the apparatus, then the procedure.

Practice prompts

A mixture contains iodine, sand and iron filings. Outline, in correct order, the three separation techniques you would use and the property each exploits.
A student claims a separating funnel can purify ethanol contaminated with water. Explain, using the concept of miscibility, why this is impossible and name the technique that will work above 95 % ethanol.

Continue your study

View this topic in your WAEC WASSCE roadmap — see where “Separation Techniques” fits in your personalised plan
Build a quick revision plan — 1-day sprint covering highest-weight topics
WAEC WASSCE exam overview — pattern, eligibility, and syllabus
All Chemistry notes — browse sibling topics in this subject

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Separation Techniques