Separation and Purification Techniques

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Separation and purification techniques are physical methods that isolate components of a mixture by exploiting differences in solubility, boiling point, particle size, density, or magnetism. NECO SSCE tests these yearly, usually as a 3–10 mark structured question in Paper II or as an objective in Paper I.

Filtration — separates an insoluble solid (residue) from a liquid (filtrate) using filter paper and a funnel.
Evaporation vs Crystallisation — evaporation drives off the solvent completely; crystallisation cools a hot saturated solution to grow pure crystals.
Distillation — simple distillation works when boiling points differ by >70 °C; fractional distillation uses a fractionating column when the gap is <25 °C.

The key chromatography formula is R_f = distance travelled by solute ÷ distance travelled by solvent front; values lie between 0 and 1.

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Choosing the Right Technique

Each technique exploits one physical property. Selecting the wrong one costs marks in NECO structured questions, so anchor your choice to the property that distinguishes the components.

Technique	Property exploited	Typical mixture
Filtration	Particle size	Sand + water
Decantation	Density / settling	Water + settled mud
Evaporation	Volatility of solvent	Salt solution
Crystallisation	Solubility vs temperature	Hot saturated sugar solution
Sublimation	Sublimable solid	NH₄Cl + NaCl mixture
Simple distillation	Boiling point (>70 °C gap)	Water + dissolved salt
Fractional distillation	Boiling point (<25 °C gap)	Ethanol + water
Separating funnel	Immiscibility + density	Oil + water
Chromatography	Differential solubility/adsorption	Dye inks
Magnetisation	Magnetism	Iron filings + sulphur

R_f and Identification

The chromatogram is the developed paper; each spot’s R_f is constant for a given solvent at a given temperature. Comparing R_f values against known standards identifies unknown solutes.

Common NECO Traps

Forgetting anti-bumping granules in the distillation flask causes uneven boiling.
Using a naked flame with a separating funnel or with ethanol is unsafe — use a water bath.
Reporting R_f > 1 is impossible because the solute cannot outrun the solvent front.

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Mechanisms Behind the Techniques

Crystallisation works because solubility of most solids decreases as temperature falls; slow cooling yields large, pure crystals because impurities stay in the mother liquor. Washing with a cold solvent removes surface impurities without re-dissolving much product. Sublimation requires a solid whose vapour pressure reaches 1 atm below its melting point — that is why NH₄Cl, iodine, naphthalene, and camphor sublime, but NaCl and sugar do not.

Fractional distillation is more efficient than simple distillation because the fractionating column provides repeated condensation–evaporation cycles on the glass beads, enriching the vapour in the more volatile component with each cycle.

Worked Calculation

A student chromatographs three inks A, B, C alongside a standard. Solvent front moves 8.0 cm; A moves 5.6 cm, B moves 2.4 cm, C moves 7.8 cm.

R_f(A) = 5.6 / 8.0 = 0.70
R_f(B) = 2.4 / 8.0 = 0.30
R_f(C) = 7.8 / 8.0 = 0.98

Comparing to standards: A matches compound X (R_f = 0.70), B matches Y (0.30), and C is a new component requiring further analysis — its R_f approaching 1 means the solvent carries it almost as readily as the front.

Adjacent Topics and Strategy

This topic links directly to salt preparation (filtration + crystallisation in Acids, Bases and Salts), the fractional distillation of liquid air (Air and Water), and chlorine/bromine separation (Periodic Table). In NECO Paper II it usually appears as Question 4 or 5 worth 5–10 marks; spend about 4 minutes sketching a labelled diagram and naming the apparatus — Liebig condenser, round-bottom flask, fractionating column, or separating funnel.

Practice Prompts

Describe, with a labelled diagram, how pure water is obtained from a mixture of sand, salt, and water. (8 marks)
A mixture contains naphthalene and sodium chloride. State the technique, the principle, and the procedure to separate them. (5 marks)

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Separation and Purification Techniques