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Detailed notes on Experimental Techniques for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Choosing the right purification method depends on the nature of the mixture. Cambridge tests: filtration (insoluble solid from liquid), crystallisation (dissolved solid), distillation (liquids), and fractional distillation (mixtures of liquids).
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Filtration removes insoluble solids. Crystallisation purifies soluble solids from solution.
Filtration:
Evaporation to dryness:
Crystallisation:
Simple distillation separates one liquid from a non-volatile solute. Fractional distillation separates miscible liquids with different boiling points.
Simple distillation:
Fractional distillation:
Key apparatus: round-bottom flask, fractionating column, Liebig condenser, conical flask as receiver, thermometer at top of fractionating column.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 6: 'Describe how you would obtain pure salt crystals from a rock salt sample' (4 marks — dissolve in water, filter, heat filtrate, cool slowly, filter crystals, dry). 'Name the process used to separate ethanol from water' (1 mark — fractional distillation). 'Explain why a fractionating column is used' (2 marks — different boiling points, repeated condensation and vaporisation). Paper 4 also asks which technique is best for specific separations.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C2); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on methods of purification , written exactly the way a tutor would explain them at the board.
Question
Describe how you would obtain pure, dry crystals of copper(II) sulfate from a solution of copper(II) sulfate.
Step-by-step solution
Step 1
Heat the solution gently to evaporate water until the solution is saturated (test: a glass rod dipped in the solution forms crystals on cooling).
Step 2
Remove from heat and allow the solution to cool slowly to room temperature. Blue copper sulfate crystals form as the solution becomes supersaturated.
Step 3
Filter the mixture through a filter funnel and filter paper; the crystals are collected on the paper and the filtrate passes through.
Step 4
Dry the crystals by pressing them between pieces of filter paper or leaving in a warm oven (below 100 °C to avoid losing water of crystallisation).
Answer
Evaporate to saturation → cool to crystallise → filter → dry between filter paper.
Examiner tip
Do not say 'boil to dryness' — this would decompose the crystals or give an impure powder. The word 'saturated' is important.
Question
Explain why simple distillation is used to obtain pure water from sea water rather than filtration.
Step-by-step solution
Step 1
In sea water, salt (NaCl) is dissolved in water; it exists as Na⁺ and Cl⁻ ions, not as solid particles.
Step 2
Filtration only separates insoluble solids from liquids; it cannot separate dissolved substances.
Step 3
Distillation heats the sea water until water evaporates; the vapour is then cooled and condensed in a separate vessel, leaving the dissolved salt behind.
Answer
Salt is dissolved (not suspended) in sea water, so filtration cannot remove it; distillation evaporates the water, which is then condensed as pure liquid, leaving the salt in the flask.
Question
Describe how fractional distillation is used to separate a mixture of ethanol (boiling point 78°C) and water (boiling point 100°C).
Step-by-step solution
Step 1
The mixture is heated in a flask. Ethanol has a lower boiling point (78 °C) and evaporates preferentially.
Step 2
The vapour rises through a fractionating column packed with glass beads. Higher boiling-point components (water vapour) condense and drain back; lower boiling-point ethanol vapour passes through.
Step 3
Ethanol vapour reaches the top of the column and enters the condenser, where it is cooled and collected as liquid ethanol.
Step 4
Once most ethanol has distilled off, the temperature rises to 100 °C and water distils over and is collected separately.
Answer
Heat the mixture; ethanol (b.p. 78 °C) evaporates first and rises through a fractionating column; condenses and is collected; then water (b.p. 100 °C) distils over.
Examiner tip
Award marks for: fractionating column, different boiling points, lower boiling point substance distils first, temperature at thermometer.
Question
A student has a mixture of sand and sodium chloride dissolved in water. Describe how to obtain a pure, dry sample of sodium chloride from this mixture.
Step-by-step solution
Step 1
Filter the mixture: sand is insoluble and is retained on the filter paper; the sodium chloride solution (filtrate) passes through.
Step 2
Evaporate the filtrate (sodium chloride solution) gently to saturation, then allow to cool to crystallise the sodium chloride.
Step 3
Filter off the crystals and dry them between filter paper.
Answer
Step 1: filter to remove sand. Step 2: evaporate and crystallise to obtain NaCl crystals. Step 3: filter and dry the crystals.
Definitions to memorise and the exact keywords mark schemes credit for methods of purification answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A separation technique that uses a filter (e.g. filter paper) to separate an insoluble solid from a liquid mixture; the solid is the residue, the liquid is the filtrate.
Obtaining a pure solid solute from a solution by evaporating the solvent until saturated, then cooling to allow crystals to form, followed by filtration and drying.
A technique that separates a liquid from a dissolved non-volatile solute, or liquids with very different boiling points, by heating, evaporation, and condensation.
A technique for separating miscible liquids with similar but different boiling points using a fractionating column; the component with the lowest boiling point distils first.
The solid material that remains on the filter paper after filtration.
The liquid that passes through the filter paper during filtration.
The traps other students keep falling into on methods of purification questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2022
Why it happens
Students think all water must be removed; they do not realise that boiling to dryness gives a powder or impure residue, not pure crystals.
How to avoid it
Evaporate until saturated (test with glass rod), then cool slowly; this gives proper crystals.
Why it happens
Students confuse dissolved (invisible in solution) with suspended (cloudy). Filtration cannot remove dissolved solutes.
How to avoid it
Filtration: insoluble solids only. Distillation/crystallisation: dissolved substances.
Why it happens
Students apply simple distillation to all liquid separations without considering boiling point differences.
How to avoid it
If boiling points differ by less than ~25 °C, use fractional distillation with a fractionating column.
The things students keep getting wrong in this sub-topic, answered.