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LearnCambridge50702026–2028Paper 2 · TheoryExperimental techniques and chemical analysis

Paper 2 · Module

Experimental techniques and chemical analysis

3 subtopics≈ 110 min in total

Start the module: Separation and purification Open graphing calculator

Separation and purification
Choose apparatus for precision: a burette or pipette for accurate volumes, a gas syringe for gas volumes, a balance for mass. Choose a separation method from the properties of the mixture: filtration removes an insoluble solid from a liquid; crystallisation gets a soluble solid from its solution; simple distillation gets a solvent from a solution; fractional distillation separates miscible liquids with different boiling points; chromatography separates dissolved substances such as dyes.
≈ 35 min
Chromatography
Paper chromatography separates soluble substances. Spots are placed on a pencil baseline above the solvent level; the solvent moves up the paper, carrying each substance a different distance depending on its solubility and attraction to the paper. A pure substance gives one spot. Substances are identified by comparing with known samples, or by the $R_f$ value $=\frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$ . Colourless substances need a locating agent.
≈ 30 min
Tests for ions and gases
Flame tests: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green. Sodium hydroxide: $\mathrm{Cu^{2+}}$ blue ppt, $\mathrm{Fe^{2+}}$ green ppt, $\mathrm{Fe^{3+}}$ red-brown ppt, $\mathrm{Al^{3+}}$ / $\mathrm{Zn^{2+}}$ white ppt (dissolves in excess), $\mathrm{NH_4^+}$ ammonia on warming. Anions: carbonate + acid → $\mathrm{CO_2}$ ; halides + acidified silver nitrate → white (Cl⁻), cream (Br⁻), yellow (I⁻); sulfate + acidified barium chloride/nitrate → white ppt. Gases: H₂ squeaky pop; O₂ relights glowing splint; CO₂ limewater milky; Cl₂ bleaches damp litmus; NH₃ turns damp red litmus blue.
≈ 45 min