CC2.2 — Separation techniques (Edexcel 1SC0)

Filtration

Separates an insoluble solid from a liquid. The mixture is poured through filter paper; the solid remains (residue); the liquid passes through (filtrate).

Crystallisation

Separates a soluble solid from a solution. Solution heated → solvent evaporates → concentration increases → solid crystallises on cooling.

Simple distillation

Separates a solvent from a solution (e.g. water from salt water). Solution heated → solvent evaporates → vapour condenses in condenser → pure liquid collected.

Fractional distillation

Separates a mixture of liquids with different boiling points (e.g. crude oil fractions; ethanol from water). A fractionating column maintains a temperature gradient — liquid with lowest boiling point distils off first.

Paper chromatography

Separates dissolved substances based on different solubilities in the solvent vs different affinities for the paper.

Rf value: $$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$

Rf is constant for a substance in a given solvent — can be used to identify unknowns by comparing with standards.

Gas chromatography (GC)

Separates components of a volatile mixture. Components elute at different times (retention times). Area under each peak ∝ amount of substance.