GCSE Chemistry · Edexcel 1CH0 · Topic 2B
Methods of separating and purifying substances
Key points
- A pure substance has a sharp melting point; an impure one melts over a range of temperatures, usually lower.
- Filtration separates an insoluble solid from a liquid. Crystallisation gets a soluble solid from a solution by evaporating some water then cooling.
- Simple distillation separates a liquid from a solution; fractional distillation separates liquids with different boiling points using a fractionating column.
- Paper chromatography: spots on a pencil line, solvent moves up the paper. . A pure substance gives one spot.
- Core practical: investigate inks using simple distillation and chromatography.
- Potable water: from fresh water by sedimentation, filtration and chlorination; from sea water by distillation (desalination), which needs a lot of energy. Water for chemical analysis must be pure (distilled or deionised).
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Most substances made in a lab or found in nature are mixtures. These notes cover how to test purity, the main separation techniques, chromatography with Rf values, the core practical on inks, and how drinking water is produced.
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