Mixtures and Separation Techniques — AQA GCSE Chemistry
Mixtures can be separated by physical methods that do not involve chemical reactions.
What is a mixture?
A mixture consists of two or more substances not chemically combined. The substances keep their own properties, so they can be separated physically without chemical reactions.
Separation techniques
| Technique | Separates | How it works |
|---|---|---|
| Filtration | insoluble solid from a liquid | the solid (residue) is trapped by the filter paper; the liquid (filtrate) passes through |
| Crystallisation | a soluble solid from its solution | the solution is evaporated until crystals form |
| Simple distillation | a solvent from a solution | the liquid evaporates, then condenses; dissolved solid is left behind |
| Fractional distillation | liquids with different boiling points | each liquid evaporates and condenses at its own boiling point |
| Chromatography | substances of different solubility | substances move different distances in a solvent |
Choosing the right method
- To get the liquid from a salt solution → simple distillation.
- To get the solid (salt) from a solution → crystallisation or evaporation.
- To separate two miscible liquids (e.g. ethanol and water) → fractional distillation.
- To separate coloured substances (e.g. dyes) → chromatography.
Exam tips
- Match the technique to what is being separated.
- Filtration = insoluble solid + liquid; crystallisation = soluble solid from solution.
- Fractional distillation separates liquids by boiling point.
- None of these are chemical reactions — they are physical separations.