Mixtures and separation techniques

What you need to know

• A mixture is two or more substances not chemically combined — each substance keeps its own chemical properties.
• Separation techniques are all physical processes: no chemical reactions occur and no new substances are formed.
• Filtration separates an insoluble solid from a liquid; crystallisation obtains a soluble solid from a solution; simple distillation separates a solvent from a solution; fractional distillation separates miscible liquids by different boiling points; chromatography separates substances by different solubilities.
• When choosing a technique, identify the key physical difference between the substances — that difference tells you which method to use.

The big picture

A mixture contains two or more substances not chemically combined, so each substance keeps its own chemical properties. Because no chemical bonds are formed or broken, mixtures can be separated using physical processes alone. The technique you choose depends on the physical properties of the substances involved — such as particle size, solubility, or boiling point. Mastering which technique to use, and why, is exactly what AQA examiners test.

The five separation techniques

Each technique exploits one key physical difference between substances — find that difference and you've found your method.

1. 1

   Filtration

   Works because the solid is INSOLUBLE — its particles are too large to pass through the filter paper, so the liquid (filtrate) drains through while the solid remains.

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2. 2

   Crystallisation

   Works because the solid is SOLUBLE — gently evaporating the solvent makes the solution saturated so pure crystals form; if the solid is less soluble when cool, cooling the saturated solution helps more crystals deposit.

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3. 3

   Simple distillation

   Works because the solvent has a lower boiling point than the dissolved solid — heating vaporises the solvent, which is then condensed and collected as a pure liquid.

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4. 4

   Fractional distillation

   Works because miscible liquids have DIFFERENT boiling points — the fractionating column allows each liquid to vaporise and condense at a different height, separating them.

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5. 5

   Chromatography

   Works because substances have DIFFERENT SOLUBILITIES in the solvent — more soluble substances are carried further by the moving solvent, so the mixture spreads into separate spots.

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Stop and think

Commit to an answer before you reveal — this is exactly the kind of question AQA asks.

A student has a mixture of two miscible liquids: ethanol (boiling point 78 °C) and water (boiling point 100 °C). Which technique should they use to separate them, and what physical property does it exploit?

Tap any cell to reveal it. Tap a column header to read one property down every item.

Memory hook

Remember the five techniques with 'Five Clever Scientists Find Chromatography': Filtration, Crystallisation, Simple distillation, Fractional distillation, Chromatography — match each to its KEY physical difference.

Check yourself

Without looking — name the technique you would use to separate ethanol from water, and state the physical property that makes it work.