Mixtures — AQA GCSE Chemistry
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Mixtures explained
A mixture contains two or more substances not chemically joined, so it can be separated by physical processes that use differences in properties.
Read the full explanation
Filtration separates an insoluble solid from a liquid using filter paper and a funnel. Crystallisation recovers a dissolved solid by evaporating some solvent then leaving crystals to form. Simple distillation separates a solvent from a solution by evaporating and condensing it. Fractional distillation separates miscible liquids with different boiling points, for example ethanol and water, using a fractionating column. Chromatography separates dissolved substances by how far they travel up a stationary phase in a solvent.
Your focus
- Name the physical process suitable for separating a given mixture.
- Explain how the chosen process separates the substances using a property difference.
- Describe the apparatus or key steps for filtration, crystallisation, simple distillation, fractional distillation or chromatography.
Mixtures exam tips
Marking Points
- Filtration separates an insoluble solid from a liquid because the solid particles are too large to pass through the filter paper pores.
- Crystallisation recovers a dissolved solid by gently evaporating some solvent to reach a saturated solution, then cooling or leaving it so crystals form.
- Simple distillation separates a solvent from a solution by heating to evaporate the solvent, then condensing the vapour back to a liquid.
- Fractional distillation separates miscible liquids with different boiling points, using a fractionating column so the liquid with the lower boiling point distils first.
- Chromatography separates dissolved substances because they have different solubilities in the solvent and different attractions to the stationary phase, so they travel different distances.
Examiner Tips
- 💡Name the process first, then state the property difference it relies on, such as solubility or boiling point.
- 💡For a mixture question, identify the states and whether the solid is dissolved before choosing a technique.
- 💡Describe the key apparatus or step, such as filter paper and funnel, a fractionating column, or a locating agent, to show understanding.
Common Mistakes
- Thinking filtration can separate a dissolved solid from a solution; the correction is that filtration only removes an insoluble solid, so crystallisation or evaporation is needed for a dissolved solid.
- Using simple distillation to separate two miscible liquids with close boiling points; the correction is that fractional distillation is used because it gives better separation.
- Believing chromatography separates substances by boiling point; the correction is that it separates by differing solubility and attraction to the stationary phase.