Mixtures — AQA GCSE Combined Science
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Mixtures explained
A mixture contains two or more substances not joined by chemical bonds.
Read the full explanation
The substances can be elements, compounds or both, and they keep their own chemical properties. For example, air is a mixture of nitrogen, oxygen, argon and carbon dioxide. Each gas behaves as it would on its own; oxygen still supports combustion, while nitrogen remains largely unreactive. The components of a mixture can be separated by physical methods such as filtration or distillation because no chemical bonds are broken. In contrast, a compound has a fixed composition and its elements are chemically combined, so its properties differ from those of the original elements. This distinction is assessed by asking you to identify mixtures and describe separation methods.
Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical reactions and no new substances are made.
A mixture contains two or more substances not chemically joined, so each keeps its own properties and can be separated by physical processes. Filtration separates an insoluble solid from a liquid, for example sand from salty water, using filter paper and a funnel. Crystallisation recovers a dissolved solid by gently evaporating solvent until crystals form. Simple distillation separates a solvent from a solution, for example pure water from salty water, by boiling then condensing the vapour. Fractional distillation separates miscible liquids with different boiling points, such as ethanol and water, using a fractionating column. Chromatography separates dissolved substances by how far they travel up a stationary phase, useful for identifying dyes. In every case no chemical reaction occurs and no new substances are made; the substances are only separated physically.
describe, explain and give examples of the specified processes of separation
A mixture contains two or more substances not chemically joined, which can be separated by physical means. Filtration separates an insoluble solid from a liquid: sand and water poured through filter paper leaves sand as residue and water as filtrate. Crystallisation recovers a dissolved solid: heat a salt solution so solvent evaporates, then cool to form crystals. Simple distillation separates a solvent from a solution, e.g. pure water from seawater: heat the solution so the solvent boils, condense the vapour and collect the distillate. Fractional distillation separates miscible liquids with different boiling points, like ethanol and water, using a fractionating column. Chromatography separates dissolved substances, like ink dyes, by their different solubilities in a solvent moving up paper.
suggest suitable separation and purification techniques for mixtures when given appropriate information.
When a question supplies information about a mixture, you must choose a technique that matches the physical properties given. If the mixture is a solid and a liquid and the solid is insoluble, filtration separates them. If the solid is dissolved, evaporating the solvent or crystallising the solution recovers it. If a pure liquid is wanted from a solution, simple distillation is suitable because the solvent boils and condenses while the solute stays behind. If two miscible liquids have different boiling points, fractional distillation separates them. If substances are dissolved and coloured or otherwise detectable, chromatography can separate and identify them. Purity can be checked by melting point or boiling point: a pure substance melts or boils sharply at one temperature, while an impure sample melts or boils over a range.
Your focus
- Define a mixture as two or more elements or compounds not chemically combined.
- Explain why each substance in a mixture keeps its chemical properties.
- Select a suitable physical separation method for a given mixture.
Show all 12 objectives
- Identify filtration, crystallisation, simple distillation, fractional distillation and chromatography as physical separation processes.
- Describe how each named process separates a given mixture, referring to a property such as solubility or boiling point.
- Explain why these separation processes are physical and do not produce new substances.
- Describe the apparatus and steps for filtration, crystallisation, simple distillation, fractional distillation and chromatography.
- Explain each separation in terms of a physical property such as solubility, boiling point or state.
- Give a suitable named example of a mixture separated by each specified process.
- Choose a suitable separation technique from information about solubility, state and boiling point.
- Explain why the chosen technique works for the mixture described.
- Describe how purity can be assessed using melting point or boiling point data.
Mixtures exam tips
Marking Points
- A mixture contains two or more elements or compounds that are not chemically combined.
- The substances in a mixture can be present in any proportion, unlike a compound which has a fixed composition.
- Each substance in a mixture retains its own chemical properties.
- Mixtures can be separated by physical processes because no chemical bonds are broken.
- Examples of mixtures include air, seawater, alloys and crude oil.
- State that a mixture contains two or more substances that are not chemically combined, so each substance keeps its own properties.
- Describe filtration as separating an insoluble solid from a liquid using filter paper, with the solid left as residue and the liquid passing through as filtrate.
- Describe crystallisation as evaporating solvent from a solution to leave the dissolved solid as crystals, without chemically changing that solid.
- Describe simple distillation as separating a solvent from a solution by evaporation followed by condensation, giving a pure solvent.
- Describe fractional distillation as separating miscible liquids with different boiling points using a fractionating column, collecting fractions at different temperatures.
- Describe chromatography as separating dissolved substances using a stationary phase and a mobile phase, where different substances travel different distances.
- Explain that these processes are physical because no new substances are made and no chemical bonds are broken or formed.
- State that a mixture is not chemically combined, so its components can be separated by physical processes without chemical reactions.
- Describe filtration as separating an insoluble solid from a liquid using filter paper and a funnel, naming the solid as the residue and the liquid as the filtrate.
- Describe crystallisation as evaporating some solvent from a solution and then cooling or leaving it so crystals of the dissolved solid form.
- Describe simple distillation as evaporating a solvent by heating, condensing the vapour, and collecting the distillate while the dissolved solute remains.
- Describe fractional distillation as separating miscible liquids by repeated evaporation and condensation in a fractionating column, using differences in boiling point.
- Describe chromatography as separating dissolved substances using a stationary phase and a moving solvent, relying on different solubilities and therefore different distances travelled.
- Give a named example for each process, such as sand and water for filtration, copper sulfate solution for crystallisation, pure water from seawater for simple distillation, ethanol and water for fractional distillation, and ink dyes for chromatography.
- Select filtration when the mixture contains an insoluble solid in a liquid, and identify the solid as residue and the liquid as filtrate.
- Select crystallisation or evaporation when a dissolved solid must be recovered from its solution, choosing crystallisation when crystals are wanted.
- Select simple distillation when a solvent must be separated from a solution and collected as a pure liquid.
- Select fractional distillation when two or more miscible liquids have different boiling points and both components are needed.
- Select chromatography when dissolved substances need separating for identification, for example dyes or sugars in a mixture.
- Use melting point or boiling point data to judge purity, recognising that a sharp value indicates a pure substance and a range indicates impurities.
- Justify the choice by referring to the property given, such as insoluble, dissolved, different boiling points or different solubilities.
Examiner Tips
- 💡When asked to distinguish a mixture from a compound, refer to chemical combination and fixed composition.
- 💡Use a named separation technique to support your answer, such as filtration for a solid–liquid mixture or distillation for a solvent–solute mixture.
- 💡Name the process first, then state the property it depends on, such as solubility for filtration or boiling point for distillation.
- 💡Use the terms residue and filtrate correctly when describing filtration, and solvent and solution when describing crystallisation.
- 💡When asked whether a change is physical or chemical, justify your answer by stating that no new substances are made.
- 💡Link each named process to a specific mixture in the question rather than describing all five processes in general terms.
- 💡Use the correct vocabulary precisely: residue, filtrate, distillate, solvent front and stationary phase all carry credit in written answers.
- 💡When asked to explain, give the reason for separation, such as different boiling points or different solubilities, not just the practical steps.
- 💡For a six-mark comparison, structure the answer around one process at a time and include a named example for each.
- 💡Underline the key property in the question, such as insoluble, dissolved or different boiling points, before choosing a technique.
- 💡Name the technique and then justify it in one sentence using the property, for example filtration works because the solid is insoluble.
- 💡If the question asks for purification as well as separation, mention how purity could be checked, such as by melting point.
- 💡Avoid listing several techniques without choosing: a clear decision with a reason scores better than a list of possibilities.
Common Mistakes
- Thinking that a mixture has fixed proportions: for example, saying that air always contains exactly 20% oxygen. Correction: the proportions in a mixture can vary, while a compound has a fixed ratio of elements.
- Confusing mixtures with compounds: for example, calling seawater a compound because it contains sodium and chloride ions. Correction: seawater is a mixture of water and dissolved salts; the ions are not chemically bonded to water molecules.
- Believing that chemical properties change when substances are mixed: for example, saying that oxygen in air no longer supports combustion. Correction: each substance keeps its chemical properties in a mixture.
- Thinking filtration separates a dissolved solid from a solution; correct this by stating filtration only removes an insoluble solid, while crystallisation or distillation is needed for a dissolved solid.
- Believing distillation involves a chemical reaction because heating is used; correct this by explaining that boiling and condensing are physical changes of state only.
- Confusing simple and fractional distillation; correct this by noting simple distillation separates a solvent from a solution, while fractional distillation separates two or more miscible liquids with different boiling points.
- Confusing filtration with evaporation: filtration only works when the solid is insoluble, so a dissolved salt passes through the filter paper. Correction: use crystallisation or distillation to recover a dissolved solid.
- Thinking distillation destroys the solute: the solute is not lost but remains in the flask, so the method recovers the solvent rather than the solid. Correction: state that the distillate is the condensed solvent and the solute stays behind.
- Believing chromatography separates by colour alone: separation depends on how strongly each substance is attracted to the stationary and mobile phases. Correction: explain separation in terms of differing solubilities in the solvent.
- Writing that fractional distillation separates liquids by filtering them: it separates miscible liquids by boiling point differences. Correction: describe repeated evaporation and condensation in the column.
- Choosing filtration for a solution of salt in water: the dissolved salt passes through the filter paper with the water. Correction: use crystallisation or evaporation to recover the salt.
- Choosing simple distillation to separate ethanol and water completely: simple distillation gives only partial separation of miscible liquids. Correction: use fractional distillation with a fractionating column.
- Treating a boiling point range as evidence of purity: a range shows impurities are present. Correction: state that a pure substance has a sharp, single melting or boiling point.
- Ignoring the information given and naming a favourite technique: the choice must follow from the stated properties. Correction: quote the relevant property before naming the method.