Mixtures — AQA GCSE Chemistry
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Mixtures explained
Mixtures covers 5 specification points, including A mixture consists of two or more elements or… and The chemical properties of each substance in….
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Open a point below for its explanation and revision guidance.
A mixture consists of two or more elements or compounds not chemically combined together.
A mixture contains two or more substances that are together in the same space but are not joined by chemical bonds. The substances keep their own identities, so a mixture can usually be separated by physical methods such as filtration, crystallisation, simple distillation, fractional distillation or chromatography. Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and other gases; seawater is a mixture of water and dissolved salts. In a mixture the components can be present in any proportion, unlike a compound which has a fixed composition by mass. No new chemical bonds form between the components, so the mixture does not have a single fixed formula.
The chemical properties of each substance in the mixture are unchanged.
When substances are mixed without chemical reaction, each substance keeps its own chemical properties. For example, the oxygen in air still supports combustion and the nitrogen still behaves as a relatively unreactive gas; the salt in seawater still gives the characteristic chloride test. This is because no new chemical bonds form between the components, so the particles remain the same and their reactions are unchanged. Physical properties such as melting point and boiling point may change because the components affect each other physically; for instance, seawater boils at a higher temperature than pure water. This principle allows mixtures to be separated and the components recovered with their original chemical identities.
Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography.
A mixture contains two or more substances not chemically joined, so it can be separated by physical processes that use differences in properties. 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.
These physical processes do not involve chemical reactions and no new substances are made.
Separation techniques such as filtration, crystallisation, simple distillation, fractional distillation and chromatography are physical processes. They separate substances that are already present in a mixture without changing their chemical identities, so no new substances are made. For example, filtering a mixture of sand and salt solution gives sand and salt solution, and evaporating the water from salt solution leaves the same sodium chloride. In contrast, a chemical reaction such as burning magnesium produces magnesium oxide, a new substance with different properties. Recognising this difference helps you choose a separation method and explain why it works.
Students should be able to: describe, explain and give examples of the specified processes of separation suggest suitable separation and purification techniques for mixtures when given appropriate information.
You need to describe how each separation process works, explain why it separates the components, and give examples. Filtration separates an insoluble solid from a liquid, for example sand from salt solution. Crystallisation recovers a dissolved solid, for example copper sulfate crystals from solution. Simple distillation separates a solvent from a solution, for example pure water from sea water. Fractional distillation separates miscible liquids with different boiling points, for example ethanol from water. Chromatography separates dissolved substances, for example dyes in ink. When given information about a mixture, choose the technique that matches the components and their properties.
Your focus
- Define a mixture as two or more elements or compounds not chemically combined.
- Compare mixtures with compounds in terms of composition, bonding and separation.
- Identify the components of common mixtures and suggest suitable separation techniques.
Show all 15 objectives
- State that the chemical properties of each substance in a mixture are unchanged.
- Explain why chemical properties are retained in a mixture in terms of unchanged particles and absence of chemical bonding.
- Use examples to distinguish unchanged chemical properties from physical properties that may vary in a mixture.
- 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.
- State that separation processes are physical and do not make new substances.
- Explain why the substances obtained from a separation are chemically unchanged.
- Distinguish a physical separation from a chemical reaction using a suitable example.
- Describe the specified separation processes and give a named example of each.
- Explain how each process separates components using differences in physical properties.
- Select and justify a suitable separation or purification technique from information about a mixture.
Mixtures exam tips
Marking Points
- Defines a mixture as two or more elements or compounds that are not chemically combined together.
- Distinguishes mixtures from compounds: mixtures have variable composition, no chemical bonds between components and can be separated by physical means.
- Gives examples of mixtures such as air, seawater, alloys and crude oil, and identifies the elements or compounds present.
- Explains that the components of a mixture retain their individual identities and can be separated using techniques matched to their physical properties.
- Recognises that no new substance is formed when a mixture is made, so the process is not a chemical reaction.
- Uses particle diagrams or descriptions to show different types of particles mixed but not bonded together.
- States that each substance in a mixture keeps its own chemical properties because no chemical bonds form between the components.
- Explains that the particles of each substance remain unchanged, so their reactions and characteristic tests are the same as when the substance is pure.
- Contrasts this with a compound, where the elements have reacted and the compound has different chemical properties from its constituent elements.
- Uses examples such as air, where oxygen still supports combustion, or a salt solution, where the salt still gives a positive flame test or precipitation reaction.
- Distinguishes chemical properties, which are unchanged, from physical properties such as boiling point, which may change in a mixture.
- Links the unchanged chemical properties to the possibility of separating a mixture and recovering the original substances.
- 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.
- Physical separation processes do not involve chemical reactions, so the substances keep their original chemical identities.
- No new substances are made during filtration, crystallisation, simple distillation, fractional distillation or chromatography.
- The separated substances can be recombined to give the original mixture because no chemical change has occurred.
- A chemical reaction, such as burning magnesium to form magnesium oxide, does make a new substance and is not a separation process.
- Changes of state during distillation or crystallisation are physical changes, not chemical reactions.
- Describe filtration as separating an insoluble solid from a liquid using filter paper and a funnel, and explain that the solid particles are too large to pass through the paper.
- Describe crystallisation as evaporating some solvent to form a saturated solution, then cooling or leaving it so crystals of the dissolved solid form.
- Describe simple distillation as heating a solution to evaporate the solvent, then condensing the vapour to collect the pure solvent.
- Describe fractional distillation as separating miscible liquids with different boiling points using a fractionating column, with the lower-boiling liquid distilling first.
- Describe chromatography as separating dissolved substances using a stationary phase and a mobile solvent, where components travel different distances.
- Suggest a suitable technique by matching the mixture information to the property difference, such as solubility, boiling point or particle size.
Examiner Tips
- 💡When asked to classify a substance, look for a fixed formula and chemical bonding to identify a compound, and variable composition with no bonding to identify a mixture.
- 💡Name the components of a named mixture and state the physical property that allows separation, such as boiling point or solubility.
- 💡Use particle diagrams to support your answer; show different particles close together but not joined.
- 💡Use a named example and state the unchanged chemical property, such as oxygen in air supporting combustion.
- 💡Contrast the mixture with a compound formed from the same elements to show the difference in chemical properties.
- 💡Avoid saying properties are always identical; specify that chemical properties are unchanged while physical properties may vary.
- 💡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.
- 💡Use the phrase no new substances are made when explaining why a separation is physical.
- 💡Contrast a separation with a named chemical reaction to show the difference clearly.
- 💡Check whether the question asks about a mixture or a compound before describing the process.
- 💡Read the mixture information carefully and name the property difference before choosing a technique.
- 💡Give a named example for each process, such as sand and salt solution for filtration or ink dyes for chromatography.
- 💡Link each step of your method to why it separates the components, rather than listing apparatus only.
Common Mistakes
- Describing a mixture as a compound because the substances are together; the correction is to state that a compound involves chemical bonding and fixed composition, whereas a mixture does not.
- Assuming a mixture always has a fixed ratio of components; the correction is to note that mixtures can be made in any proportion.
- Thinking that a mixture cannot be separated because it looks uniform; the correction is to recall that physical separation methods can separate even apparently uniform mixtures such as salt solution.
- Claiming that a mixture has the average properties of its components; the correction is to state that each component keeps its own chemical properties, although measured physical properties may vary with composition.
- Confusing a mixture with a compound and saying new chemical properties appear; the correction is to explain that new properties arise only when elements react chemically to form a compound.
- Assuming that because a mixture looks different from its components, its chemical behaviour has changed; the correction is to test or recall that the components still react in their characteristic ways.
- 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.
- Saying distillation makes a new substance because the liquid changes to a gas; the correction is that changes of state are physical, so the substance is unchanged.
- Confusing separation with a chemical reaction; the correction is that separation only divides a mixture, whereas a reaction forms new products.
- Thinking crystallisation changes the dissolved solid into a different compound; the correction is that the solid recrystallises as the same substance.
- Choosing filtration for a dissolved solid; the correction is to use crystallisation or evaporation because the solid is not insoluble.
- Using simple distillation for two miscible liquids with similar boiling points; the correction is to use fractional distillation for better separation.
- Describing chromatography without mentioning the stationary phase and mobile phase; the correction is to state that the solvent moves through the stationary phase and carries components different distances.