Revision notes for AQA GCSE Chemistry Mixtures. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Mixtures
What you'll learn
- What a mixture is, and how it differs from a compound.
- Why separation methods are physical processes, not chemical reactions.
- How filtration, crystallisation, simple distillation, fractional distillation and chromatography work.
- How to choose a suitable separation technique for an unfamiliar mixture.
Before mixtures: pure substances, elements and compounds
An element is a substance made from only one type of atom. For example, copper is made from copper atoms only.
A compound is made when atoms of two or more different elements are chemically combined. For example, water is a compound made from hydrogen and oxygen chemically bonded together.
A pure substance contains only one substance. In GCSE Chemistry, this might be one element, like oxygen, or one compound, like sodium chloride.
What is a mixture?
A mixture contains more than one substance, but the substances are not chemically combined with each other.
Definition
Mixture
A mixture consists of two or more elements or compounds that are not chemically combined together. The chemical properties of each substance in the mixture are unchanged.
For example, air is a mixture of gases including nitrogen, oxygen, carbon dioxide and argon. The oxygen in air still behaves like oxygen — it has not reacted with nitrogen to become a new substance.
Key Idea
The big difference
In a compound, atoms are chemically bonded and a new substance is made. In a mixture, substances are just together in the same place, so they can usually be separated by physical methods.
Mixture or compound?
You can often tell the difference by asking: has a chemical reaction happened?
If no new substance has formed, it is probably a mixture. If a new substance with new properties has formed, it is probably a compound.
Example
Deciding whether iron and sulfur are a mixture or a compound
A student mixes iron filings with sulfur powder. Another student heats iron and sulfur strongly until they react to form iron sulfide.
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In the unheated sample, the iron filings and sulfur powder are only mixed together. The iron still behaves like iron, so it can be attracted by a magnet.
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Because the substances have kept their original properties and no new substance has formed, the unheated sample is a mixture.
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In the heated sample, iron and sulfur react together. The product, iron sulfide, does not behave like a simple mixture of iron and sulfur.
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Because a new substance with different properties has formed, iron sulfide is a compound, not a mixture.
Common Mistake
Mixtures are not chemically bonded
Do not say the substances in a mixture are “chemically joined” or “bonded together”. They are present together, but not chemically combined.
Physical processes and chemical reactions
A physical process changes the arrangement, state or position of substances, but does not make new substances.
A chemical reaction makes new substances. Chemical bonds are broken and made, and the products have different chemical properties from the reactants.
Definition
Physical separation
A physical separation process separates substances in a mixture without a chemical reaction. No new substances are made.
Separation works because the substances in a mixture have different physical properties, such as:
- whether they dissolve in a solvent
- particle size
- boiling point
- how strongly they are attracted to a surface
- how far they travel in a solvent
Use this decision tree when choosing between the main GCSE separation techniques.
Filtration
Filtration separates an insoluble solid from a liquid.
Definition
Insoluble
An insoluble substance does not dissolve in a particular solvent. For example, sand is insoluble in water.
In filtration, the mixture is poured through filter paper in a funnel.
- The insoluble solid is trapped by the filter paper. This is called the residue.
- The liquid passes through the filter paper. This is called the filtrate.
Filtration works because the solid particles are too large to pass through the tiny holes in the filter paper, but the liquid particles can pass through.
Examples of filtration
You could use filtration to separate:
- sand from water
- soil from water
- a precipitate from a reaction mixture
Tip
Filtration clue
If the question says the solid is insoluble, filtration is usually the first separation method to think of.
Crystallisation
Crystallisation separates a dissolved solid from a solution by forming crystals.
Definition
Solution, solute and solvent
A solution is made when a substance dissolves. The dissolved substance is the solute, and the liquid that does the dissolving is the solvent.
For example, in salt water:
- salt is the solute
- water is the solvent
- salt water is the solution
To make crystals, you usually heat the solution gently so some solvent evaporates. The solution becomes more concentrated. When the solution is concentrated enough, crystals start to form as it cools.
How to crystallise a dissolved solid
- Pour the solution into an evaporating basin.
- Heat gently to evaporate some solvent.
- Stop heating when crystals start to appear, or when the solution is nearly saturated.
- Leave the solution to cool so more crystals form.
- Filter and dry the crystals.
Common Mistake
Do not heat to dryness
In school practicals, you usually do not heat a solution until all the water has gone. Hot crystals may spit out, decompose, or become contaminated. Heat gently, then leave the solution to cool and crystallise.
Example
Obtaining salt crystals from salty water
You have a beaker of salty water and want to collect solid salt crystals.
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Salt is dissolved in the water, so filtration alone will not work; dissolved salt particles pass through filter paper with the water.
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The useful product is the dissolved solid, so you need to remove some solvent and make the solution more concentrated.
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Heat the solution gently in an evaporating basin until some water evaporates and crystals begin to form.
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Leave the concentrated solution to cool, then filter and dry the salt crystals.
Simple distillation
Simple distillation separates a liquid from a solution. It is useful when you want to collect the solvent, not the dissolved solid.
Definition
Boiling point
The boiling point is the temperature at which a liquid boils and changes into a gas.
In simple distillation:
- The solution is heated.
- The solvent boils and turns into vapour.
- The vapour passes into a condenser.
- The condenser cools the vapour, turning it back into liquid.
- The collected pure liquid is called the distillate.
For example, simple distillation can be used to get pure water from salt water. The water evaporates and condenses; the salt stays behind because it has a much higher boiling point and does not evaporate with the water.
Common Mistake
Crystallisation vs simple distillation
Use crystallisation if you want the dissolved solid. Use simple distillation if you want the solvent as a pure liquid.
Example
Choosing how to collect pure water from salt water
A sample contains sodium chloride dissolved in water. You need to collect pure water.
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The sodium chloride is dissolved, so filtration will not separate it from the water.
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The desired product is the water, which is the solvent. Crystallisation would mainly collect the salt instead.
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Heat the solution so water boils and leaves as vapour, while sodium chloride remains in the flask.
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Cool the water vapour in a condenser so it condenses into pure liquid water.
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The best method is simple distillation.
Fractional distillation
Fractional distillation separates two or more liquids that are mixed together and have different boiling points.
Definition
Miscible liquids
Miscible liquids mix together completely. For example, ethanol and water are miscible.
Fractional distillation is like simple distillation, but it uses a fractionating column. This column helps separate liquids with different boiling points more effectively.
The liquid with the lowest boiling point evaporates most easily. It rises up the column first, reaches the condenser, cools and is collected.
Examples of fractional distillation
Fractional distillation can be used to separate:
- ethanol and water
- different substances in crude oil
- liquid air into nitrogen and oxygen
Example
Separating ethanol and water
A mixture contains ethanol and water. Ethanol boils at 78 °C and water boils at 100 °C.
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Compare the boiling points: ethanol has the lower boiling point, so it evaporates more easily.
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When the mixture is heated, ethanol vapour rises through the fractionating column more readily than water vapour.
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The ethanol vapour enters the condenser, cools, and becomes liquid ethanol.
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The ethanol is collected first because it has the lower boiling point.
Tip
Distillation clue
If the mixture contains liquids with different boiling points, think about distillation. Use simple distillation for a solvent from a solution, and fractional distillation for separating mixed liquids.
Chromatography
Chromatography separates substances dissolved in a solvent. At GCSE, it is often used to separate coloured substances such as inks, dyes or food colourings.
Definition
Chromatography
Chromatography is a separation method where substances move through a material at different speeds, so they spread out and separate.
In paper chromatography:
- The paper is the stationary phase because it stays still.
- The solvent is the mobile phase because it moves up the paper.
- A pencil line is drawn near the bottom of the paper.
- A spot of the mixture is placed on the pencil line.
- The solvent travels up the paper and carries the substances with it.
- Different substances travel different distances, so separate spots form.
The diagram shows the key parts of a paper chromatography setup and the separated spots after the solvent has moved up the paper.
Common Mistake
Do not use pen for the baseline
The baseline should be drawn in pencil, not ink. Ink may dissolve in the solvent and interfere with the chromatogram.
Why substances separate in chromatography
Substances separate because they have different attractions to the paper and different solubilities in the solvent.
A substance that is more soluble in the solvent, and less strongly attracted to the paper, travels further up the paper. A substance that is less soluble, or more strongly attracted to the paper, travels a shorter distance.
Choosing the best separation method
When an exam question asks you to suggest a method, focus on the properties of the substances in the mixture.
| Mixture type | Best method | Why it works |
|---|---|---|
| Insoluble solid and liquid | Filtration | Solid particles cannot pass through filter paper |
| Dissolved solid in solution, collecting the solid | Crystallisation | Solvent evaporates and solute forms crystals |
| Dissolved solid in solution, collecting the solvent | Simple distillation | Solvent evaporates, then condenses as pure liquid |
| Two or more miscible liquids | Fractional distillation | Liquids have different boiling points |
| Dissolved coloured substances | Chromatography | Substances travel different distances in a solvent |
Example
Separating a mixture of sand, salt and water
A mixture contains sand, salt and water. You want to obtain dry salt crystals.
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Sand is insoluble in water, but salt is dissolved. Start by filtering the mixture to remove the sand as the residue.
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The filtrate is now salt solution, containing dissolved salt in water.
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Because the required product is solid salt, use crystallisation rather than simple distillation.
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Heat the salt solution gently to evaporate some water, then leave it to cool so salt crystals form.
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Filter and dry the crystals to obtain the salt.
Key Idea
Separation depends on differences
Separation methods work because substances in a mixture have different physical properties. The substances themselves are not chemically changed.
Exam technique
In the exam
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Identify what type of mixture you have: insoluble solid, dissolved solid, mixed liquids, or dissolved colours.
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Say which substance is being collected. This helps you choose between crystallisation and simple distillation.
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Explain the method using a physical property, such as solubility, particle size, boiling point, or distance travelled in a solvent.
Self review
Check yourself
- Why can mixtures usually be separated by physical processes?
- Which method would you use to collect pure water from copper sulfate solution?
- Why must the solvent level be below the pencil baseline in paper chromatography?
A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes
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