Why is "Calling everyday 'pure' products (orange juice, milk, fresh air) pure substances" a common mistake in Pure Substance vs Mixture?

Why it happens: Students use the everyday meaning of 'pure' (natural / nothing added) instead of the chemistry meaning. How to avoid it: In chemistry, **pure = one substance only**. Orange juice, milk and air all contain several substances, so they are **mixtures**. Source: Edexcel Chemistry examiner reports — recurring error

Why is "Saying an impurity raises the melting point (or lowers the boiling point)" a common mistake in Pure Substance vs Mixture?

Why it happens: Students assume an impurity always makes a substance 'worse' in the same direction. How to avoid it: Learn the directions: an impurity **LOWERS the melting point** but **RAISES the boiling point**. They go opposite ways. Source: Edexcel Chemistry examiner reports

Why is "Concluding 'impure' without quoting the data" a common mistake in Pure Substance vs Mixture?

Why it happens: Students give the right verdict but forget that the marks are for the reasoning. How to avoid it: Always **quote the melting range and compare it with the pure value** — e.g. 'melts over 71–78 °C, lower than the pure 80 °C, so impure'. Source: Edexcel Chemistry examiner reports

Why is "Forgetting that a mixture melts over a RANGE, not at one temperature" a common mistake in Pure Substance vs Mixture?

Why it happens: Students learn the pure value and assume mixtures also melt at a single temperature. How to avoid it: Remember: **pure = sharp/single temperature; mixture = range**. The range itself is the key evidence of impurity. Source: Edexcel Chemistry examiner reports

Why is "Thinking 'pure' just means clean, clear or colourless" a common mistake in Pure Substance vs Mixture?

Why it happens: In daily life clear, clean liquids are described as 'pure'. How to avoid it: Purity is about **how many substances are present**, not how it looks. A clear, colourless salt solution is still a mixture. Source: Edexcel Chemistry examiner reports

Why is "Saying only elements can be pure (or that compounds are mixtures)" a common mistake in Pure Substance vs Mixture?

Why it happens: Students confuse 'pure' with 'element' and 'mixture' with 'compound'. How to avoid it: A **single compound is pure** too. Pure water (H₂O) is a pure substance; a compound is only impure if other substances are mixed in. Source: Edexcel Chemistry examiner reports

Elements, compounds and mixturesEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Pure Substance vs Mixture

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Pure Substance vs Mixture — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

What 'pure' really means in chemistry, why a pure substance melts and boils at a sharp, fixed temperature, and how to use melting-point data to decide whether a substance is pure or a mixture. Covers Double Award Chemistry spec 1.7 and 1.9, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

1.7 — Understand the difference between a pure substance and a mixture in terms of melting and boiling points.
1.9 — Use melting point data to determine the purity of a substance.

What 'pure' means in chemistry (spec 1.7)

Chemically pure = ONE substance only — a single element or single compound.

In everyday language 'pure' usually means 'natural' or 'nothing added' — like 'pure orange juice' or 'pure spring water'. But in chemistry the word means something much stricter.

A pure substance is made of only ONE substance — a single element or a single compound — with nothing else mixed in.

Pure water is only H₂O molecules.
Pure copper is only copper atoms.
Pure sodium chloride is only NaCl.

A mixture (also called an impure substance) contains two or more substances that are not chemically bonded together. Each substance in the mixture keeps its own properties.

Sea water = water + dissolved salts → a mixture.
Air = nitrogen + oxygen + argon + carbon dioxide → a mixture.
'Pure' orange juice = water + sugars + citric acid + pulp + vitamins → a mixture, even though the label says 'pure'!

This is a classic exam trap. If a question gives you an everyday 'pure' product (orange juice, milk, fresh air), in chemistry it is almost always a mixture. Pure, to a chemist, means one substance — measured by a sharp melting and boiling point.

Pure substance = single element or single compound, nothing else.
Mixture = two or more substances not chemically bonded (impure).
Everyday 'pure' (e.g. orange juice) is usually a chemical mixture.
Chemical purity is judged by sharp melting/boiling points.

See the full worked example for pure substance vs mixture →

Pure vs mixture: melting and boiling points (spec 1.7)

Pure = sharp fixed temperature; mixture = melts/boils over a range.

The most important difference for the exam is how a pure substance and a mixture melt and boil.

A pure substance melts and boils at a sharp, fixed temperature — one exact value.

Pure ice melts at exactly 0 °C and pure water boils at exactly 100 °C (at normal pressure).
Pure sodium chloride melts at 801 °C; pure ethanol boils at 78 °C.
On a heating curve, the temperature is flat (constant) at a single value while it melts.

A mixture (impure substance) melts and boils over a RANGE of temperatures, not at one fixed value.

An impurity LOWERS the melting point and makes it melt over a range (it starts melting below the pure value).
An impurity RAISES the boiling point and makes it boil over a range (it boils above the pure value, and the temperature keeps rising as it boils).

A pure substance melts at one sharp temperature (flat line); a mixture melts over a range and begins below the pure melting point.

Real-life example. Salt is spread on icy roads in winter because the salt water (a mixture) freezes/melts below 0 °C, so the ice melts even in cold weather. The impurity (salt) lowers the melting point.

Pure: sharp, fixed melting and boiling point (one exact value).
Mixture/impure: melts and boils over a RANGE.
Impurity lowers the melting point; raises the boiling point.
Salt on icy roads: impurity lowers the melting point of water.

See the full worked example for pure substance vs mixture →

Using melting-point data to judge purity (spec 1.9)

Compare the measured m.p. with the known pure value AND check it is sharp.

Examiners give you a melting-point measurement and ask whether a substance is pure. You make TWO checks:

Is the melting point SHARP (a single temperature) or a RANGE?
- Sharp single value → likely pure.
- A range (e.g. "melts between 48 °C and 53 °C") → impure / a mixture.
Does it match the known value for the pure substance?
- Same as the data-book value → pure.
- Lower than the data-book value → impure (impurities lower the m.p.).

Both clues point the same way: a pure substance gives a sharp melting point at the expected temperature.

Substance	Pure m.p.	Measured	Verdict
Sample A	80 °C	melts sharply at 80 °C	Pure
Sample B	80 °C	melts over 71–78 °C	Impure (lower + range)
Sample C	80 °C	melts over 78–80 °C	Slightly impure (small range)

Pure: sharp melting point at the known value. Impure: lower melting point over a range.

Why this matters. In a pharmaceutical or chemistry lab, measuring the melting point of a sample in a thin tube is one of the first quick tests of purity — a pure drug must give a sharp melting point at the correct temperature.

Check 1: is the m.p. sharp (pure) or a range (impure)?
Check 2: does it match the known pure value (pure) or is it lower (impure)?
Impurities both lower the m.p. AND spread it over a range.
Sharp m.p. at the data-book value = pure substance.

See the full worked example for pure substance vs mixture →

Why a mixture melts and boils over a range

Impurity particles disrupt the regular structure, so it melts at a lower, spread-out temperature.

For top marks you may be asked to explain why a mixture does not have a sharp melting point.

In a pure substance, all the particles are the same and arranged in a regular pattern, held by forces of attraction of equal strength. They all break free at the same temperature, so melting happens at one sharp value.

In a mixture, the different particles disrupt the regular arrangement. The forces between particles are no longer all the same, so the particles break free at different temperatures — the substance melts gradually over a range rather than all at once.

Boiling. As a mixture (such as salt water) boils, the water boils off first, so the remaining solution becomes more concentrated in dissolved solid. A more concentrated solution boils at an even higher temperature, so the boiling temperature keeps rising during boiling — there is no single fixed boiling point.

Key summary:

Pure substance → identical particles, equal forces → one sharp melting/boiling temperature.
Mixture → different particles disrupt the structure → melts/boils over a range.

Pure: identical particles + equal forces → sharp melting point.
Mixture: different particles disrupt the structure → melts over a range.
Impurity lowers the melting point and broadens it.
As a solution boils, it gets more concentrated, so the b.p. rises.

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). Purity appears as: stating the difference between a pure substance and a mixture in terms of melting/boiling points (2–3 marks); reading a melting-point data table to decide whether a sample is pure or impure (2–4 marks); and explaining why impurities lower the melting point and broaden the range. The biggest discriminators are remembering that pure = SHARP/FIXED while impure = a RANGE, and not falling for the 'everyday pure' trap (e.g. orange juice).

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

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Step-by-step worked examples — Pure Substance vs Mixture

Step-by-step solutions to past-paper-style questions on pure substance vs mixture, written exactly the way a tutor would explain them at the board.

1Classify as pure substance or mixture (Getting started)
Getting started• 4WSD0/1C style — short answer• purity, spec-1.7
Question
State whether each of the following is a pure substance or a mixture: (a) distilled water (H₂O only); (b) sea water; (c) air. (3 marks)
Step-by-step solution
1. Step 1
  (a) Distilled water is only H₂O with nothing else → pure substance.
2. Step 2
  (b) Sea water is water + dissolved salts → mixture.
3. Step 3
  (c) Air is nitrogen + oxygen + argon + carbon dioxide → mixture.
Answer
(a) pure substance; (b) mixture; (c) mixture.
Examiner tip
A pure substance contains only one substance. Anything with two or more substances not chemically bonded — including most everyday liquids — is a mixture.
2The 'everyday pure' trap (Getting started)
Getting started• 4WSD0/1C style — short answer• purity, spec-1.7, common-error
Question
A carton is labelled 'pure orange juice'. Explain why, in chemistry, orange juice is NOT a pure substance. (2 marks)
Step-by-step solution
1. Step 1
  Chemistry definition (1). In chemistry a pure substance is a single substance (one element or one compound) with nothing else mixed in.
2. Step 2
  Apply it (1). Orange juice contains many substances (water, sugars, citric acid, vitamins, pulp), so it is a mixture, not a pure substance.
Answer
In chemistry 'pure' means a single substance. Orange juice contains many different substances (water, sugars, acids, etc.) that are not chemically bonded, so it is a mixture, not a pure substance.
Examiner tip
The everyday meaning of 'pure' (natural / nothing added) is not the chemistry meaning (one substance). Always answer with the chemistry definition.
3Difference in melting/boiling behaviour (Building confidence)
Building confidence• 4WSD0/1C style — structured• purity, spec-1.7, melting-point
Question
Describe how the melting and boiling points of a pure substance differ from those of a mixture. (3 marks)
Step-by-step solution
1. Step 1
  Pure (1). A pure substance melts and boils at a sharp, fixed (single) temperature.
2. Step 2
  Mixture (1). A mixture melts and boils over a range of temperatures.
3. Step 3
  Effect of impurity (1). An impurity lowers the melting point and raises the boiling point (and spreads them over a range).
Answer
A pure substance has a sharp, fixed melting and boiling point (one temperature). A mixture melts and boils over a range of temperatures; impurities lower the melting point and raise the boiling point.
Examiner tip
The two marking words are 'sharp/fixed' for pure and 'range' for a mixture. Saying a mixture 'has a higher melting point' is wrong — impurities LOWER the melting point.
4Use melting-point data to decide purity (Building confidence)
Building confidence• 4WSD0/1C style — structured• purity, spec-1.9, data
Question
Pure aspirin melts sharply at 135 °C. A student makes a sample of aspirin and finds it melts over the range 128–132 °C. State, with two reasons, whether the student's sample is pure. (3 marks)
Step-by-step solution
1. Step 1
  Conclusion (1). The sample is impure (not pure).
2. Step 2
  Reason 1 (1). It melts over a range (128–132 °C), not at a single sharp temperature — a pure substance melts sharply.
3. Step 3
  Reason 2 (1). The melting point is lower than the pure value of 135 °C — impurities lower the melting point.
Answer
The sample is impure. It melts over a range (128–132 °C) rather than at one sharp temperature, and the melting point is lower than the pure value of 135 °C — both signs of an impure substance.
Examiner tip
Both pieces of evidence (range AND lower than the known value) score. A bare 'impure' with no use of the data gets no marks — you must quote the range and the comparison.
5Explain why a mixture melts over a range (Stretch)
Stretch• 4WSD0/1C style — extended• purity, spec-1.7, reasoning
Question
Explain, in terms of the particles, why a pure substance melts at a sharp temperature but a mixture melts over a range. (4 marks)
Step-by-step solution
1. Step 1
  Pure (1). In a pure substance all the particles are the same and arranged regularly, held by forces of equal strength.
2. Step 2
  Pure result (1). They all break free at the same temperature, so melting happens at one sharp value.
3. Step 3
  Mixture (1). In a mixture different particles disrupt the regular arrangement, so the forces between particles are not all the same.
4. Step 4
  Mixture result (1). The particles break free at different temperatures, so the mixture melts gradually over a range.
Answer
In a pure substance all particles are identical with equal forces of attraction, so they all break free at the same temperature, giving a sharp melting point. In a mixture the different particles disrupt the structure, so the forces vary and particles break free at different temperatures, giving a melting range.
Examiner tip
Full marks need both halves: identical particles/equal forces → sharp; different particles disrupt structure → range. Vague answers like 'mixtures are less pure' score nothing.
6Rank samples by purity from data (Stretch)
Stretch• 4WSD0/1C style — extended• purity, spec-1.9, data
Question
Pure naphthalene melts sharply at 80 °C. Three samples are tested: P melts at 80 °C; Q melts over 72–78 °C; R melts over 77–80 °C. Identify which sample is pure and place the impure samples in order from least pure to most pure, explaining your reasoning. (4 marks)
Step-by-step solution
1. Step 1
  Pure sample (1). P melts sharply at exactly 80 °C → it is pure.
2. Step 2
  Q and R are impure (1). Both melt over a range and below 80 °C, so both contain impurities.
3. Step 3
  Compare ranges (1). Q melts over a wider range (72–78 °C, a 6 °C spread, starting much lower); R melts over a narrower range (77–80 °C, a 3 °C spread, closer to 80 °C).
4. Step 4
  Order (1). More impurity = lower start + wider range. So least pure = Q, then R, then P (pure).
Answer
P is pure (sharp m.p. at 80 °C). Q and R are impure (range, below 80 °C). Q is the least pure (wider range, 72–78 °C, lower start); R is more pure (narrower range, 77–80 °C). Order least → most pure: Q, R, P.
Examiner tip
The discriminator is using BOTH how far the melting point is lowered AND how wide the range is. The more impure a sample, the lower and broader its melting range.

Model Answers — Pure Substance vs Mixture

High-scoring sample answers for pure substance vs mixture on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style2 marks
(a) State what is meant by a pure substance in chemistry. (b) Give one example of a pure substance and one example of a mixture. (2 marks)
Model answer
(a) A pure substance is a single substance — a single element or a single compound — with nothing else mixed in. (1)

(b) Pure example: pure water (H₂O) / copper / oxygen. Mixture example: sea water / air / orange juice. (1)
Why this scores
The definition must say 'single / only one substance (element or compound)'. Any genuine single-substance example and any genuine mixture example score.
Question 2
4WSD0/1C style2 marks
Pure ice melts sharply at 0 °C. State what the word 'sharply' tells you about how a pure substance melts, and what this tells you about the substance's purity. (2 marks)
Model answer
'Sharply' means it melts at one single, fixed temperature (0 °C) rather than over a range. (1)

This is evidence that the substance is pure — pure substances have sharp, fixed melting points. (1)
Why this scores
1 mark for 'single/fixed temperature' and 1 mark for linking a sharp melting point to purity. A mixture would melt over a range.
Question 3
4WSD0/1C style4 marks
Explain the effect that an impurity has on (a) the melting point and (b) the boiling point of a substance. (4 marks)
Model answer
(a) Melting point.

An impurity lowers the melting point (it melts below the pure value). (1)

It also makes the substance melt over a range of temperatures rather than at one sharp value. (1)

(b) Boiling point.

An impurity raises the boiling point (it boils above the pure value). (1)

It boils over a range, and the temperature keeps rising during boiling as the solution becomes more concentrated. (1)
Why this scores
Watch the directions: impurity LOWERS m.p. but RAISES b.p. Students often get them the wrong way round. The 'range' point is awarded separately from the direction point.
Question 4
4WSD0/1C style4 marks
Pure ethanoic acid melts sharply at 17 °C. A sample of ethanoic acid is found to melt over the range 11–15 °C. Use this data to determine whether the sample is pure, giving two reasons. (4 marks)
Model answer
The sample is impure (it is a mixture). (1)

Reason 1: it melts over a range (11–15 °C) rather than at one sharp temperature; a pure substance melts at a single fixed temperature. (1)

Reason 2: the melting point is lower than the pure value of 17 °C; impurities lower the melting point. (1)

Both observations are consistent with an impure sample. (1)
Why this scores
You must use the actual numbers from the question. Quoting the range (11–15 °C) and comparing it with the pure value (17 °C) is what earns the marks — a bare conclusion does not.
Question 5
4WSD0/1C style — extended6 marks
Explain, in terms of particles and the forces between them, why a pure substance has a sharp melting point but a mixture melts over a range of temperatures. (6 marks)
Model answer
Pure substance.

All the particles are the same (identical). (1)

They are arranged regularly and held by forces of attraction of equal strength. (1)

So all the particles break free at the same temperature, giving a single, sharp melting point. (1)

Mixture.

A mixture contains different particles mixed together. (1)

These different particles disrupt the regular arrangement, so the forces between particles are not all the same (some weaker, some stronger). (1)

The particles therefore break free at different temperatures, so the mixture melts gradually over a range — and the impurity lowers the temperature at which melting begins. (1)
Why this scores
Top band requires the contrast: identical particles + equal forces → all melt together (sharp); different particles disrupt the structure + unequal forces → melt at different temperatures (range). Linking impurity to a LOWER starting temperature secures the final mark.
Question 6
4WSD0/1C style — extended8 marks
A student prepares a sample of a white solid and wants to check its purity. Pure samples of the solid melt sharply at 119 °C. The student's sample melts over 110–116 °C. (a) Describe how the student could measure the melting point. (b) State whether the sample is pure and explain how the data shows this. (c) Suggest how the student could obtain a purer sample. (8 marks)
Model answer
(a) Measuring the melting point.

Place a small amount of the solid in a melting-point tube (thin capillary tube). (1)

Heat it slowly (e.g. in a heating block or oil bath) and watch with a thermometer. (1)

Record the temperature at which it starts to melt and the temperature at which it has completely melted. (1)

(b) Is the sample pure?

The sample is impure. (1)

It melts over a range (110–116 °C), not at a single sharp temperature. (1)

The melting point is lower than the pure value (119 °C), and impurities lower the melting point. (1)

(c) Obtaining a purer sample.

Recrystallise the solid: dissolve it in the minimum hot solvent, filter, then cool so pure crystals form, leaving impurities in solution. (1)

Re-measure the melting point — a purer sample will melt sharply and closer to 119 °C. (1)
Why this scores
Part (b) must quote the range and compare with the pure value. Part (c) rewards a named purification method (recrystallisation/crystallisation) and the idea of re-testing the melting point to confirm improved purity.

Key Definitions and Keywords — Pure Substance vs Mixture

Definitions to memorise and the exact keywords mark schemes credit for pure substance vs mixture answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Pure substance
Examiner keyword
A substance made of only one substance — a single element or a single compound — with nothing else mixed in. It has a sharp, fixed melting and boiling point.
Mixture
Examiner keyword
Two or more substances that are not chemically bonded together, present in any proportion. Each substance keeps its own properties and can be separated by physical means.
Impure substance
Examiner keyword
Another word for a mixture — a substance that contains more than one substance, so it melts and boils over a range of temperatures.
Melting point
Examiner keyword
The temperature at which a solid turns into a liquid. A pure substance has a sharp, fixed melting point; a mixture melts over a range.
Boiling point
The temperature at which a liquid turns into a gas throughout. A pure substance boils at a sharp, fixed temperature; a mixture boils over a range.
Impurity
Examiner keyword
A small amount of another substance mixed in. Impurities lower the melting point and raise the boiling point, and spread both over a range.
Sharp melting point
Examiner keyword
Melting that occurs at a single, fixed temperature rather than over a range — evidence that a substance is pure.
Melting range
A spread of temperatures over which a substance melts (e.g. 71–78 °C). A melting range is evidence that a substance is impure / a mixture.
Purity test (melting point)
A quick lab test: a substance is pure if it melts sharply at its known melting point; it is impure if it melts over a range and/or below the expected value.
Physical separation
Separating the substances in a mixture without a chemical reaction (e.g. filtration, distillation), possible because they are not chemically bonded.

Common Mistakes and Misconceptions — Pure Substance vs Mixture

The traps other students keep falling into on pure substance vs mixture questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Calling everyday 'pure' products (orange juice, milk, fresh air) pure substances
Edexcel Chemistry examiner reports — recurring error
Why it happens
Students use the everyday meaning of 'pure' (natural / nothing added) instead of the chemistry meaning.
How to avoid it
In chemistry, pure = one substance only. Orange juice, milk and air all contain several substances, so they are mixtures.
✕Saying an impurity raises the melting point (or lowers the boiling point)
Edexcel Chemistry examiner reports
Why it happens
Students assume an impurity always makes a substance 'worse' in the same direction.
How to avoid it
Learn the directions: an impurity LOWERS the melting point but RAISES the boiling point. They go opposite ways.
✕Concluding 'impure' without quoting the data
Edexcel Chemistry examiner reports
Why it happens
Students give the right verdict but forget that the marks are for the reasoning.
How to avoid it
Always quote the melting range and compare it with the pure value — e.g. 'melts over 71–78 °C, lower than the pure 80 °C, so impure'.
✕Forgetting that a mixture melts over a RANGE, not at one temperature
Edexcel Chemistry examiner reports
Why it happens
Students learn the pure value and assume mixtures also melt at a single temperature.
How to avoid it
Remember: pure = sharp/single temperature; mixture = range. The range itself is the key evidence of impurity.
✕Thinking 'pure' just means clean, clear or colourless
Edexcel Chemistry examiner reports
Why it happens
In daily life clear, clean liquids are described as 'pure'.
How to avoid it
Purity is about how many substances are present, not how it looks. A clear, colourless salt solution is still a mixture.
✕Saying only elements can be pure (or that compounds are mixtures)
Edexcel Chemistry examiner reports
Why it happens
Students confuse 'pure' with 'element' and 'mixture' with 'compound'.
How to avoid it
A single compound is pure too. Pure water (H₂O) is a pure substance; a compound is only impure if other substances are mixed in.

Past paper style quiz

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Pure Substance vs Mixture

Detailed Notes

What you’ll learn

How it’s examined

1Classify as pure substance or mixture (Getting started)

2The 'everyday pure' trap (Getting started)

3Difference in melting/boiling behaviour (Building confidence)

4Use melting-point data to decide purity (Building confidence)

5Explain why a mixture melts over a range (Stretch)

6Rank samples by purity from data (Stretch)

Pure substance

Mixture

Impure substance

Melting point

Boiling point

Impurity

Sharp melting point

Melting range

Purity test (melting point)

Physical separation

✕Calling everyday 'pure' products (orange juice, milk, fresh air) pure substances

✕Saying an impurity raises the melting point (or lowers the boiling point)

✕Concluding 'impure' without quoting the data

✕Forgetting that a mixture melts over a RANGE, not at one temperature

✕Thinking 'pure' just means clean, clear or colourless

✕Saying only elements can be pure (or that compounds are mixtures)

Past paper style quiz