Why is "Defining diffusion as just 'particles spreading out'" a common mistake in Diffusion & Dilution?

Why it happens: It captures the idea loosely but misses the marking points. How to avoid it: State the **direction** (high → low concentration) AND the **cause** (random particle movement). Both are needed. Source: Edexcel Chemistry examiner reports

Why is "Saying the ring forms nearer HCl because hydrogen chloride is slower/heavier without linking to ammonia" a common mistake in Diffusion & Dilution?

Why it happens: Students name the wrong molecule or forget to compare. How to avoid it: Explain it from the ammonia: **NH₃ is lighter → diffuses faster → travels further**, so they meet nearer the HCl end. Source: Edexcel Chemistry examiner reports — recurring

Why is "Thinking diffusion is caused by density, gravity or one gas 'sinking'" a common mistake in Diffusion & Dilution?

Why it happens: Heavier gases are expected to fall, so students explain spreading by density. How to avoid it: Diffusion is caused by the **random movement of particles**, not density — gases diffuse upwards as well as downwards. Source: Edexcel Chemistry examiner reports

Why is "Saying the colour spreads because the water was stirred or moved" a common mistake in Diffusion & Dilution?

Why it happens: Students assume mixing always needs an external push. How to avoid it: Stress that **no stirring is needed** — the particles move on their own. That is the whole point of the experiment. Source: Edexcel Chemistry examiner reports

Why is "Saying the colour gets paler because the coloured particles 'disappear' or are 'used up'" a common mistake in Diffusion & Dilution?

Why it happens: A fading colour looks like the substance is vanishing. How to avoid it: The particles are not destroyed — the **same particles are spread through a larger volume**, so the colour is more dilute. Source: Edexcel Chemistry examiner reports

Why is "Stopping at 'higher temperature gives particles more energy' without linking to faster diffusion" a common mistake in Diffusion & Dilution?

Why it happens: Students give the first step of the chain only. How to avoid it: Complete the chain: more energy → particles **move faster** → they **diffuse faster**. Source: Edexcel Chemistry examiner reports

States of matterEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Diffusion & Dilution

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Diffusion & Dilution — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

How the spreading of coloured solutions and the diffusion of gases give direct evidence that particles are moving — including the classic ammonia + hydrogen chloride experiment and why lighter molecules diffuse faster (spec 1.3). Assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

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

1.3 — Understand how the results of experiments involving the dilution of coloured solutions and diffusion of gases can be explained.
(Linked) — Know that diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random movement of particles.

What is diffusion? (spec 1.3)

Particles spread from high to low concentration because they move randomly.

Diffusion is the net (overall) movement of particles from where they are more concentrated to where they are less concentrated. It happens because all particles in liquids and gases are constantly moving in random directions.

Because the movement is random, over time the particles spread out and mix until they are evenly spread — no stirring or pushing is needed.

Random particle movement spreads the colour evenly through the liquid.

Diffusion is fastest in gases (particles far apart and fast), slower in liquids, and does not happen in solids (particles are locked in fixed positions).

Diffusion = net movement, high → low concentration.
Caused by random motion of particles.
Needs no stirring — happens on its own.
Fast in gases, slower in liquids, none in solids.

See the full worked example for diffusion & dilution →

Dilution of a coloured solution — evidence particles move

A purple crystal colouring still water proves liquid particles move and mix.

A simple experiment: drop a crystal of purple potassium manganate(VII) into a beaker of still water and leave it (do not stir).

What you see: the purple colour slowly spreads out from the crystal until, after some time, the whole beaker is an even, paler purple.

Explanation (the marks):

The crystal dissolves, releasing coloured particles into the water.
Both the water particles and the coloured particles are moving randomly.
The coloured particles diffuse from where they are concentrated (around the crystal) to where there are none, until they are evenly spread.
The colour becomes paler because the same coloured particles are now spread through a much larger volume — this is dilution.

The fact that the colour spreads without stirring is direct evidence that particles are constantly moving.

Purple manganate(VII) spreads through still water with no stirring.
Coloured particles diffuse from high to low concentration.
Even colour at the end = particles evenly spread.
Paler colour = same particles in a bigger volume (dilution).

See the full worked example for diffusion & dilution →

Diffusion of gases: the NH₃ + HCl experiment

A white ring forms nearer the HCl because ammonia diffuses faster.

This is the most-tested diffusion experiment. A long glass tube has a cotton-wool plug soaked in concentrated ammonia solution at one end and a plug soaked in concentrated hydrochloric acid at the other. The tube is sealed and left.

The ring forms nearer the HCl end — proof that the lighter ammonia diffused faster.

What you see: after a few minutes a white ring of solid ammonium chloride appears inside the tube — and it forms nearer the hydrochloric acid end.

Explanation (the marks):

Both substances give off gases that diffuse along the tube and meet in the middle, reacting to form white ammonium chloride: ammonia + hydrogen chloride → ammonium chloride (NH₃ + HCl → NH₄Cl).
The ring forms nearer the HCl end because the ammonia molecules (NH₃) are lighter than the hydrogen chloride molecules (HCl), so ammonia diffuses faster and travels further before they meet.

Key rule: at the same temperature, lighter molecules diffuse faster than heavier molecules. (Mᵣ of NH₃ = 17; Mᵣ of HCl = 36.5, so NH₃ moves faster.)

White ring of ammonium chloride forms where the gases meet.
Ring is nearer the HCl end.
NH₃ is lighter (Mᵣ 17) than HCl (Mᵣ 36.5), so it diffuses faster.
Lighter molecules diffuse faster at the same temperature.

See the full worked example for diffusion & dilution →

What affects the rate of diffusion?

Mass of the molecule and temperature both change diffusion speed.

Two factors decide how fast diffusion happens:

Factor	Effect	Why
Mass of the molecule (Mᵣ)	lighter = faster	at the same temperature, lighter molecules move faster
Temperature	higher = faster	particles gain more energy and move faster, so they spread more quickly

Worked logic for "which travels further": compare the relative molecular masses. The molecule with the smaller Mᵣ is lighter, moves faster, and travels further in a given time.

A higher temperature speeds up all diffusion because the particles have more kinetic energy and collide and spread out faster.

Lighter molecule (smaller Mᵣ) → faster diffusion → travels further.
Higher temperature → faster diffusion.
Compare Mᵣ values to predict which gas diffuses faster.

How it’s examined

On Chemistry Paper 1 (4WSD0/1C) diffusion appears as a describe-and-explain question: explain why a colour spreads through still water, or explain the position of the white ring in the ammonia/hydrogen chloride tube. The high-value mark is always the comparison of molecular masses to justify which gas diffuses faster. Phrase answers in terms of particles moving randomly and going from high to low concentration.

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 — Diffusion & Dilution

Step-by-step solutions to past-paper-style questions on diffusion & dilution, written exactly the way a tutor would explain them at the board.

1Define diffusion (Getting started)
Getting started• 4WSD0/1C style — short answer• diffusion, spec-1.3
Question
What is meant by the term diffusion? (2 marks)
Step-by-step solution
1. Step 1
  Direction (1). Diffusion is the (net) movement of particles from a region of higher concentration to a region of lower concentration.
2. Step 2
  Cause (1). It happens because the particles are moving randomly (on their own — no stirring needed).
Answer
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random movement of the particles.
Examiner tip
Both marks need the direction (high → low concentration) AND the cause (random movement). 'Particles spreading out' alone is too vague.
2Colour spreading in still water (Getting started)
Getting started• 4WSD0/1C style — short answer• dilution, spec-1.3
Question
A purple crystal of potassium manganate(VII) is placed in still water. After an hour the whole beaker is pale purple, even though it was not stirred. Explain this observation. (3 marks)
Step-by-step solution
1. Step 1
  Dissolving (1). The crystal dissolves, releasing coloured particles into the water.
2. Step 2
  Diffusion (1). The coloured particles diffuse from where they are concentrated to where there are none, because both they and the water particles move randomly.
3. Step 3
  Result (1). They spread until evenly distributed; the colour is paler because the same particles now fill a much larger volume.
Answer
The crystal dissolves and its coloured particles diffuse through the water, moving randomly from high to low concentration until they are spread evenly. No stirring is needed because the particles move on their own; the colour is paler because the particles are now spread through a larger volume.
Examiner tip
Credit is for: dissolving, diffusion (high → low / random movement), even spreading. The 'no stirring needed' detail shows particles move by themselves.
3The white ring experiment (Building confidence)
Building confidence• 4WSD0/1C style — structured• diffusion, gases, spec-1.3
Question
Cotton wool soaked in ammonia solution is put in one end of a long tube, and cotton wool soaked in concentrated hydrochloric acid in the other. A white ring forms inside the tube, nearer the hydrochloric acid end. Explain why the ring forms, and why it forms nearer the acid end. (4 marks)
Step-by-step solution
1. Step 1
  Why a ring (1 + 1). Both ends give off gases that diffuse along the tube; where they meet they react to form white ammonium chloride (NH₃ + HCl → NH₄Cl).
2. Step 2
  Why nearer the acid (1). The ammonia molecules (NH₃) are lighter than the hydrogen chloride molecules (HCl).
3. Step 3
  Link to distance (1). Because ammonia is lighter it diffuses faster, so it travels further before the gases meet — the meeting point (ring) is nearer the HCl end.
Answer
Ammonia gas and hydrogen chloride gas diffuse along the tube and react where they meet to form a white ring of ammonium chloride. The ring forms nearer the hydrochloric acid end because ammonia molecules are lighter than hydrogen chloride molecules, so ammonia diffuses faster and travels further along the tube.
Examiner tip
The discriminating mark is 'ammonia is lighter → diffuses faster → travels further'. Just saying 'ammonia is faster' without the reason (lighter molecules) limits the marks.
4Predict which gas diffuses faster (Building confidence)
Building confidence• 4WSD0/1C style — structured• diffusion, Mr, spec-1.3
Question
Two gases are released at the same time from the same point: hydrogen (H₂) and carbon dioxide (CO₂). State which diffuses faster and explain your answer. (Relative atomic masses: H = 1, C = 12, O = 16.) (3 marks)
Step-by-step solution
1. Step 1
  Mᵣ of H₂ (1). = 2 × 1 = 2.
2. Step 2
  Mᵣ of CO₂ (1). = 12 + (2 × 16) = 44.
3. Step 3
  Conclusion (1). Hydrogen has the smaller Mᵣ, so it is lighter; lighter molecules diffuse faster, so hydrogen diffuses faster.
Answer
Hydrogen diffuses faster. Mᵣ(H₂) = 2 and Mᵣ(CO₂) = 44, so hydrogen molecules are much lighter; at the same temperature lighter molecules move faster, so hydrogen diffuses faster.
Examiner tip
Showing both Mᵣ values is the safest way to earn the explanation mark. The rule 'lighter = faster' must be stated, not assumed.
5Effect of temperature on diffusion (Stretch)
Stretch• 4WSD0/1C style — extended• diffusion, temperature, spec-1.3
Question
A drop of food colouring is added to a beaker of cold water and to an identical beaker of hot water. The colour spreads faster in the hot water. Explain why, in terms of particles. (3 marks)
Step-by-step solution
1. Step 1
  More energy (1). In hot water the particles have more kinetic energy.
2. Step 2
  Faster movement (1). So the water particles and the coloured particles move faster.
3. Step 3
  Faster diffusion (1). Faster-moving particles spread out and mix more quickly, so the colour diffuses faster in the hot water.
Answer
In the hot water the particles have more kinetic energy and move faster. Because the particles move faster, the coloured particles diffuse (spread from high to low concentration) more quickly, so the colour spreads faster.
Examiner tip
Tie the steps together: more energy → faster movement → faster diffusion. Stopping at 'particles have more energy' without linking to faster spreading loses a mark.
6Diffusion as evidence for the particle model (Stretch)
Stretch• 4WSD0/1C style — extended• diffusion, particle-theory, spec-1.3, reasoning
Question
Explain how the spreading of a smell (e.g. perfume) across a still room provides evidence that gases are made of moving particles. (4 marks)
Step-by-step solution
1. Step 1
  Observation (1). The perfume can be smelt across the room even though the air is still and nobody fanned it.
2. Step 2
  Particles released (1). The perfume gives off gas particles into the air at the bottle.
3. Step 3
  Random movement (1). These particles must be moving by themselves, randomly, to travel across the room.
4. Step 4
  Diffusion (1). They diffuse from high concentration (at the bottle) to low concentration (across the room) and mix with the air — showing gas particles are constantly moving.
Answer
Perfume particles are released into the air and can be smelt across a still room without any fanning. The only way they can travel is by moving on their own — they diffuse from where they are concentrated (the bottle) to where they are not (the rest of the room), mixing with the air. This shows that gases are made of particles in constant random motion.
Examiner tip
Top answers stress that the air was 'still' (no draught), so the spreading must be due to the particles' own random movement — that is the evidence for moving particles.

Model Answers — Diffusion & Dilution

High-scoring sample answers for diffusion & dilution 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 style3 marks
Complete the sentence: 'During diffusion, particles move from a region of __________ concentration to a region of __________ concentration.' Then state why no stirring is needed. (3 marks)
Model answer
Particles move from a region of higher concentration to a region of lower concentration. (2)

No stirring is needed because the particles are constantly moving on their own (randomly). (1)
Why this scores
1 mark for each blank (higher, lower) and 1 mark for random/constant particle movement. The whole idea of diffusion is that it is spontaneous.
Question 2
4WSD0/1C style2 marks
In which state of matter — solid, liquid or gas — does diffusion happen fastest? Explain your answer. (2 marks)
Model answer
Diffusion is fastest in a gas. (1)

This is because gas particles are far apart and move quickly (with the most energy), so they spread out fastest; in solids the particles are fixed in place and cannot diffuse. (1)
Why this scores
1 mark for 'gas'; 1 mark for the particle reason (far apart / fast moving). Stating that solids do not diffuse is a useful supporting point.
Question 3
4WSD0/1C style4 marks
A purple crystal of potassium manganate(VII) is dropped into a beaker of water and left without stirring. Describe and explain what is observed over the next hour. (4 marks)
Model answer
Observation. The purple colour gradually spreads out from the crystal until the whole beaker becomes an even, pale purple. (1)

Explanation.

The crystal dissolves, releasing coloured particles into the water. (1)

The coloured particles diffuse — moving randomly from where they are concentrated (around the crystal) to where there are none — until they are evenly spread. (1)

The colour is paler at the end because the same number of coloured particles are now spread through a much larger volume of water (dilution). (1)
Why this scores
Marks for: colour spreads/even; dissolving; diffusion (high → low / random); paler = spread through larger volume. The word 'diffuse' with a direction is essential.
Question 4
4WSD0/1C style4 marks
In the ammonia and hydrogen chloride diffusion experiment, a white solid forms inside the tube. (a) Name the white solid and the type of reaction. (b) Explain why it forms nearer the hydrochloric acid end of the tube. (4 marks)
Model answer
(a) The white solid is ammonium chloride (NH₄Cl), formed by a neutralisation reaction between ammonia (a base) and hydrogen chloride (an acid). (2)

(b) Both gases diffuse along the tube and meet where the ring forms. Ammonia molecules (NH₃, Mᵣ = 17) are lighter than hydrogen chloride molecules (HCl, Mᵣ = 36.5), so ammonia diffuses faster and travels further before the two gases meet. The meeting point is therefore nearer the hydrochloric acid end. (2)
Why this scores
(a) 1 mark for ammonium chloride, 1 for neutralisation/acid–base. (b) 1 mark for 'ammonia is lighter', 1 for 'diffuses faster/travels further'. Quoting the Mᵣ values strengthens the answer.
Question 5
4WSD0/1C style — extended6 marks
Two gas jars are connected, one containing bromine vapour (Br₂, brown) below a jar of colourless air. When the cover is removed the brown colour spreads upwards into the air jar until both jars are the same pale brown, even though bromine is denser than air. Explain these observations fully, in terms of particles. (6 marks)
Model answer
The colour spreads (diffusion).

The bromine and air particles are constantly moving in random directions. (1)

The brown bromine particles diffuse from the lower jar (high concentration) into the upper jar (low concentration). (1)

At the same time air particles diffuse downwards, so the two gases mix. (1)

Why it happens even though bromine is denser.

Diffusion is driven by the random movement of particles, not by density — so bromine spreads upwards against gravity. (1)

The final state.

Mixing continues until the particles are evenly spread through both jars. (1)

Both jars then look the same pale brown because the bromine particles are now shared between a larger total volume (more dilute). (1)
Why this scores
Indicative content: random particle movement; diffusion high → low concentration in both directions (mixing); diffusion not driven by density; even spread; equal/paler colour from larger volume. The 'denser but still spreads up' point is the discriminator.
Question 6
4WSD0/1C style — extended6 marks
A student claims that warming the ammonia and hydrogen chloride tube will make the white ring appear sooner. Use the particle model to explain whether the student is correct, and state one variable that should be kept the same for a fair comparison. (6 marks)
Model answer
The student is correct.

At a higher temperature the gas particles have more kinetic energy. (1)

So the ammonia and hydrogen chloride particles move faster. (1)

Faster-moving particles diffuse faster along the tube. (1)

They therefore meet sooner, so the white ring of ammonium chloride appears in a shorter time. (1)

Fair comparison.

Keep the length of the tube the same (so the gases travel the same distance). (1)

Also keep the concentration of the ammonia and hydrochloric acid solutions the same, so only temperature is changed. (1)
Why this scores
Indicative content: higher temperature → more KE → faster movement → faster diffusion → ring sooner; plus a sensible control variable (tube length and/or solution concentration). A clear causal chain scores the upper band.

Key Definitions and Keywords — Diffusion & Dilution

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

Diffusion
Examiner keyword
The net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random movement of the particles.
Concentration
A measure of how many particles of a substance are present in a given volume. Particles diffuse from high to low concentration.
Dilution
Examiner keyword
Spreading a fixed amount of dissolved (coloured) particles through a larger volume, so the solution becomes less concentrated (e.g. a paler colour).
Random motion
Examiner keyword
The continuous movement of particles in liquids and gases in all directions, which causes diffusion without any stirring.
Rate of diffusion
Examiner keyword
How fast particles spread out. It increases with temperature and is faster for lighter molecules (smaller relative molecular mass).
Relative molecular mass (Mᵣ)
Examiner keyword
The sum of the relative atomic masses of all the atoms in a molecule. At the same temperature, molecules with a smaller Mᵣ diffuse faster.
Ammonium chloride (NH₄Cl)
The white solid formed when ammonia gas and hydrogen chloride gas meet and react (NH₃ + HCl → NH₄Cl) in the diffusion tube experiment.
Ammonia (NH₃)
An alkaline gas, Mᵣ = 17. Being lighter than hydrogen chloride, it diffuses faster, so the white ring forms nearer the acid end of the tube.

Common Mistakes and Misconceptions — Diffusion & Dilution

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

✕Defining diffusion as just 'particles spreading out'
Edexcel Chemistry examiner reports
Why it happens
It captures the idea loosely but misses the marking points.
How to avoid it
State the direction (high → low concentration) AND the cause (random particle movement). Both are needed.
✕Saying the ring forms nearer HCl because hydrogen chloride is slower/heavier without linking to ammonia
Edexcel Chemistry examiner reports — recurring
Why it happens
Students name the wrong molecule or forget to compare.
How to avoid it
Explain it from the ammonia: NH₃ is lighter → diffuses faster → travels further, so they meet nearer the HCl end.
✕Thinking diffusion is caused by density, gravity or one gas 'sinking'
Edexcel Chemistry examiner reports
Why it happens
Heavier gases are expected to fall, so students explain spreading by density.
How to avoid it
Diffusion is caused by the random movement of particles, not density — gases diffuse upwards as well as downwards.
✕Saying the colour spreads because the water was stirred or moved
Edexcel Chemistry examiner reports
Why it happens
Students assume mixing always needs an external push.
How to avoid it
Stress that no stirring is needed — the particles move on their own. That is the whole point of the experiment.
✕Saying the colour gets paler because the coloured particles 'disappear' or are 'used up'
Edexcel Chemistry examiner reports
Why it happens
A fading colour looks like the substance is vanishing.
How to avoid it
The particles are not destroyed — the same particles are spread through a larger volume, so the colour is more dilute.
✕Stopping at 'higher temperature gives particles more energy' without linking to faster diffusion
Edexcel Chemistry examiner reports
Why it happens
Students give the first step of the chain only.
How to avoid it
Complete the chain: more energy → particles move faster → they diffuse faster.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

The ring forms nearer the HCl end — proof that the lighter ammonia diffused faster.

What you see: after a few minutes a white ring of solid ammonium chloride appears inside the tube — and it forms nearer the hydrochloric acid end.

Explanation (the marks):

Both substances give off gases that diffuse along the tube and meet in the middle, reacting to form white ammonium chloride: ammonia + hydrogen chloride → ammonium chloride (NH₃ + HCl → NH₄Cl).
The ring forms nearer the HCl end because the ammonia molecules (NH₃) are lighter than the hydrogen chloride molecules (HCl), so ammonia diffuses faster and travels further before they meet.

Key rule: at the same temperature, lighter molecules diffuse faster than heavier molecules. (Mᵣ of NH₃ = 17; Mᵣ of HCl = 36.5, so NH₃ moves faster.)

Factor

Effect

Why

Mass of the molecule (Mᵣ)

lighter = faster

at the same temperature, lighter molecules move faster

Temperature

higher = faster

particles gain more energy and move faster, so they spread more quickly

Diffusion & Dilution

Detailed Notes

What you’ll learn

How it’s examined

1Define diffusion (Getting started)

2Colour spreading in still water (Getting started)

3The white ring experiment (Building confidence)

4Predict which gas diffuses faster (Building confidence)

5Effect of temperature on diffusion (Stretch)

6Diffusion as evidence for the particle model (Stretch)

Diffusion

Concentration

Dilution

Random motion

Rate of diffusion

Relative molecular mass (Mᵣ)

Ammonium chloride (NH₄Cl)

Ammonia (NH₃)

✕Defining diffusion as just 'particles spreading out'

✕Saying the ring forms nearer HCl because hydrogen chloride is slower/heavier without linking to ammonia

✕Thinking diffusion is caused by density, gravity or one gas 'sinking'

✕Saying the colour spreads because the water was stirred or moved

✕Saying the colour gets paler because the coloured particles 'disappear' or are 'used up'

✕Stopping at 'higher temperature gives particles more energy' without linking to faster diffusion

Past paper style quiz

Diffusion & Dilution

Detailed Notes

What you’ll learn

How it’s examined

1Define diffusion (Getting started)

2Colour spreading in still water (Getting started)

3The white ring experiment (Building confidence)

4Predict which gas diffuses faster (Building confidence)

5Effect of temperature on diffusion (Stretch)

6Diffusion as evidence for the particle model (Stretch)

Diffusion

Concentration

Dilution

Random motion

Rate of diffusion

Relative molecular mass (Mᵣ)

Ammonium chloride (NH₄Cl)

Ammonia (NH₃)

✕Defining diffusion as just 'particles spreading out'

✕Saying the ring forms nearer HCl because hydrogen chloride is slower/heavier without linking to ammonia

✕Thinking diffusion is caused by density, gravity or one gas 'sinking'

✕Saying the colour spreads because the water was stirred or moved

✕Saying the colour gets paler because the coloured particles 'disappear' or are 'used up'

✕Stopping at 'higher temperature gives particles more energy' without linking to faster diffusion

Past paper style quiz