Diffusion Exam Questions
Practice IGCSE Chemistry exam questions on diffusion: definitions, the bromine and ammonia-HCl demonstrations, and the effect of molecular mass on diffusion rate.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover every angle the 0620 examiners use for diffusion. Work through each one before checking the mark scheme — the act of writing the answer, not reading the answer, is what builds the skill.
Question 1 (2 marks, Core)
Define diffusion.
Mark scheme
- The net movement of particles from a region of higher concentration to a region of lower concentration [1]
- As a result of the random motion of the particles [1]
Examiner note: “Spreading out” alone is not sufficient. The direction (high → low) and the cause (random motion) are separate marking points.
Question 2 (3 marks, Core)
A gas jar of bromine vapour is placed below a gas jar of air and the cover slip is removed. After 30 minutes, both jars are an even brown colour.
(a) Explain why the bromine colour eventually becomes even throughout both jars. (2)
(b) State what this experiment provides evidence for. (1)
Mark scheme
(a) Bromine particles move randomly [1] and spread from the region of higher concentration (lower jar) to the region of lower concentration (upper jar) by diffusion [1]
(b) Evidence that particles are in constant random motion / evidence for kinetic particle theory [1]
Examiner note: Saying “bromine rises” implies directed movement and will not score. The movement is random — the net effect is spreading from high to low concentration. Stating it moves “upwards against gravity” is a valid supporting point but not required.
Question 3 (2 marks, Core)
A crystal of potassium manganate(VII) is dropped into a beaker of cold water. After several hours, the water is uniformly purple. The experiment is repeated with warm water.
Predict and explain the difference in the time taken for the colour to spread.
Mark scheme
- The colour spreads faster in warm water / takes less time [1]
- Because at a higher temperature the particles have more kinetic energy and move faster, so diffusion is faster [1]
Examiner note: “Heat makes it faster” without a particle-level explanation scores 0. Link temperature → kinetic energy → speed of particles.
Question 4 (3 marks, Supplement)
Cotton wool soaked in concentrated ammonia solution is placed at end A of a glass tube. Cotton wool soaked in concentrated hydrochloric acid is placed at end B. After several minutes, a white ring forms inside the tube.
(a) Name the white solid. (1)
(b) Explain why the ring forms closer to end B than to end A. (2)
Mark scheme
(a) Ammonium chloride [1]
(b) Ammonia has a lower relative molecular mass than hydrogen chloride (Mr: NH₃ = 17, HCl = 36.5) [1], so ammonia molecules move/diffuse faster and travel further along the tube in the same time [1]
Examiner note: Simply saying “ammonia is lighter” earns 1 mark. The second mark requires the consequence: moves faster / travels further. Calculating Mr values is not required but strengthens the answer.
Question 5 (2 marks, Supplement)
Hydrogen bromide (HBr) and ammonia (NH₃) are released simultaneously at opposite ends of a tube at the same temperature. Predict which end the white ring will form closer to, and explain your answer.
(Mr: HBr = 81, NH₃ = 17)
Mark scheme
- The ring forms closer to the HBr end [1]
- NH₃ has a lower Mr so its molecules move faster and travel further along the tube [1]
Examiner note: This is a transfer question — same principle as the HCl version, with different Mr values. The logic is identical: lighter gas diffuses faster, ring forms closer to the heavier gas.
Question 6 (3 marks, Core)
Explain why diffusion is faster in gases than in liquids.
Mark scheme
- In gases, particles are far apart / have large spaces between them [1]
- Gas particles move at high speeds / move freely [1]
- In liquids, particles are close together, so a diffusing particle collides frequently and makes slow progress [1]
Examiner note: A comparison is required. Writing only about gases or only about liquids limits you to 2 marks maximum. The question asks “why faster”, so both states must feature.
Question 7 (4 marks, Supplement)
A student sets up the ammonia–hydrochloric acid tube experiment at 20 °C and measures the distance from each end to the white ring.
| Distance from ammonia end | Distance from HCl end |
|---|---|
| 35.0 cm | 21.5 cm |
(a) Calculate the total length of the tube. (1)
(b) The student repeats the experiment at 40 °C. State and explain two changes you would expect to observe. (3)
Mark scheme
(a) 35.0 + 21.5 = 56.5 cm [1]
(b) The ring forms in less time [1] because at higher temperature both sets of molecules have more kinetic energy and move faster [1]. The ring still forms closer to the HCl end [1] because the ratio of Mr values has not changed — NH₃ is still lighter than HCl.
Examiner note: Many candidates incorrectly predict the ring moves to the centre at higher temperature. Temperature increases the speed of both gases equally (same average kinetic energy), so the ratio of distances stays the same. Only the total time changes.
Question 8 (2 marks, Core)
A student opens a bottle of perfume at the front of a classroom. After a few minutes, students at the back of the room can smell it.
Explain, in terms of particles, why this happens.
Mark scheme
- Perfume particles move randomly in all directions / diffuse through the air [1]
- They spread from the region of high concentration (near the bottle) to regions of lower concentration (back of room) [1]
Examiner note: “The smell travels through the air” is not a particle explanation. Name the particles and describe their random movement and the concentration gradient.
What to revise if you scored below 6
If you dropped marks on Questions 1–3, revisit the diffusion Core notes and memorise the two-part definition. If Questions 4–5 or 7 were difficult, work through the Mr calculations until you can calculate the molecular mass of any compound in under 30 seconds — the tube question is an Mr question dressed in a gas-jar diagram.
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