Who this is for: Cambridge IGCSE Chemistry (0620) students who can define diffusion but struggle to explain why it happens or which factors speed it up in exam answers.
What query it owns: how to understand and revise diffusion in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the diffusion revision-guide angle, while Tutopiya’s Diffusion subtopic page owns the learning resource and the free Diffusion quiz owns the practice.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by random particle movement. It appears throughout Cambridge IGCSE Chemistry (0620) — from gases mixing to iodine crystals subliming in a jar. Examiners reward answers that link kinetic energy, concentration gradient and state of matter. This guide shows how to answer diffusion questions with full explain marks.

Key takeaways

• Diffusion = net movement down a concentration gradient due to random particle movement.
• Faster in gases than liquids — particles are further apart with more kinetic energy.
• Temperature increases rate (more kinetic energy); greater surface area increases contact.
• Explain answers need random movement + concentration difference — not just “particles spread out”.
• Test with the Diffusion quiz after the Diffusion notes.

What is diffusion in Cambridge IGCSE Chemistry?

Diffusion is the spreading of particles of a substance so they mix with particles of another substance, occurring in liquids and gases where particles are free to move. In Cambridge IGCSE Chemistry (0620) you must define diffusion, explain it using the particle model, and predict how temperature and state affect its rate.

The full notes and diagrams are on Tutopiya’s Diffusion subtopic page.

Factors affecting the rate of diffusion

Factor	Effect on rate	Particle-model reason
Higher temperature	Faster	Particles have more kinetic energy; move faster
Gas vs liquid	Faster in gas	Particles further apart; collide less; move more freely
Steeper concentration gradient	Faster initially	Greater difference drives net movement
Larger surface area	Faster (practical contexts)	More particles can enter/leave at once

How to answer diffusion questions — step by step

1. Define diffusion if the command word is define — include concentration gradient and random movement.
2. Identify the states involved — gas-gas, gas-liquid, etc.
3. For explain questions, state particles move randomly; more particles start in the high-concentration region; net movement is to the low-concentration region until evenly mixed.
4. For rate questions, link temperature or state to kinetic energy and collision frequency.
5. Confirm with the free Diffusion quiz.

Diffusion in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical stem
Define	Precise definition	”Define diffusion.”
Explain	Mechanism with particles	”Explain how bromine vapour spreads through a gas jar.”
Describe	What is observed	”Describe what happens to the colour of the gas jar over time.”
Suggest	Apply to new context	”Suggest why a gas leak is detected more quickly in a warm room.”

Worked exam-style stems (how to answer the wording)

1. “Define diffusion.” Target: the net movement of particles from a region of higher concentration to a region of lower concentration. Define = syllabus wording — avoid “spreading” alone without concentration gradient.
2. “Explain why a crystal of potassium manganate(VII) turns water purple slowly at the bottom first.” Target: particles move randomly in the liquid; net movement from high concentration (near crystal) to low concentration (rest of water) until evenly distributed. Reward: random movement + concentration difference.
3. “Bromine vapour and air are in connected gas jars. Explain why the brown colour spreads.” Target: bromine and air particles move randomly; net movement from higher bromine concentration until uniform mixture. Link to Solids, Liquids and Gases particle movement in gases.

Practise more stems on the States of Matter topical past paper questions.

How diffusion connects to the particle model

Diffusion depends on the particle ideas from Solids, Liquids and Gases. Solids do not show diffusion at IGCSE level because particles are fixed. Liquids and gases allow net movement down a gradient. The Cambridge IGCSE Chemistry resource hub links both subtopics and the Solids, Liquids and Gases quiz for particle-model revision.

Common mistakes students make

• Defining diffusion as “particles spreading” without concentration gradient.
• Saying particles move in one direction — movement is random; net movement is down the gradient.
• Confusing diffusion with osmosis (osmosis is water through a partially permeable membrane).
• Ignoring state of matter when comparing rates.
• Skipping the Diffusion quiz after reading notes.

When you need more support

If explain-style diffusion questions still score partial marks, drill the States of Matter topical past paper questions and book a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

Does diffusion happen in solids? Not at IGCSE level — particles in solids are fixed; diffusion is examined in liquids and gases.

Why is diffusion faster at higher temperature? Particles have more kinetic energy and move faster, so mixing occurs more quickly.

What is the difference between diffusion and osmosis? Diffusion is net movement of any particles down a concentration gradient; osmosis is specifically the movement of water through a partially permeable membrane.

How do I revise diffusion effectively? Read Diffusion notes, practise define/explain stems, take the Diffusion quiz, then cross-check particle movement on the Solids, Liquids and Gases subtopic.

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