What is diffusion and how does it occur in different states of matter?

Diffusion is the process by which particles spread from an area of higher concentration to an area of lower concentration. In gases, diffusion occurs rapidly due to the high kinetic energy and large spaces between particles. In liquids, diffusion is slower as particles are closer together and move less freely. In solids, diffusion is very slow because particles are tightly packed and can only vibrate in place. Understanding diffusion is essential for Cambridge IGCSE Chemistry as it explains how substances mix and interact at the molecular level.

Why is diffusion important in biological systems?

Diffusion is crucial in biological systems because it allows for the movement of substances such as oxygen, carbon dioxide, and nutrients across cell membranes. This process is vital for cellular respiration and nutrient uptake. In the Cambridge IGCSE curriculum, understanding diffusion helps explain how cells maintain homeostasis and how substances are transported in living organisms.

How does temperature affect the rate of diffusion?

Temperature significantly impacts the rate of diffusion. As temperature increases, particles gain kinetic energy and move more rapidly, leading to a faster rate of diffusion. Conversely, at lower temperatures, particles move more slowly, and diffusion occurs at a reduced rate. This concept is important in Cambridge IGCSE Chemistry as it helps explain how temperature influences the behavior of particles in different states of matter.

Can diffusion occur in solids, and if so, how?

Yes, diffusion can occur in solids, but it happens at a much slower rate compared to liquids and gases. In solids, particles are tightly packed and can only vibrate in fixed positions. Over time, atoms or molecules can move from one position to another, allowing diffusion to occur. This slow process is relevant in Cambridge IGCSE Chemistry when discussing the movement of atoms in solid materials, such as alloys.

What are some real-life examples of diffusion?

Real-life examples of diffusion include the spreading of perfume in a room, the mixing of food coloring in water, and the exchange of gases in the lungs. These examples illustrate how diffusion allows substances to spread and mix naturally. In the Cambridge IGCSE Chemistry curriculum, such examples help students understand the practical applications of diffusion in everyday life and scientific processes.

Why is "Saying diffusion needs energy input" a common mistake in Diffusion?

Why it happens: Confusing with active transport. How to avoid it: Diffusion is PASSIVE — driven by random thermal motion of particles.

Why is "Inverting the relationship between rate and $M_{r}$" a common mistake in Diffusion?

Why it happens: Memory slip. How to avoid it: Heavier particles move SLOWER. So bigger M_r → slower diffusion. Source: 0620/42 — recurring

Why is "Using $r \propto 1/M_{r}$ instead of $1/\sqrt{M_{r}}$" a common mistake in Diffusion?

Why it happens: Forgetting the square root. How to avoid it: Graham's law uses √(M_r) — at equal kinetic energy, v 1/√(m).

Why is "Saying 'heat makes molecules go faster' without 'kinetic energy'" a common mistake in Diffusion?

Why it happens: Lazy phrasing. How to avoid it: Examiners want 'increase in kinetic energy' as the link from temperature to motion.

States of MatterCambridge IGCSE Chemistry (0620)

Diffusion

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Diffusion — Cambridge IGCSE 0620 Chemistry Extended (2026)

Net movement of particles from high concentration to low. Faster in gases, faster at higher temperature, faster for lighter molecules. The classic NH₃-HCl tube experiment.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.2 — Describe diffusion in terms of particle motion.
1.2 — Explain why diffusion is faster in gases than in liquids.
1.2 — Compare rates of diffusion of different gases (Extended).

Diffusion: net particle movement

Particles move from where there are many to where there are fewer.

Diffusion. The net movement of particles from a region of HIGH concentration to a region of LOW concentration. The particles themselves move randomly; the NET (overall) flow is from high to low because there are more particles to move OUT of the crowded region than IN.

Why does it happen? Particles in fluids have kinetic energy → constant random motion → they spread out into available space.

No diffusion in solids. Particles vibrate in fixed positions; they don't migrate (under normal conditions).

Worked qualitative. A drop of dye placed in a glass of water: the colour spreads out. Eventually, the entire water is uniformly coloured (equilibrium reached). At this point diffusion still occurs at a particle level, but there's no NET movement.

Random particle motion spreads particles out until concentration is uniform.

Common examples.

Smelling perfume from across a room (gas diffusion).
Dye spreading in water (liquid diffusion).
Tea brewing (water-soluble compounds diffusing out of leaves).

Net movement: high → low concentration.
Driven by random KE of particles.
Stops when concentration uniform (no NET movement).
No diffusion in solids.

What changes the rate of diffusion

Faster in gases, at higher $T$ , for lighter molecules.

Three big factors.

State of matter. Gases diffuse fastest (particles have most freedom). Liquids slower. Solids: practically no diffusion.
Temperature. Higher $T$ → particles have more kinetic energy → move faster → diffuse faster.
Mass of the diffusing particle. At the same $T$ , lighter particles move faster (KE = $\frac{1}{2}mv^{2}$ , so smaller $m$ → larger $v$ for the same KE).

Worked qualitative. Place soaked NH₃ cotton wool at one end of a long glass tube and HCl-soaked cotton wool at the other end. A white ring of solid NH₄Cl forms where the gases meet. The ring forms CLOSER to the HCl end — because:

$M_r(\text{NH}_{3}) = 17$ . $M_r(\text{HCl}) = 36.5$ .
NH₃ is lighter → moves faster → travels FURTHER along the tube before meeting HCl.

This is the classic Cambridge gas-diffusion experiment.

Lighter NH₃ diffuses faster, so the white NH₄Cl ring forms closer to the HCl end.

Cambridge tip. When asked to compare diffusion rates of two gases, mention both relative molecular masses AND the fact that lighter = faster.

Faster: gas > liquid > solid.
Faster: higher $T$ .
Faster: lighter $M_r$ .
NH₃ travels further than HCl in the classic tube.

How it’s examined

Diffusion is a 2-3 mark item on most Paper 2s (define, give an example) and a 4-5 mark item on Paper 4 (the classic NH₃/HCl tube). Examiner reports flag students saying particles 'stop' at equilibrium — they don't, they just balance net movement.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.2); 0620/42 Oct/Nov 2024 — Q2 (NH₃/HCl tube); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Diffusion

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

1Define diffusion
Core• definition
Question
Define diffusion in particle terms.
Step-by-step solution
1. Step 1
  Random motion of particles spreads them from a region of high concentration to one of low concentration.
2. Step 2
  Continues until particles are evenly distributed (equilibrium).
Answer
Net random movement of particles from higher to lower concentration until evenly mixed.
2Compare diffusion rates
Extended• Adapted from 0620/42 May/Jun 2024 Q2• rate
Question
$\text{H}_{2}$ ( $M_{r} = 2$ ) and $\text{O}_{2}$ ( $M_{r} = 32$ ) diffuse at room temperature. Which diffuses faster, and by what ratio?
Step-by-step solution
1. Step 1
  $r \propto \dfrac{1}{\sqrt{M_{r}}}$ at the same temperature.
2. Step 2
  $\dfrac{r_{H_{2}}}{r_{O_{2}}} = \sqrt{\dfrac{32}{2}} = \sqrt{16} = 4$ .
Answer
$\text{H}_{2}$ diffuses $4$ times faster than $\text{O}_{2}$ .
3Effect of temperature on diffusion
Extended• temperature
Question
Explain why diffusion is faster at higher temperatures.
Step-by-step solution
1. Step 1
  Higher temperature → particles have more kinetic energy.
2. Step 2
  Faster motion → mix and spread out more quickly.
Answer
Higher temperature increases particle kinetic energy → faster random motion → faster diffusion.
4Direction of diffusion in a classic demo
Core• demo
Question
A bottle of perfume is opened in one corner. Where would the smell appear first?
Step-by-step solution
1. Step 1
  Perfume particles diffuse outward from the source.
2. Step 2
  First detected nearest to the bottle, then further away.
Answer
Closest to the bottle first, then progressively further away.

Key Formulae — Diffusion

The formulae you need to memorise for diffusion on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Rate of diffusion (gases)
$r \propto \dfrac{1}{\sqrt{M_{r}}}$
$M_{r}$
relative molecular mass
When to use
Comparing two gases at the same temperature and pressure.

Key Definitions and Keywords — Diffusion

Definitions to memorise and the exact keywords mark schemes credit for diffusion answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Diffusion
Examiner keyword
Net random movement of particles from a region of higher concentration to a region of lower concentration.
Concentration gradient
Examiner keyword
Difference in concentration between two regions; drives diffusion.

Common Mistakes and Misconceptions — Diffusion

The traps other students keep falling into on diffusion questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying diffusion needs energy input
Why it happens
Confusing with active transport.
How to avoid it
Diffusion is PASSIVE — driven by random thermal motion of particles.
✕Inverting the relationship between rate and $M_{r}$
0620/42 — recurring
Why it happens
Memory slip.
How to avoid it
Heavier particles move SLOWER. So bigger $M_{r}$ → slower diffusion.
✕Using $r \propto 1/M_{r}$ instead of $1/\sqrt{M_{r}}$
Why it happens
Forgetting the square root.
How to avoid it
Graham's law uses $\sqrt{M_{r}}$ — at equal kinetic energy, $v \propto 1/\sqrt{m}$ .
✕Saying 'heat makes molecules go faster' without 'kinetic energy'
Why it happens
Lazy phrasing.
How to avoid it
Examiners want 'increase in kinetic energy' as the link from temperature to motion.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Video lesson

Short walkthrough of the concepts students most often get stuck on.

Diffusion — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Diffusion

Short Study Notes in the form of Slides

Detailed Notes

What you’ll learn

How it’s examined

1Define diffusion

2Compare diffusion rates

3Effect of temperature on diffusion

4Direction of diffusion in a classic demo

Rate of diffusion (gases)

Diffusion

Concentration gradient

✕Saying diffusion needs energy input

✕Inverting the relationship between rate and MrM_{r}Mr​

✕Using r∝1/Mrr \propto 1/M_{r}r∝1/Mr​ instead of 1/Mr1/\sqrt{M_{r}}1/Mr​​

✕Saying 'heat makes molecules go faster' without 'kinetic energy'

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Diffusion — frequently asked questions

✕Inverting the relationship between rate and $M_{r}$

✕Using $r \propto 1/M_{r}$ instead of $1/\sqrt{M_{r}}$