Diffusion

CIE IGCSE Chemistry· 1.2· 12 min read

1. Core: Definition of Diffusion★☆☆☆☆⏱ 3 min

📘 Definition

Diffusion

The net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.

Diffusion is a direct consequence of the kinetic particle theory: all particles in gases, liquids, and (very slowly) solids are in constant random motion, so they spread out to fill available space without needing external force.

📐 Worked Example

A student sprays deodorant at the front of a classroom. After 10 seconds, students at the back of the room can smell the deodorant. Explain this observation using diffusion.

1
Identify the moving particles: deodorant gas particles released from the spray.
2
State the concentration gradient: high concentration of deodorant near the spray, low concentration at the back of the room.
3
Link to random motion: gas particles move randomly, so they spread from the high concentration region to the low concentration region until evenly distributed.

Exam tip:

Always reference random particle motion and concentration gradient in your explanation to get full marks for diffusion questions.

2. Core: Evidence for Diffusion in Gases and Liquids★★☆☆☆⏱ 4 min

Diffusion is visible in experiments with coloured or strongly scented substances, which provide clear evidence for the movement of particles that are too small to see individually.

Gas example: When a glass jar of red-brown bromine gas is placed upside down on a jar of air, the brown colour spreads evenly through both jars over several minutes, showing gas particles diffuse freely.
Liquid example: A small crystal of purple potassium manganate(VII) dropped into a beaker of water slowly spreads to turn the entire solution purple, without stirring, showing liquid diffusion.

📐 Worked Example

Explain why diffusion happens much faster in gases than in liquids.

1
Compare particle arrangement and energy: Gas particles have much higher kinetic energy than liquid particles, with large spaces between them and almost no attractive forces.
2
Compare movement: Gas particles move freely at high speed, while liquid particles are closer together and collide more frequently, slowing their net movement.

Exam tip:

You do not need to recall specific experiment details, but you must be able to use given experimental observations to explain diffusion.

3. Extended Only: Diffusion Rate and Relative Molecular Mass★★★☆☆Extended only⏱ 4 min

📘 Definition

Relative Molecular Mass

$M_{r}$

The sum of the relative atomic masses of all atoms in a molecule. Lighter molecules have lower $M_{r}$ values.

At the same temperature, gas particles with a smaller relative molecular mass have a higher average speed, so they diffuse faster than gas particles with a larger relative molecular mass. This is a qualitative comparison only: lighter gas particles diffuse faster than heavier ones.

📐 Worked Example

Hydrogen chloride ( $H C l$ , $M_{r} = 36.5$ ) and ammonia ( $N H_{3}$ , $M_{r} = 17$ ) gases are released at opposite ends of a long glass tube. A white ring of ammonium chloride forms where the gases meet. Explain why the ring forms closer to the hydrogen chloride end of the tube.

1
Compare $M_{r}$ values: $N H_{3}$ has a lower $M_{r}$ than $H C l$ , so $N H_{3}$ molecules are lighter.
2
Link to diffusion rate: Lighter $N H_{3}$ particles diffuse faster than heavier $H C l$ particles at the same temperature.
3
Explain position: The faster-moving $N H_{3}$ travels further along the tube in the same time, so the reaction happens closer to the $H C l$ end.

Exam tip:

Extended questions often ask you to predict which gas diffuses faster when given their formulae: first calculate $M_{r}$ for each, then state the lighter one diffuses faster.

4. Structured Exam Answer Practice★★★☆☆⏱ 3 min

Exam questions for this topic often ask you to apply diffusion concepts to unfamiliar scenarios, rather than just recall definitions. Always structure your answer to include all required marking points.

✓ Quick check

A student adds a drop of blue food colouring to a beaker of warm water and another drop to a beaker of cold water. The colour spreads faster in the warm water. Explain why.
Reveal answer
Particles in warm water have higher kinetic energy than particles in cold water, so they move faster. This means the food colouring particles diffuse down the concentration gradient more quickly in warm water.

5. Common Pitfalls

Wrong move:

Saying diffusion is particles "moving to where there is empty space" instead of down a concentration gradient.

Why:

Marks are awarded for referencing the concentration gradient, not just empty space.

Correct move:

Always state particles move from higher to lower concentration down a concentration gradient.

Wrong move:

Explaining diffusion in solids is as fast as in liquids or gases.

Why:

Diffusion in solids is extremely slow because particles are tightly packed and only vibrate in fixed positions.

Correct move:

Specify that observable diffusion only occurs in gases and liquids.

Wrong move:

(Extended) Stating heavier molecules diffuse faster because they have more momentum.

Why:

At the same temperature, all gas particles have the same average kinetic energy, so lighter molecules move faster.

Correct move:

Link diffusion rate directly to relative molecular mass: lower $M_{r}$ = faster diffusion.

Wrong move:

Forgetting to mention random particle motion in explanations.

Why:

Random motion of particles is the underlying cause of diffusion, so it is a required marking point for most 2+ mark questions.

Correct move:

Include the phrase "random motion of particles" in all diffusion explanations.

Wrong move:

Claiming diffusion requires energy input from a cell or external source.

Why:

Diffusion is a passive process that happens spontaneously due to particle motion.

Correct move:

State that diffusion is passive and requires no external energy input.

6. Quick Reference Cheatsheet

Concept	Core Only	Extended Only
Diffusion Definition	Net movement of particles from high to low concentration, down a concentration gradient, due to random motion	Same as core, plus a smaller $M_{r}$ means faster diffusion (qualitative)
Rate Comparison	Gases > Liquids >> Solids	Lower $M_{r}$ = faster gas diffusion at same temperature
Key Marking Points	Concentration gradient, random particle motion	All core points, plus $M_{r}$ comparison
Common Experiment Contexts	Bromine gas diffusion, potassium manganate(VII) in water	Ammonia + hydrogen chloride glass tube experiment

7. Frequently Asked

Is diffusion a passive or active process?

Diffusion is passive: it happens spontaneously due to random particle motion, no external energy input is required.

Do I need to recall specific diffusion experiments for the exam?

You do not need to memorise experiment details, but you must be able to explain given experimental observations using diffusion concepts.

Going deeper

syllabusCIE IGCSE Chemistry 0620 2026-2028 Spec

What's Next

Now that you have mastered diffusion concepts for CIE IGCSE Chemistry 0620, you are ready to move on to the next topics in the States of Matter unit, including changes of state and detailed properties of solids, liquids and gases. Understanding diffusion is also foundational for later topics such as movement of substances in and out of cells (for combined science learners) and gas properties in the quantitative chemistry unit. Make sure you practice structured answer writing for diffusion questions, as these are commonly tested in both Paper 3 (Core) and Paper 4 (Extended) theory exams, and are worth 2-3 marks per question. Revisit the common pitfalls section before your exam to avoid easy mark losses.