Diffusion

Summary: Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by random molecular motion. It explains how gases mix, how smells travel, and is fundamental to understanding particle theory in chemistry. Tags: igcse chemistry particulate-nature Created: 2026-07-14 Last Updated: 2026-07-17

Diffusion is the net movement of particles (atoms, molecules, or ions) from a region of higher concentration to a region of lower concentration, occurring as a direct consequence of the constant random motion described by the kinetic particle theory. It is a passive process — no external energy input is required, because the particles’ own kinetic energy drives their movement. Diffusion happens fastest in gases, where particles are far apart and move freely at high speeds; it is slower in liquids and negligible in solids. The rate of diffusion depends on temperature (hotter = faster), the relative molecular mass of the diffusing particles (lighter = faster), and the steepness of the concentration gradient. Everyday evidence includes the spread of perfume across a room and the gradual mixing of a coloured solution into water. Diffusion provides direct experimental evidence that matter is made of tiny particles in constant random motion.

1. Definition of Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient). It occurs because of the random motion of particles.

Key points about diffusion:

• Particles are in constant, random motion
• Particles collide with each other and with the walls of the container
• Over time, particles spread out to fill the available space evenly
• No energy input is required — diffusion is a passive process

2. Diffusion and Kinetic Particle Theory

Diffusion is explained by the kinetic particle theory:

1. All particles (atoms, molecules, ions) possess kinetic energy
2. Particles move randomly in all directions
3. Particles collide with each other and bounce off
4. The net effect is movement from where particles are more concentrated to where they are less concentrated — simply because there are more particles on the high-concentration side to randomly move across

Analogy: Imagine a room full of people (high concentration) with a door to an empty corridor (low concentration). People move randomly. Some will wander through the door into the corridor. Over time, people spread evenly between both spaces. No one is directing them — it is just a statistical consequence of random movement.

3. Factors Affecting the Rate of Diffusion

4. Effect of Relative Molecular Mass on Diffusion Rate

At the same temperature, all gas particles have the same average kinetic energy. Since kinetic energy = ½mv², particles with a lower mass must be moving at a higher speed. This means the rate of diffusion is inversely proportional to the square root of the relative molecular mass:

Rate of diffusion ∝ 1 / √Mr

The lighter the gas molecule, the faster it diffuses.

Example: Comparing hydrogen (H₂, Mr = 2) and oxygen (O₂, Mr = 32):

Rate(H₂) / Rate(O₂) = √(Mr(O₂) / Mr(H₂)) = √(32/2) = √16 = 4

Hydrogen diffuses 4 times faster than oxygen. See Relative Molecular Mass for more detail.

5. The Ammonia and Hydrogen Chloride Experiment

This is the classic IGCSE demonstration of diffusion:

Setup:

• Two cotton wool balls are soaked — one in concentrated ammonia solution (NH3(aq)), the other in concentrated hydrochloric acid (HCl(aq))
• They are placed at opposite ends of a glass tube and the ends are stoppered

Observations:

• A white ring of ammonium chloride (NH4Cl) forms inside the tube
• The ring forms closer to the HCl end than the NH3 end

Equation: NH3(g) + HCl(g) → NH4Cl(s)

Explanation:

• NH3 (Mr = 17) and HCl (Mr = 36.5) diffuse through the air in the tube
• NH3 is lighter → diffuses faster → travels further before they meet
• The ring forms where the two gases meet and react

Conclusion: The position of the white ring demonstrates that lighter molecules diffuse faster than heavier molecules.

6. Bromine Diffusion Demonstration

Another common IGCSE demonstration:

• A gas jar of bromine vapour (Br2, red-brown) is placed mouth-to-mouth with a gas jar of air
• After several minutes, both jars appear uniform in colour
• This shows that bromine molecules diffuse into the air jar, and air molecules diffuse into the bromine jar

7. Diffusion in Liquids

Diffusion occurs in liquids but is much slower than in gases because:

• Particles in liquids are closer together
• There are more frequent collisions that slow down net movement

Example: A crystal of potassium manganate(VII) (KMnO4, purple) placed at the bottom of a beaker of water will slowly spread to produce a uniform purple solution. This can take hours or days.

8. Diffusion and Dissolving

When a solid dissolves:

• Particles leave the surface of the solid and enter the liquid
• These particles then diffuse through the liquid
• Stirring/heating speeds up dissolving by speeding up diffusion

9. Diffusion vs. Convection

Key Concepts from Past Papers

Definitions You MUST Know (Exact Mark Scheme Wording)

• Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration, due to random particle motion
• Concentration gradient: the difference in concentration between two regions

Recurring Mark Scheme Answers

• Any two from: (particles move in) random motion
• (particles) collide
• (particles) move from a region of high concentration to low concentration
• Alternative phrases for collide: “down a concentration gradient”
• Molecules in (constant) movement / molecules collide / molecules travel
• (Movement of) molecules is random / haphazard / in every direction

Keywords from Past Papers

molecules, particles, concentration, movement, spread, collide, move, random, diffusion, out, down, gradient, constant, travel, every

Related Notes

• States of Matter — Particle arrangement in solids, liquids, and gases
• Kinetic Particle Theory — The theory that explains diffusion
• Relative Molecular Mass — Effect of Mr on diffusion rate
• Gas — Properties of gases
• Atomic Structure — Relative masses of atoms and molecules
• Rates of Reaction — Effect of temperature on particle collisions
• IGCSE-Chem-Index — Full IGCSE Chemistry index

Sources

• OpenStax Chemistry 2e — [Chapter 9: Gases (Effusion and Diffusion)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Diffusion], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 1.2: Diffusion, Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 14: Diffusion and Effusion], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q66(c)(i) (0m)
• 0620/33 May/June 2021: Q22(d)(ii) (3m)
• 0620/33 May/June 2023: Q88(c)(v) (0m)
• 0971/31 May/June 2018: Q33(a)(i) (3m)
• 0971/31 May/June 2021: Q22(f)(ii) (3m)
• 0971/32 May/June 2023: Q88(c)(iii) (0m)

Common Misconceptions