DiffusionCambridge IGCSE Chemistry: Revision notes
Section 1
What Is Diffusion?
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until the particles are evenly spread out.
Diffusion happens because particles are in constant, random motion. It occurs fastest in gases, more slowly in liquids, and (for practical purposes) does not happen in solids because particles cannot move from place to place.
- No stirring or outside force is needed — diffusion happens on its own
- It continues until particles are evenly distributed throughout the available space
- It is explained using the kinetic particle theory: all matter is made of particles that are constantly moving
A drop of perfume released in one corner of a room can eventually be smelt across the whole room — the scent particles diffuse through the air.
Section 2
Why Does Particle Arrangement Determine How Fast Diffusion Happens?
How quickly diffusion can occur depends on how the particles are arranged and how freely they can move in each state of matter.
| State | Particle separation | Arrangement | Motion |
|---|---|---|---|
| Solid | Very close together | Fixed, regular pattern | Vibrate in place only |
| Liquid | Close together | Random, no fixed pattern | Move around each other, slide past |
| Gas | Far apart | Random, spread out | Move freely at high speed in all directions |
- In a gas, particles are far apart and move quickly and randomly in all directions, so diffusion is fast
- In a liquid, particles are still close together, so diffusion is much slower than in a gas
- In a solid, particles are fixed in position and only vibrate, so diffusion does not occur
Don't say gas particles diffuse fast just because they are 'lighter' — it is because they are far apart, move freely, and have high kinetic energy, allowing large, fast, random movement.
Section 3
How Does Relative Molecular Mass Affect the Rate of Diffusion?
At the same temperature, all gas particles have the same average kinetic energy, but this does not mean they move at the same speed.
- Gases made of lighter particles (lower relative molecular mass) move faster on average and diffuse more quickly
- Gases made of heavier particles (higher relative molecular mass) move more slowly on average and diffuse more slowly
- This means a lighter gas will spread across a room, or through another gas, faster than a heavier gas released under the same conditions
Ammonia (NH3, Mr = 17) diffuses faster than hydrogen chloride gas (HCl, Mr = 36.5). In a classic tube experiment, the white ring of ammonium chloride forms closer to the HCl end, because the lighter ammonia molecules travel further in the same time.
Section 4
How Do Temperature and Pressure Affect the Volume of a Gas?
Kinetic particle theory also explains how gases respond to changes in temperature and pressure.
- Increasing temperature: particles gain kinetic energy and move faster, colliding with the container walls more often and with greater force — if the gas is free to expand, its volume increases
- Increasing pressure (at constant temperature): squeezing the same number of particles into a smaller space increases the frequency of collisions with the walls, so unless the container is fixed, the volume decreases
- These effects work in both directions: cooling a gas or reducing pressure has the opposite effect on volume
Link every temperature/pressure/volume question back to particle motion and collision frequency with the container walls — examiners reward explanations, not just the rule.
Must Know
- Diffusion is the net movement of particles from high to low concentration, caused by random particle motion
- Diffusion is fastest in gases, slower in liquids, and does not occur in solids
- Gases with a lower relative molecular mass diffuse faster than gases with a higher relative molecular mass
- All gas particles at the same temperature have the same average kinetic energy, but lighter particles move faster
- Increasing temperature increases gas volume (particles move faster, collide harder/more often)
- Increasing pressure decreases gas volume (particles are forced closer together)
That's the notes covered.
Carry on to the next subtopic.