Diffusion
The movement of particles from high to low concentration, and the effect of relative molecular mass.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.2.1Describe and explain diffusion in terms of the random motion of particles.
- 1.2.2Explain why gases with a lower relative molecular mass diffuse faster.Supplement
6 minute read
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It happens in liquids and gases, where particles are free to move past one another, but not in solids.
No external force or stirring is needed — diffusion happens purely because particles move randomly in all directions, so over time they spread out and mix until evenly distributed.
Diffusion and relative molecular mass
At a given temperature, all particles have the same average kinetic energy. Since Ek = ½mv², a lighter particle must be moving faster than a heavier one to have the same kinetic energy. This means gases with a lower relative molecular mass diffuse faster than gases with a higher relative molecular mass.
This is demonstrated by the classic ammonia and hydrogen chloride experiment: cotton wool soaked in ammonia solution (Mr 17) is placed at one end of a tube, and cotton wool soaked in hydrochloric acid (Mr 36.5) at the other. A white ring of ammonium chloride forms where the two gases meet — and because ammonia is lighter and diffuses faster, the ring forms nearer the hydrochloric acid end.
Rate of diffusion
Diffusion is faster at higher temperatures, because particles have more kinetic energy and move faster.
Think of it like this
Diffusion is like a drop of ink spreading through still water: no one stirs it, but the random jostling of the water molecules gradually carries the ink particles outward until the colour is evenly spread through the whole glass.
Common misconceptions
- Thinking diffusion needs stirring or another external force. It happens by itself, driven entirely by the random motion of particles.
- Believing heavier gas particles diffuse faster because they have "more energy". At the same temperature all particles share the same average kinetic energy, so it is the lighter particles that move faster.
- Assuming the white ring in the ammonia/HCl experiment forms exactly in the middle. It forms closer to the heavier, slower-diffusing gas (HCl), because the lighter ammonia travels further in the same time.
In the exam
- When comparing diffusion rates, always compare relative molecular masses explicitly and state that the lighter particles move faster — do not just say one gas "diffuses better".
- Diffusion questions about solids expect the answer "diffusion does not happen (or is negligible) in solids, because the particles are not free to move from place to place".