Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, until they are evenly spread out. It happens because particles are always moving randomly.
Why diffusion happens
Particles in gases and liquids move continuously in random directions. Where the particles are crowded together they spread into the emptier space, giving a net flow down the concentration gradient. This continues until the particles are evenly mixed; diffusion then appears to stop, even though the particles keep moving. No stirring or shaking is needed — the particles do it themselves.
Factors affecting the rate
Diffusion is fastest in gases, where particles move quickly and are far apart, slower in liquids and negligible in solids. Raising the temperature gives particles more kinetic energy, so they move and mix faster. The relative molecular mass Mr also matters: at the same temperature, lighter molecules travel faster and so diffuse faster.
Key idea
The rate of diffusion increases with temperature and decreases with relative molecular mass: rate ∝ 1/√Mr. The lighter the molecule, the faster it diffuses.
Worked example
Cotton wool soaked in concentrated ammonia (NH₃, Mr = 17) and concentrated hydrochloric acid (HCl, Mr = 36.5) are placed at opposite ends of a glass tube. A white ring of ammonium chloride (NH₄Cl) forms nearer the HCl end, because the lighter NH₃ molecules diffuse faster and travel a greater distance before the two gases meet.
Remember
- Diffusion does not need any stirring or external force.
- Higher temperature → faster diffusion.
- Smaller Mr → faster diffusion.