1.2 Diffusion
- Syllabus
- 0620–2026–2027
- Topic
- 1.2
- Level
- —
Learning objectives
Explain diffusion with moving particles
Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration because the particles move randomly.
| Scale | What happens |
|---|---|
| particle scale | every particle moves randomly in changing directions and collides with other particles |
| concentration scale | more particles start in the higher-concentration region, so more cross out of it than cross back |
| final pattern | spreading continues until particles are evenly distributed; random motion still continues |
At higher temperature, particles have more average kinetic energy and move faster, so diffusion occurs faster. Diffusion is usually slower through a liquid than through a gas because liquid particles are much closer together.
Perfume first evaporates to form gas particles, then those particles diffuse through air. A soluble coloured crystal first dissolves, then its particles diffuse through the liquid.
Particles do not choose to move down a concentration gradient. Brownian motion is the irregular movement of visible particles bombarded by smaller particles; it is not another name for diffusion.
Explain how molecular mass affects gas diffusion
At the same temperature, gas particles with lower relative molecular mass, Mᵣ, have a higher average speed and therefore diffuse faster than particles with higher Mᵣ.
| Comparison under the same conditions | Diffusion result |
|---|---|
| lower Mᵣ | higher average particle speed; faster diffusion |
| higher Mᵣ | lower average particle speed; slower diffusion |
| equal Mᵣ | same expected diffusion rate |
Calculate each gas's Mᵣ from its formula, compare the totals, then state both links: lower Mᵣ → faster particles → faster diffusion. The explanation must compare the two gases, not just name one mass.
When ammonia and hydrogen chloride diffuse from opposite ends of a tube, ammonia has the lower Mᵣ and travels farther in the same time. Their solid product therefore forms nearer the hydrogen chloride end.
Keep temperature and other conditions the same when comparing molecular mass. Chemical reactivity, acidity and whether a gas is an element or compound do not determine its diffusion rate.