Topic 2 of 4
Diffusion: spreading without stirring
Connect everyday observations to random particle motion.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Pure onlyDiffusion: spreading without stirring
Connect everyday observations to random particle motion.
Diffusion is the net spreading of particles from a region of higher concentration to one of lower concentration because particles move randomly. Individual particles travel in all directions; more leave the concentrated region than enter it. Eventually a uniform distribution develops, although particles keep moving.
Perfume and cooking aromas reach you as gas particles spread through air. Soluble coloured substances from tea or coffee spread through water, providing evidence that particles in liquids move too. Dissolving releases particles into the water; diffusion distributes them. Stirring and convection can also spread substances, so an experiment comparing diffusion should avoid stirring and large temperature gradients.
| Change | Prediction | Particle explanation |
|---|---|---|
| Higher temperature | Faster diffusion | Greater average kinetic energy gives faster particle movement. |
| Lower molecular mass at the same temperature | Generally faster diffusion in a gas | Lighter molecules move faster on average at the same temperature. No quantitative diffusion law is required. |
Worked example
Use evidence without overclaiming
A coloured solution slowly spreads through still water. What does this support?
- There is no stirring to carry the colour through the whole beaker.
- The dissolved particles spread from the concentrated region into surrounding water.
- Random movement of particles explains the spreading.
The observation supports particle motion in a liquid. It does not show individual atoms directly.