PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure
Physics 6.18 Diffusion of gases and liquids — Random Particle Motion and Temperature
5 min read 6 parts Lesson 18 of 19
Lesson summary
Physics 6.18 Diffusion of gases and liquids — Random Particle Motion and Temperature — Alhussan International Academy — Physics (Stages 7–9) notes: Use the particle theory to explain diffusion in gases and liquids and describe the evidence for it.Diffusion = the random movement a…
- No stirring needed
- Everyday example
- Size and mass of particles
- State of the substances
- Bromine demonstration
- Potassium manganate(VII)
Part 1 of 6What diffusion is
- Diffusion = the random movement and mixing of particles.
- It results in a net movement from a region of high concentration to one of low concentration.
- No stirring needed — Particles in gases and liquids are moving, so you do not need to move or stir a substance to make diffusion happen.
- Everyday example — Food particles evaporate from cooking, move randomly among air particles and spread through a house until they reach your nose.
Part 2 of 6Factors affecting the speed of diffusion
- TemperatureParticles in a warmer gas or liquid move faster, so diffusion is faster.
- Size and mass of particlesThis also affects the speed of mixing.
- State of the substancesDiffusion is fastest in gases, slower in liquids and almost absent in solids.
Part 3 of 6Diffusion in gases
- Fast — A gas particle can travel a long distance before it hits another particle.
- Bromine demonstration — When the lid between a jar of air and a jar of bromine gas is removed, the gases diffuse into each other until the mixture is even.
Part 4 of 6Diffusion in liquids
- Potassium manganate(VII) — A crystal placed in water slowly spreads its purple colour through the whole beaker.
- Slower than in gases — Liquid particles are closer, collide more often and have stronger forces between them.
Part 5 of 6Diffusion and temperature
- Warm food — Its particles diffuse more quickly than those from cool food.
- Hotter liquids and gases — Particles move faster, so diffusion is faster.
| State | Particle movement | Rate of diffusion |
|---|---|---|
| Gas | Fast, long free paths | Fastest |
| Liquid | Slower, frequent collisions | Slow |
| Solid | Vibrate only | Almost none |
Part 6 of 6Brownian motion as evidence
- Robert Brown — In 1828 he suspended pollen grains in water and saw them move quickly in a random motion through a microscope.
- Explanation — The grains are pushed around by the random movements of water particles.
- Water particles are tiny compared with pollen grains but move very fast, on average faster than 1600 km/h at 20 °C.
- Uneven pushes from many fast, invisible particles make the grain jiggle.
- Evidence — Smoke particles in air also show Brownian motion, supporting the particle model.
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