1Brownian motion
Look at smoke through a microscope and the specks jiggle about at random. This is Brownian motion, named after Robert Brown, who saw it in 1827 with tiny bits from pollen grains in water.
Brownian motion
Lesson 29.3 notes
Brownian motion and diffusion
Evidence that particles move, and how gases and liquids spread and mix.
2What causes it
The specks are hit by air particles far too small to see. The hits don't balance, so each speck is pushed one way, then another. It's evidence that particles exist and move.
3Hotter, faster
Warm the air and its particles move faster. They hit the specks harder and more often, so the specks jiggle more.
4Diffusion
Spray perfume and soon you smell it across the room. Its particles move at random, bumping into air particles, and spread out: diffusion.
diffusion
5High to low concentration
Particles spread from where they're crowded, a high concentration, to where there are few, until they're evenly spread. They keep moving even then.
concentration
6Diffusion in liquids
Drop ink into water and its colour slowly spreads. Liquid particles are close together, so diffusion is slower than in a gas. It's faster in hot water than cold.
7A race in a tube
Ammonia and hydrogen chloride gases diffuse from opposite ends of a tube. Where they meet, a white ring forms: nearer the hydrogen chloride, because ammonia particles move faster.
Explore: Diffusion lab
Watch a smoke speck being knocked about, and show the air particles hitting it. Then lift the barrier between two gases and see them mix, hot or cold.
Common mistake: Particles don't 'want' to spread out
Particles have no aims: they just move at random. There are more of them where it's crowded, so more move out than in, until they're evenly spread.
Remember
- Brownian motion: specks of smoke jiggle at random because invisible, fast-moving air particles hit them unevenly.
- Diffusion: particles spread from where there are lots of them (high concentration) to where there are few (low concentration).
- Diffusion is faster in gases than in liquids, and faster when it's hotter.
Going further at GCSE