What are osmosis and diffusion?

Osmosis and diffusion: π = iMRT. Molecules jostle and spread out by themselves: that is diffusion. When a membrane lets water through but blocks salt or sugar, water moves towards the saltier side, pushing with a pressure that grows with how many particles are dissolved.

Read how it works: What are osmosis and diffusion? · The history of osmosis and diffusion · More boxes on Glassbox

Chapters in this interactive model

1. Water crosses to the saltier side: A U-tube with a membrane that lets water through but not sugar or salt, and a drop of dye spreading in still water.
2. Push harder than π and osmosis runs backwards: An RO purifier squeezes water out of salty water, and seawater leaves you drier than before.
3. Every cell in you is an osmosis machine: Red blood cells swell and shrink, oxygen diffuses into blood, and a pinch of salt and sugar saves lives.
4. Salt, sugar and a thirsty plant: Pickles and jam keep because microbes can't hold on to their water, and a plant stands up because its cells are pumped full.
5. Where the simple story breaks: Diffusion is hopeless over a few centimetres, strong solutions stray from the law, and water doesn't go where the water is.