What are osmosis and diffusion?
Water crosses a membrane towards salt with a pressure strong enough to hold up a 280 m column. Watch a U-tube rise, push seawater through RO, burst red blood cells, pickle a cucumber, and find out why diffusion needs a heart to help it.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Water chases the dissolved stuff. A semipermeable membrane lets water through but not salt or sugar. Water crosses towards the side with more dissolved particles, and the pressure it pushes with is π = iMRT: seawater's is about 27 bar.
- Push back and it runs backwards. Press on the salty side harder than π and water is squeezed out through the membrane instead. That is reverse osmosis, in every RO purifier and desalination plant.
- Your cells live by it. Red blood cells burst in pure water and shrivel in seawater, so a drip is 0.9% saline. Kidneys save water and the gut absorbs it by osmosis, which is why ORS works.
- Salt keeps food, water holds plants up. Brine and jam pull water out of microbes so they can't grow. Plant cells pump themselves full of water until their walls push back, and wilt when the soil is too salty.
- The limits: slow, non-ideal, and a myth. Diffusion time grows as distance squared: a cell takes milliseconds, a centimetre hours. Strong solutions stray from π = iMRT. And water flows to where there is less water, not more.
| Term | Meaning |
|---|---|
| Diffusion | The spreading of molecules from where there are more to where there are fewer, by random jostling. A cloud grows as √(2Dt). |
| Semipermeable membrane | A sheet that lets water through but blocks dissolved salt or sugar. |
| Osmosis | The net flow of water through a semipermeable membrane towards the side with more dissolved particles. |
| Osmotic pressure (π) | The pressure needed to stop water flowing in: π = iMRT. About 27 bar for seawater, 7.5 bar for blood. |
| Reverse osmosis | Pushing on salty water harder than its osmotic pressure, so pure water is squeezed out through a membrane. |
| Tonicity | Whether a liquid makes cells swell (hypotonic), stay the same (isotonic) or shrink (hypertonic). |
| Water activity (aw) | How available the water in a food is, from 1.0 down. Most bacteria need more than 0.91. |
| Turgor pressure | The push of a plant cell's contents against its wall, about 10 bar, that keeps it firm. |
From a swollen pig's bladder in 1748 to the membranes that water cities.
- 1748 · A pig's bladder swells and bursts (Abbé Jean-Antoine Nollet, Paris, France)
- 1855 · Fick writes the law of diffusion (Adolf Fick, Zurich, Switzerland)
- 1877 · Pfeffer measures osmotic pressure (Wilhelm Pfeffer, Basel, Switzerland)
- 1886 · π = iMRT: dissolved stuff acts like a gas (Jacobus Henricus van 't Hoff, Amsterdam, Netherlands)
- 1901 · The first Nobel Prize in Chemistry (Jacobus Henricus van 't Hoff, Stockholm, Sweden)
- 1960 · A membrane that desalts seawater (Sidney Loeb and Srinivasa Sourirajan, University of California, Los Angeles, USA)
- 1971 · ORS in the refugee camps (Dilip Mahalanabis, Bangaon, West Bengal, India)
- 1978 · The most important medical advance of the century? (The Lancet, London, England)
The full story, with 20 moments, charts, people and 31 sources: glassbox.how/e/osmosisclear/history. The data lives in history.json.
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