Diffusion chamber sim
Drop purple dye into either half of a divided tank, set the bath temperature, lift the partition and step the stopwatch on 30 s at a time, taking readings until both halves level off at the mean, while a close-up shows the particles still crossing both ways.
Movement of substances · covers the O-Level, IGCSE, GCSE topic
What the sim shows
A clear glass tank stands in a thermostatic water bath. A removable partition divides it into two equal halves. Purple potassium permanganate is dropped into one or both halves; the rest is water. Lift the partition and the dye particles start to cross the opening. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. The word net matters. Particles move randomly in all directions all the time. Where there are more of them, more happen to wander away than towards, so the overall drift is down the gradient. Nothing pushes them. Because it needs no push, diffusion is a passive process: it requires no energy from respiration. Once the concentration is equal everywhere, there is no gradient and no net movement. But the particles do not stop. They still move back and forth, just as many one way as the other. Model: the two halves are each treated as evenly mixed, linked through the opening. Rate of net movement = k × (difference in concentration), so the difference falls by the same share every second and both lines level off at the mean. k is larger for a bigger opening, a shorter distance and a higher temperature: warmer particles have more kinetic energy, so they move faster (D ∝ T/η, Stokes–Einstein, with the viscosity of water). Concentrations are in arbitrary units (au): one drop of stock dye adds 2 au to a half. Time is speeded up: a real unstirred tank this size takes months to even out. On this page, at 20 °C with the opening clear, the difference halves in about 42 s. In a real tank the colour spreads out from the gap as a fading front; here each half is drawn evenly mixed, as the model treats it. The close-up shows 3 dye particles for each au in each half; water particles are not drawn.
- The partition's grip: drag it up to open the gap between the halves
- Purple: potassium permanganate solution; clear: water
- Close-up: each dot is a dye particle; water particles are not drawn
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