Lesson overview
Diffusion: How does a smell cross a room?
Put a pad soaked in ammonia solution in a long tank of still air and find out which damp litmus strips turn blue first. Then try the pad in the middle, and look at the particles of the gas.
Suggested sequence
- Lesson 1 · A smell you can't see. Diffusion in gases and liquids, explained with randomly moving particles. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Lighter, faster. Why lighter gas particles diffuse faster (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: A smell you can't see
- Describe diffusion as the net movement of particles from a higher to a lower concentration, in gases and in liquids
- Explain diffusion in terms of kinetic particle theory: random motion and collisions, with no intended direction
Part 2: Lighter, faster
- Explain diffusion in terms of kinetic particle theory: random motion and collisions, with no intended direction
- Describe the effect of relative molecular mass on the rate of diffusion of gases, and predict where a reaction ring forms
- Explain the effect of relative molecular mass on the rate of diffusion of gases, using the average speed of the particles
Misconceptions to listen for
- “A gas fills its container at once.” — Not a long tank of still air. The strips turned one after another, so the ammonia took time to reach each one.
- “There is less ammonia once it has spread out.” — There is the same amount, spread through more space. Its concentration near the pad falls, but none leaves the closed tank.
- “Every ammonia particle moves away from the pad.” — Each one moves at random, so some head back towards the pad. The gas spreads away from the pad only because more particles cross from the crowded side.
- “The statement that some particles move back towards the pad is wrong, because the ammonia only moves away from the pad.” — Single particles move in every direction, including back towards the pad. Only the net movement is away from the pad.
- “The statement about more particles crossing from the pad side is wrong, because with random motion the same number must cross each way.” — Random motion means no direction is favoured for any one particle. But there are more particles on the pad side, so more of them cross from there.
- “The ammonia spreads because the particles push each other away from the crowded centre.” — Gas particles are far apart and only affect each other when they collide. The spreading comes from random motion plus there being more particles at the centre.
- “In the end, the particles stop moving.” — The particles of a gas never stop moving. When the concentration is even, as many cross each way, so nothing more changes.
- “Diffusion is slower in a liquid because the particles in a liquid move more slowly.” — The reason to give is spacing, not speed. Liquid particles are close together, so a particle collides almost all the time and gets very little distance before changing direction.
- “Stirring speeds up diffusion.” — Stirring moves the whole liquid and carries the particles along. That spreading is not diffusion, which needs no stirring at all.
- “The dye spreads out because it wants to fill the dish.” — No particle aims anywhere. Dye particles move at random; more cross outwards than back because there are more of them near the hole.
Scope
Diffusion explained with the random movement of particles (1.2.1) and the effect of relative molecular mass on the rate of diffusion of gases (Supplement 1.2.2). The experiment is a virtual version of a teacher demonstration with concentrated ammonia; time is schematic. Graham’s law as an equation is not part of this module.