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wise otter.Chemistry
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K324 / 2027
Particles and Atoms

Lesson 2 of 4

Diffusion: spreading without stirring

Connect everyday observations to random particle motion.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can use evidence for particle movement.Syllabus

    Syllabus K324, 2.1(b). Use evidence for particle movement

  • I can explain everyday diffusion.Syllabus

    Syllabus K324, 2.1(c). Explain everyday diffusion

  • I can predict diffusion-rate changes.Syllabus

    Syllabus K324, 2.1(d). Predict diffusion-rate changes

Diffusion: spreading without stirring

Connect everyday observations to random particle motion.

Make a guess. You are not marked.

Cotton wool soaked in ammonia is put at one end of a long tube and cotton wool soaked in hydrochloric acid at the other. A white ring of ammonium chloride forms where the gases meet. Where is the ring?

  1. Exactly in the middle of the tube
  2. Nearer the hydrochloric acid end
  3. Nearer the ammonia end of the tube
Show the answer

Nearer the hydrochloric acid end

Ammonia molecules (Mr 17) are lighter than hydrogen chloride molecules (Mr 36.5), so they diffuse faster and travel further before the gases meet.

Diffusion is the net spreading of particles from a region of higher concentration to one of lower concentration because particles move randomly. Individual particles travel in all directions; more leave the concentrated region than enter it. Eventually a uniform distribution develops, although particles keep moving.

Perfume and cooking aromas reach you as gas particles spread through air. Soluble coloured substances from tea or coffee spread through water, providing evidence that particles in liquids move too. Dissolving releases particles into the water; diffusion distributes them. Stirring and convection can also spread substances, so an experiment comparing diffusion should avoid stirring and large temperature gradients.

Check your understanding

Which two changes each make a gas diffuse faster?

  1. Raising the temperature, or using a gas of higher relative molecular mass
  2. Lowering the temperature, or using a gas of higher relative molecular mass
  3. Raising the temperature, or using a gas of lower relative molecular mass
Show the answer

Raising the temperature, or using a gas of lower relative molecular mass

Higher temperature gives particles more kinetic energy on average, so they move faster. At the same temperature, lighter molecules move faster on average. No diffusion formula is needed.

Worked exampleUse evidence without overclaiming

A coloured solution slowly spreads through still water. What does this support?

Try it first. Then show the worked stepsHide the worked steps
  1. There is no stirring to carry the colour through the whole beaker.
  2. The dissolved particles spread from the concentrated region into surrounding water.
  3. Random movement of particles explains the spreading.
Answer

The observation supports particle motion in a liquid. It does not show individual atoms directly.

Check your understanding

Why does perfume generally spread faster in warmer air?

  1. The warmth makes the liquid produce more perfume particles to spread.
  2. The particles expand in the heat, so they spread over a larger space.
  3. The particles have more kinetic energy on average, so they move faster.
Show the answer

The particles have more kinetic energy on average, so they move faster.

A higher temperature means a greater average kinetic energy, so particles move and mix faster. The particles do not change size, and no new perfume particles are made.