Explain why smoke particles suspended in still air move in a random, zigzag path.

Cambridge International IGCSE Physics (0625) — 2.1 Kinetic particle model of matter · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When smoke is introduced into a glass cell containing still air and observed under a microscope with a light source, the tiny smoke particles are seen to move continuously and randomly in all directions.

Model answer (4 marks)

Air molecules are in continuous, rapid random motion.
When they collide with the suspended smoke particles, the impacts are unequal in number and force from different directions at any instant.
These unbalanced collisions give the smoke particles a random, zig‑zag trajectory.
This phenomenon is known as Brownian motion – the smoke particles are much larger and more massive than the air molecules, so they are moved by the fast‑moving air molecules.

Examiner tips

  • Use the term "Brownian motion". Show the chain: air molecules move → collide with smoke particles → unequal forces → random path. Mention size/mass difference to justify why smoke particles are affected.
  • common_mistakes
  • :
  • Failing to mention Brownian motion. Omitting the unequal collision explanation. Using vague terms like "random motion" without linking to collisions.

Mark scheme (4 marks)

  1. Air molecules are in continuous/rapid random motion
  2. Air molecules collide with the smoke particles
  3. The collisions are unequal/unbalanced in number or force from different directions at any instant
  4. This is Brownian motion — the smoke particles are much more massive/larger than air molecules so they are moved by the light, fast-moving air molecules

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