# 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

> 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.

## 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

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
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