# A perfume bottle is opened at one end of a room. After a short time, a person standing at the other end of the room can smell the perfume. Using kinetic theory, explain why the perfume smell spreads through the room and why it takes some time to reach the other end.

> WJEC A-Level Physics (Wales) — 3.3 Kinetic theory · Explain · 5 marks

## Mark scheme (5 marks)

1. Perfume particles / molecules are in constant random motion
2. Perfume particles collide with air particles / molecules and spread out in all directions
3. This spreading of particles from a region of higher concentration to lower concentration is called diffusion
4. The perfume particles must travel a long / indirect path because they undergo many collisions with air molecules
5. The overall (net) movement of perfume particles across the room therefore takes time / is slow compared to the speed of individual particles

## Key terms

- [kinetic theory](https://www.gradenine.co.uk/glossary/kinetic-theory)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-perfume-bottle-is-opened-at-4bf0fbbc) · Published by Druglandscape Ltd.