# A student places a drop of ink into a glass of still water at room temperature. Over several minutes, the ink spreads throughout the water without being stirred. Explain, using kinetic theory, why the ink spreads through the water.

> Eduqas A-Level Physics — 3.3 Kinetic theory · Explain · 5 marks

## Mark scheme (5 marks)

1. Water (and ink) particles are in continuous random motion
2. The ink particles collide with water particles (and vice versa)
3. These collisions cause the ink particles to move in different/unpredictable directions
4. Over time, ink particles spread throughout the water (this process is called diffusion)
5. No energy input (stirring) is needed because the particles already have kinetic energy / are already moving

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Physics](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-places-a-drop-of-df93a081) · Published by Druglandscape Ltd.