# A student places a drop of ink into a beaker of still water at room temperature. Over the next few minutes, the ink slowly spreads throughout the water until the colour is uniform. Explain why the ink spreads through the water, and why the process happens more quickly if the water is heated.

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

## Mark scheme (5 marks)

1. Water molecules are in continuous random motion
2. Water molecules collide with ink particles, pushing them in random directions (Brownian motion / diffusion)
3. Over time this results in the ink becoming evenly spread / uniform throughout the water
4. When the water is heated, the water molecules have more kinetic energy and move faster
5. Faster-moving water molecules collide with ink particles more frequently / with greater force, so the ink spreads more quickly

## 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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