# A student investigates how the size of a cube affects the rate of diffusion into it. She uses agar cubes of different sizes soaked in acid. Explain why the smallest cube becomes fully coloured by the indicator solution more quickly than the largest cube.

> AQA A-Level Biology (7402) — 3.3.1 Surface Area to Volume Ratio · Explain · 5 marks

> Agar cubes of different sizes are placed in an indicator solution. The indicator diffuses into the agar from the outer surface. The smallest cube (1 cm sides) becomes fully coloured much faster than the largest cube (4 cm sides).

## Mark scheme (5 marks)

1. The smallest cube has a larger surface area to volume ratio
2. A larger surface area to volume ratio means more diffusion can occur relative to the volume of the cube
3. In the largest cube the diffusion distance to the centre is greater
4. Diffusion is a slow/passive process so a longer distance takes more time
5. In the smallest cube the diffusion distance to the centre is short so the entire cube becomes coloured quickly

## Key terms

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

## Related

- [Revision notes for AQA A-Level Biology (7402)](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-student-investigates-how-the-size-784c9c17) · Published by Druglandscape Ltd.