CAIE A Level Biology 9700 · Cell membranes and transport
A student prepared agar blocks of different sizes, each containing a pH indicator that turns colourless in acid. They placed each block into a dilute acid solution and recorded the time taken for the entire block to become colourless.
| Cube side length / cm | Time for complete colour change / min |
|---|---|
| 0.5 | 2 |
| 1.0 | 8 |
| 2.0 | 32 |
Explain the relationship between the size of the agar block and the time taken for the acid to diffuse completely through it. Use the data to support your answer.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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