# A student is investigating the effect of pH on the rate of enzyme activity using lipase and milk. Describe how the student should make their results more reliable, and explain why each step helps.

> OCR GCSE Biology A: Gateway Science (J247) — B7 Practical skills · Describe · 4 marks

> A student adds lipase solution to full-fat milk and measures how long it takes for the milk to change from white to clear at different pH values. They carry out the experiment once at each pH.

## Mark scheme (4 marks)

1. The student should repeat the experiment at each pH value (at least three times)
2. Repeating allows anomalous results to be identified and a mean (average) to be calculated, increasing reliability
3. Keep all controlled variables constant, such as temperature, volume of lipase, and volume/concentration of milk
4. Controlling these variables ensures that only pH is changing, so any difference in the rate of reaction is due to pH alone / ensures a fair test

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-effect-63449f7a) · Published by Druglandscape Ltd.