# A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter to measure the absorbance of a solution over time. Explain why the student should use a water bath rather than placing the reaction tubes directly in a beaker of water on a hot plate, and describe one additional step the student should take to ensure the results are reliable.

> OCR A-Level Biology A (H420) — 1.1 Practical skills assessed in a written examination · Explain · 4 marks

> A student is measuring the rate of an enzyme-controlled reaction at five different temperatures (20°C, 30°C, 40°C, 50°C, 60°C). The enzyme breaks down a coloured substrate, causing the solution to become less coloured over time. Absorbance is recorded every 30 seconds using a colorimeter.

## Mark scheme (4 marks)

1. A water bath provides a more stable / uniform temperature throughout the tube
2. A hot plate creates a temperature gradient / uneven heating, so the temperature at the reaction site may not match the set temperature
3. Pre-incubate / equilibrate the enzyme and substrate solutions separately in the water bath at the required temperature before mixing
4. Repeat the experiment at each temperature and calculate a mean / identify and exclude anomalous results

## Key terms

- [colorimeter](https://www.gradenine.co.uk/glossary/colorimeter)
- [water bath](https://www.gradenine.co.uk/glossary/water-bath)
- [absorbance](https://www.gradenine.co.uk/glossary/absorbance)
- [reliable](https://www.gradenine.co.uk/glossary/reliable)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-the-effect-of-654afb3c) · Published by Druglandscape Ltd.