# A student is investigating the effect of temperature on the rate of enzyme activity using a colorimeter to measure the colour change of a solution over time. Explain why the student should use a water bath rather than placing the reaction tube directly into a beaker of hot water on a hot plate, and describe two additional steps the student should take to ensure the investigation produces valid results.

> OCR A-Level Biology A (H420) — 1.2 Practical skills assessed in the practical endorsement · Explain · 4 marks

> A student sets up an investigation using an enzyme that breaks down a coloured substrate into a colourless product. They intend to measure absorbance readings at regular intervals using a colorimeter. They plan to test five different temperatures between 20 °C and 60 °C.

## Mark scheme (4 marks)

1. A water bath provides a more stable / uniform temperature throughout the reaction mixture, whereas a hot plate creates uneven heating / temperature gradients.
2. This is important because temperature is the independent variable and must be precisely controlled to ensure the test is valid / to prevent uncontrolled temperature variation affecting the rate of reaction.
3. The student should calibrate / zero the colorimeter using a cuvette containing the buffer solution / water (a blank) before taking readings, to account for background absorbance.
4. The student should pre-incubate / allow the enzyme solution and substrate solution to reach the target temperature separately in the water bath before mixing them, to ensure the reaction begins at the correct temperature.

## Key terms

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

## 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-is-investigating-the-effect-d1f9118a) · Published by Druglandscape Ltd.