# A student is investigating how the concentration of hydrogen peroxide affects the rate of decomposition of hydrogen peroxide by catalase, an enzyme found in celery. The student collects the gas produced over a fixed time period at each concentration. Describe how the student should carry out this investigation to make it a fair test, and explain why a control variable must be kept constant.

> OCR A-Level Biology B — Advancing Biology (H422) — 1.1 Practical skills assessed in a written examination · Describe · 5 marks

> Catalase is an enzyme found in many living tissues, including celery. It catalyses the decomposition of hydrogen peroxide into water and oxygen. A student wants to find out whether increasing the concentration of hydrogen peroxide increases the rate of this reaction.

## Mark scheme (5 marks)

1. Change only the concentration of hydrogen peroxide (independent variable) and keep all other variables constant
2. Measure the volume of oxygen / gas collected over a fixed time period (dependent variable)
3. Keep temperature constant throughout the investigation
4. Keep the mass / volume / surface area of celery the same at each concentration
5. Explanation: if a control variable changes it could affect the rate of reaction / enzyme activity, making it impossible to know whether the concentration of hydrogen peroxide caused any change

## Key terms

- [catalase](https://www.gradenine.co.uk/glossary/catalase)
- [enzyme](https://www.gradenine.co.uk/glossary/enzyme)
- [control variable](https://www.gradenine.co.uk/glossary/control-variable)
- [fair test](https://www.gradenine.co.uk/glossary/fair-test)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" 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-how-the-58e34875) · Published by Druglandscape Ltd.