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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. Vary only the concentration of hydrogen peroxide – this is the independent variable.
2. Measure the volume of oxygen produced over a fixed time – this is the dependent variable.
3. Keep the temperature constant throughout the experiment.
4. Use the same mass/volume and surface area of celery for each trial.
5. Explanation: if a control variable (e.g. temperature, celery amount) changes, it can alter enzyme activity and the reaction rate, so any observed change could not be attributed solely to hydrogen‑peroxide concentration.
2. Measure the volume of oxygen produced over a fixed time – this is the dependent variable.
3. Keep the temperature constant throughout the experiment.
4. Use the same mass/volume and surface area of celery for each trial.
5. Explanation: if a control variable (e.g. temperature, celery amount) changes, it can alter enzyme activity and the reaction rate, so any observed change could not be attributed solely to hydrogen‑peroxide concentration.
Examiner tips
- State clearly which variable is independent and which is dependent. Keep the temperature and celery mass/area constant to avoid confounding effects. Use a fixed time interval for all trials to allow direct comparison of volumes.
Common mistakes
- Changing the amount of celery or its surface area between trials. Failing to keep the temperature constant. Not specifying which variable is independent and which is dependent.
Mark scheme (5 marks)
- Change only the concentration of hydrogen peroxide (independent variable) and keep all other variables constant
- Measure the volume of oxygen / gas collected over a fixed time period (dependent variable)
- Keep temperature constant throughout the investigation
- Keep the mass / volume / surface area of celery the same at each concentration
- 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 in this question
catalase · enzyme · control variable · fair test
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Practical skills assessed in a written examination questions
- A student is investigating the effect of light intensity on the rate of photosyn…
- A student is investigating the effect of substrate concentration on the rate of …
- A student is investigating the effect of pH on the rate at which lipase breaks d…
- A student is investigating the effect of temperature on the rate of reaction of …