A student is investigating the effect of temperature on the rate of enzyme activity using starch and amylase. Explain why the student should use a controlled variable in this investigation and give two examples of variables that should be controlled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student adds amylase solution to starch solution at different temperatures and tests samples with iodine solution at regular intervals to determine how quickly the starch is broken down.
Model answer (4 marks)
A controlled variable is one that is kept constant throughout the experiment.
Keeping variables constant ensures that any change in the rate of reaction can be attributed only to the independent variable, which in this case is temperature.
Examples of controlled variables:
1. The volume (or concentration) of the amylase solution.
2. The volume (or concentration) of the starch solution (or the pH of the solution).
Keeping variables constant ensures that any change in the rate of reaction can be attributed only to the independent variable, which in this case is temperature.
Examples of controlled variables:
1. The volume (or concentration) of the amylase solution.
2. The volume (or concentration) of the starch solution (or the pH of the solution).
Examiner tips
- Use the definition of a controlled variable first. Explain why keeping variables constant isolates the effect of temperature. Give two specific examples that match the mark scheme.
Common mistakes
- Confusing controlled variables with the independent or dependent variable. Listing variables that are actually the independent or dependent variable. Using vague terms like "the same amount" without specifying volume or concentration.
Mark scheme (4 marks)
- A controlled variable is a variable that remains constant throughout the experiment
- Controlled variables are kept constant so that any change in the rate of reaction is only due to the independent variable (temperature)
- First correct example of a controlled variable, e.g. volume / concentration of amylase solution
- Second correct example of a controlled variable, e.g. volume / concentration of starch solution OR pH
Key terms in this question
Related
- All OCR GCSE Biology A: Gateway Science (J247) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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