A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter to measure the decolouration of a dye. Explain why it is important that the student uses a water bath to maintain the temperature of the reaction mixture, rather than placing the tubes directly in a room-temperature environment.

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

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student is investigating how temperature affects the rate of an enzyme-controlled reaction. They use a colorimeter to record how quickly a coloured substrate is broken down at different temperatures (20°C, 30°C, 40°C, 50°C). They are deciding whether to use a water bath or simply leave the tubes on the bench.

Model answer (4 marks)

A water bath gives a uniform temperature throughout the reaction mixture, so every enzyme molecule experiences the same thermal environment.

Temperature is the independent variable that determines the rate of enzyme activity; if the temperature fluctuates, the reaction rate will change unpredictably, acting as a confounding factor.

Room temperature is not constant and will not reliably match the target temperatures (20 °C, 30 °C, 40 °C, 50 °C), so the independent variable cannot be set precisely.

Using a water bath therefore ensures the investigation is a fair test – temperature is the only variable that changes, making the results valid and reproducible.

Examiner tips

  • Use the word ‘confounding variable’ to show understanding of experimental control.
  • Show the link between uniform temperature and enzyme kinetics.
  • Mention reproducibility and validity as key assessment criteria.

Common mistakes

  • Saying the water bath is ‘faster’ or ‘more accurate’ without explaining uniformity.
  • Forgetting that room temperature can vary and is not the target temperature.
  • Claiming the water bath is unnecessary because the colourimeter measures colour change directly.

Mark scheme (4 marks)

  1. A water bath provides a consistent/uniform temperature throughout the reaction mixture
  2. Temperature affects the rate of enzyme activity / enzyme kinetics, so uncontrolled fluctuations would act as a confounding variable
  3. Room temperature fluctuates / is not reliably at the target temperature, so the independent variable cannot be precisely set
  4. This ensures the investigation is a fair test / results are valid and reproducible, as temperature is the only variable being changed

Key terms in this question

colorimeter · water bath · decolouration · rate of enzyme activity

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