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 · View as Markdown

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

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.

Model answer (4 marks)

A water bath gives a uniform, stable temperature throughout the reaction mixture, whereas a hot plate causes temperature gradients and uneven heating.

Because temperature is the independent variable, it must be precisely controlled; uncontrolled variation would make the rate of reaction unreliable.

The student should 1) calibrate the colourimeter with a blank cuvette (buffer or water) to zero background absorbance.

2) Pre‑incubate the enzyme and substrate separately in the water bath until they reach the target temperature before mixing, so the reaction starts at the correct temperature.

Examiner tips

  • Use the word ‘calibrate’ or ‘zero’ when describing the colourimeter step. Mention ‘pre‑incubate’ or ‘allow to reach temperature’ for the second step. Show the water bath gives uniform temperature and explain why this matters for the independent variable.

Common mistakes

  • Writing that the hot plate is better for heating. Forgetting to mention the need to zero the colourimeter. Not stating that the enzyme and substrate must be pre‑incubated separately.

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 in this question

colorimeter · water bath · valid

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