A student is investigating the effect of pH on the rate of enzyme activity using lipase and milk. Describe how the student should make their results more reliable, and explain why each step helps.

OCR GCSE Biology A: Gateway Science (J247) — B7 Practical skills · Describe · 4 marks · View as Markdown

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

A student adds lipase solution to full-fat milk and measures how long it takes for the milk to change from white to clear at different pH values. They carry out the experiment once at each pH.

Model answer (4 marks)

1. Repeat the experiment at each pH value at least three times.
2. Calculate a mean (average) time for each pH to identify any anomalous runs.
3. Keep all other variables constant – e.g. temperature, volume of lipase, volume and concentration of milk.
4. By controlling these factors only pH changes, so any difference in reaction rate can be attributed to pH alone, giving a fair test and more reliable results.

Examiner tips

  • Use the command word ‘describe’ – list each step and explain its purpose. Show the link between controlling variables and a fair test. Include the idea of averaging to reduce random error.
  • common_mistakes
  • :
  • Failing to mention the need to repeat trials. Not specifying which variables must be kept constant. Using vague terms like ‘make it better’ without explaining why.

Mark scheme (4 marks)

  1. The student should repeat the experiment at each pH value (at least three times)
  2. Repeating allows anomalous results to be identified and a mean (average) to be calculated, increasing reliability
  3. Keep all controlled variables constant, such as temperature, volume of lipase, and volume/concentration of milk
  4. Controlling these variables ensures that only pH is changing, so any difference in the rate of reaction is due to pH alone / ensures a fair test

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