A student is investigating how the volume of oxygen produced changes over time when hydrogen peroxide decomposes in the presence of a catalyst. Describe how the student should carry out a fair test to compare the effect of two different catalysts on the rate of this reaction, and explain why controlling variables is important in this investigation.

OCR GCSE Chemistry A: Gateway Science (J248) — C7 Practical activity skills · Describe · 4 marks · View as Markdown

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

Hydrogen peroxide solution decomposes to form water and oxygen. The rate at which oxygen is produced can be used as a measure of the rate of reaction. A student wants to compare how effectively manganese(IV) oxide and iron(III) oxide act as catalysts for this decomposition.

Model answer (4 marks)

1. Use the same volume and concentration of hydrogen peroxide for both trials.
2. Use the same mass (or surface area) of catalyst in each trial.
3. Keep the temperature constant for both trials.
4. Measure the volume of oxygen collected at regular time intervals for each catalyst.

Controlling variables is important because it ensures that any difference in the rate of reaction is due only to the type of catalyst used, making the comparison valid and fair.

Examiner tips

  • Use numbered points to show each controlled variable; mention the independent and dependent variables explicitly; keep the language concise and use exam‑style terminology such as ‘mass of catalyst’ and ‘volume of oxygen’.

Common mistakes

  • Changing more than one variable at a time; not specifying which variable is independent; failing to state why controlling variables matters.

Mark scheme (4 marks)

  1. Keep one variable the same (e.g. volume/concentration of hydrogen peroxide, mass/surface area of catalyst, temperature) to make the test fair
  2. Change only the independent variable — the type of catalyst used (manganese(IV) oxide or iron(III) oxide)
  3. Measure the dependent variable in the same way for both catalysts — e.g. measure the volume of oxygen collected at regular time intervals
  4. Controlling variables is important because it means any difference in the rate of reaction can be attributed to the catalyst (independent variable) alone / ensures results are valid and a fair comparison can be made

Key terms in this question

fair test · catalyst

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