A student carries out an investigation into the effect of temperature on the rate of reaction between sodium thiosulfate solution and hydrochloric acid. The student wants to make their results as reliable as possible. Describe how the student should make their investigation reliable and explain why each step is important.

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).

Sodium thiosulfate solution reacts with hydrochloric acid to produce a yellow precipitate of sulfur. The reaction is timed until a cross drawn on paper beneath the flask can no longer be seen.

Model answer (4 marks)

1. Repeat each experiment at least twice at each temperature and calculate a mean time.
2. Keep all other variables constant – use the same concentration of sodium thiosulfate, the same volume of each solution, the same size of cross and the same type of paper.
3. Use the same observer to judge when the cross disappears, or apply a clear, consistent end‑point criterion.
4. Repeating the trials and controlling variables ensures that any anomalous results are recognised and that the measured times are reliable and comparable across temperatures.

Examiner tips

  • Use bullet points or short numbered sentences to show each step clearly.
  • Mention the purpose of each step – why it improves reliability.
  • Show the calculation of a mean time if you include data.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Failing to state that the mean time should be calculated.
  • Not recognising the need to keep concentrations and volumes constant.
  • Using different observers or inconsistent end‑point criteria.

Mark scheme (4 marks)

  1. The student should repeat each experiment at least once (at each temperature) and calculate a mean time
  2. Repeating allows anomalous results to be identified and/or increases the reliability / confidence in the results
  3. Keep all other variables constant / control variables (e.g. concentration of sodium thiosulfate, volume of solutions, same cross size/paper)
  4. Use the same observer / judge to decide when the cross has disappeared, OR use a consistent end-point criterion throughout the investigation

Related

More Practical activity skills questions

▶ Try answering this question with AI marking (free) →