# 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

> 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.

## 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

- [Revision notes for OCR GCSE Chemistry A: Gateway Science (J248)](https://www.gradenine.co.uk/learn)
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