# A student is investigating how temperature affects the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid. The reaction produces a yellow precipitate of sulfur, which makes the solution go cloudy. The student places a conical flask on a piece of paper with a black cross drawn on it and measures the time taken for the cross to disappear when viewed from above. Explain how the student should carry out this investigation to make it a fair test, and describe one safety precaution they should take.

> OCR A-Level Chemistry B: Salters (H433) — 1.1 Practical skills assessed · Explain · 5 marks

> The student heats the sodium thiosulfate solution to different temperatures before adding a fixed volume of hydrochloric acid each time. Sulfur dioxide gas is also produced during this reaction.

## Mark scheme (5 marks)

1. Keep the concentration / volume of sodium thiosulfate the same each time
2. Keep the concentration / volume of hydrochloric acid the same each time
3. Only change the temperature (independent variable) and measure the time for the cross to disappear (dependent variable)
4. View the cross from the same distance / height / angle each time to ensure consistent judgement of when the cross disappears
5. Carry out the investigation in a well-ventilated area / fume cupboard because sulfur dioxide is a toxic gas

## Key terms

- [fair test](https://www.gradenine.co.uk/glossary/fair-test)
- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-temperature-61479ced) · Published by Druglandscape Ltd.