# A student is investigating the rate of reaction between sodium thiosulfate solution and hydrochloric acid. The student repeats the experiment using a higher concentration of sodium thiosulfate solution. Explain why the reaction is faster when a higher concentration of sodium thiosulfate solution is used.

> OCR GCSE Chemistry A: Gateway Science (J248) — C5.1 Monitoring chemical reactions · Explain · 4 marks

> When sodium thiosulfate solution reacts with hydrochloric acid, a yellow precipitate of sulfur forms, making the mixture go cloudy. The student uses this cloudiness to judge when the reaction is complete.

## Mark scheme (4 marks)

1. Increasing concentration means particles are closer together (in solution)
2. Particles collide more often / frequency of collisions increases
3. There are more successful collisions per unit time / higher rate of successful collisions
4. Therefore the rate of reaction increases

## Key terms

- [concentration](https://www.gradenine.co.uk/glossary/concentration)
- [rate of reaction](https://www.gradenine.co.uk/glossary/rate-of-reaction)

## Related

- [Revision notes for OCR GCSE Chemistry A: Gateway Science (J248)](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-the-rate-88847c84) · Published by Druglandscape Ltd.