# A student adds a piece of marble (calcium carbonate) to hydrochloric acid. The student notices that the reaction slows down over time and eventually stops. Explain why the rate of reaction decreases as the reaction proceeds.

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

> Marble chips react with hydrochloric acid to produce carbon dioxide gas. A student observes that bubbles are produced rapidly at first but the rate of bubble production slows noticeably over time.

## Mark scheme (4 marks)

1. The concentration of hydrochloric acid decreases as the reaction proceeds
2. The reacting particles are further apart / less crowded in solution
3. Collisions between reacting particles become less frequent / fewer successful collisions per second
4. Therefore the rate of reaction decreases

## Key terms

- [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-adds-a-piece-of-752a639e) · Published by Druglandscape Ltd.