# A student is investigating the rate of a reaction between marble chips (calcium carbonate) and hydrochloric acid by measuring the loss in mass over time. Explain how the student should carry out this investigation to obtain valid and reliable results, and describe how they would use their results to reach a conclusion.

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

> Marble chips react with hydrochloric acid to produce carbon dioxide gas, which escapes from the reaction vessel, causing a measurable loss in mass. The student wants to investigate how the concentration of hydrochloric acid affects the rate of reaction.

## Mark scheme (5 marks)

1. Control variables — the student must keep at least one named variable constant (e.g. mass/size of marble chips, temperature, or total volume of acid)
2. Method of measuring loss in mass — place the flask on a balance and record mass at regular time intervals
3. Repeat the experiment and calculate a mean to improve reliability
4. Change the independent variable systematically — use at least three different concentrations of hydrochloric acid
5. Conclusion — a steeper/faster loss in mass indicates a faster rate of reaction, so a higher concentration of acid gives a greater rate of reaction

## Key terms

- [valid](https://www.gradenine.co.uk/glossary/valid)
- [reliable](https://www.gradenine.co.uk/glossary/reliable)

## 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-the-rate-c52086aa) · Published by Druglandscape Ltd.