# A student carries out an experiment in which dilute hydrochloric acid is added to solid calcium carbonate. Explain the type of reaction that occurs and describe the role of the acid in terms of the ions present in solution.

> OCR A-Level Chemistry B: Salters (H433) — 2.3 Acid-base and redox reactions · Explain · 5 marks

> Calcium carbonate is an ionic compound found in limestone. When dilute hydrochloric acid is added to calcium carbonate, a reaction takes place that produces a gas, a salt, and water. The ionic equation for the reaction is: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂.

## Mark scheme (5 marks)

1. The reaction is an acid-base (neutralisation) reaction
2. The acid provides hydrogen ions (H⁺) in solution
3. The carbonate ion (CO₃²⁻) acts as the base by accepting / reacting with the H⁺ ions
4. Carbon dioxide gas is produced as the carbonate ion is broken down / decomposed by the acid
5. A salt (calcium chloride) and water are also formed, showing the acid has been neutralised

## 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)
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