# Calcium carbonate (CaCO₃) decomposes when heated to form calcium oxide (CaO) and carbon dioxide (CO₂). Explain, using ideas about elements, compounds, and mixtures, why calcium carbonate is a compound and why the products of this reaction are not a mixture.

> Edexcel A-Level Chemistry (9CH0) — Topic 1 Foundations in chemistry · Explain · 5 marks

> Calcium carbonate is a white solid found in limestone. When strongly heated, it breaks down into two substances: calcium oxide and carbon dioxide.

## Mark scheme (5 marks)

1. Calcium carbonate is a compound because it contains two or more elements chemically combined / bonded together
2. The elements in calcium carbonate are present in fixed proportions / a fixed ratio
3. A mixture contains two or more substances that are NOT chemically combined / bonded
4. Calcium oxide and carbon dioxide are separate / distinct compounds (each with their own chemical formula/bonds)
5. The products (calcium oxide and carbon dioxide) are not in fixed proportions to each other / are not chemically bonded to each other, so they could form a mixture if collected together, BUT each individual product is itself a compound not a mixture

## Key terms

- [compound](https://www.gradenine.co.uk/glossary/compound)
- [mixture](https://www.gradenine.co.uk/glossary/mixture)
- [element](https://www.gradenine.co.uk/glossary/element)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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/calcium-carbonate-caco-decomposes-when-heated-26807e1e) · Published by Druglandscape Ltd.