# Copper carbonate decomposes when heated according to the balanced equation: CuCO₃ → CuO + CO₂. A student heats a sample of copper carbonate and notices that the total mass of the reaction vessel and its contents decreases during the reaction. Using the law of conservation of mass, explain why the mass appears to decrease, and describe what happens to the number of atoms of each element during this reaction.

> WJEC A-Level Chemistry (Wales) — 3.1 Introduction to organic chemistry · Explain · 5 marks

> Copper carbonate is a green powder that decomposes on heating to form copper oxide and carbon dioxide gas.

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. Carbon dioxide is a gas and escapes from / leaves the reaction vessel
3. The mass of products equals the mass of reactants (total mass is conserved)
4. The number of atoms of each element (copper, carbon, oxygen) is the same on both sides of the equation
5. The apparent decrease in mass is because the CO₂ gas is not measured / not captured in the vessel

## Key terms

- [conservation of mass](https://www.gradenine.co.uk/glossary/conservation-of-mass)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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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