# Calcium oxide (CaO) is made by heating calcium carbonate (CaCO₃) in a kiln. The equation for this reaction is: CaCO₃ → CaO + CO₂. The relative atomic masses (Ar) are: Ca = 40, C = 12, O = 16. Explain why the mass of solid remaining in the kiln is less than the mass of calcium carbonate that was added. Use the relative formula masses of the substances in the equation to support your answer.

> AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks

## Mark scheme (4 marks)

1. Carbon dioxide (CO₂) is a gas / is released into the atmosphere / escapes from the kiln
2. The law of conservation of mass still holds / no atoms are lost or made / total mass of products equals mass of reactants
3. Correct relative formula mass of CaCO₃ = 100 (or CaO = 56, or CO₂ = 44)
4. Links the difference in Mr values to mass loss — the mass of CO₂ (Mr = 44) accounts for the difference between the mass of CaCO₃ (Mr = 100) and CaO (Mr = 56)

## Key terms

- [relative formula mass](https://www.gradenine.co.uk/glossary/relative-formula-mass)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-oxide-cao-is-made-by-c6fa2e28) · Published by Druglandscape Ltd.