# Zinc carbonate decomposes when heated to form zinc oxide and carbon dioxide gas. A student heats a sample of zinc carbonate and finds that the mass of the solid decreases during the reaction. Explain why the mass of the solid decreases, and state the law that describes what happens to the total mass of all the substances present, including the gas produced.

> Eduqas A-Level Chemistry — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks

> The equation for the reaction is: ZnCO₃(s) → ZnO(s) + CO₂(g)

## Mark scheme (5 marks)

1. The mass of the solid decreases because carbon dioxide gas is released / escapes from the solid
2. Carbon dioxide is a gas so it is no longer part of the solid being measured / it escapes into the surroundings
3. The total mass of all substances (solid + gas) remains the same / the mass of products equals the mass of reactants
4. This is described by the law of conservation of mass
5. No atoms are lost or made during the reaction / atoms are rearranged but the number of each type of atom stays the same

## Related

- [Revision notes for Eduqas A-Level Chemistry](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/zinc-carbonate-decomposes-when-heated-to-61ec0bf3) · Published by Druglandscape Ltd.