# Calcium burns in oxygen to form calcium oxide. A student investigates this reaction and states that the law of conservation of mass means the total mass of products must equal the total mass of reactants. Explain what is meant by the law of conservation of mass and describe how the student could use the balanced equation 2Ca + O₂ → 2CaO to show that the masses of reactants and products are consistent with this law.

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

> Calcium (relative atomic mass = 40) reacts with oxygen gas (relative atomic mass of O = 16) according to the equation: 2Ca + O₂ → 2CaO. A student burns a sample of calcium completely in excess oxygen and collects all of the calcium oxide formed.

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. The mass of the products equals the mass of the reactants
3. The relative formula mass of 2Ca is correctly stated as 2 × 40 = 80
4. The relative formula mass of O₂ is correctly stated as 32, giving total reactant mass of 80 + 32 = 112
5. The relative formula mass of 2CaO is 2 × (40 + 16) = 112, which equals the total reactant mass, confirming conservation of mass

## Key terms

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

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-burns-in-oxygen-to-form-0c0767b8) · Published by Druglandscape Ltd.