# A student dissolves iron sulfide in excess dilute sulfuric acid inside a sealed container placed on a balance. The reading on the balance does not change during the reaction. Explain why the mass stays the same, and explain what would happen to the balance reading if the container were left open during the reaction.

> AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · Explain · 4 marks

## Mark scheme (4 marks)

1. No atoms are lost or made during a chemical reaction (law of conservation of mass)
2. So the total mass of products equals the total mass of reactants
3. In an open container a gas (hydrogen sulfide) would escape / be released into the atmosphere
4. The balance reading would decrease because the mass of the escaped gas is not measured

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