A student heats copper carbonate powder in a crucible. The solid turns black and a gas is released. After heating, the student notices that the mass of the crucible contents has decreased. Explain why the total mass of solid in the crucible has decreased, even though the law of conservation of mass applies to this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The copper carbonate decomposes:
1. CuCO₃(s) → CuO(s) + CO₂(g)
2. The CO₂ is a gas and escapes into the surroundings.
3. Because the CO₂ leaves the crucible, the mass of the solid left (CuO) is less than the original mass of CuCO₃.
4. The law of conservation of mass is not violated – the total mass of all products (CuO + CO₂) equals the initial mass of CuCO₃, but only the solid part is weighed.
1. CuCO₃(s) → CuO(s) + CO₂(g)
2. The CO₂ is a gas and escapes into the surroundings.
3. Because the CO₂ leaves the crucible, the mass of the solid left (CuO) is less than the original mass of CuCO₃.
4. The law of conservation of mass is not violated – the total mass of all products (CuO + CO₂) equals the initial mass of CuCO₃, but only the solid part is weighed.
Examiner tips
- Use the correct decomposition equation, include the gas product.
- Explain that the gas leaves the crucible, so its mass is not counted.
- Show that the remaining solid mass is less, but total mass is conserved.
- Use UK spelling and correct terminology (decomposes, escapes).
Common mistakes
- Writing CuCO₃ → CuO + O₂ instead of CO₂.
- Forgetting to mention that CO₂ is a gas that escapes.
- Claiming the law of conservation of mass is broken.
Mark scheme (4 marks)
- The copper carbonate decomposes / breaks down into copper oxide and carbon dioxide
- Carbon dioxide is a gas and escapes / is released into the surroundings
- The mass of solid decreases because the mass of carbon dioxide that escaped is no longer being measured / weighed
- The law of conservation of mass still holds because the total mass of all products (solid copper oxide + carbon dioxide gas) equals the mass of the original copper carbonate
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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