Copper carbonate decomposes when heated to form copper oxide and carbon dioxide. Explain why the total mass of products formed is always equal to the mass of copper carbonate that decomposed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student heats a sample of copper carbonate in an open crucible and measures the mass of the solid remaining after heating. The student notices the solid mass decreases during heating.
Model answer (4 marks)
1. In the decomposition of copper carbonate, the atoms present in the solid reactant are the same atoms that appear in the products.
2. Because atoms are neither created nor destroyed, the total mass of the reactants must equal the total mass of the products – this is the law of conservation of mass.
3. During heating, carbon dioxide is produced as a gas and leaves the crucible, so it is no longer part of the solid mass measured.
4. Therefore the mass of the remaining copper oxide plus the mass of the CO₂ gas released together equals the original mass of copper carbonate that decomposed.
2. Because atoms are neither created nor destroyed, the total mass of the reactants must equal the total mass of the products – this is the law of conservation of mass.
3. During heating, carbon dioxide is produced as a gas and leaves the crucible, so it is no longer part of the solid mass measured.
4. Therefore the mass of the remaining copper oxide plus the mass of the CO₂ gas released together equals the original mass of copper carbonate that decomposed.
Examiner tips
- Use the phrase "conservation of mass" and mention that atoms are conserved.
- Explain that CO₂ is a gas that escapes, so its mass is not counted in the solid residue.
- Show the mass balance: CuCO₃ → CuO + CO₂.
- Keep the answer short and directly address each point of the mark scheme.
Common mistakes
- Failing to mention that CO₂ is a gas that leaves the crucible.
- Confusing the mass of the solid residue with the total mass of all products.
- Using vague language like "the same amount of mass" without linking to conservation of atoms.
Mark scheme (4 marks)
- Atoms are conserved / no atoms are created or destroyed during a chemical reaction
- The total mass of reactants equals the total mass of products (conservation of mass)
- Carbon dioxide is a gas that is released / escapes into the surroundings
- The mass of copper oxide plus the mass of carbon dioxide gas released equals the original mass of copper carbonate
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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