A student adds excess marble chips (calcium carbonate) to dilute sulfuric acid. At first bubbles are produced, but the reaction soon stops even though marble chips are still present. Suggest why the reaction stops, and explain what the student could do to make the reaction continue.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium carbonate reacts with dilute sulfuric acid: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Calcium sulfate is an insoluble solid.
Model answer (4 marks)
Calcium sulfate forms a solid precipitate on the surface of the marble chips. This insoluble layer coats the calcium carbonate and blocks the dilute acid from reaching the unreacted solid. As a result the reaction stops even though marble chips are still present.
To resume the reaction the student should break up the coated chips – for example by crushing or grinding them. This removes the calcium‑sulfate coating and exposes fresh calcium‑carbonate surface, allowing the acid to react again.
To resume the reaction the student should break up the coated chips – for example by crushing or grinding them. This removes the calcium‑sulfate coating and exposes fresh calcium‑carbonate surface, allowing the acid to react again.
Examiner tips
- Use the exact phrase ‘insoluble solid/precipitate’ to show understanding of solubility. Explain the blocking effect of the precipitate. State the practical solution (crushing/grinding) and why it works. Keep the answer concise – 4 marks only.
- common_mistakes
- :
- Saying the reaction stops because the acid is used up – the acid is still present. Forgetting to mention that calcium sulfate is insoluble. Using vague terms like ‘something blocks the reaction’ instead of ‘precipitate coats the surface’.
Mark scheme (4 marks)
- Calcium sulfate (an insoluble solid/precipitate) forms on the surface of the marble chips
- The insoluble calcium sulfate layer prevents the acid from reaching the calcium carbonate
- The student could crush/grind the marble chips (to break up the coating and expose fresh surface)
- This exposes fresh (unreacted) calcium carbonate surface so the acid can react again
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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