A student prepares a sample of copper(II) sulfate solution by adding dilute sulfuric acid to an excess of copper(II) carbonate powder. She then heats the mixture and observes that the fizzing stops before all the solid has dissolved. She filters the mixture to obtain a clear blue solution. Explain why the student added an excess of copper(II) carbonate and why she filtered the mixture at the end.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Copper(II) carbonate reacts with dilute sulfuric acid in a neutralisation reaction to produce copper(II) sulfate, water and carbon dioxide.
Model answer (4 marks)
The excess copper(II) carbonate guarantees that all the dilute sulphuric acid is consumed, so the final solution contains only copper(II) sulfate and not any residual acid. If acid remained, the copper(II) sulfate would be contaminated and the solution would be impure.
After the reaction the mixture still contains unreacted solid carbonate. Filtration removes this excess solid, leaving a clear blue filtrate that is a pure copper(II) sulfate solution.
After the reaction the mixture still contains unreacted solid carbonate. Filtration removes this excess solid, leaving a clear blue filtrate that is a pure copper(II) sulfate solution.
Examiner tips
- Use the word ‘excess’ to show you understand the purpose of the extra carbonate. Mention that the filtrate is the desired pure salt solution. Keep the answer concise – 4 marks can be earned with two short points.
Common mistakes
- Saying the carbonate is added to ‘increase the yield’ instead of to consume all acid. Forgetting to explain why filtration is needed (removing unreacted solid).
Mark scheme (4 marks)
- Excess copper(II) carbonate is added to ensure all the acid is used up / reacted
- If acid remained in solution, the final salt solution would be impure / contaminated with acid
- Filtration is carried out to remove the excess / unreacted copper(II) carbonate solid
- This leaves only the copper(II) sulfate solution in the filtrate / gives a pure salt solution
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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