A student adds excess zinc carbonate powder to dilute hydrochloric acid. The mixture fizzes and eventually the fizzing stops. State and explain what the student would observe about the final mixture, and describe how the student could obtain a pure, dry sample of zinc chloride from this mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc carbonate reacts with hydrochloric acid to produce zinc chloride, water and carbon dioxide.
Model answer (4 marks)
The student will see a solid of zinc carbonate left in the mixture after the fizzing stops, because the acid has been completely used up.
The excess zinc carbonate remains in the final mixture as a solid.
To obtain a pure, dry sample of zinc chloride:
1. Filter the mixture to remove the solid zinc carbonate.
2. Evaporate or gently heat the filtrate until all the water has left, giving dry zinc chloride crystals.
The excess zinc carbonate remains in the final mixture as a solid.
To obtain a pure, dry sample of zinc chloride:
1. Filter the mixture to remove the solid zinc carbonate.
2. Evaporate or gently heat the filtrate until all the water has left, giving dry zinc chloride crystals.
Examiner tips
- Use the word "solid" to show the unreacted zinc carbonate. Explain that the fizzing stops when the acid is consumed. Show the two steps – filtration then evaporation – to get dry ZnCl₂.
- common_mistakes
- :
- Saying the acid is still present after the fizzing stops. Failing to mention that the solid is zinc carbonate, not zinc chloride. Not describing the evaporation step to dry the product.
Mark scheme (4 marks)
- Unreacted / excess zinc carbonate remains visible (as a solid) in the final mixture
- Zinc carbonate is in excess so the hydrochloric acid is completely used up / neutralised, leaving unreacted zinc carbonate
- Filter the mixture to remove the excess zinc carbonate solid
- Evaporate / heat the filtrate (zinc chloride solution) to obtain dry zinc chloride crystals
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "State, Explain and Describe" question →
- Decode the mark scheme abbreviations →
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