Zinc nitrate solution is neutral. When zinc oxide powder is added to dilute nitric acid, a salt solution is formed. Explain why zinc oxide is described as a base, and describe how you could obtain pure, dry crystals of zinc nitrate from the resulting solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc oxide is a white powder that does not dissolve in water. It reacts with dilute nitric acid to form zinc nitrate solution and water only.
Model answer (4 marks)
Zinc oxide is a base because it reacts with the H⁺ ions in dilute nitric acid, accepting protons and forming a salt (zinc nitrate) and water:
ZnO + 2 HNO₃ → Zn(NO₃)₂ + H₂O
To obtain pure, dry crystals of zinc nitrate, add excess ZnO to the acid so that all H⁺ is neutralised. Filter the mixture to remove the unreacted ZnO. Evaporate the filtrate (or heat it gently) to remove water and concentrate the solution. Allow the concentrated solution to cool; zinc nitrate will crystallise, giving pure, dry crystals.
ZnO + 2 HNO₃ → Zn(NO₃)₂ + H₂O
To obtain pure, dry crystals of zinc nitrate, add excess ZnO to the acid so that all H⁺ is neutralised. Filter the mixture to remove the unreacted ZnO. Evaporate the filtrate (or heat it gently) to remove water and concentrate the solution. Allow the concentrated solution to cool; zinc nitrate will crystallise, giving pure, dry crystals.
Examiner tips
- Use the reaction equation to show ZnO accepts H⁺ and forms salt + water; this demonstrates its basic nature.
- Explain that excess ZnO guarantees complete neutralisation, a point the scheme rewards.
- Describe filtration to remove solid ZnO, then evaporation and cooling to crystallise the salt.
Mark scheme (4 marks)
- A base is a substance that neutralises an acid (accepting / reacting with H⁺ ions / reacting with an acid to form a salt and water only)
- Add excess zinc oxide to the acid to ensure all the acid is neutralised / reacted
- Filter the mixture to remove excess unreacted zinc oxide
- Evaporate / heat the filtrate (zinc nitrate solution) to remove water / concentrate the solution, then allow to cool so that zinc nitrate crystals form
Key terms in this question
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain / Describe" question →
- Decode the mark scheme abbreviations →
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