A student adds dilute hydrochloric acid to separate aqueous solutions of silver nitrate, barium chloride, and calcium nitrate. Describe the observations the student would make and explain what these observations show about the ions present in each solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Add dilute HCl to AgNO₃:
1. A white precipitate appears.
2. The precipitate is AgCl, confirming Ag⁺ is present.
Add dilute HCl to BaCl₂:
3. No visible change; no precipitate.
Add dilute HCl to Ca(NO₃)₂:
4. No visible change; no precipitate.
These observations show that chloride ions precipitate Ag⁺ but not Ba²⁺ or Ca²⁺; a different reagent would be required to detect the latter ions.
1. A white precipitate appears.
2. The precipitate is AgCl, confirming Ag⁺ is present.
Add dilute HCl to BaCl₂:
3. No visible change; no precipitate.
Add dilute HCl to Ca(NO₃)₂:
4. No visible change; no precipitate.
These observations show that chloride ions precipitate Ag⁺ but not Ba²⁺ or Ca²⁺; a different reagent would be required to detect the latter ions.
Examiner tips
- Use the command word ‘describe’ – list observations first, then explain. Mention the identity of the precipitate (AgCl) and the lack of precipitate for Ba²⁺ and Ca²⁺. Show the link between the ion present and the reaction with chloride.
- common_mistakes
- :
- Failing to state that the precipitate is AgCl. Saying ‘no change’ for all solutions. Not explaining why Ba²⁺ and Ca²⁺ do not precipitate with chloride.
Mark scheme (5 marks)
- A white precipitate forms when dilute hydrochloric acid is added to silver nitrate solution
- The white precipitate with silver nitrate is silver chloride, showing silver ions (Ag⁺) are present
- No precipitate / no visible change when dilute hydrochloric acid is added to barium chloride solution
- No precipitate / no visible change when dilute hydrochloric acid is added to calcium nitrate solution
- The absence of a precipitate with barium chloride and calcium nitrate shows that dilute hydrochloric acid / chloride ions alone do not precipitate barium or calcium ions; a different reagent (e.g. dilute sulfuric acid) would be needed to distinguish them
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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