Cation Tests
IGCSE Edexcel Chemistry
2.47 Tests for NH4+, Cu2+, Fe2+, Fe3+
Key Concept:
Sodium hydroxide forms characteristic precipitates with some metal ions and releases ammonia from ammonium ions.
Section A: Observations
1. Describe the test and positive result for ammonium ions.
[2]
2. State the precipitate color formed with sodium hydroxide for: Cu2+, Fe2+, Fe3+.
[3]
3. Iron(II) hydroxide changes color on standing in air. What happens and why?
[2]
Total marks: 7
Mark Scheme
1.
Add NaOH and warm; ammonia gas released; turns damp red litmus paper blue [2]
2.
Cu2+ blue ppt, Fe2+ green ppt, Fe3+ brown ppt [3]
3.
Green ppt turns brown as Fe2+ oxidises to Fe3+ in air [2]