Learning focus
Develop safe and reproducible practical methods, record precise observations and measurements, process evidence and evaluate experimental quality.
Method
Add aqueous ammonia dropwise and observe precipitate colour, then add excess. Compare with the official qualitative-analysis table.
White precipitates
Al³⁺ gives white precipitate insoluble in excess. Zn²⁺ gives white precipitate soluble in excess to a colourless solution. Ca²⁺ gives no precipitate or a very slight white precipitate.
Copper and chromium
Cu²⁺ gives light blue precipitate that dissolves in excess to a dark blue solution. Cr³⁺ gives green precipitate insoluble in excess.
Iron ions
Fe²⁺ gives green precipitate insoluble in excess and it turns brown near the surface on standing. Fe³⁺ gives red-brown precipitate insoluble in excess.
Practical or data skill
Use NaOH and NH3 observations together to identify six unknown cation solutions.
Examination tip
The dark blue solution in excess ammonia is a distinctive Cu²⁺ result.
Review questions and suggested answers
Question 1
Which ion gives a dark blue solution in excess ammonia?
Suggested answer
Cu²⁺.
Question 2
How does Zn²⁺ behave in excess ammonia?
Suggested answer
Its white precipitate dissolves to a colourless solution.
Question 3
How does Al³⁺ differ?
Suggested answer
Its white precipitate is insoluble in excess ammonia.
