Learning focus
Develop safe and reproducible practical methods, record precise observations and measurements, process evidence and evaluate experimental quality.
Adding reagent
Add aqueous sodium hydroxide dropwise and record any precipitate colour. Then add excess to test whether it dissolves. Use a fresh portion for ammonium testing if needed.
White precipitates
Al³⁺ and Zn²⁺ give white precipitates soluble in excess. Ca²⁺ gives a white precipitate insoluble in excess. These observations must be combined with ammonia tests or flame tests.
Coloured precipitates
Cr³⁺ gives green precipitate soluble in excess. Cu²⁺ gives light blue insoluble precipitate. Fe²⁺ gives green insoluble precipitate that browns near the surface; Fe³⁺ gives red-brown insoluble precipitate.
Ammonium
NH₄⁺ releases ammonia on warming with aqueous sodium hydroxide. Test with damp red litmus paper.
Practical or data skill
Build an identification key beginning with precipitate colour and then solubility in excess NaOH.
Examination tip
Always state what happens in excess reagent when it is part of the identification.
Review questions and suggested answers
Question 1
Which ions give white precipitates soluble in excess NaOH?
Suggested answer
Al³⁺ and Zn²⁺.
Question 2
What colour is the Cu²⁺ precipitate?
Suggested answer
Light blue.
Question 3
Which ion gives ammonia on warming?
Suggested answer
NH₄⁺.
