A few drops of alkali can reveal which metal is hiding in a salt solution. Learn the colours to look for.
Adding sodium hydroxide to a salt solution first gives a precipitate of the metal hydroxide. The precipitate colour, and whether it dissolves in excess alkali, identifies the metal ion. Cu²⁺ gives pale blue and Fe²⁺ gives dirty green. Fe³⁺ gives reddish brown, while Zn²⁺ and Pb²⁺ give white precipitates.
What do you see when NaOH solution is added drop by drop to copper sulphate solution?
A pale blue precipitate of copper hydroxide forms, and it does not dissolve in excess NaOH
Write the equation for the reaction of CuSO₄ with NaOH.
CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄
How can you tell an Fe²⁺ salt from an Fe³⁺ salt using NaOH?
Fe²⁺ gives a dirty green precipitate and Fe³⁺ gives a reddish brown precipitate
With ammonium hydroxide the same metals give the same coloured precipitates, but the behaviour in excess differs. Cu(OH)₂ and Zn(OH)₂ dissolve in excess NH₄OH, giving an inky blue and a colourless solution respectively. Pb(OH)₂ and the iron hydroxides do not dissolve.
Zinc and lead salts both give a white precipitate with NaOH that dissolves in excess. How do you tell them apart?
Add NH₄OH: the zinc hydroxide precipitate dissolves in excess, but the lead hydroxide precipitate does not
How is the ammonium ion identified?
Warm the salt with NaOH solution: ammonia gas is given off, which turns moist red litmus blue and forms dense white fumes with HCl
1. What colour is the precipitate when NaOH is added to copper sulphate solution?
2. What happens to copper hydroxide on adding excess NH₄OH?
3. Which salt solution gives a dirty green precipitate with NaOH?
4. Which salt solution gives a reddish brown precipitate with NaOH?
5. Both Zn²⁺ and Pb²⁺ salts give white precipitates with NaOH. How can you distinguish them using NH₄OH?
6. Does lead hydroxide dissolve in excess NaOH solution?
7. How do you test for the ammonium ion in a salt?
8. Write the equation for FeSO₄ with NaOH.
9. Which two hydroxide precipitates dissolve in neither excess NaOH nor excess NH₄OH?
10. A salt gives a white precipitate with NaOH that dissolves in excess, and a white precipitate with NH₄OH that also dissolves in excess. Which cation is present?