Ammonia vs NaOH Showdown
Tell ions apart by what dissolves in excess ammonia.
What this lesson covers
Why it matters
Two alkalis, one twist. With ammonia solution some metal hydroxides redissolve in excess — and that is exactly how you tell them apart.
Predict first
What happens to Cu(OH)₂ when you add EXCESS ammonia solution?
With NH₄OH, the hydroxides of Cu²⁺ and Zn²⁺ dissolve in excess (Cu → deep-blue solution, Zn → colourless), while Fe²⁺, Fe³⁺, Pb²⁺ and Al³⁺ hydroxides stay as precipitates.
- It dissolves to a deep-blue solution — correct
- It stays as a blue precipitate
- It turns into a white solid
What you do
Sort each ion by what its hydroxide does in EXCESS ammonia solution.
Check yourself
A blue precipitate dissolves in excess ammonia to give a deep-blue solution. The ion is…
Cu²⁺ forms a deep-blue soluble complex in excess ammonia.
Excess ammonia helps distinguish Zn²⁺ from Pb²⁺ because…
Both give white precipitates, but only Zn(OH)₂ dissolves in excess ammonia.
- Cu²⁺ — correct
- Fe²⁺
- Pb²⁺
- Zn(OH)₂ dissolves but Pb(OH)₂ does not — correct
- both dissolve equally
- neither dissolves