Testing for Aqueous Cations
Chemistry: Cambridge International GCSE Supplement, topic 12.5.5
Testing for Aqueous Cations
Aqueous solutions of metal compounds contain metal ions. We can test what these metal ions are when they react with a sodium hydroxide solution (NaOH) to produce different coloured precipitates.
Tests for cations
- Copper (II) ions (Cu2+) form a blue precipitate when reacted with NaOH.
- Iron (II) ions (Fe2+) form a green precipitate when reacted with NaOH.
- Iron(III) ions (Fe3+) form a brown precipitate when reacted with NaOH.
Tests for cations 2
- Chromium (III) ions (Cr3+) form a green precipitate when reacted with NaOH.
- The green precipitate dissolves in excess NaOH.
Zinc, calcium or aluminium?
- Zinc (Zn2+), calcium (Ca2+) and aluminium (Al3+) are three cations that form white precipitates when a few drops of NaOH are added.
- Luckily, there is a way to tell which is which!
,w_800.png)
Test for Zn, Ca and Al
- The precipitate formed from zinc and aluminium ions will redissolve following the addition of excess NaOH.
- The precipitate formed from calcium ions will remain insoluble.
,w_800.png)
Test for Zn, Ca and Al cont.
- Zinc and aluminium ions will both form a white precipitate with a few drops of ammonia.
- The precipitate formed from zinc ions will redissolve following the addition of excess ammonia.
- The aluminium precipitate will remain insoluble when excess ammonia is added.
,w_800.png)
Testing for ammonium ions
- Ammonium (NH4+) ions form water and ammonia when reacted with NaOH. Ammonia gas evolves out of the solutions.
- You then test for ammonia gas which will turn damp red litmus paper from red to blue.
Test yourself on Testing for Aqueous Cations
I test an aqueous solution by adding a few drops of sodium hydroxide. A white precipitate forms. Following the addition of excess sodium hydroxide, the white precipitate remains. What cations are present in the aqueous solution?
I test an unknown solution with a few drops of sodium hydroxide. A blue precipitate forms. What ions are present in the unknown solution?
I test an unknown solution with a few drops of sodium hydroxide. A brown precipitate forms. What ions are present in the unknown solution?
I add a few drops of sodium hydroxide to an unknown solution. A white precipitate forms that redissolves with an excess of sodium hydroxide.
I then add a few drops of ammonia to another sample of the same solution. A white precipitate forms that redissolves with an excess of ammonia.
What cations are present in the unknown solution?