Test for cations
Testing for Ammonium ions
Some salts contain the Ammonium ion (NH4+).
To identify it the salts should be dissolved in water (all Ammonium salts are soluble) and a little Sodium Hydroxide should be added.
The mixture is then gently warmed over a Bunsen.
A faint smell of Ammonia may be detected but the test is to hold some damp red Litmus paper over the test-tube.
Any Ammonia gas will dissolve and change the Litmus to a blue colour
Ammonium ion + Hydroxide ion --> Ammonia gas + Water
NH4 +(aq) + OH-(aq) --> NH3 (g) + H2O (l)
Testing for Iron, Copper and Zinc ions
All dissolved metal ions from the Transition Block of the Periodic Table precipitate with Sodium Hydroxide.
A precipitate may form when two soluble ionic substances are mixed.
In a solution the anions and cations are split up, so the anion from one solution will sometimes collide with the cation from the other.
If they happen to form an insoluble substance it will fall to the bottom of the test tube.
Transition metal hydroxides are all insoluble and colourful, so this is a good way to identify the metal ion that was dissolved.
Generally: Mx+(aq) + xOH-(aq) --> M(OH)x(s)
Copper (II): Cu2+(aq) + 2OH-(aq) --> Cu(OH)2(s) Blue Precipitate
Iron (II): Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s) Green Precipitate
Iron (III): Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s) Brown Precipitate
Zn (II): Zn2+(aq) + 2OH-(aq) --> Zn(OH)2(s) White Precipitate
Iron (III) left.
Copper (II) 2nd left
The remaining precipitates are for Chromium and Cobalt ions which are not part of the iGCSE syllabus.
Videos
Copper (II) test
https://www.youtube.com/watch?v=KLGRf4AdKQM
Iron (II) test
https://www.youtube.com/watch?v=bivr2CzlTF4
Iron (III) Test
https://www.youtube.com/watch?v=gj7ju9wRtdY
Ammonium ion test.
https://www.youtube.com/watch?v=b28pw1q2qdo
Zinc ion test
http://www.youtube.com/watch?v=JjPNDnoxYN4