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) --> NH_{3 (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: M^x+_(aq) + xOH^-_(aq) --> M(OH)_x(s)

Copper (II): Cu²⁺_(aq) + 2OH^-_(aq) --> Cu(OH)_2(s) Blue Precipitate

_{Iron (II): Fe}²⁺_(aq) + 2OH^-_(aq) --> Fe(OH)_2(s) Green Precipitate

_{Iron (III): Fe}³⁺_(aq) + 3OH^-_(aq) --> Fe(OH)_3(s) Brown Precipitate

_{Zn (II): Zn}²⁺_(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.