Question

(i) Name the other ion formed when ammonia dissolves in water.
(ii) Give one test that can be used to detect the presence of the ion produced.

Answer 1

Aqueous solution of ammonia $\left({\mathrm{NH}}_3\right)$:

• Aqueous solution of ammonia is called ammonium hydroxide, $\left({\mathrm{NH}}_4\mathrm{OH}\right).$ It is a weak base as it dissociates partially to give hydroxyl ions.
• ${\mathrm{NH}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)$
• ${\mathrm{NH}}_4\mathrm{OH}\rightleftharpoons {\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}$

The presence of hydroxy ions can be detected by:

Reaction with iron(III) chloride$\left({\mathrm{FeCl}}_3\right)$:

• When ammonium hydroxide is treated with iron(III) chloride solution, it produces a reddish brown precipitate of ferric hydroxide, which is insoluble in excess of ${\mathrm{NH}}_4\mathrm{OH}$
• ${\mathrm{FeCl}}_3\left(\mathrm{aq}\right)+3{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)⟶\mathrm{Fe}(\mathrm{OH}{)}_3\left(\mathrm{s}\right)\downarrow +3{\mathrm{NH}}_4\mathrm{Cl}(\mathrm{aq})$

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1. The other ion formed when ammonia dissolves in water is hydroxyl ion.
2. The presence of the ion produced can be done by reacting ammonium hydroxide with iron(III) chloride.