What happens when ammonia solution is added first dropwise and then in excess to the following solutions:

(i) CuSO₄

(ii) ZnSO₄

(iii) FeCl₃

Write balanced equations for these reactions.

(i) When ammonia solution is added dropwise to cupper sulphate, a pale blue ppt of copper hydroxide is obtained.

$\underset{\text{blue}}{{\text{CuSO}$4}} + 2\text{NH}4\text{OH} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}2} ↓ + \underset{\text{colourless in solution}}{(\text{NH}4)2\text{SO}4}

On adding excess of ammonia solution, the ppt dissolves and a deep blue solution is obtained.

$\text{Cu(OH)}$2 + (\text{NH}4)2\text{SO}4 + 2\text{NH}4\text{OH} \longrightarrow \underset{\underset{\text{copper (II) sulphate}}{\text{Tetraammine}}}{[(\text{Cu(NH}3)4]\text{SO}4} + 4\text{H}_2\text{O}

(ii) When ammonia solution is added dropwise to zinc sulphate, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless solution}}{{\text{ZnSO}$4}} + 2\text{NH}4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}2↓} + \underset{\text{colourless in solution}}{(\text{NH}4)2\text{SO}4}

On adding excess of ammonia solution, the ppt dissolves and a colourless solution is obtained.

$\text{Zn(OH)}$2 + (\text{NH}4)2\text{SO}4 + 2\text{NH}4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}3)4]\text{SO}4} + 4\text{H}_2\text{O}

(iii) When ammonia solution is added dropwise to iron (III) chloride, a reddish brown ppt. of Fe(OH)₃ is obtained.

$\underset{\text{yellow solution}}{{\text{FeCl}$3}} + 3\text{NH}4\text{OH} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}3↓} + \underset{\text{colourless in solution}}{3\text{NH}4\text{Cl}}

Excess of ammonia solution addition doesn't dissolve ppt.