Question

State your observations when Ammonium hydroxide solution is added drop by drop and then in excess to each of the following solutions:
Copper sulphate solution.
Zinc sulphate solution.

Answer 1

Part 1: Addition of Ammonium hydroxide with Copper sulphate solution:-

1. Ammonium hydroxide is added drop by drop in the Copper sulphate solution.
2. A pale blue precipitate of $\mathrm{Cu}{\left(\mathrm{OH}\right)}_2$is observed.
3. Upon excess addition of Ammonium hydroxide, this precipitate the turns into deep blue colour.
4. Balanced chemical equation : $$\begin{gathered} \underset{\mathrm{Copper}\mathrm{sulphate}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{sulphate}}{2{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Copper}\mathrm{hydroxide}}{\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)(\downarrow )(\mathrm{pale}\mathrm{blue})} \\ \underset{\mathrm{Copper}\mathrm{sulphate}}{\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)(\downarrow )(\mathrm{pale}\mathrm{blue})}+\underset{\mathrm{Ammonium}\mathrm{sulphate}}{2{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4(\mathrm{aq}})+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\left[\mathrm{excess}\right]}\to \underset{\mathrm{Water}}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{Tetraaminecopper}\left(\mathrm{II}\right)\mathrm{sulphate}}{\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)} \end{gathered}$$

Part 2: Addition of Ammonium chloride with Zinc sulphate solution:-

1. Ammonium hydroxide is added drop by drop in the Zinc sulphate solution.
2. A gelatinous white precipitate is formed.
3. Upon addition of excess Ammonium hydroxide, it further becomes colourless solution.
4. Balanced chemical equation : $$\begin{gathered} \underset{\mathrm{Zinc}\mathrm{sulphate}}{{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{sulphate}}{2{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Zinc}\mathrm{hydroxide}}{\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)(\downarrow )(\mathrm{gelatinous}\mathrm{white})} \\ \underset{\mathrm{Zinc}\mathrm{sulphate}}{\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)(\downarrow )(\mathrm{gelatinous}\mathrm{white})}+\underset{\mathrm{Ammonium}\mathrm{sulphate}}{2{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4(\mathrm{aq}})+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\left[\mathrm{excess}\right]}\to \underset{\mathrm{Water}}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{Tetraaminezinc}\left(\mathrm{II}\right)\mathrm{sulphate}}{\left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)} \end{gathered}$$

Therefore, when Ammonium hydroxide solution is added drop by drop and then in excess to Copper sulphate solution it forms a pale blue ppt and then turns deep blue, whereas, when Ammonium hydroxide solution is added drop by drop and then in excess to Zinc sulphate solution it forms a white gelatinous ppt and then turns colorless.