When a few drops of sodium hydroxide solution is added to copper nitrate solution, light blue colour ppt. of copper hydroxide is formed. This ppt. is insoluble in excess $\mathrm{NaOH}$ . $\; \mathrm{Cu} (\mathrm{NO}_3)_2\mathrm{(aq)} + 2 \mathrm{NaOH} \rightarrow$ $\; \underset{\text{Light Blue}}{\mathrm{Cu}(\mathrm{OH})_2\mathrm{(s)}} + 2 \mathrm{NaNO}_3\mathrm{(aq)}$