(b) $\mathrm{Cu}^{2+}$ ion reacts with ammonia solution to form a deep blue coloured complex because of the property of ammonia to form complex compounds. Ammonia acts as a lewis base due to presence of lone pair of electrons on the nitrogen atom. Hence, it is able to form coordinate bond with electron deficient molecules or a number of transition metal cations.$\mathrm{Cu}^{2+}(\mathrm{aq}) + 4 \mathrm{NH}_3 (\mathrm{aq}) \xrightarrow[\mathrm{Cu}(\mathrm{NH}_3)_4]{2+}$