What is observed when adding ammonia to $C{{u}^{2+}}$ solution and why does this happen?

Hint: Copper is a transition metal that has an oxidation state of +2. It is a d block element and has the ability to form colored complexes due to d –d transition. The $C{{u}^{2+}}$ ions can be used in the form of copper sulfate $CuS{{O}_{4}}$that will act as Lewis acid and ammonia will act as a Lewis base.

Complete answer:
As we know, the complexes of d- block elements are able to emit colors. This is due to the fact that these metals contain unpaired electrons in the d – orbital. When the ligand approaches to form a complex with these metals, the electrons get excited and move to higher energy levels as of empty d – orbital. They absorb energy and move to higher energy levels, and come back to lower levels and emit energy in the visible region which is characterized as colors.
Now, when copper sulfate solution is added to liquid ammonia solution $\left( N{{H}_{4}}OH \right)$, then initially a precipitate is formed of copper (II) hydroxide which is basic and blue in color due to the presence of ions of copper (II) complex, like$Cu{{({{H}_{2}}O)}_{4}}{{(OH)}_{2}}$, the reaction for blue precipitate formation is,
$CuS{{O}_{4}}(aq)+N{{H}_{4}}OH(aq)\to {{(N{{H}_{4}})}_{2}}S{{O}_{4}}(aq)+Cu{{(OH)}_{2}}(s)\downarrow (blue)$
Further, when more ammonia is added, the solid gets dissolved in the solution and the ammonia acts as a ligand for copper and the color of the solution becomes dark – blue. The reaction is:
$Cu{{({{H}_{2}}O)}_{4}}{{(OH)}_{2}}(aq)+4N{{H}_{4}}OH(aq)\to {{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}O)}_{2}}]}^{2+}}(aq)+2{{H}_{2}}O(l)+2O{{H}^{-}}(aq)$
This is a type of ligand exchange reaction and is also an example of Lewis acid – base reaction.
Hence, addition of ammonia to $C{{u}^{2+}}$ solution imparts a blue color solution that turns dark blue when more ammonia solution is added.

Note:
Ammonia solution is made by making ammonia into an aqueous solution and liquefying it by adding water, the reaction is, $N{{H}_{3}}+{{H}_{2}}O\rightleftharpoons N{{H}_{4}}^{+}+O{{H}^{-}}$, these are all in equilibrium and a clear solution of ammonium hydroxide is obtained as $N{{H}_{4}}OH$. The complex of copper (II) in ammonia solution is called copper (II) tetramine sulfate.