When excess aqueous ammonia is added to aqueous copper(II) sulphate, the deep blue colour arises from

A $[Cu (H X_{2} O) X_{6}] X^{2 +}$ , whose colour is intensified by $NH X_{3}$
B $[Cu (NH X_{3}) X_{4}] X^{2 +}$ , in which $NH X_{3}$ donates its lone pair as a Lewis base
C $Cu (OH) X_{2}$ precipitate that does not dissolve in excess $NH X_{3}$
D $[Cu (NH X_{3}) X_{4}] X^{2 +}$ , in which $NH X_{3}$ accepts an electron pair as a Lewis acid

Answer: B

A pale blue precipitate of

Cu (OH) X_{2}

forms first and then dissolves in excess

NH X_{3}

to give deep blue

[Cu (NH X_{3}) X_{4}] X^{2 +}

. The nitrogen lone pair of

NH X_{3}

is donated to

Cu X^{2 +}

, so

NH X_{3}

acts as a Lewis base (ligand).