Ammonia forms the complex ion $[Cu {({NH}_{3})}_{4}]^{2 +}$ with copper ions in alkaline solutions but not in acidic solutions. The reason is

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In acidic solutions protons coordinate with ammonia molecules forming ${NH}_{4}^{+}$ ions and ${NH}_{3}$ molecules are not available.

Copper hydroxide is an amphoteric substance.

In acidic solutions hydration protects copper ions.

In alkaline solutions $Cu {(OH)}_{2}$ insoluble is precipitated which is soluble in excess of any alkali

answer is B.

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Detailed Solution

Ammonia is Base, to form ammonium ions in the acidic medium it reacts with a proton.
No free ammonia molecules are present.
No co -ordination with Cu(II) ions can happen to form a complex ion.
Ammonium ions are formed when ammonia reacts with a proton.
These ions have a higher charge density than the nitrogen atom in ammonia which means that it attracts electrons better so that it possesses a negative charge. It also has a higher vibration frequency than that of ammonia and this helps in the formation of ammonium ions.

Ammonium ions are formed when ammonia is dissolved in water and reacts with the presence of H⁺ ions.

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