What is the colour of product when ammonia reacts with a solution of Cu2+?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Deep blue
Ammonia is a Lewis base with a lone pair on nitrogen. When excess ammonia is added to a solution of \( \ce{Cu^2+} \), the water ligands are replaced and the tetraamminecopper(II) complex ion is formed, which has an intense deep blue colour:
\( \displaystyle \ce{[Cu(H2O)4]^2+ + 4NH3 -> [Cu(NH3)4]^2+ + 4H2O} \)
This colour is a common test for \( \ce{Cu^2+} \) ions; a little ammonia first gives a pale blue precipitate of \( \ce{Cu(OH)2} \), which dissolves in excess ammonia to give the deep blue solution.