JEE Main 2021 · 26 August · morning · Single correct · +4 / −1
Which one of the following when dissolved in water gives coloured solution in nitrogen atmosphere?
- (a) CuCl₂
- (b) AgCl
- (c) ZnCl₂
- (d) Cu₂Cl₂
Answer and solution
Answer (a) CuCl₂
Solution
Chemistry ma’am · 3 steps
Find the metal ion and its d-count. Colour needs a partly filled d-subshell, so d10 ions stay colourless.
- Why colour appears. A transition metal ion looks coloured when a d electron absorbs visible light and jumps to a higher d orbital. That d–d transition needs a partly filled d-subshell, so d⁰ and d¹⁰ ions are colourless.
- Why nitrogen. The nitrogen atmosphere keeps out air. Without oxygen, Cu⁺ cannot be oxidised to Cu²⁺, so each salt shows the colour of its own ion.
Compound Metal ion d configuration Colour CuCl₂ Cu²⁺ 3d⁹, 1 unpaired electron blue AgCl Ag⁺ 4d¹⁰, full colourless (and insoluble) ZnCl₂ Zn²⁺ 3d¹⁰, full colourless Cu₂Cl₂ Cu⁺ 3d¹⁰, full colourless (and nearly insoluble) - So. Only Cu²⁺ has a partly filled d-subshell, so CuCl₂ gives the coloured (blue) solution. The answer is (a).
The idea to keep. Colour in transition metal ions comes from d–d transitions. Work out the oxidation state, write the d-count, and remember that d⁰ and d¹⁰ ions are colourless.
Answer from the NTA final answer key. Solution by PrepWiser.