Which one of the following ions is colourless in aqueous solution?

The correct answer is Option (4) → Zn²⁺

A transition metal ion is coloured when a d electron can be excited from a lower to a higher energy d orbital by absorbing visible light. This needs a partly filled d subshell; an ion with 3d⁰ or 3d¹⁰ cannot do it and is colourless.

• Configuration of Zn: Z = 30, [Ar]3d¹⁰4s².

• Electrons removed: the two 4s electrons, giving \( \ce{Zn^2+} \): 3d¹⁰.

• Result: the 3d subshell is full, so no d–d transition is possible and \( \ce{Zn^2+} \) is colourless.

Why the other options are incorrect

Option (1) Ti³⁺ — Incorrect. Ti (Z = 22) is [Ar]3d²4s²; \( \ce{Ti^3+} \) is 3d¹, and its aqueous solution is purple.

Option (2) Cu²⁺ — Incorrect. Cu (Z = 29) is [Ar]3d¹⁰4s¹; \( \ce{Cu^2+} \) is 3d⁹, and its aqueous solution is blue.

Option (3) Ni²⁺ — Incorrect. Ni (Z = 28) is [Ar]3d⁸4s²; \( \ce{Ni^2+} \) is 3d⁸, and its aqueous solution is green.

NCERT (2026-27 edition):
4.3.10 Formation of Coloured Ions – “When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed (Unit 5). This frequency generally lies in the visible region. … In aqueous solutions where water molecules are the ligands, the colours of the ions observed are listed in Table 4.8.”

Table 4.8: Colours of Some of the First Row (aquated) Transition Metal Ions