Zn²⁺ salts are white while Cu²⁺ salts are blue.

Solution

Chemistry ma’am · 3 steps

Check the d-electrons first: a full d¹⁰ shell has no room for a d-d jump, a d⁹ shell does.

1. Where colour comes from. In a transition metal ion, ligands split the d orbitals into two sets of slightly different energy. An electron can absorb visible light and jump to the higher set, and we see the complementary colour.

   Ion	Configuration	d-d transition possible?	Colour
   Zn²⁺	[Ar] 3d¹⁰	No, every d orbital is full	white (colourless)
   Cu²⁺	[Ar] 3d⁹	Yes, one d orbital is half-filled	blue

2. Why each one. With 3d¹⁰, Zn²⁺ has no vacancy in the upper d set, so no visible light is absorbed. With 3d⁹, Cu²⁺ absorbs red-orange light, and hydrated salts like CuSO₄.5H₂O look blue.
3. So. The statement is true: Zn²⁺ salts are white and Cu²⁺ salts are blue.

The idea to keep. Colour in transition metal ions needs a d-d jump, so it needs partly filled d orbitals. Ions with d⁰ or d¹⁰, like Sc³⁺, Zn²⁺ and Cu⁺, are colourless.

Answer from the NTA final answer key. Solution by PrepWiser.