The aqueous solution containing which one of the following ions will be colourless ?
(Atomic no. : Sc = 21, Fe = 26, Ti = 22, Mn = 25)

The color of an aqueous solution of metal ions depends on the presence of unpaired electrons in the d orbitals of the ion, which can absorb light in the visible spectrum. Sc3+ (scandium(III)) has no unpaired electrons in its d orbitals (since it's a d0 ion), making it colorless. Fe2+ (iron(II)) has unpaired electrons and is typically green or pale green in aqueous solution. Ti3+ (titanium(III)) also has unpaired electrons and is purple in aqueous solution. Mn2+ (manganese(II)) has unpaired electrons and is pink or pale pink in aqueous solution. Therefore, the ion that will result in a colorless aqueous solution is Sc3+.