Which of the following will not be coloured?

The correct answer is Option (1) → TiO₂

Colour in transition metal compounds usually comes from d–d transitions, which need at least one d electron; a d⁰ oxide is coloured only if a charge-transfer band falls in the visible region.

• Option (1) TiO₂ — Not coloured. Ti is +4; \( \ce{Ti^{4+}} \) is [Ar] 3d⁰. No d electron, no d–d transition, and the charge-transfer band of \( \ce{TiO2} \) lies in the ultraviolet, so it is white (the white pigment in paints).

• Option (2) CrO₃ — Coloured. Cr is +6, 3d⁰, yet it is deep red because of strong O → Cr charge transfer in the visible region.

• Option (3) MnO₂ — Coloured. Mn is +4, [Ar] 3d³, with three unpaired d electrons; the oxide is black/dark brown.

• Option (4) V₂O₅ — Coloured. V is +5, 3d⁰, but it is orange due to charge transfer in the visible region.

NCERT (2026-27 edition):
Formation of Coloured Ions – …