Which of the following oxides is amphoteric and reacts with both NaOH and HCl, but does NOT disproportionate in acidic or alkaline medium?
(A) CrO₃
(B) Mn₂O₇
(C) PbO₂
(D) VO₂
Correct: D
CrO₃ is acidic and dissolves in base but does not react with acid (it is already in the highest oxidation state +6 for Cr). Mn₂O₇ is acidic and disproportionate in presence of water (it is highly unstable). PbO₂ is amphoteric: it reacts with HCl (reducing to Pb²⁺) and with NaOH (forming plumbite, [Pb(OH)₆]²⁻), but it does disproportionate only under specific conditions; however, PbO₂ does disproportionate in acidic medium (2PbO₂ + 4H⁺ → 2Pb²⁺ + O₂ + 2H₂O). VO₂ (vanadium(IV) oxide) is amphoteric: it dissolves in acid to form VO²⁺ and in base to form vanadate (VO₃²⁻ or [V(OH)₆]²⁻). VO₂ does not disproportionate under normal acidic or alkaline conditions. The best answer is D (VO₂).