What is the change in oxidation number of sulfur in the reaction SO₂ + H₂O → H₂SO₃?
S (+4 in SO₂) remains +4 in H₂SO₃. Change = 0 (not a redox reaction).
Ref: NCERT Class 11 Chemistry > Chapter 7: Redox Reactions > Topic: Classical and Electronic Concept of Redox and Oxidation Number