The presence of which reactant is the best indicator of an oxidation-reduction reaction?

H₂ is an element in its free form, so each hydrogen atom has an oxidation state of 0. In reactions, hydrogen gas commonly changes to a different oxidation state, indicating electron transfer and an oxidation-reduction reaction. H₂SO₄ and NaOH more commonly indicate acid-base reactions, while KNO₃ alone is not as clear an indicator.

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