In which reaction does the oxidation number of hydrogen change? A)HCl (aq)+ NaOH (aq)→ NaCl (aq)+ H₂O (l) B)2Na (s)+ 2H₂O (l)→ 2NaOH (aq)+ H₂ (g) C)CaO (s)+ H₂O (l)→ Ca(OH)₂ (s) D)2HClO₄ (aq)+ CaCO₃ (s)→ Ca(ClO₄)₂ (aq)+ H₂O (l)+ CO₂ (g) E)SO₂(g)+ H₂O (l)→ H₂SO₃ (aq)

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In reaction B, hydrogen goes from being part of a molecule (H2O) where it has an oxidation state of +1 to forming H2 gas, where its oxidation state is 0. This change in oxidation state indicates a change in the oxidation number of hydrogen.

In Which Reaction Does the Oxidation Number of Hydrogen Change