In which reaction does the oxidation number of oxygen increase? in which reaction does the oxidation number of oxygen increase? 2so2 (g) + o2 (g) → 2so3 (g) mgo (s) + h2o (l) → mg(oh)2 (s) 2h2o (l) → 2h2 (g) + o2 (g) ba(no3)2 (aq) + k2so4 (aq) → baso4 (s) + 2kno3 (aq) hcl (aq) + naoh (aq) → nacl (aq) + h2o (l)?