The oxidation state of sulphur in sulphur dioxide is +4. It can be oxidised to +6 oxidation state or reduced to +2. Therefore, sulphur dioxide acts as a reducing agent as well as oxidising agent. Similarly, the oxidation state of oxygen in hydrogen peroxide is –1. It can be oxidised to O2 (zero oxidation state) or reduced to H2O or OH– (–2 oxidation state) and therefore, acts as reducing as well as oxidising agents.
However, both ozone and nitric acid can only decrease their oxidation number and therefore, act only as oxidising agents.