Sulphur dioxide and hydrogen peroxide can act as an oxidising as well as a reducing agents in their reactions, while ozone and nitric acid act only as an oxidants. Why?

Explanation

(a) `SO_(2)` : In `SO_(2)`, oxidation number of S is +4. Oxidation number of S ranges from -2 to +6. Therefore, in `SO_(2)` oxidation number of S can vary. S can acts as an oxidation agent or as a reducing agent.
(b) `H_(2)O_(2)` : In `H_(2)O_(2)` oxidation number of O is -1. Generally minimum oxidation number of oxygen is -2 and maximum oxidation number of oxygen is 0. (Exception : In `O_(2)F_(2)` oxidation number of oxygen is +1.)
So, oxidation number of oxygen of `H_(2)O_(2)` is increasing by -1 to 0 and decreases by -1 to -2. So, `H_(2)O_(2)` act as oxidising agent and reducing agent.
( c) `O_(3)` : Oxidation number of oxygen in `O_(3)` is zero. Here oxidation number of oxygen reduced from zero to -1 or -2. Therefore, it acts as an oxidizing agent.
(d) `HNO_(3)` : Nitrogen is `HNO_(3)` oxidation number is +5. Therefore, its oxidation number it can only decreased. Therefore, `HNO_(3)` can only used as an oxidizing agent.