While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agents in their reactions, ozone and nitric acid act only as oxidants. Why ?

Explanation

In `SO_(2)`, the O.S. of S is + 4. Sulphur can exhibit any oxidation state in between-2 and + 6. Therefore, in `SO_(2)`, it can decrease as well as increase its oxidation state, i.e., it can undergo reduction as well as oxidation. This is why `SO_(2)` can act both as an oxidising and a reducing agent. Same is the situation with hydrogen peroxide. In `H_(2)O_(2)`, O atom is in the O.S. of -1. Oxygen can exhibit any oxidation state in between - 2 and 0 (+ 2 also when in combination with F). Therefore, in `H_(2)O_(2)`, it is free to decrease or increase its oxidation state. This is why `H_(2)O_(2)` also acts both as an oxidising and as a reducing agent.
In `O_(3)`, the O.S. of O is zero. It can only decrease its oxidation state either to -1 or to -2, but can not increase its oxidation state. Therefore, `O_(3)` acts only as an oxidant. Same is the situation with nitric acid. In `HNO_(3)`, the O.S. of N is +5, which is the highest O.S. of N. Therefore, it can only decrease its O.S. This is why `HNO_(3)` also acts only as an oxidant.