Nitrogen can have oxidation number from -3 to +5. The oxidation number of nitrogen in `HNO_(3)` is +5. Thus, increase in oxidation number beyond +5 cannot occur. Hence, `HNO_(3)` cannot act as reducing agent. The oxidation number of nitrogen in `HNO_(3)` can only decreasel thus it acts as an oxidising agent. In `HNO_(3)` the oxidation number of nitrogen is +3. Thus, it can increase or decrease within the range -3 to +5. Hence it can act as an oxidising as well as reducing agent.
