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Explain why $HN{{O}_{3}}$ acts only as oxidising agent while $HN{{O}_{2}}$ can act both as a reducing agent and an oxidising agent ?

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Hint: Oxidising agent is a substance that can oxidise other substances or it is a species that can accept electrons or donate protons. While reducing agents are those which reduce substances and can lose electrons or accept protons. The ability of a substance to act as oxidising agent and reducing agent depends on its oxidation state. If it can change its oxidation state to a more positive state then it can oxidise substances and it can change to more negative, then it can reduce substances.

Complete answer:
- Reducing agents are those compounds which can reduce other compounds and themselves get oxidised. Oxidising agents are those substances which can oxidise other substances and themselves get reduced.
- Oxidising and reducing property of a substance depends on the oxidation state of the substance.
- Oxidation state of nitrogen ranges from -3 to +5. In nitric acid, the oxidation state of nitrogen is +5, that is the maximum oxidation state nitrogen can acquire in a compound. So, it can only get reduced to a number less than +5 till -3. Therefore, nitric acid can only act as an oxidising agent because it itself gets reduced. In case of nitrous acid, the oxidation state of nitrogen is +2. So it can get oxidised to +5 state and also get reduced to -3 state. Thus, it can act as both reducing agent and oxidising agent.

Note: Usually oxidation and reduction occurs simultaneously. When one of the reagents gets oxidised, the other reagent gets reduced. Such a reaction is called a redox reaction.