Question

why hno3 can only work as oxidising agent

Answer 1

HNO3 can be reducedonly and hence acts as an oxidizing agent only. ... Thus oxidation state of nitrogen present in HNO2 can be increased or decreased and hence itcan act as an oxidizing agent or as areducing agent.

Answer 2

HNO3 can act both as an oxidizing agent and a normal acid depending on the reactant. Salts such as carbonates, sulphites react with hno3 to form co2 and so2. However sulphides react with hno3 to generate both a precipitate of sulphur as hydrogen sulphide is oxidized by the nitric acid. It also reacts with metals like copper, mercury and silver(only nitic acid can react with silver and mercury because of its strong oxidizong property) to form nitrogen oxides and the corresponding nitrates and non metals like sulphur and phosphorus to give sulphuric acid and phosphoric acid respectively.

So, HNO3 can act both as an oxidizing agent as well as a normal acid depending on reactants.

Hope it helped !!