Question

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?

Answer 1

In sulphur dioxide $(SO_2)$, the oxidation number (O.N.) of $S$ is $+4$ and the range of the O.N. that $S$ can have is from $+6$ to $-2$.
Therefore, $SO_2$ can act as an oxidising as well as a reducing agent.
In hydrogen peroxide $(H_2O_2)$, the O.N. of $O$ is $-1$ and the range of the O.N. that $O$ can have is from $0$ to $-2$. $O$ can sometimes also attain the oxidation numbers $+1$ and $+2$.

Hence, $H_2O_2$ can act as an oxidising as well as a reducing agent.

In $ozone$ $(O_3)$, the O.N. of $O$ is zero and the range of the O.N. that $O$ can have is from $0$ to $-2$. Therefore, the O.N. of $O$ can only decrease in this case. Hence, $O_3$ acts only as an oxidant.
In nitric acid $(HNO_3)$, the O.N. of $N$ is $+5$ and the range of the O.N. that $N$ can have is from $+5$ to $-3$. Therefore, the O.N. of $N$ can only decrease in this case.

Hence, $HNO_3$ acts only as an oxidant.