8.8 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 ?

In sulphur dioxide (${\mathrm{SO}}_{2}$), 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, ${\mathrm{SO}}_{2}$ can act as an oxidising as well as a reducing agent. In hydrogen peroxide (${\mathrm{H}}_{2}{\mathrm{O}}_{2}$), 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, ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ can act as an oxidising as well as a reducing agent.
In ozone (${\mathrm{O}}_{3}$), 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, O3 acts only as an oxidant.
In nitric acid (${\mathrm{HNO}}_{3}$), 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, HNO3 acts only as an oxidant.