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(SO₂), 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, SO₂ can act as an oxidizing as well as a reducing agent.

Let the oxidation number of S in SO₂ be x.

X + 2(-2) = 0 {O.N. of O = -2}

X = + 4

In hydrogen peroxide (H₂O₂), 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, H₂O₂ can act as an oxidizing as well as a reducing agent.

Let the oxidation number of O in H₂O₂ be x.

2(+ 1) + 2x = 0 {O.N. of H = + 1}

X = -1

In ozone(O₃), 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 can act only as an oxidant.

In nitric acid(HNO₃), the O.N. of N is + 5 and the range of the O.N. that N cam 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.

Let the O.N. of N in HNO₃ be ‘x’.

1 + x + 3(-2) = 0 {O.N. of H = + 1 and of O = -2}

X = + 5