$\text{S}{{\text{O}}_{2}}+2{{\text{H}}_{2}}\text{O}\to {{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}+\,\underset{nascent\,\,\,hydrogen}{\mathop{2H}}\,$

Thus, $\mathrm{SO}_{2}$ in presence of moisture is used as bleaching agent. This is due to the reducing nature of $\mathrm{SO}_{2}-$ For delicate articles

                Coloured matter $+\mathrm{H} \longrightarrow$ Colourless matter

Similarly, $\mathrm{Cl}_{2}$ acts as bleaching agent in presence of moisture

                            $\text{C}{{\text{l}}_{2}}+{{\text{H}}_{2}}\text{O}\to 2\text{HCl}+[\text{O}]$

                 Coloured matter $+\,[O]\,\to $ Colourless matter