Question

Bleaching action of chlorine is permanent while bleaching action of sulphur dioxide is temporary, why?

Answer 1

Hint : We must know the difference lies in the oxidation and reduction of colour to become colourless. And we must know the reaction of chlorine and sulphur dioxide with water.

Complete step by step solution :
Let’s start the answer with answering why the bleaching action of chlorine is permanent? Well, the answer to this question is oxidation. Due to the involvement of the process of oxidation, permanent bleaching happens. When chlorine reacts with water (${H_2}O$) it produces nascent oxygen. This nascent oxygen combines with colour agents and turns them colourless. The nascent oxygen is a powerful oxidizing agent hence, the bleaching action is permanent. Given below are the chemical reactions for bleaching action by chlorine:
${\text{C}}{{\text{l}}_2} + {{\text{H}}_2}{\text{O}} \to {\text{HCl + HClO}}$
The $HClO$is highly unstable results in release of nascent oxygen
${\text{HClO}} \to {\text{HCl + [O]}}$
Now, this nascent oxygen [O] reacts with colours and turns them colourless.
${\text{Colour + [O] }} \to {\text{ Colourless}}$
Now, let’s discuss the bleaching action of sulphur dioxide. In case of sulphur dioxide instead of oxidation, reduction happens which leads to temporary bleaching. Sulphur dioxide removes the oxygen (reduces) from colour and turns it colourless. Due to the presence of oxygen in the atmosphere, after some time the colour absorbs the atmospheric oxygen and again gains its colour leads to a temporary loss of colour or temporary bleaching. Given below is the chemical equation of the temporary bleaching by sulphur dioxide.
${\text{S}}{{\text{O}}_2}{\text{ + 2}}{{\text{H}}_2}{\text{O }} \to {\text{ }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{ + 2[H]}}$
When Sulphur dioxide reacts with water it produces nascent hydrogen which leads to reduction.
${\text{Colour + 2[H] }} \to {\text{ Colourless}} \\\ {\text{Colourless + }}{{\text{O}}_2}({\text{Atmospheric Oxygen) }} \to {\text{ Colour}} \\\$
So like this we have discussed why the bleaching action of chlorine is permanent and Sulphur dioxide is temporary.

Note : We must know that the bleaching agents are used in many industries. Some of the famous industries include textile, design, salons, paper, etc. all these industries use either permanent or temporary bleaching processes in their manufacturing or service oriented works.