Question

Why bleaching action of Chlorine is permanent while that of sulfur dioxide is temporary?

Answer 1

Bleaching:

• Bleaching is the process of removing ingested or naturally occurring coloring agents from raw textile material to decolorize it.

Effect of Chlorine in bleaching:

• Chemical reaction followed by bleaching using chlorine is as follows,

$\underset{\mathrm{Chlorine}}{\underbrace{{\mathrm{Cl}}_2\left(\mathrm{l}\right)}}+\underset{\mathrm{Water}}{\underbrace{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}\to \underset{\mathrm{Hydrochloric}\mathrm{Acid}}{\underbrace{\mathrm{HCl}\left(\mathrm{l}\right)}}+\underset{\mathrm{Hypochlorous}\mathrm{Acid}}{\underbrace{\mathrm{HClO}\left(\mathrm{l}\right)}}$

• Due to the process of oxidation, the bleaching action of chlorine is permanent.
• Water and chlorine combine to create nascent oxygen.
• After then, the oxygen reacts with the colored substance to turn it colorless. It has a strong oxidizing effect.
• After the formation of hydrochloric acid and hypochlorous acid, the hypochlorous acid dissociates and forms nascent oxygen because of its instability.
• The following reaction illustrates the entire procedure,

$\underset{\mathrm{Hypochlorous}\mathrm{Acid}}{\underbrace{\mathrm{HClO}\left(\mathrm{l}\right)}}\to \underset{\mathrm{Hydrochloric}\mathrm{Acid}}{\underbrace{\mathrm{HCl}\left(\mathrm{l}\right)}}+\underset{\mathrm{Nascent}\mathrm{Oxygen}}{\underbrace{\left[\mathrm{O}\right]\left(\mathrm{g}\right)}}$

Effect of Sulfur Dioxide in bleaching:

• Chemical reaction followed by bleaching using Sulfur Dioxide is as follows,

$\underset{\mathrm{Sulfur}\mathrm{Dioxide}}{\underbrace{{\mathrm{SO}}_2\left(\mathrm{g}\right)}}+\underset{\mathrm{Water}}{\underbrace{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}⟶\underset{\mathrm{Sulphuric}\mathrm{Acid}}{\underbrace{{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)}}+\underset{\mathrm{Nascent}\mathrm{Hydrogen}}{\underbrace{2\left[\mathrm{H}\right]}\left(\mathrm{g}\right)}$

• Due to the process of reduction, the bleaching action of chlorine is temporary.
• Sulfur dioxide renders the colored material colorless by removing oxygen.
• The substance regains its color when atmospheric oxygen gradually replaces the lost oxygen.

Hence the bleaching action of Chlorine is permanent while that of Sulfur Dioxide is temporary