Question

Bleaching action of $SO_2$ is permanent:
A. True
B. False

Answer 1

Bleaching is the loss of colour. When a coloured material undergoes reduction the bleaching effect is temporary. However, when a coloured material undergoes oxidation, the bleaching effect is permanent.

Complete step by step answer:
When we talk about bleaching a substance, we usually refer to clothes or anything which has a colour to it
Bleaching a substance would mean that there is a chemical reaction which causes the substance to lose its colour.
Now, there are two possible reactions which make up for entire chemical reactions.
Reduction and oxidation.
In case o f reduction, Oxygen combines with the coloured material and forms a colourless compound.
For example, $C{l_2}$
Chlorine gas in water give hypochlorite
$C{l_2} + {H_2}O \to HCl + HClO$
Hypochlorite readily breaks down to give nascent oxygen.
$HClO \to [O] + HCl$
Nascent oxygen is atomic oxygen which can be used for oxidation
Coloured substance $+ [O] \to$ colourless substance.
This type of bleaching is permanent.
Now, in case of reduction,
Atomic hydrogen or nascent hydrogen attacks the coloured substance to form colourless substances, by removing oxygen.
$S{O_2} + {H_2}O \to {H_2}S{O_4} + 2[H]$
Nascent hydrogen then reacts with colourless substances.
Coloured substance $+ [H] \to$ colourless substance.
Now, it is very much possible that atmospheric oxygen can slowly over time start reacting with the molecule and the colour will reappear.
Hence, this type of bleaching is permanent.

So, the statement is false.

Note: Sulphur dioxide only acts as a bleaching agent in the presence of moisture. This is due to the liberation of nascent hydrogen when sulphur dioxide comes in contact with water in the atmosphere. Bleaching due to reduction is a very temporary form of bleaching.