Question

Bleaching of flowers by $C{{l}_{2}}$ is permanent while bleaching by $S{{O}_{2}}$ is temporary. Why?

Answer 1

Hint:
Here, one of the bleaching agents acts as an oxidizing agent for the colouring dyes whereas the other acts as a reducing agent. The oxidised due lasts permanently as it cannot revert back but the reduced dye can convert back reacting with the atmospheric oxygen.

Complete step by step answer:
-We know that bleaching a substance means ripping off its colour and hence making it colourless.
-There are various elements which are used as a bleaching agent but the most common bleaching agent is chlorine. This is due to the fact that the bleaching effect caused by chlorine is permanent which is not the case for every other bleaching agent out there for example, bleaching by sulphur dioxide is temporary.
-This difference is caused due to the difference in the mechanism of reaction of these elements which we will discuss below-
-Bleaching through chlorine is based on oxidation whereas through sulphur dioxide is based on reduction.
-When chlorine is passed through any coloured substance in presence of moisture or water it gives us hypochlorous acid and hydrochloric acid. The hypochlorous acid is unstable thus further dissociates into hydrochloric acid and a nascent oxygen atom. This oxygen acts as an oxidising agent and reacts with the colouring dyes and makes the material colourless permanently. We can write the reactions as-
     \[\begin{align}
  & C{{l}_{2}}(g)+{{H}_{2}}O(l)\to HOCl+HCl \\
 & HOCl\to HCl+\left[ O \right] \\
 & \left[ O \right]+colouredmaterial\to colourless \\
\end{align}\]
-But when we use sulphur dioxide in presence of water for bleaching, the dyes are reduced because sulphur dioxide acts as a reducing agent. It decolourises the substance but it does not last very long because the atmospheric oxygen oxidizes the dyes and makes the material coloured again. We can write the reaction as-
     \[S{{O}_{2}}+{{H}_{2}}O\to HS{{O}_{3}}^{-}+{{H}^{+}}\]
-Therefore, bleaching of flowers by $C{{l}_{2}}$ is permanent while bleaching by $S{{O}_{2}}$ is temporary.

Note:
Apart from decolourising dyes, as chlorine in presence of water forms hypochlorous acid $\left( HOCl \right)$ and hypochlorite ion $\left( OC{{l}^{-}} \right)$, it acts as a disinfectant. Chlorine also kills pathogens and bacteria by breaking the chemical bonds in the molecules but as chlorine is a harsh bleaching agent, still sulphur dioxide is still used in bleaching of papers and clothes.