Question: Give reasons why a paper written in ordinary ink is bleached by chlorine whereas it cannot bleach pr...

Question

Give reasons why a paper written in ordinary ink is bleached by chlorine whereas it cannot bleach printed ink.

Answer 1

Solution

Bleaching means the removal of color from a substance to make it colorless. Common ink does not contain carbon, but printed ink contains carbon content. A good oxidizing agent can be a good bleaching agent.

Complete step by step answer:
A chemical that bleaches another substance is called a bleaching agent. Chlorine can remove ordinary ink used on a paper because it is a good oxidizing agent, and therefore, a good bleaching agent.
The chromophores present in ink break down on coming in contact with chlorine because chlorine oxidizes them into a substance that does not absorb light in the visible spectrum, or it can be said that absorbs light in the range of wavelength not visible to our eyes. This makes the paper appear colorless on adding chlorine to it. Printed ink, on the other hand, contains carbon. Chlorine, on reacting with carbon present in printed ink
Chlorine cannot oxidize carbon, therefore, does not remove printed ink on paper. The bleaching action of chlorine can be explained through the following steps:
The paper from which ink is to be removed has to be first dipped in water.
Chlorine is then added to water. The chlorine reacts with the water molecule to form hydrochloric acid $(HCl)$ and hypochlorous acid $(HClO)$ as follows:
$4Cl_2+H_2O \rightarrow HCl + HClO$
The hypochlorous acid formed is highly unstable and immediately decomposes into hydrochloric acid and nascent oxygen as follows:
$HClO \rightarrow HCl+[O]$
It is this nascent oxygen which acts as a strong oxidizing agent and oxidizes the chromophore present in ink to change into a colorless substance.

Note: A chromophore is a color imparting chemical present in a substance.
It is to be noted that chlorine can act as a bleaching agent only in the presence of moisture or water.
Other than chlorine, some other chemical substances are also used as bleaching agents, such as sulfur dioxide $(SO_2)$ and hydrogen peroxide $(H_2O_2)$ .