The bleaching action of chlorine gas is effective due to the presence of

Analysis of the Question

This question tests your understanding of the chemical properties of chlorine gas, specifically its role as a bleaching agent. You are asked to identify the substance that must be present for chlorine to perform its bleaching function.

Concept Explanation

Chlorine ($Cl_2$) acts as a bleaching agent through an oxidation reaction. However, dry chlorine gas does not bleach. For chlorine to bleach, it must react with water to form two acids: hydrochloric acid ($HCl$) and hypochlorous acid ($HOCl$).

The chemical equation for this reaction is:

The hypochlorous acid ($HOCl$) formed is unstable and readily decomposes to release nascent oxygen (a single atom of oxygen, $[O]$), which is a very powerful oxidizing agent:

The nascent oxygen then reacts with the colored pigments in the material (like dyes or fabrics) to oxidize them into colorless substances. Therefore, without water, the hypochlorous acid cannot be formed, and the bleaching process cannot occur.

Step-by-Step Solution

1. Identify the bleaching agent: Chlorine gas ($Cl_2$) is the primary agent, but it requires a medium to react.
2. Examine the reaction: Chlorine reacts with water to produce hypochlorous acid ($HOCl$).
3. Determine the active component: The hypochlorous acid releases nascent oxygen, which is responsible for the actual bleaching (removal of color).
4. Conclusion: Since the entire process starts with the reaction between chlorine and water, water is the essential component required for the bleaching action to take place.

Identification of the Answer

Comparing this to the provided options:

• A. Hydrogen chloride is a product of the reaction, not the cause of bleaching.
• B. Water is the essential reactant that enables the formation of the bleaching agent.
• C. Air is not required for this specific chemical reaction.
• D. Oxygen is the byproduct of the decomposition of $HOCl$, but it is the water that initiates the process.

Exam Tip

Always remember: "Dry chlorine does not bleach." If you see a question about chlorine bleaching, immediately look for "water" or "moisture" as the key requirement. If the question asks about the substance responsible for the bleaching, it is "nascent oxygen," but if it asks what must be present for the action to be effective, it is "water."