Among the following substances, which one is most commonly used as a bleaching agent for whitening fabrics and pulp and for disinfection purposes?

Introduction / Context:
Bleaching agents are chemicals used to remove colour from substances such as fabrics, paper pulp, and sometimes even to disinfect water. Different bleaching agents work through different chemical mechanisms, usually involving oxidation. In everyday life and industry, one particular element and its compounds are widely used because of their strong oxidising and germicidal properties. This question asks you to identify the most commonly used bleaching agent among the listed substances.

Given Data / Assumptions:

• The options include chlorine, ethyl alcohol, carbon dioxide, sodium chloride, and calcium carbonate.
• We are focusing on bleaching and disinfection, not on other uses like carbonation or drinking.
• Chlorine can be used directly as a gas or through compounds such as bleaching powder and sodium hypochlorite.
• We assume typical industrial and domestic bleaching and water treatment applications.

Concept / Approach:
Chlorine is a powerful oxidising agent and is widely used for bleaching wood pulp in the paper industry and for disinfecting drinking water and swimming pools. When dissolved in water, chlorine forms hypochlorous acid, which can oxidise coloured organic compounds, turning them colourless. Compounds derived from chlorine, such as bleaching powder and sodium hypochlorite, are also common household and industrial bleaches. Alcohol, carbon dioxide, sodium chloride, and calcium carbonate do not serve as primary bleaching agents. Therefore, chlorine is the best answer.

Step-by-Step Solution:
Step 1: Consider the general role of a bleaching agent: it should be able to remove colour by breaking down coloured molecules, usually by oxidation. Step 2: Recall that chlorine gas and its compounds like sodium hypochlorite are widely used in bleaching and disinfection. Step 3: Recognise that chlorine in water forms hypochlorous acid, a strong oxidising agent that can attack chromophore groups in dyes and pigments. Step 4: Compare this with ethyl alcohol, which is mainly used as a solvent and antiseptic, not as a primary bleaching agent. Step 5: Note that carbon dioxide, common salt, and calcium carbonate have other uses such as carbonation, seasoning, and building materials, but are not standard bleaching agents.

Verification / Alternative check:
In industrial practice, chlorine based compounds are standard for bleaching wood pulp and textiles. Household bleach often contains sodium hypochlorite, a chlorine based compound that releases active chlorine species in solution. Water treatment plants use chlorine to disinfect water by killing microorganisms. These uses align with bleaching and germicidal action. Alcohols are used for disinfection on skin and surfaces but not for bleaching fabrics or pulp. Carbon dioxide is used in carbonation and fire extinguishers, common salt and calcium carbonate have mineral uses, but neither removes colour in the way chlorine does. This confirms chlorine as the correct answer.

Why Other Options Are Wrong:
Ethyl alcohol is an effective antiseptic and solvent but does not serve as a typical bleaching agent for fabrics or pulp. Carbon dioxide is a relatively inert gas used in beverages and fire suppression and has no bleaching role. Sodium chloride is common table salt; it is used in food and sometimes in brine solutions but not as a bleaching agent. Calcium carbonate is the main component of limestone and chalk and is used in construction and as a filler in paper, not to bleach it. These substances therefore do not match the standard profile of a bleaching agent.

Common Pitfalls:
Some learners may confuse disinfection and bleaching and assume that ethyl alcohol might be used for both, leading them to choose alcohol. Others may focus on the word "chloride" in sodium chloride and incorrectly think that any chloride compound must act like chlorine gas. To avoid these errors, remember that bleaching agents are strong oxidisers, and in everyday chemistry chlorine and its compounds are the classic example used for whitening and disinfection on a large scale.

Final Answer:
The most commonly used bleaching agent among the given options is chlorine.