Bleaching powder is added in water used for drinking purpose. The role of bleaching powder in this case is

1. Salts and Common Chemicals from Sodium Chloride (basic)

Welcome to your first step in mastering Applied Everyday Chemistry! To understand the chemicals in our kitchen and industries, we must start with a single, humble hero: Sodium Chloride (NaCl), or common salt. It is fascinating to note that while sodium is a highly reactive metal and chlorine is a toxic gas, they chemically combine in a 1:1 ratio to form a harmless compound essential for life Science, Class VIII, Nature of Matter, p.124.

In the world of chemistry, common salt is much more than a seasoning; it is a vital raw material. Through various industrial processes, NaCl is transformed into several multi-purpose chemicals that define our modern life. The most prominent members of this "chemical family" include:

• Bleaching Powder (CaOCl₂): Produced by the action of chlorine gas on dry slaked lime (calcium hydroxide). While it is widely used in the textile industry, its most critical role in public health is acting as a disinfectant for drinking water, effectively killing harmful bacteria and pathogens Science, Class X, Acids, Bases and Salts, p.30-31.
• Baking Soda (NaHCO₃): Known chemically as sodium hydrogencarbonate. It is a mild non-corrosive base. When heated during cooking, it releases CO₂ gas, which makes bread and cakes rise, becoming soft and spongy Science, Class X, Acids, Bases and Salts, p.31.
• Washing Soda (Na₂CO₃.10H₂O): Sodium carbonate is used extensively in the glass, soap, and paper industries. Locally, it is a powerful cleaning agent and is uniquely used to remove the permanent hardness of water Science, Class X, Acids, Bases and Salts, p.32.

Chemical Name	Common Name	Primary Everyday Use
Sodium Hydrogencarbonate	Baking Soda	Antacids and Baking
Calcium Oxychloride	Bleaching Powder	Disinfecting Water
Sodium Carbonate	Washing Soda	Removing Water Hardness

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.30-32; Science, Class VIII (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.124

2. The Chlor-Alkali Process: Source of Chlorine (basic)

To understand how we get the chemicals used in our daily lives, we must start with a very familiar substance: common salt (sodium chloride). Beyond the kitchen, salt serves as a vital raw material for the chemical industry. The most important method for processing it is the Chlor-Alkali process. In this setup, electricity is passed through a concentrated aqueous solution of sodium chloride, commonly known as brine. As electricity flows, the salt decomposes into its constituent parts through a process of electrolysis Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 30.

The name "Chlor-Alkali" is a clever shorthand for the two main products formed during the reaction: Chlor stands for chlorine gas, and Alkali refers to sodium hydroxide (NaOH). It is important to remember that an alkali is specifically a base that dissolves in water Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 24. During this process, three valuable products are generated at different points in the electrolytic cell:

• Chlorine gas (Cl₂): Released at the anode (the positive electrode). This gas is the primary ingredient for making disinfectants and bleaching agents.
• Hydrogen gas (H₂): Released at the cathode (the negative electrode). It is often used as fuel or for making ammonia for fertilizers.
• Sodium Hydroxide (NaOH): This remains in the solution near the cathode. It is a strong base used in making soaps, detergents, and paper.

The overall chemical reaction can be represented as:
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24

3. Water Quality Standards: pH and TDS (intermediate)

When we evaluate the quality of water, we are essentially checking if it is fit for its intended use—be it drinking, irrigation, or sustaining aquatic life. Two of the most fundamental parameters used globally and in India are pH and TDS (Total Dissolved Solids). These standards are primarily formulated by the Bureau of Indian Standards (BIS), the national body responsible for ensuring the quality and safety of products through schemes like the ISI mark Indian Economy, Agriculture, p.326.

pH (Potential of Hydrogen) is a scale from 0 to 14 that measures the concentration of hydrogen ions in a liquid. A value of 7 is considered neutral, like pure water. Values below 7 indicate acidity, while those above 7 indicate alkalinity Geography of India, Soils, p.3. For humans, maintaining a stable pH is vital; our bodies function within a narrow range of 7.0 to 7.8. In the environment, pH shifts can be catastrophic. For instance, when rain becomes acidic (pH less than 5.6) and flows into rivers, it lowers the water's pH, making it difficult for fish and other aquatic organisms to survive Science, Acids, Bases and Salts, p.26. For drinking water, the BIS (IS 10500) generally mandates a range of 6.5 to 8.5.

Total Dissolved Solids (TDS) refers to the combined content of all inorganic and organic substances dissolved in water. These primarily include minerals like Calcium, Magnesium, Sodium, and Potassium, as well as carbonates and chlorides. While some dissolved minerals are essential for health, high TDS levels can lead to a salty or bitter taste and may indicate the presence of harmful contaminants like arsenic or fluoride. Conversely, very low TDS (often seen in over-processed RO water) might leave the water "flat" and devoid of necessary minerals. The monitoring of these levels in our water bodies is a key function of state and central pollution control boards under various environmental acts Environment, Environmental Pollution, p.77.

Parameter	What it Measures	Impact of Deviation
pH	Hydrogen ion concentration (0–14 scale)	Values below 5.6 (acid rain) can kill aquatic life; extreme values cause corrosion or scale.
TDS	Dissolved minerals and salts (mg/L)	High levels affect taste and can indicate pollution; low levels may lack essential minerals.

Sources: Indian Economy, Agriculture, p.326; Geography of India, Soils, p.3; Science, Acids, Bases and Salts, p.26; Environment, Environmental Pollution, p.77

4. Hardness of Water and Treatment Methods (intermediate)

In our daily lives, we often distinguish between "soft" and "hard" water based on how it interacts with soap. From a scientific perspective, water hardness is caused by the presence of dissolved salts of calcium and magnesium — specifically their hydrogencarbonates (bicarbonates), chlorides, and sulphates. When you try to wash clothes with soap in hard water, you will notice that it doesn't foam easily; instead, it forms a sticky, white curdy precipitate known as scum. This happens because the soap reacts with the calcium and magnesium ions to form insoluble substances, wasting the soap and making cleaning difficult Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.76.

Hardness is generally classified into two types, depending on which salts are present:

Type of Hardness	Caused by...	Common Treatment
Temporary Hardness	Calcium and Magnesium Hydrogencarbonates	Can often be removed by simple boiling or adding lime.
Permanent Hardness	Chlorides and Sulphates of Calcium and Magnesium	Requires chemical treatments like adding washing soda or ion-exchange resins.

Beyond removing minerals, treating water for consumption requires making it biologically safe. This is where Bleaching Powder, chemically known as Calcium Hypochlorite [Ca(ClO)₂], plays a crucial role. It is produced by the action of chlorine gas on dry slaked lime [Ca(OH)₂] Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. In the context of water treatment, its primary function is not to soften the water, but to act as a powerful disinfectant. When added to water, it releases chlorine which kills bacteria, viruses, and other pathogens, ensuring the water is germ-free and safe to drink Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.76; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31

5. Bleaching Powder: Preparation and Properties (intermediate)

To understand bleaching powder, we must first look at its chemical identity and its creation. Known scientifically as calcium hypochlorite, it is represented by the formula CaOCl₂ (though its actual structure is more complex). The production process is a classic example of industrial chemistry: it is manufactured by the action of chlorine gas on dry slaked lime [Ca(OH)₂]. Interestingly, the chlorine used here isn't just pulled from thin air; it is a byproduct of the electrolysis of brine (aqueous sodium chloride), showing how different chemical processes are interconnected in industry Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 30. Slaked lime itself is the product of a vigorous combination reaction between quicklime (CaO) and water Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p. 6. Beyond its production, the properties of bleaching powder make it indispensable for public health and industry. Its primary characteristic is its strength as an oxidizing agent. In the textile and paper industries, it is used to remove color from fabrics and wood pulp. However, for a student of applied chemistry, its most vital role is in water treatment. When added to water, bleaching powder acts as a potent disinfectant. It releases chlorine, which effectively 'sterilizes' the water by killing harmful pathogens such as bacteria and germs, making it safe for drinking Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 31.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.30-31; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6

6. Disinfection Mechanism in Water Treatment (exam-level)

At its core, **disinfection** in water treatment is the process of neutralizing or killing pathogenic microorganisms to prevent waterborne diseases. The most common chemical agent used for this in everyday municipal treatment is **bleaching powder**, scientifically known as **calcium hypochlorite [Ca(ClO)₂]**. This compound is manufactured by passing chlorine gas over dry **slaked lime [Ca(OH)₂]**. While we often associate bleaching powder with the textile industry, its role in public health is far more critical: it acts as a powerful source of chlorine for water purification Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 30. When added to water, bleaching powder releases **chlorine**, which functions as a potent **oxidizing agent**. This chlorine penetrates the cell walls of harmful microbes like bacteria, viruses, and protozoa, disrupting their metabolic processes and effectively 'sterilizing' the water. This ensures the water is safe for consumption and free from biological contaminants Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 31. It is important to distinguish this from 'softening' water; while certain sodium compounds are used to remove permanent hardness, chlorine-based compounds are specifically targeted at biological safety Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 33. The effectiveness of chlorine stems from its high chemical reactivity. In environmental chemistry, we see this same reactivity in the stratosphere, where free chlorine atoms act as a catalyst to break down **ozone (O₃)** molecules into oxygen. While this catalytic cycle is destructive to the ozone layer, in the context of water treatment, that same 'aggressive' nature of chlorine is our greatest ally in destroying the pathogens that threaten human health Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p. 268.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30-33; Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.268

7. Solving the Original PYQ (exam-level)

Now that you have mastered the chemistry of salts, specifically calcium hypochlorite, you can see how the building blocks of chemical reactions apply to public health. You recently learned that bleaching powder is produced by the action of chlorine on dry slaked lime. This question tests your ability to identify the specific functional role of this compound when applied to drinking water. While you know its chemical structure and general properties, the UPSC expects you to connect its oxidizing power to the elimination of biological threats in a municipal context.

To arrive at the correct answer, follow the logic of water purification: when bleaching powder is added to water, it releases chlorine, which acts as a potent disinfectant. This chemical process effectively kills harmful pathogens, such as bacteria and viruses, making the water safe for human consumption. This specific application is highlighted in Science, Class X (NCERT), which notes its use for making drinking water free from germs. Therefore, the role is (A) of disinfectant, as the primary objective in treating drinking water is sterilization rather than aesthetic cleaning.

The other options are classic UPSC distractors. Option (B) is a trap because it uses the literal name of the substance; while it is a bleaching agent in the textile industry, that is not its purpose in water. Option (C) is too broad, as "impurities" often refers to physical sediments which are removed via filtration, not chemical addition. Finally, Option (D) is incorrect because bleaching powder is basic in nature and would not be used to decrease pH. Always focus on the specific context—drinking water—to distinguish between a substance's general properties and its specialized utility.