Match List-I with List-II and select the correct answer using the code given below the Lists: List-I (Name)

1. Introduction to Acids, Bases, and Salts (basic)

To understand chemistry in our daily lives, we must first look at the most common chemical 'personalities': **Acids** and **Bases**. Acids are generally sour-tasting substances that turn blue litmus paper red. They are found naturally in many of the foods we consume, such as acetic acid in vinegar, citric acid in lemons, and lactic acid in curd Science, Class X, p.28. Bases, conversely, are bitter to the taste and feel soapy to the touch, turning red litmus paper blue. These two are chemical opposites; when they are mixed in the right proportions, they undergo a neutralization reaction to produce water and a salt Science, Class X, p.21.

It is a common misconception that all acids are dangerous. In reality, we distinguish between mineral acids (like Hydrochloric acid, HCl), which are strong and fully ionize, and organic acids (like acetic acid), which are weak and only partially ionize Science, Class X, p.73. The product of their interaction—the salt—is not always just 'table salt.' In chemistry, a salt is a general term for any ionic compound formed during neutralization. The nature of this salt (whether it is acidic, basic, or neutral) depends on the 'strength' of the parent acid and base. For example, a salt derived from a strong acid and a strong base will be neutral, possessing a pH value of 7 Science, Class X, p.29.

Below is a quick reference to help you distinguish between the two based on their physical and chemical properties:

Feature	Acids	Bases
Taste	Sour	Bitter
Touch	May cause a burning sensation	Slippery/Soapy feel
Litmus Paper	Turns Blue to Red	Turns Red to Blue
Key Reaction	Acid + Base → Salt + Water (Neutralization)

Sources: Science, Class X, Acids, Bases and Salts, p.21; Science, Class X, Acids, Bases and Salts, p.28; Science, Class X, Acids, Bases and Salts, p.29; Science, Class X, Carbon and its Compounds, p.73

2. The pH Scale and Its Everyday Importance (basic)

To understand the chemistry that surrounds us, we must first master the pH scale, which is the standard yardstick for measuring how acidic or basic a substance is. The term 'pH' comes from the German word potenz, meaning 'power,' and it specifically measures the concentration of hydrogen ions (H⁺) in a solution Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p. 25. Think of it as an inverse relationship: the more hydrogen ions you have, the more acidic the solution is, and the lower the pH value will be. The scale typically runs from 0 to 14, where 7 is perfectly neutral (like pure water). Values below 7 indicate acidity, while values above 7 indicate alkalinity or basicity Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p. 34. In everyday life, pH isn't just a laboratory number; it is a critical boundary for survival. For instance, our human body functions within a very narrow window of 7.0 to 7.8. If this balance shifts too far, our metabolic activities can fail Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p. 26. Similarly, the environment is highly sensitive. When atmospheric pollution causes the pH of rain to drop below 5.6, we call it acid rain, which can devastate aquatic ecosystems by making river water too acidic for fish to survive Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p. 26. Agriculture also relies heavily on these values. Different crops require specific soil conditions to thrive. While a 'neutral' soil is generally around a pH of 7.2, some soils can become as acidic as 3.0, requiring chemical intervention to restore fertility Geography of India, Majid Husain, Soils, p. 3. Whether it is the antacid you take for a sour stomach or the lime a farmer spreads on a field, the goal is the same: neutralization, where acids and bases react to bring the pH back to a healthy equilibrium.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25, 26, 34; Geography of India, Majid Husain, Soils, p.3

3. Industrial Chemistry: The Chlor-alkali Process (intermediate)

The Chlor-alkali process is a cornerstone of industrial chemistry, serving as the primary method for producing three of the most important chemicals in the modern world: Chlorine (Cl₂), Hydrogen (H₂), and Sodium Hydroxide (NaOH). The process begins with brine, which is simply a concentrated aqueous solution of sodium chloride (NaCl). When an electric current is passed through this solution—a process known as electrolysis—the salt decomposes into its constituent parts and reacts with water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. The name 'Chlor-alkali' is derived from the products themselves: 'chlor' for chlorine and 'alkali' for sodium hydroxide, which is a strong base. During electrolysis, the movement of ions is specific and predictable. Chlorine gas is formed at the anode (the positive electrode), while hydrogen gas is released at the cathode (the negative electrode). Sodium hydroxide solution remains and concentrates near the cathode Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. This process is chemically represented as:
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ Historically, this process often utilized mercury cells, but due to the high toxicity of mercury and its tendency to bioaccumulate, global environmental standards like the Minamata Convention now require nations to phase out or reduce mercury use in manufacturing processes, specifically including chlor-alkali production Environment, Shankar IAS Academy (ed 10th), International Organisation and Conventions, p.411. The versatility of these products is immense. Chlorine is used for water treatment, PVC production, and the creation of bleaching powder (Calcium oxychloride, CaOCl₂) by reacting it with dry slaked lime Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33. Sodium hydroxide is vital for degreasing metals and manufacturing soaps and paper. In India, the foundations of such industrial chemical advancements were championed by Acharya Prafulla Chandra Ray, the 'Father of Modern Indian Chemistry', who established India's first pharmaceutical and chemical works in 1901 Science-Class VII, NCERT (Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.17.

Product	Location at Electrolysis	Common Industrial Uses
Chlorine (Cl₂)	Anode (+)	Water disinfection (killing bacteria), PVC, Bleaching powder.
Hydrogen (H₂)	Cathode (-)	Fuels, margarine production, ammonia for fertilizers.
Sodium Hydroxide (NaOH)	Near Cathode	Soap making, paper industry, degreasing metals.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33; Environment, Shankar IAS Academy (ed 10th), International Organisation and Conventions, p.411; Science-Class VII, NCERT (Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.17

4. Water Chemistry: Hardness and Treatment (intermediate)

When we speak of hard water, we aren't talking about ice! In chemistry, hardness refers to the presence of dissolved mineral salts—specifically Calcium (Ca²⁺) and Magnesium (Mg²⁺) ions. When rain falls, it is naturally soft, but as it flows over rocks like limestone (calcium carbonate), it dissolves these minerals. The most visible sign of hard water is its refusal to form a rich lather with soap; instead, it reacts with soap to form a sticky, insoluble precipitate called 'scum'.

Hardness is generally classified into two categories based on the anions involved:

Type of Hardness	Cause (Salts of Ca and Mg)	Primary Treatment Method
Temporary Hardness	Bicarbonates (e.g., Ca(HCO₃)₂)	Simple boiling or adding Slaked Lime (Clark’s process).
Permanent Hardness	Chlorides and Sulphates (e.g., MgCl₂, CaSO₄)	Chemical treatment using Washing Soda (Sodium Carbonate).

To tackle permanent hardness, we rely on Sodium Carbonate (Na₂CO₃·10H₂O), commonly known as Washing Soda. It works by reacting with the dissolved calcium and magnesium chlorides or sulphates to form insoluble carbonates that precipitate out, effectively 'softening' the water so it can be used efficiently in glass, soap, and paper industries Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. Beyond softening, general water treatment often involves Bleaching Powder (CaOCl₂) for disinfection and Slaked Lime (Ca(OH)₂) for neutralizing acidity or treating sewage.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32

5. Sodium Compounds: Baking and Washing Soda (exam-level)

In our journey through everyday chemistry, two sodium salts stand out for their incredible utility in our kitchens and industries: Baking Soda and Washing Soda. While they look similar, their chemical structures and behaviors are distinct. Baking Soda (Sodium hydrogencarbonate, NaHCO₃) is a mild, non-corrosive basic salt. When it is heated or mixed with water, it releases Carbon Dioxide (CO₂) gas. This reaction is the secret behind the fluffiness of cakes and bread, as the escaping gas bubbles cause the dough to rise Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31. Interestingly, its solubility in water is temperature-dependent; at 70 °C, water can dissolve significantly more baking soda than at room temperature Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.138.

Washing Soda (Sodium carbonate, Na₂CO₃·10H₂O), on the other hand, is obtained by heating baking soda and then recrystallizing the product. It is a vital industrial chemical used in the manufacture of glass, soap, and paper. One of its most critical applications is the removal of permanent hardness of water, making it indispensable for laundry and industrial boilers Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. It also serves as a precursor for other important sodium compounds like Borax.

Both of these compounds share a common chemical signature: they are carbonates. When they react with acids (like ethanoic acid), they produce a salt, water, and carbon dioxide. This CO₂ gas can be identified by passing it through lime water, which turns milky Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.74. This property makes baking soda effective in soda-acid fire extinguishers, where the reaction with an acid generates enough CO₂ to smother a fire.

Feature	Baking Soda (NaHCO₃)	Washing Soda (Na₂CO₃·10H₂O)
Nature	Mildly basic, edible antacid	Highly alkaline cleaning agent
Key Use	Leavening agent in food	Removing permanent water hardness
Heat Reaction	Decomposes to Na₂CO₃, H₂O, and CO₂	Loses water of crystallization

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.74; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.138

6. Calcium Compounds: Lime and Bleaching Powder (exam-level)

Calcium compounds are foundational to both ancient architecture and modern industry. We begin with Calcium Oxide (CaO), commonly known as Quicklime. It is a white, caustic solid produced by heating limestone. When you add water to quicklime, it reacts vigorously in an exothermic combination reaction—releasing significant heat—to form Calcium Hydroxide (Ca(OH)₂), or Slaked Lime Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6. This slaked lime is a versatile base used in neutralizing acidic soils and in the process of whitewashing walls.

The chemistry of whitewashing is particularly elegant. When a solution of slaked lime is applied to walls, it reacts slowly with the Carbon Dioxide (CO₂) in the air to form a thin, shiny layer of Calcium Carbonate (CaCO₃). This reaction (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O) explains why a whitewashed wall takes two to three days to develop its characteristic bright, hard finish Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. Interestingly, this is the same chemical compound that makes up marble.

Beyond construction, calcium compounds play a critical role in sanitation through Bleaching Powder, chemically known as Calcium Oxychloride (CaOCl₂). It is manufactured by the action of chlorine gas on dry slaked lime Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. Because it releases chlorine, it serves as a powerful disinfectant for drinking water and a bleaching agent for textiles and paper pulp. It is often referred to as 'chloride of lime'.

Common Name	Chemical Name	Formula	Primary Use
Quicklime	Calcium Oxide	CaO	Manufacturing cement/glass
Slaked Lime	Calcium Hydroxide	Ca(OH)₂	Whitewashing, soil treatment
Bleaching Powder	Calcium Oxychloride	CaOCl₂	Disinfecting water, bleaching textiles

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6-7; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30

7. Solving the Original PYQ (exam-level)

This question is a perfect synthesis of the Chemical Nomenclature of Common Salts you recently mastered. It requires you to bridge the gap between industrial common names and their precise chemical identities—specifically focusing on Sodium and Calcium derivatives. By recalling the building blocks of salts from NCERT Class 10 Science, you can see that UPSC is testing your precision in distinguishing between compounds that sound similar but serve vastly different industrial functions.

To arrive at the correct answer, use a systematic mapping approach. First, link Bleaching powder with its unique oxygen-chlorine structure, Calcium oxychloride (A-4). Next, distinguish between the two sodium salts: remember that Baking soda (B-1) contains "Hydrogen" making it Sodium hydrogen carbonate, while Washing soda (C-2) is the basic Sodium carbonate. Finally, Slaked lime (D-3) is identified as Calcium hydroxide, the product of lime reacting with water. This sequence leads logically to Option (a).

The other options represent classic UPSC "distractor" patterns. The examiners often swap the identities of Sodium compounds (Baking vs. Washing) or Calcium compounds (Slaked lime vs. Quicklime) to exploit common mnemonic confusions. Options (b), (c), and (d) are wrong because they mismatch these specific functional groups—for instance, confusing a carbonate with a hydroxide—which would be a critical error in a laboratory or industrial setting. Accuracy in chemical formulas is the key to avoiding these traps.