Which one of the following is used in soda-acid fire extinguishers ?

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

Welcome to your first step in mastering applied chemistry! To understand how chemicals work in our daily lives, we must start with the two most common 'personalities' in the chemical world: Acids and Bases. At their simplest, acids are substances that taste sour and turn blue litmus paper red, while bases are bitter, feel soapy to the touch, and turn red litmus paper blue Science-Class VII, Exploring Substances, p.18. However, for the UPSC, we look deeper at their molecular behavior: acids are defined by their ability to generate Hydrogen ions (H⁺) in solution, whereas bases produce Hydroxyl ions (OH⁻) Science, Class X, Chapter 2, p.22, 26.

The intensity of these substances is measured by their strength. A strong acid, like Hydrochloric acid (HCl), releases a high concentration of H⁺ ions, while a weak acid, like the acetic acid in vinegar, releases fewer ions Science, Class X, Chapter 2, p.26. We can compare their fundamental differences here:

What happens when these two opposites meet? They undergo a Neutralization reaction. In this process, the H⁺ from the acid and the OH⁻ from the base combine to form Water (H₂O), effectively 'canceling' each other out. The remaining parts of the compounds form a Salt. A classic example is the reaction between Sodium Hydroxide and Hydrochloric acid to produce common table salt: NaOH + HCl → NaCl + H₂O Science, Class X, Chapter 2, p.21. This reaction is almost always exothermic, meaning it releases heat energy Science-Class VII, Exploring Substances, p.18.

Sources: Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18; Science, Class X, Acids, Bases and Salts, p.21; Science, Class X, Acids, Bases and Salts, p.22; Science, Class X, Acids, Bases and Salts, p.26

2. Chemicals from Common Salt (NaCl) (basic)

Common salt, or Sodium Chloride (NaCl), is far more than just a seasoning for our food; it serves as a foundational raw material for an entire suite of industrial and household chemicals. In chemical industries, when electricity is passed through a concentrated aqueous solution of sodium chloride (commonly called brine), it undergoes decomposition. This industrial milestone is known as the Chlor-alkali process, named so because of its two primary products: Chlor for chlorine gas and alkali for sodium hydroxide (NaOH) Science, Class X (NCERT 2025 ed.), Chapter 2, p.30.

Each byproduct of this process fuels different applications. The chlorine gas released at the anode is used to manufacture Bleaching Powder (CaOCl₂) by reacting it with dry slaked lime [Ca(OH)₂] Science, Class X (NCERT 2025 ed.), Chapter 2, p.33. Meanwhile, the sodium hydroxide produced is essential for making soaps, detergents, and paper. Interestingly, modern international environmental standards, like the Minamata Convention, now emphasize phasing out mercury-based technologies traditionally used in these chlor-alkali plants to prevent environmental contamination Environment, Shankar IAS Academy (ed 10th), Chapter 20, p.411.

Beyond these, sodium chloride is a precursor for two other vital compounds used in our homes: Baking Soda (Sodium Hydrogen Carbonate) and Washing Soda (Sodium Carbonate). While baking soda is famous for making fluffy cakes or acting as a mild antiseptic, washing soda plays a critical role in the glass and soap industries. One of its most distinctive chemical properties is its ability to remove the permanent hardness of water, making it indispensable for domestic cleaning Science, Class X (NCERT 2025 ed.), Chapter 2, p.32.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.32; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.33; Environment, Shankar IAS Academy (ed 10th), International Organisation and Conventions, p.411

3. Reactions of Metal Carbonates and Hydrogen Carbonates (intermediate)

In our journey through everyday chemistry, we encounter substances that define our environment—from the limestone of our buildings to the baking soda in our kitchens. These are Metal Carbonates (like Calcium Carbonate, CaCO₃) and Metal Hydrogen Carbonates (also known as bicarbonates, like Sodium Hydrogen Carbonate, NaHCO₃). While they may look like simple white powders, they harbor a specific chemical property: when they encounter an acid, they react vigorously to produce a salt, water, and—most importantly—Carbon Dioxide gas (CO₂).

This reaction is a fundamental rule in chemistry. Whether it is a carbonate or a hydrogencarbonate, the products remain consistent. You can observe this rule in action here:

A classic example is the reaction of limestone, chalk, or marble—all of which are different physical forms of Calcium Carbonate—with an acid. This general reaction can be summarized as: Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21.

The CO₂ gas produced in these reactions has a unique signature: it turns lime water (calcium hydroxide solution) milky because it forms insoluble calcium carbonate. However, a fascinating shift occurs if you continue passing excess CO₂ through that milky solution. The milkiness disappears because a new substance, Calcium Hydrogen Carbonate (Ca(HCO₃)₂), is formed, which is soluble in water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21. This same chemical dance is what drives ocean acidification; as the ocean absorbs more atmospheric CO₂, it shifts the balance between carbonate and bicarbonate ions, impacting marine life Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21; Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.264

4. Everyday Chemistry: Antacids and pH Balance (intermediate)

In our daily lives, one of the most direct applications of chemistry occurs right inside our stomachs. To facilitate digestion, our stomach secretes Hydrochloric acid (HCl). While this acid is essential for breaking down food, its overproduction—often due to spicy food or stress—leads to a condition known as acidity or indigestion, causing pain and irritation. To restore pH balance, we turn to substances known as antacids.

Antacids are essentially mild bases that work on the principle of neutralization. When an antacid reacts with the excess stomach acid, it produces a salt and water, thereby reducing the acidity level. One of the most common household ingredients used for this purpose is Sodium hydrogen carbonate (NaHCO₃), popularly known as baking soda. Because it is alkaline, it effectively neutralizes the excess HCl in the stomach Science, Class X, Acids, Bases and Salts, p.31. Another widely used antacid is Magnesium hydroxide [Mg(OH)₂], often called 'Milk of Magnesia', which is a mild base specifically used to treat gastric upset.

The chemistry of these substances is quite versatile. For instance, when Sodium hydrogen carbonate reacts with an acid in the stomach, it doesn't just neutralize the acid; it also produces Carbon dioxide (CO₂) gas, which is why you might experience burping after taking a baking soda solution. This same chemical logic—the reaction between a carbonate and an acid to release CO₂—is what makes bread rise when baking powder is used and what allows certain fire extinguishers to smother flames by displacing oxygen Science, Class X, Acids, Bases and Salts, p.31.

Sources: Science, Class X, Acids, Bases and Salts, p.31; Science, Class X, Acids, Bases and Salts, p.20

5. Bleaching Agents and Disinfectants (intermediate)

In the realm of applied chemistry, bleaching agents and disinfectants are essential for both industrial hygiene and household sanitation. At the heart of this topic is Bleaching Powder (chemically known as Calcium Oxychloride, CaOCl₂). It is produced by the action of chlorine gas on dry slaked lime [Ca(OH)₂]. It is important to distinguish slaked lime from 'quick lime' (Calcium Oxide); slaked lime is formed when quick lime reacts vigorously with water in a combination reaction, releasing significant heat Science, Class X, p.6.

The functionality of bleaching powder is two-fold: it acts as a powerful oxidizing agent and a potent disinfectant. When used as a bleach, it removes color from materials like cotton and linen in the textile industry or wood pulp in paper manufacturing through the process of oxidation Science, Class X, p.31. This same oxidative power allows it to destroy the cellular structures of bacteria and viruses, making it indispensable for treating drinking water to make it free from germs Science, Class X, p.31.

In everyday life, we see these chemicals applied across various sectors. For instance, in the textile industry—one of the oldest and most widespread industries—bleaching is a critical step in preparing cotton, flax (linen), and even synthetic fibers for dyeing or finishing Certificate Physical and Human Geography, Manufacturing Industry, p.279. Beyond the factory, bleaching powder is a staple in laundry for whitening clothes and in public health for maintaining the safety of municipal water supplies.

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31; Certificate Physical and Human Geography, GC Leong, Manufacturing Industry, p.279

6. Hydrated Salts: Plaster of Paris and Gypsum (intermediate)

When we think of crystals, we often imagine dry, solid structures. However, many salt crystals contain a fixed number of water molecules chemically combined in every formula unit of the salt. This is known as water of crystallization. These salts are called hydrated salts. A classic example is Copper Sulphate (CuSO₄·5H₂O). While it appears dry, it contains five molecules of water for every one molecule of copper sulphate, which gives it a distinct blue color. If you heat these crystals, the water is driven off, and the salt turns white and powdery Science, class X (NCERT 2025 ed.), Chapter 2, p.32.

Two of the most economically and medically important hydrated salts are Gypsum and Plaster of Paris (POP). They are both forms of calcium sulphate but differ in the amount of water they hold within their crystal lattice. Gypsum is the naturally occurring mineral, whereas Plaster of Paris is its processed derivative used extensively in construction and orthopedics.

The transformation between these two is a precise chemical process. When Gypsum is heated to exactly 373 K (100°C), it loses three-fourths of its water of crystallization to become Plaster of Paris. The "hemihydrate" formula (½H₂O) suggests that two formula units of CaSO₄ share one molecule of water. The most remarkable property of POP is its reversibility: when you add water to POP powder, it undergoes a hydration reaction, re-incorporating water molecules to turn back into a hard solid mass of Gypsum Science, class X (NCERT 2025 ed.), Chapter 2, p.32.

Sources: Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.32

7. Properties and Uses of Baking Soda (exam-level)

Sodium Hydrogen Carbonate (NaHCO₃), commonly known as baking soda, is a versatile chemical staple in both kitchens and industries. At its core, it is a mild, non-corrosive basic salt. One of its fascinating physical properties is its solubility: while it dissolves in water, its solubility is highly temperature-dependent. As the water temperature increases from 20°C to 70°C, the amount of baking soda that can be dissolved increases significantly Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.138.

In the culinary world, we must distinguish between baking soda and baking powder. Baking powder is actually a mixture of baking soda and a mild edible acid, such as tartaric acid. When this mixture is heated or moistened, a chemical reaction occurs that releases Carbon Dioxide (CO₂) gas. It is these tiny bubbles of CO₂ that get trapped in dough, causing bread or cake to 'rise' and become soft and spongy Science, Class X, Acids, Bases and Salts, p.31. Beyond baking, its alkaline nature makes it a perfect antacid; it works by neutralizing the excess hydrochloric acid in our stomachs, providing instant relief from acidity.

Safety and industrial applications also rely on the specific way baking soda reacts with acids. For instance, when it reacts with ethanoic acid (acetic acid) or sulphuric acid, it vigorously produces a salt, water, and CO₂ Science, Class X, Carbon and its Compounds, p.74. This exact principle is utilized in soda-acid fire extinguishers. In these devices, the reaction between the carbonate and the acid generates a massive volume of CO₂ gas, which helps smother flames by cutting off the oxygen supply Science, Class X, Acids, Bases and Salts, p.31.

Sources: Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.138; Science, Class X, Acids, Bases and Salts, p.31; Science, Class X, Carbon and its Compounds, p.74

8. Mechanism of Soda-Acid Fire Extinguishers (exam-level)

To understand how a soda-acid fire extinguisher works, we must first look at the basic chemistry of combustion. A fire requires three things: fuel, heat, and oxygen. The primary goal of this specific extinguisher is to displace oxygen by creating a heavy blanket of non-combustible gas. This is achieved through a classic acid-base reaction between sodium hydrogen carbonate (commonly known as baking soda) and dilute sulphuric acid.

Inside the extinguisher, these two components are kept separate until the moment of need. The sodium hydrogen carbonate is dissolved in water in the main container, while the dilute sulphuric acid is stored in a small, sealed glass ignition tube or a separate internal bottle Science, class X (NCERT 2025 ed.), Chapter 2, p. 36. When you activate the extinguisher (usually by striking a plunger or tilting the device), the acid mixes with the soda solution. This triggers an immediate, vigorous chemical reaction:

2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂

This reaction produces carbon dioxide (CO₂), water, and a salt (sodium sulphate). The CO₂ gas is produced in such large volumes that it creates significant pressure, forcing a stream of liquid and gas out through the nozzle. Because CO₂ is denser than oxygen, it settles over the burning material like a blanket, cutting off the atmospheric oxygen supply and effectively "smothering" the flames. Additionally, the water produced in the reaction helps in cooling the fuel below its ignition temperature.

It is important to note that sodium hydrogen carbonate is chosen for this role because it is a mild, non-corrosive basic salt that reacts efficiently with acids to release CO₂ Science, class X (NCERT 2025 ed.), Chapter 2, p. 31. Other common substances, like sodium chloride (table salt), do not have this property and would be useless in this mechanism.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.36; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.31

9. Solving the Original PYQ (exam-level)

This question perfectly bridges the gap between theoretical chemistry and real-world application. Having just mastered the reactions of acids and bases, you know that metal hydrogen carbonates react with acids to produce a salt, water, and—most importantly—carbon dioxide gas. In the context of a fire extinguisher, this CO2 acts as a non-combustible blanket that displaces oxygen, effectively smothering the fire. This practical application of chemical properties is exactly what the UPSC expects you to identify from your study of NCERT Class X Science, Chapter 2: Acids, Bases and Salts.

To arrive at the correct answer, you must think through the mechanics of the device. The 'soda-acid' type requires a stable, alkaline powder that can release a high volume of gas instantly upon contact with an acid (typically sulphuric acid). Sodium hydrogen carbonate (also known as baking soda) is the ideal candidate because it is safe to handle and highly efficient at releasing CO2. Therefore, the correct answer is (B) Sodium hydrogen carbonate. Remember: the 'soda' in the name refers to the sodium-based carbonate, not a liquid drink!

UPSC often includes 'distractor' chemicals to test the depth of your conceptual clarity. Sodium chloride is common table salt and is chemically neutral, meaning it won't produce the necessary gas. Calcium hydroxide (slaked lime) is a base but is primarily used in construction and soil treatment, not as a gas-evolver in extinguishers. Acetic acid is an organic acid; while it would react with a carbonate, it is an acid itself, not the 'soda' component the question asks for. Don't be misled by familiar names; always focus on the functional role the chemical plays in the reaction.