During explosion of gunpowder, which of the following gases are evolved in the proportion of 1:3?

1. Basics of Chemical Reactions and Balancing (basic)

A chemical reaction is a process where substances, known as reactants, undergo a transformation to form new substances called products. These products possess entirely different chemical properties from the original materials. For instance, when a magnesium ribbon is burnt in the presence of oxygen, it transforms into a white powder known as magnesium oxide. While we can describe this in a sentence, scientists use chemical equations as a shorthand to represent these changes precisely Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2.

The most critical rule in writing these equations is the Law of Conservation of Mass. This law dictates that mass can neither be created nor destroyed during a chemical reaction. Consequently, the total mass of the elements in the products must equal the total mass of the elements in the reactants. In practical terms, this means the number of atoms of each element must be identical on both sides of the arrow Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.4. If an equation does not satisfy this, it is called a skeletal equation and must be balanced.

To balance an equation, we use the hit-and-trial method. We adjust the coefficients (the numbers placed in front of a chemical formula) until the atom counts match. It is vital to remember that we never change the subscripts (the small numbers within a formula, like the '2' in H₂O), as doing so would change the identity of the substance itself. Finally, to provide a complete picture, we include physical states: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5.

Component	Description	Example (Burning Magnesium)
Reactants	Substances that start the reaction	Mg (Magnesium) and O₂ (Oxygen)
Products	New substances formed	MgO (Magnesium Oxide)
Balanced Equation	Atom counts are equal on both sides	2Mg(s) + O₂(g) → 2MgO(s)

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.4; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.5

2. Oxidizing Agents and Reducing Agents (basic)

At the heart of many chemical transformations, from the rusting of an iron nail to the firing of a rocket, is a process called Redox (Reduction-Oxidation). To understand this, think of a chemical reaction as a trade. If one substance gains oxygen, it is oxidised; if it loses oxygen, it is reduced Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12. However, these processes never happen in isolation—they are two sides of the same coin.

To identify the "players" in this trade, we use the terms Oxidising Agents and Reducing Agents. Their names describe what they do to others, not what happens to themselves:

• Oxidising Agent: This is a substance that has the power to add oxygen to another substance. Because it gives its oxygen away (or facilitates the process), the agent itself ends up being reduced Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71. Common examples include Potassium Permanganate (KMnO₄) and Nitric Acid.
• Reducing Agent: This substance "steals" oxygen from another. By removing oxygen from a compound, it reduces that compound. In the process of taking that oxygen, the reducing agent itself becomes oxidised Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.51. Carbon (C) is a classic reducing agent used in industry to extract pure metals from their oxides.

In everyday applications, like the combustion of fuels or explosives, we look for a potent oxidising agent to provide the "spark" of oxygen needed for a rapid reaction. For instance, in organic chemistry, substances like acidified potassium dichromate are used specifically because they are capable of adding oxygen to alcohols to turn them into acids Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71. Understanding who is giving and who is taking oxygen is the first step in mastering applied chemistry.

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.51; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

3. Combustion and Rapid Oxidation (intermediate)

At its core, combustion is a chemical process in which a substance reacts with an oxidizing agent (usually oxygen) to release energy in the form of heat and light. This is a classic example of an exothermic reaction, meaning it releases heat into the surroundings Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. While we often think of combustion as a fire we can see, it is essentially a form of rapid oxidation. This differentiates it from "slow oxidation" processes like respiration or the rusting of iron, which also release energy but at a much slower pace and without a flame.

For combustion to occur, three specific conditions must be met simultaneously, often referred to as the "fire triangle":

• Fuel: A combustible substance (like wood, charcoal, or kerosene) Science-Class VII, Changes Around Us: Physical and Chemical, p.62.
• Oxidizer: Usually atmospheric oxygen (O₂), though in specialized mixtures like gunpowder, the oxygen is supplied by a chemical compound.
• Ignition Temperature: The minimum temperature at which a substance catches fire. A substance like paper won't burn just by sitting in air; it must be heated until it reaches this specific threshold Science-Class VII, Changes Around Us: Physical and Chemical, p.64.

In applied chemistry, we often manipulate these reactions to create explosions—which are simply extremely rapid combustion reactions occurring in a confined space. A primary example is gunpowder (black powder). Unlike a campfire that draws oxygen from the air, gunpowder contains its own oxidizer: Potassium Nitrate (KNO₃). When ignited, the KNO₃ decomposes to release oxygen, which then reacts violently with the fuel (charcoal and sulfur). This reaction produces a massive volume of hot gases, specifically Nitrogen (N₂) and Carbon dioxide (CO₂). In a standard balanced reaction (2KNO₃ + S + 3C → K₂S + N₂ + 3CO₂), these gases are produced in a 1:3 molar ratio. It is the sudden expansion of these specific gases that provides the explosive force.

Feature	Slow Oxidation (Respiration)	Rapid Oxidation (Combustion)
Speed	Slow and controlled	Fast and often uncontrollable
Energy Release	Heat released gradually	Heat and light released quickly
Temperature	Occurs at body/ambient temperature	Requires reaching Ignition Temperature

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.62; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.64

4. Nitrogen Compounds: From Fertilizers to Explosives (intermediate)

To understand nitrogen's journey from food to fire, we must look at its paradoxical nature. In the atmosphere, nitrogen exists as N₂, a relatively inert gas because of its strong triple bond. This stability makes it perfect for preservation; it is pumped into chip packets to prevent rancidity (oxidation of fats) and into electric bulbs to prevent the tungsten filament from burning up in the presence of oxygen Physical Geography by PMF IAS, Earths Atmosphere, p.272. However, when nitrogen is 'fixed' into compounds like nitrates (NO₃⁻) or ammonium (NH₄⁺), it becomes the fuel for both life and destruction. In agriculture, nitrogen is a limiting nutrient. Although it makes up 16% of all proteins, plants cannot 'breathe' N₂ from the air; it must be converted into chemical forms through nitrogen fixation—either by soil bacteria, lightning, or industrial fertilizer production Environment, Shankar IAS Acedemy, Functions of an Ecosystem, p.19. These fertilizers, specifically those containing Potassium Nitrate (KNO₃) or Ammonium Nitrate, are the backbone of modern food security. Yet, these same nitrates are also the core ingredients of gunpowder (black powder). When we look at explosives, the chemistry shifts from slow growth to rapid expansion. In gunpowder, Potassium Nitrate acts as a powerful oxidizer. When ignited with sulfur and charcoal, a violent chemical reaction occurs, releasing massive volumes of gaseous Nitrogen (N₂) and Carbon Dioxide (CO₂). Because nitrogen wants to return to its stable N₂ gas form, it releases a tremendous amount of energy in the process. The standard combustion of gunpowder can be represented as: 10KNO₃ + 3S + 8C → 2K₂CO₃ + 3K₂SO₄ + 6CO₂ + 5N₂. This sudden conversion from solid salt to hot, expanding gas is what creates the explosive force. While nitrogen salts are useful, nitrogen's byproduct in combustion engines—Nitrogen Dioxide (NO₂)—is a reddish-brown toxic gas that contributes to respiratory issues and acid rain Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.116. Thus, the story of nitrogen is one of balance: it preserves our food, feeds our crops, powers our defenses, and challenges our environmental health.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.272; Environment, Shankar IAS Acedemy, Functions of an Ecosystem, p.19; Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.116

5. Modern Explosives: RDX, TNT, and Dynamite (exam-level)

In the world of applied chemistry, an explosive is a substance that undergoes a rapid chemical transformation, producing a sudden release of energy, heat, and a large volume of gas. While traditional gunpowder (Black Powder) was a simple mechanical mixture of Potassium Nitrate, Sulfur, and Charcoal, modern explosives are sophisticated chemical compounds where the oxidizer and fuel are often packed into the same molecule for maximum efficiency.

Let’s break down the three giants of modern explosive technology:

• Dynamite: Patented by Alfred Nobel in 1867, dynamite revolutionized construction and mining. Its active ingredient is Nitroglycerin. On its own, nitroglycerin is a highly unstable liquid that explodes with the slightest shock. Nobel stabilized it by soaking it into an absorbent material called Kieselguhr (diatomaceous earth). This transformed a dangerous liquid into a manageable solid brick.
• TNT (Trinitrotoluene): If dynamite was about power, TNT was about stability. TNT (C₇H₅N₃O₆) is remarkably safe to handle; it can be melted and poured into shells without exploding. It requires a detonator to go off. Because of its predictable nature, it is the international standard for measuring explosive power (the "megaton").
• RDX (Research Department eXplosive): Chemically known as Cyclotrimethylenetrinitramine or Cyclonite, RDX is significantly more powerful than TNT. It is often mixed with plasticizers to create "plastic explosives" like C-4, which can be molded by hand. It is primarily used for military demolition and high-velocity shells.

Explosive	Key Characteristic	Primary Use
Dynamite	Nitroglycerin-based; stabilized by an absorbent.	Mining, tunneling, and demolition.
TNT	High chemical stability; safe handling.	Military shells, standard of explosive yield.
RDX	Highly powerful; base for plastic explosives.	Military demolition and high-explosive warheads.

In India, industrial explosives, including gunpowder and matchboxes, are regulated under specific industrial licenses due to their hazardous nature, as noted in Indian Economy, Nitin Singhania, p.379. Furthermore, many high-energy chemicals are strictly monitored or restricted internationally to prevent environmental and health risks, a concept echoed in the regulation of industrial chemicals under international conventions Environment, Shankar IAS Academy, p.407.

Sources: Indian Economy, Nitin Singhania, Indian Industry, p.379; Environment, Shankar IAS Academy, International Organisation and Conventions, p.407

6. Chemistry and Composition of Gunpowder (exam-level)

Gunpowder, historically known as "black powder," is one of the most significant inventions in the history of chemistry. Unlike many modern explosives which are single chemical compounds, gunpowder is a physical mixture of three distinct substances: Potassium Nitrate (KNO₃), Charcoal (C), and Sulfur (S). In the study of matter, we classify it as a non-uniform mixture because its components can be physically separated, much like the iron and sulfur mixtures discussed in Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.126. Due to its explosive properties, it is classified under industrial explosives and regulated as a hazardous chemical Indian Economy, Nitin Singhania, Indian Industry, p.379.

Each component in the mixture plays a vital role in the chemical reaction. Potassium Nitrate (often called saltpeter) acts as the oxidizer; it provides the oxygen required for the fuel to burn rapidly, even in an enclosed space like a gun barrel. Charcoal acts as the primary fuel, providing carbon. Sulfur serves as a sensitizer; it lowers the temperature required to ignite the mixture and increases the speed of combustion. When ignited, these solids undergo a rapid redox reaction, a concept fundamental to chemical equations Science, Class X NCERT, Chemical Reactions and Equations, p.14, transforming into solid salts and expanding gases.

The explosive force of gunpowder comes from the sudden generation of heat and the rapid expansion of gases. While the actual chemistry is complex, a standard simplified model of the reaction is: 2KNO₃ + S + 3C → K₂S + N₂ + 3CO₂. In this specific theoretical model, we observe that for every one molecule of Nitrogen (N₂) gas produced, three molecules of Carbon dioxide (CO₂) are released. This yields a precise molar ratio of 1:3 between Nitrogen and Carbon dioxide. It is important to note that free oxygen is not a product of this reaction; instead, the oxygen from the nitrate is consumed to oxidize the carbon and sulfur.

Component	Chemical Identity	Primary Role
Saltpeter	Potassium Nitrate (KNO₃)	Oxidizer (Provides Oxygen)
Charcoal	Carbon (C)	Fuel (Reducing Agent)
Sulfur	Sulfur (S)	Lowers ignition point / Accelerant

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.126; Indian Economy, Nitin Singhania, Indian Industry, p.379; Science, Class X NCERT, Chemical Reactions and Equations, p.14

7. Solving the Original PYQ (exam-level)

This question tests your ability to apply stoichiometry and chemical composition to real-world explosives. Having learned that gunpowder is a mixture of Potassium Nitrate (KNO₃), Sulfur (S), and Charcoal (C), you can deduce that the explosion is a rapid oxidation-reduction reaction. The key is identifying the specific gases produced: carbon from the charcoal reacts with oxygen to form CO₂, while Nitrogen is released from the nitrate source as N₂ gas. Understanding these building blocks allows you to predict the chemical behavior of the mixture under combustion.

To find the specific 1:3 ratio, we look at the standard simplified reaction: 2KNO₃ + S + 3C → K₂S + N₂ + 3CO₂. As you analyze the coefficients, you can clearly see that for every one mole of Nitrogen (N₂) evolved, three moles of Carbon dioxide (CO₂) are generated. This logical progression from components to balanced products leads us directly to the correct answer, (C) Nitrogen : Carbon dioxide. As your coach, I recommend always visualizing the reaction products when dealing with fuels and oxidizers; this helps you identify the primary gases evolved without needing to memorize every complex equation.

UPSC frequently uses distractor options and reversal traps to test your precision. Options (A) and (B) are incorrect because Oxygen is the oxidizer that gets consumed during the explosion, not evolved as a major gaseous product. Option (D) is a classic order trap; while it lists the correct gases, it reverses the proportion to 3:1. In the high-pressure environment of the exam, always double-check the order of the ratio to ensure it matches the sequence of the gases mentioned. General Science for Civil Services.