Which one of the following gases is supporter of combustion?

1. Composition of the Earth's Atmosphere (basic)

The Earth's atmosphere is not just empty space; it is a complex, dynamic mixture of gases, water vapor, and suspended particles. Think of it as a protective chemical envelope that makes life possible. By volume, the atmosphere is remarkably consistent in its lower layers, dominated by Nitrogen (78.08%) and Oxygen (20.95%) Majid Hussain, Environment and Ecology, p.6. These are known as permanent gases because their proportions remain nearly constant across the globe. However, this composition changes as we go higher: for instance, oxygen becomes almost negligible at a height of 120 km, while Carbon dioxide and water vapor are only found up to 90 km from the surface NCERT Class XI, Fundamentals of Physical Geography, p.64.

From a chemical perspective, the specific roles of these gases are fascinating. While Oxygen (O₂) is the essential 'supporter of combustion'—the chemical process where substances react to release heat—it is actually Nitrogen (N₂) that keeps our world from burning up. Nitrogen is relatively inert, meaning it doesn't easily react; its primary job in our air is to dilute the oxygen, preventing spontaneous fires and controlling the rate of combustion PMF IAS, Physical Geography, p.272. Other gases, like Argon (0.93%) and Carbon dioxide (0.036%), though present in smaller amounts, play massive roles in Earth's temperature and chemical balance.

Gas Type	Major Constituent	Percentage (Approx)	Key Chemical Role
Permanent	Nitrogen (N₂)	78.08%	Inert diluent; controls combustion.
Permanent	Oxygen (O₂)	20.95%	Essential for respiration & combustion.
Permanent	Argon (Ar)	0.93%	Noble gas; chemically inactive.
Variable	Carbon dioxide (CO₂)	0.036%	Greenhouse gas; used in photosynthesis.

Beyond gases, the atmosphere contains water vapor and dust particles (like sea salt, pollen, and soot), which are vital for weather phenomena like cloud formation. Interestingly, while Hydrogen (H₂) is a highly combustible fuel, it is only found in trace amounts (0.00005%) in our atmosphere, as it is too light to be easily held by Earth's gravity in large quantities PMF IAS, Physical Geography, p.271.

Sources: Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.6; Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.64; Physical Geography by PMF IAS, Earth's Atmosphere, p.271-272

2. Physical vs. Chemical Changes (basic)

In our daily lives, we witness matter transforming constantly—ice melts, iron nails rust, and wood burns. To master chemistry for the UPSC, we must distinguish these transformations into two fundamental categories: Physical and Chemical changes. The core distinction lies in whether the identity of the substance itself changes.

A Physical Change occurs when a substance undergoes a change in its physical properties such as shape, size, color, or state (solid, liquid, gas), but no new substance is formed. For example, when you fold a handkerchief or freeze water into ice, the chemical makeup remains exactly the same—it is still cotton or H₂O. These changes are often, though not always, reversible Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 59, 69.

In contrast, a Chemical Change is a process where one or more new substances are formed through a chemical reaction. This change is usually accompanied by the evolution of heat, light, or the production of a gas. A classic example is combustion (burning), where a substance reacts with oxygen to produce heat and new products like CO₂ or ash Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 68, 70. Other examples include the rusting of iron (forming iron oxide) or the curdling of milk, where the original substance is permanently transformed into something with entirely different properties.

To help you distinguish them quickly, let’s look at this comparison:

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are created.
Properties Affected	Shape, size, state, or appearance.	Chemical composition and reactivity.
Examples	Melting wax, cutting vegetables, folding paper.	Burning magnesium, rusting, cooking food.

Interestingly, some complex natural processes involve both. For instance, the weathering of rocks to form soil involves physical changes (erosion by wind and water) and chemical changes (minerals reacting with rainwater) Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 68.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 68, 69, 70

3. Carbon Dioxide: Greenhouse Effect and Global Warming (intermediate)

To understand the Greenhouse Effect, think of Earth’s atmosphere as a thermal blanket. While Carbon Dioxide (CO₂) is naturally part of our carbon cycle—circulating between the atmosphere, oceans, and living organisms—it plays a unique role in regulating temperature Environment, Shankar IAS Academy (ed 10th), Climate Change, p.255. When solar radiation reaches Earth, the surface absorbs it and radiates it back as long-wave infrared radiation (heat). Most gases allow this heat to escape into space, but CO₂ molecules act like a one-way filter: they are transparent to incoming sunlight but absorb and re-emit the outgoing heat back toward the surface. This trapped energy is what we call the Greenhouse Effect. While CO₂ is essential for keeping Earth habitable, an excess of it leads to Global Warming. We use CO₂ as the universal benchmark to measure the impact of all other greenhouse gases through a metric called Global Warming Potential (GWP). CO₂ is assigned a GWP of 1, serving as the baseline Environment, Shankar IAS Academy (ed 10th), Climate Change, p.260. Even though gases like Methane (CH₄) absorb more energy pound-for-pound, CO₂ is the primary concern because of its sheer volume and long-lasting presence in our atmosphere. This is why total emissions are often calculated in CO₂ equivalents Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.425. A stark lesson in the power of this effect can be seen on Venus. The atmosphere of Venus is composed almost entirely of CO₂, creating such an intense greenhouse effect that surface temperatures are hot enough to melt lead, making human life there highly improbable Physical Geography by PMF IAS, The Solar System, p.28. In the laboratory, we see CO₂ produced through various chemical reactions, such as when metal carbonates (like limestone or marble) react with acids to release salt, water, and CO₂ gas Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21.

Sources: Environment, Shankar IAS Academy (ed 10th), Climate Change, p.255; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.260; Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.425; Physical Geography by PMF IAS, The Solar System, p.28; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21

4. Nitrogen: Inertness and the Nitrogen Cycle (intermediate)

To understand nitrogen, we must first look at its personality: it is the 'silent moderator' of our atmosphere. While oxygen is highly reactive and essential for life and fire, Nitrogen (N₂) makes up about 78.08% of the air and is chemically inert under normal conditions Physical Geography by PMF IAS, Earths Atmosphere, p.271. This inertness stems from the incredibly strong triple bond holding the two nitrogen atoms together, which requires significant energy to break. Because of this, nitrogen acts as a crucial 'diluter' of oxygen. Without it, the oxygen in our atmosphere would be so concentrated that even a small spark could lead to uncontrollable, spontaneous combustion Physical Geography by PMF IAS, Earths Atmosphere, p.272. We use this property daily—for instance, chip packets are flushed with nitrogen to prevent the oxidation (rancidity) of oils, and electric bulbs use it to keep the tungsten filament from burning up Physical Geography by PMF IAS, Earths Atmosphere, p.272.

However, there is a fascinating biological paradox: while nitrogen is a fundamental building block of life—constituting about 16% of all proteins and being essential for DNA—living organisms cannot use the 'inert' nitrogen gas we breathe Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19. To enter the food chain, nitrogen must be 'fixed', or converted into reactive chemical forms like ammonia (NH₃), nitrites (NO₂⁻), or nitrates (NO₃⁻). This conversion happens through three main pathways: biological fixation by specialized soil bacteria and blue-green algae (often found in the root nodules of legumes like peas and beans), atmospheric fixation via the high energy of lightning, and industrial fixation in fertilizer factories Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Environment and Ecology, Majid Hussain, Basic Concepts, p.20.

Process	Mechanism	Outcome
Combustion Control	Diluting O₂ in the atmosphere	Prevents rapid, uncontrollable fires
Nitrogen Fixation	Bacteria, Lightning, or Industry	Converts N₂ into usable nitrates/ammonia
Preservation	Displacing O₂ in packaging	Prevents oxidation and food spoilage

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271-272; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.20, 116

5. Oxidation and Reduction (Redox) Reactions (intermediate)

In the world of chemistry, reactions often involve a "tug-of-war" over specific atoms or electrons. The most fundamental way to understand this is through Oxidation and Reduction. While these terms might sound technical, they describe processes we see every day, from the rusting of an iron gate to the way our bodies extract energy from food.

At its most basic level, Oxidation is defined as the gain of oxygen or the loss of hydrogen by a substance. Conversely, Reduction is the loss of oxygen or the gain of hydrogen Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13. Because these two processes are essentially the reverse of each other, they almost always occur simultaneously. If one substance loses oxygen (is reduced), that oxygen must go somewhere—it is picked up by another substance (which is oxidized). This dual process is why we call them Redox reactions (Reduction-Oxidation) Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12.

Consider the industrial process of extracting metals from their ores. For instance, when zinc oxide is heated with carbon, the zinc oxide loses oxygen to become pure zinc metal—it is reduced. Meanwhile, the carbon gains that oxygen to become carbon monoxide—it is oxidised Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13. In this scenario, carbon acts as a reducing agent because it facilitates the reduction of the metal oxide. In advanced metallurgy, highly reactive metals like Sodium (Na) or Aluminium (Al) are often used as powerful reducing agents to displace less reactive metals from their compounds Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51.

Process	Oxygen Transfer	Hydrogen Transfer
Oxidation	Gain of Oxygen	Loss of Hydrogen
Reduction	Loss of Oxygen	Gain of Hydrogen

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12-13; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51

6. Respiration: The Biological 'Combustion' (intermediate)

When we look at a burning candle or a roaring campfire, we are witnessing combustion—a chemical process where a substance (fuel) reacts with oxygen to release energy, primarily as heat and light. Oxygen is the critical supporter of combustion; without it, the fire triangle (fuel, heat, and oxygen) collapses and the flame dies out Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.62-63. In our atmosphere, nitrogen acts as a buffer, diluting oxygen to prevent the world from spontaneously catching fire, while carbon dioxide is used to extinguish flames by displacing oxygen Physical Geography by PMF IAS, Chapter 20: Earths Atmosphere, p.272.

Interestingly, your body performs a remarkably similar process called respiration. Think of respiration as a highly controlled, "slow combustion." Instead of wood or wax, your cells use glucose (a simple sugar) as fuel. In aerobic respiration, oxygen is used to break down glucose in the mitochondria to release the energy needed for life Science, class X (NCERT 2025 ed.), Life Processes, p.87. While combustion is sudden and uncontrolled, respiration happens in small, enzyme-regulated steps so that the energy released can be captured in molecules like ATP, rather than just burning the cell!

The chemical logic remains the same: Fuel + Oxygen → Carbon Dioxide + Water + Energy. However, there are key differences in how this energy is managed:

Feature	Combustion	Respiration (Aerobic)
Speed	Fast, sudden reaction.	Slow, multi-step process.
Temperature	Occurs at high temperatures.	Occurs at body temperature.
Control	Non-enzymatic; uncontrolled.	Controlled by specific enzymes.
Energy Form	Mostly heat and light.	Chemical energy (ATP) and some heat.

It is also vital to note that life has a "backup plan." In the absence of oxygen, some organisms (like yeast) or even our muscles during intense exercise perform anaerobic respiration. This process, also known as fermentation, breaks down glucose into ethanol or lactic acid, releasing much less energy because the fuel isn't "burned" completely without oxygen Science, class X (NCERT 2025 ed.), Life Processes, p.87.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.62-63; Science, class X (NCERT 2025 ed.), Life Processes, p.87; Physical Geography by PMF IAS, Chapter 20: Earths Atmosphere, p.272

7. The Fire Triangle and Combustion Chemistry (intermediate)

At its heart, combustion is a high-temperature exothermic chemical reaction between a fuel and an oxidant (usually atmospheric oxygen) that releases energy in the form of heat and light. To understand how fire starts and persists, we use the concept of the Fire Triangle. This model teaches us that three essential 'sides' must coexist for a fire to happen: Fuel, Oxygen, and Heat. If any one of these is removed, the fire is extinguished Science-Class VII . NCERT(Revised ed 2025), Chapter 5, p.64.

The first side, fuel (a combustible substance), provides the material that burns. While fuels like wood or charcoal are common, some substances are so reactive—like Sodium (Na) and Potassium (K)—that they ignite spontaneously upon contact with air and must be stored in kerosene Science, class X (NCERT 2025 ed.), Chapter 3, p.42. The second side is Heat, specifically enough heat to reach the ignition temperature—the minimum temperature at which a substance catches fire. The final side is Oxygen, the primary supporter of combustion. It is important to distinguish between substances that help a fire burn and those that do not:

Role	Substance	Function
Supporter	Oxygen (O₂)	Mandatory for burning; its absence causes incomplete combustion or extinction.
Non-Supporter (Inert)	Nitrogen (N₂)	Acts as a diluent in the atmosphere, preventing rapid, uncontrollable combustion Physical Geography by PMF IAS, Chapter 20, p.272.
Non-Supporter (Extinguisher)	Carbon Dioxide (CO₂)	Heavier than oxygen, it smothers flames by cutting off the air supply.

Understanding these principles is vital for disaster management. For instance, soda-acid fire extinguishers work by generating CO₂ through the reaction of sodium hydrogencarbonate (NaHCO₃) and sulphuric acid (H₂SO₄) to break the fire triangle Science, class X (NCERT 2025 ed.), Chapter 2, p.36. Conversely, in the absence of oxygen, we can perform pyrolysis—the chemical decomposition of organic material through heat—which allows us to create fuels like charcoal and gas without actually 'burning' the material in the traditional sense Environment, Shankar IAS Academy (ed 10th), p.86.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.64; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.42; Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.272; Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.36; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.86

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of chemical changes and atmospheric composition, this question serves as a direct application of the Fire Triangle concept. In your recent modules, you learned that combustion is not just 'burning,' but a specific chemical reaction where a substance reacts with an oxidizing agent to release energy. As highlighted in Science-Class VII . NCERT(Revised ed 2025), for any fire to exist, you need fuel, heat, and a supporter of combustion. This question tests your ability to identify which specific atmospheric gas fulfills that third role.

To arrive at the correct answer, you must distinguish between a substance that is combustible and one that is a supporter. While many gases are present in the air, only Oxygen provides the chemical environment necessary for a fuel to oxidize and release heat. As noted in Science , class X (NCERT 2025 ed.), even a small reduction in the oxygen supply can lead to incomplete combustion or cause a flame to extinguish entirely. Therefore, because it is the mandatory component that facilitates the burning of other substances, (D) Oxygen is the correct answer.

UPSC frequently uses high-energy or fire-related gases as distractors to test your conceptual clarity. A common trap is Hydrogen; remember that while hydrogen is highly combustible (it is an excellent fuel), it does not support the combustion of other materials. Similarly, Nitrogen acts as a diluent to control combustion rather than support it, as explained in Physical Geography by PMF IAS. Finally, Carbon dioxide is the ultimate non-supporter, often used in fire extinguishers because it displaces oxygen to smother the reaction. Always double-check if the question is asking for the 'fuel' or the 'supporter' before marking your choice.