Match List 1 (Gas) with List II (Main chemical composition) and select the correct answer using the codes given below the Lists

ListI | List II
A.  Water gas | 1. CH4
B.  Producer gas | 2. CO + H2
C.  Coal gas | 3. R, + CH4+CO
D .Gobargas | 4. CO + N2

1. Fundamentals of Fuels and Calorific Value (basic)

To understand applied chemistry, we must start with the energy that powers our world: fuels. At its simplest, a fuel is any substance that releases energy, typically in the form of heat, when it undergoes a chemical reaction called combustion. For combustion to occur, three essential components must meet—often called the 'Fire Triangle': the fuel itself, a supply of oxygen, and enough heat to reach the substance's ignition temperature Science-Class VII, Changes Around Us: Physical and Chemical, p.64. Most fuels we use daily are carbon-based, meaning they are either pure carbon or complex carbon compounds that react with oxygen to release CO₂ and energy Science, class X, Carbon and its Compounds, p.69.

Not all fuels are created equal; we measure their efficiency using Calorific Value. This is defined as the amount of heat energy produced by the complete combustion of 1 kg of fuel (usually expressed in kJ/kg). A 'good' fuel is one that has a high calorific value, is easily transportable, and burns cleanly. If you notice the bottom of a cooking vessel turning black, it is a sign of incomplete combustion, meaning the fuel isn't burning efficiently or the air holes are blocked, leading to soot (unburnt carbon) deposits Science, class X, Carbon and its Compounds, p.77.

In modern energy systems, gaseous fuels are highly preferred because they are easier to transport via pipelines and burn more completely than solids. Natural Gas, primarily composed of Methane (CH₄), is a versatile fuel used in power generation, as CNG (Compressed Natural Gas) for transport, and PNG (Piped Natural Gas) for cooking Contemporary India II: Textbook in Geography for Class X, p.115. Beyond fossil fuels, we also produce Biofuels from organic materials like sugar beet, cassava, or even damaged food grains, offering a renewable alternative to traditional energy sources Indian Economy, Nitin Singhania, Infrastructure, p.465.

Fuel Category	Common Examples	Key Characteristic
Solid	Coal, Wood	Higher ash content, lower efficiency.
Liquid	Petrol, Kerosene	High energy density, easy to store.
Gaseous	CNG, Biogas, LPG	Highest efficiency, cleanest combustion.

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.64; Science, class X, Carbon and its Compounds, p.69, 77; Contemporary India II: Textbook in Geography for Class X, Mineral and Energy Resources, p.115; Indian Economy, Nitin Singhania, Infrastructure, p.465

2. Hydrocarbons: Methane and Alkanes (basic)

At the heart of organic chemistry are hydrocarbons—compounds made entirely of carbon and hydrogen atoms. Think of them as the "skeletons" of almost all organic molecules. The simplest family of hydrocarbons is the alkanes. These are known as saturated hydrocarbons because every carbon atom is bonded to the maximum possible number of hydrogen atoms using only single bonds. Because they are "full" or saturated, they are generally less reactive than their "unsaturated" cousins (alkenes and alkynes) which contain double or triple bonds Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65.

The simplest and most famous alkane is Methane (CH₄). To understand its structure, we look at Carbon’s tetravalency: a carbon atom has four valence electrons and needs four more to achieve a stable configuration. In methane, one carbon atom shares electrons with four hydrogen atoms. This molecule is the primary constituent of Compressed Natural Gas (CNG) and biogas (commonly known in India as Gobar gas) Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. In the kitchen or the industry, methane is valued as a fuel because it burns cleanly with a high heat output.

Beyond its use as a fuel, methane plays a critical role in our environment. It is produced through anaerobic processes—chemical breakdowns that occur in the absence of oxygen. Common sources include the digestive tracts of livestock, waterlogged rice fields, and wetlands Environment and Ecology, Majid Hussain, Climate Change, p.11. While it is a valuable energy source, it is also a potent greenhouse gas, contributing significantly to global warming by trapping heat in the atmosphere Environment, Shankar IAS Academy, Climate Change, p.256.

Type of Hydrocarbon	Bond Type	Example
Alkane (Saturated)	Single (C-C)	Methane, Ethane
Alkene (Unsaturated)	Double (C=C)	Ethene (Ethylene)
Alkyne (Unsaturated)	Triple (C≡C)	Ethyne (Acetylene)

Sources: Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60, 65; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Climate Change, p.11; Environment, Shankar IAS Academy (10th ed.), Climate Change, p.256

3. Combustion and the formation of CO and CO₂ (intermediate)

At its heart, combustion is a high-temperature chemical reaction between a fuel (a combustible substance) and an oxidant, usually atmospheric oxygen, that releases energy in the form of heat and light. As we explore in Science-Class VII, Changes Around Us: Physical and Chemical, p.62, substances like wood, kerosene, and LPG are combustible, but they require oxygen to burn. However, the quality of this burning—and the gases it produces—depends almost entirely on the amount of oxygen available during the process. When there is a plentiful supply of oxygen, we achieve complete combustion. In this state, the carbon in the fuel reacts fully to form Carbon Dioxide (CO₂). This is characterized by a clean, blue flame, which you might see on a well-adjusted gas stove Science, class X, Carbon and its Compounds, p.69. Conversely, when oxygen is limited, incomplete combustion occurs. Instead of CO₂, the reaction produces Carbon Monoxide (CO) and unburnt carbon particles, which we see as soot. This results in a yellow, smoky flame. This distinction is critical in everyday life: CO₂ is a naturally occurring greenhouse gas FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, World Climate and Climate Change, p.96, whereas CO is a highly toxic, colorless, and odorless 'silent killer' that can build up in poorly ventilated spaces.

Feature	Complete Combustion	Incomplete Combustion
Oxygen Supply	Abundant/Sufficient	Limited/Restricted
Primary Carbon Product	Carbon Dioxide (CO₂)	Carbon Monoxide (CO)
Flame Color	Blue (Clean)	Yellow (Sooty)
Energy Yield	Higher	Lower

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.62-63; Science, class X, Carbon and its Compounds, p.69; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, World Climate and Climate Change, p.96

4. Coal Processing and Destructive Distillation (intermediate)

To understand how we get useful energy from coal, we must look beyond simply burning it. When coal—specifically bituminous coal—is heated to high temperatures (1000°C to 1400°C) in a closed vessel in the absence of air, it undergoes a process called Destructive Distillation. Unlike regular combustion which turns coal into ash and smoke, this 'chemical baking' breaks coal down into three distinct states of matter: solid coke, liquid coal tar, and gaseous coal gas. This process is essential because raw coal often contains high amounts of volatile matter and moisture that reduce its efficiency in specialized industrial applications GC Leong, Fuel and Power, p.265. The solid residue left behind is Coke. It is a hard, grey, and porous material that is much richer in carbon than the original coal. Because it burns with high heat and very little smoke, it is the primary fuel used in blast furnaces to extract iron from its ores Majid Husain, Energy Resources, p.1. Simultaneously, the gases released during this heating are collected and purified. The most significant is Coal Gas, which is a potent mixture of Hydrogen (H₂), Methane (CH₄), and Carbon Monoxide (CO). In industrial chemistry, we also create other 'synthetic' fuel gases by reacting hot coke with different substances. It is crucial to distinguish these based on their chemical 'partners':

Fuel Gas	Production Method	Primary Composition
Coal Gas	Destructive distillation of coal	H₂ + CH₄ + CO
Water Gas	Passing steam over red-hot coke	CO + H₂
Producer Gas	Passing air over red-hot coke	CO + N₂

Sources: Geography of India ,Majid Husain, (McGrawHill 9th ed.), Energy Resources, p.1; Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), Fuel and Power, p.265

5. Petroleum-Based Gases: LPG and CNG (exam-level)

In our journey through applied chemistry, understanding the fuels that power our kitchens and cars is essential. Petroleum-based gases are primarily hydrocarbons — compounds made of hydrogen and carbon. Depending on their molecular weight and how they are processed, they serve different purposes. The two most common forms we encounter are LPG and CNG.

1. Liquefied Petroleum Gas (LPG): This is the gas used in our household cylinders. It is a mixture of flammable hydrocarbon gases, primarily Butane (C₄H₁₀) and Propane (C₃H₈). Under moderate pressure, these gases turn into liquids, making them easy to transport in cylinders. As we see in the study of carbon compounds, butane and propane are part of the 'alkane' series, characterized by single bonds between carbon atoms Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. An interesting fact for your exams: LPG is naturally odorless, so a pungent-smelling chemical called Ethyl Mercaptan is added to help detect leaks.

2. Compressed Natural Gas (CNG): Unlike LPG, which is derived during petroleum refining, CNG is simply Natural Gas stored at very high pressure. Its primary component is Methane (CH₄), the simplest hydrocarbon with just one carbon atom Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. Natural gas is often found trapped with petroleum deposits underground and is released when crude oil is brought to the surface Contemporary India II: Textbook in Geography for Class X (2022 ed.), Print Culture and the Modern World, p.115. It is considered a 'cleaner' fuel because it produces fewer pollutants upon combustion compared to petrol or diesel.

Feature	LPG (Liquefied Petroleum Gas)	CNG (Compressed Natural Gas)
Primary Component	Butane (C₄H₁₀) and Propane (C₃H₈)	Methane (CH₄)
Source	Refining of crude oil	Naturally occurring with petroleum deposits
Main Use	Cooking and heating (Domestic/Industrial)	Transport fuel (Buses, Taxis, Autos)

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64; Contemporary India II: Textbook in Geography for Class X (2022 ed.), Print Culture and the Modern World, p.115

6. Bio-Energy: Biogas and Gobar Gas (intermediate)

At its heart, Biogas is a renewable energy source produced through the anaerobic decomposition of organic matter. 'Anaerobic' simply means 'in the absence of oxygen.' When shrubs, farm waste, animal dung, and even human waste are left to rot in a sealed environment (like a biogas digester), specialized bacteria break down the complex organic molecules. This biological process not only manages waste but transforms it into a high-efficiency fuel NCERT, Contemporary India II, Print Culture and the Modern World, p.117. While the process can occur naturally in marshes or rice fields, man-made biogas plants capture this gas for domestic and industrial use.

The chemical composition of biogas is dominated by Methane (CH₄), which typically makes up 50-75% of the mixture and serves as the primary combustible component. The remainder is mostly Carbon Dioxide (CO₂), with trace amounts of Nitrogen and Hydrogen Sulphide. In rural India, these systems are famously known as 'Gobar Gas' plants because they primarily utilize cattle dung (gobar). This setup provides what we call 'twin benefits': it generates clean energy for cooking and lighting, while the leftover fermented residue (slurry) serves as an improved quality organic manure, richer in nutrients than raw dung NCERT, Contemporary India II, Print Culture and the Modern World, p.117.

Compared to traditional fuels like kerosene, charcoal, or burning raw dung cakes, biogas is significantly more thermally efficient. It burns with a clean blue flame, reducing indoor air pollution and respiratory issues. Furthermore, the decomposition process helps lower the Carbon-to-Nitrogen (C:N) ratio of the waste, making the resulting compost more effective for agriculture Environment, Shankar IAS Academy, Agriculture, p.364. This technology is supported at the national level through the National Biogas and Manure Management Programme (NBMMP), managed by the Ministry of New and Renewable Energy Indian Economy, Nitin Singhania, Infrastructure, p.453.

Feature	Direct Burning (Dung Cakes)	Biogas (Gobar Gas)
Efficiency	Low thermal efficiency	High thermal efficiency
Byproduct	Ash (limited nutrient value)	Nutrient-rich organic slurry (manure)
Environment	High smoke/CO emissions	Clean burning; reduces GHG emissions

Sources: NCERT, Contemporary India II, Print Culture and the Modern World, p.117; Environment, Shankar IAS Academy, Agriculture, p.364; Indian Economy, Nitin Singhania, Infrastructure, p.453; Environment and Ecology, Majid Hussain, Climate Change, p.11

7. Industrial Gases: Water Gas and Producer Gas (exam-level)

In industrial chemistry, fuel gases are tailored mixtures designed for specific heating and power needs. Two of the most important are Water Gas and Producer Gas. The fundamental difference between them lies in whether steam or air is passed over red-hot coke (carbon).

Water Gas is a high-energy mixture of Carbon Monoxide (CO) and Hydrogen (H₂). It is produced by passing steam (H₂O) over red-hot coke at very high temperatures (C + H₂O → CO + H₂). Because both CO and H₂ are combustible, water gas has a high calorific value and is often used in the synthesis of methanol or as a fuel in industrial furnaces. In contrast, Producer Gas is formed by passing air over red-hot coke. Since air is predominantly Nitrogen (~78%) as noted in Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.66, the resulting gas is a mixture of Carbon Monoxide (CO) and Nitrogen (N₂). Because Nitrogen is inert and does not burn, it acts as a 'diluent,' making producer gas cheaper to manufacture but significantly lower in heating value than water gas.

Other common industrial fuels include Coal Gas and Gobar Gas (Biogas). Coal gas is a byproduct of the destructive distillation of coal and contains a complex mix of Hydrogen, Methane (CH₄), and CO. Biogas, on the other hand, is produced through the anaerobic decomposition of organic waste. Its primary combustible component is Methane (CH₄), which is also a trace gas in our atmosphere Physical Geography by PMF IAS, Earths Atmosphere, p.271. Understanding these compositions is vital because it explains why some gases (like Water Gas) are used for high-heat applications, while others (like Producer Gas) are used where cost-effectiveness is the priority.

Fuel Gas	Primary Composition	Production Method
Water Gas	CO + H₂	Steam over red-hot coke
Producer Gas	CO + N₂	Air over red-hot coke
Coal Gas	H₂ + CH₄ + CO	Destructive distillation of coal
Gobar Gas	CH₄ + CO₂	Anaerobic decay of biomass

Sources: Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.66; Physical Geography by PMF IAS, Earths Atmosphere, p.271

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of industrial chemistry and fuel gasification, this question serves as a perfect synthesis of your learning. In our previous modules, we discussed how the chemical composition of a fuel gas depends entirely on its source and the medium of its production—whether it is steam, air, or anaerobic decomposition. This PYQ tests your ability to distinguish between these processes, specifically focusing on the calorific value and the presence of non-combustible diluents like Nitrogen.

To arrive at the correct answer (B), follow a logical elimination path. Start with the most familiar term: Gobar gas (Biogas), which is primarily Methane (CH4) (D-1). Next, look at the coke-based gases. Think of the reactants: Water gas is produced using steam (H2O), so it must contain Hydrogen (CO + H2) (A-2). In contrast, Producer gas is made by passing air over red-hot coke; since air is mostly nitrogen, the resulting gas is a mixture of CO + N2 (B-4). Finally, Coal gas, a product of the destructive distillation of coal, is a complex mixture of H2, CH4, and CO (C-3). This step-by-step mapping confirms the sequence 2-4-3-1.

UPSC frequently uses the similarity between Water gas and Producer gas as a distractor trap, as seen in options (A) and (C). The common mistake is to overlook the Nitrogen component in Producer gas, which is the key reason it has a lower heating value than Water gas. By identifying the oxidizing agent (steam vs. air), you can easily avoid these traps. This level of conceptual precision is what separates a prepared candidate from the rest. For a deeper dive into these reactions, refer to NCERT Class 10 Science: Carbon and its Compounds.