Assertion (A): The main component of natural gas is butane. Reason (R): Butane is easily liquified under pressure

1. Introduction to Hydrocarbons: The Alkane Series (basic)

Welcome to your first step in mastering everyday chemistry! To understand the fuels that power our world, we must first meet the hydrocarbons. As the name suggests, these are compounds made up entirely of carbon and hydrogen. Within this group, the simplest family is the Alkanes. These are known as saturated hydrocarbons because every carbon atom is linked to others by single bonds only, ensuring that the valency of every atom is fully satisfied Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65.

The alkane series follows a predictable pattern where each member differs from the next by a —CH₂ unit. This is called a homologous series. The first four members are gases at room temperature and are critical to our energy needs: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), and Butane (C₄H₁₀) Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. Methane is the simplest of all, consisting of a single carbon atom bonded to four hydrogen atoms. As we add more carbon atoms to the chain, the physical properties of the substance change, such as its boiling point and how easily it can be turned into a liquid.

From a chemical standpoint, alkanes are known for being fairly unreactive or inert under normal conditions Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71. Because all their bonds are single and "satisfied," they don't easily react with other substances. However, they do undergo substitution reactions. For example, in the presence of sunlight, a chlorine atom can replace a hydrogen atom in an alkane. In our daily lives, their most important reaction is combustion—they burn in the presence of oxygen to release significant amounts of heat and light, which is why they are the primary components of natural gas and petroleum fuels.

Name	Formula	Carbon Atoms	Common Use
Methane	CH₄	1	Main component of Natural Gas (CNG)
Butane	C₄H₁₀	4	Main component of Lighter fluid and LPG

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.63, 64, 65, 71

2. Natural Gas: Composition and Primary Sources (basic)

To understand natural gas, we must first look at its organic origin. Imagine millions of years ago, where tiny marine organisms and plants were buried under thick layers of silt and mud. Under intense heat and pressure, this organic matter underwent a chemical transformation to become the fossil fuels we use today. Because natural gas is less dense (lighter) than both water and crude oil, it naturally migrates upward and gets trapped in the 'anticlines' or folds of rock, often sitting right on top of oil deposits Geography of India, Energy Resources, p.9. While it is often found alongside oil, many large fields contain gas alone.

Chemically, natural gas is not a single substance but a mixture of gaseous hydrocarbons. The undisputed heavyweight here is Methane (CH₄), which typically accounts for 80% to 90% of the total volume Environment and Ecology, Distribution of World Natural Resources, p.15. Other components include heavier hydrocarbons like Ethane (C₂H₆), Propane (C₃H₈), and Butane (C₄H₁₀). While these other gases are present, they are considered minor constituents or 'Natural Gas Liquids' (NGLs) when separated.

In the context of India, our natural gas wealth is concentrated in specific maritime and inland pockets. The Bombay High and Bassein oilfields are the crown jewels, holding about 75% of the country's reserves Geography of India, Energy Resources, p.16. From an environmental perspective, natural gas is often hailed as a 'bridge fuel' because it burns much cleaner than coal; however, we must remain cautious because methane is also a potent greenhouse gas if leaked directly into the atmosphere Environment, Climate Change, p.256.

Sources: Geography of India, Energy Resources, p.9; Environment and Ecology, Distribution of World Natural Resources, p.15; Geography of India, Energy Resources, p.16; Environment, Climate Change, p.256

3. Liquefied Petroleum Gas (LPG): The Butane-Propane Mix (intermediate)

When we turn on our kitchen stoves, we are utilizing Liquefied Petroleum Gas (LPG), a sophisticated mixture of hydrocarbons. Unlike the gas found in large cross-country pipelines, LPG is primarily composed of Butane (C₄H₁₀) and Propane (C₃H₈). These molecules belong to the alkane homologous series, where each successive member differs by a –CH₂– unit Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.66. In a typical domestic cylinder, butane is usually the dominant component because of its unique physical properties that make it ideal for portable storage.

The magic of LPG lies in the word "Liquefied." To store a large volume of energy in a small metal cylinder, the gas must be turned into a liquid. This depends on a concept called Critical Temperature—the temperature above which a gas cannot be liquefied, no matter how much pressure you apply. Butane has a high critical temperature of about 152°C (425 K), which is well above room temperature. This means we can easily turn it into a liquid just by applying moderate pressure at ordinary temperatures. In contrast, Methane (CH₄), the primary component of Natural Gas, has a very low critical temperature (-82.6°C), making it extremely difficult to liquefy without expensive cryogenic cooling Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.15.

It is a common point of confusion in competitive exams to mix up LPG with Natural Gas. While they are both fossil fuels, their compositions are distinct:

Feature	Liquefied Petroleum Gas (LPG)	Natural Gas (CNG/LNG)
Primary Component	Butane (C₄H₁₀) and Propane (C₃H₈)	Methane (CH₄) — usually 80% to 90%
Source	Refining of crude oil or processed from natural gas liquids	Found in gas fields, often above oil traps
Ease of Liquefaction	High (Liquid at room temp under pressure)	Low (Requires high pressure or extreme cold)

Finally, you might notice a strong smell when a cylinder leaks. Interestingly, both butane and propane are naturally odorless. For safety, a pungent-smelling chemical called Ethyl Mercaptan is added to LPG so that even the smallest leak can be detected by the human nose, preventing potential accidents.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64-66; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.15

4. CNG, LNG, and PNG: Forms of Natural Gas (intermediate)

To understand the different forms of natural gas, we must first look at its chemical identity. Natural gas is not a single element but a mixture of gaseous hydrocarbons. Its "hero" ingredient is Methane (CH₄), which typically accounts for 80 to 90 percent of its volume Environment and Ecology, Majid Hussain, p.15. Other components include Ethane (C₂H₆), Propane (C₃H₈), and Butane (C₄H₁₀). While Methane is the dominant gas, these other heavier hydrocarbons are often referred to as Natural Gas Liquids (NGLs).

The different names we use—CNG, LNG, and PNG—refer primarily to how the gas is processed and transported, rather than a change in its chemical formula. Compressed Natural Gas (CNG) is natural gas stored at very high pressure (200–250 bar) to reduce its volume, making it viable for vehicles. Piped Natural Gas (PNG) is the same gas delivered directly to homes and industries through a network of pipelines Indian Economy, Nitin Singhania, p.447. However, Liquefied Natural Gas (LNG) is unique; it is cooled to -162 °C to turn it into a liquid, shrinking its volume by 600 times for long-distance sea transport.

A fascinating scientific detail lies in why we must freeze it so deeply. Every gas has a critical temperature—the temperature above which it cannot be liquefied, no matter how much pressure you apply. Methane has a very low critical temperature (-82.6 °C), meaning it refuses to become a liquid at room temperature just by squeezing it. In contrast, Butane has a high critical temperature (152 °C), which is why it liquefies easily under modest pressure in your kitchen LPG cylinder. This is a key reason why LNG infrastructure requires advanced cryogenic technology and temperature-controlled supply chains Indian Economy, Nitin Singhania, p.417.

Form	Full Name	Primary State	Key Characteristic
CNG	Compressed Natural Gas	Gas (High Pressure)	Used in transport; eco-friendly alternative to petrol/diesel.
LNG	Liquefied Natural Gas	Liquid (Cryogenic)	Reduced volume (1/600th) for easy overseas shipping.
PNG	Piped Natural Gas	Gas (Low Pressure)	Continuous supply to kitchens/factories via pipelines.

In India, natural gas is a pillar of the economy, with nearly 40% of it fueling the production of chemical fertilizers and 30% dedicated to power generation Geography of India, Majid Husain, p.17. Most of this production is managed by Public Sector Undertakings (PSUs), which control over 80% of the market Indian Economy, Nitin Singhania, p.447.

Sources: Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15; Geography of India, Majid Husain, Energy Resources, p.17; Indian Economy, Nitin Singhania, Infrastructure, p.447; Indian Economy, Nitin Singhania, Food Processing Industry in India, p.417

5. Physics of Liquefaction: Critical Temperature and Pressure (intermediate)

To understand the liquefaction of gases, we must look at the battle between thermal energy (which keeps molecules zooming apart) and intermolecular forces (which try to pull them together). In a gas, molecules have high kinetic energy and move freely. To turn them into a liquid, we must either lower the temperature to slow them down or increase the pressure to force them closer together. However, physics dictates a strict boundary for this process called the Critical Temperature (T꜀). This is the temperature above which a gas cannot be liquefied, no matter how much pressure you apply. Above this point, the molecules possess too much kinetic energy for any amount of compression to bind them into a liquid state. Associated with this is Critical Pressure (P꜀), which is the minimum pressure required to liquefy a gas exactly at its critical temperature. Every substance has a unique T꜀ based on its molecular structure. For instance, methane (CH₄), the primary component of natural gas Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15, has a very low critical temperature of approximately -82.6 °C. Because this is far below normal room temperature, methane cannot be liquefied at ambient temperatures simply by pumping it into a tank; it requires cryogenic cooling to become Liquefied Natural Gas (LNG). In contrast, larger hydrocarbons like butane (C₄H₁₀), which contains four carbon atoms Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64, have much stronger intermolecular attractions. Butane has a critical temperature of about 152 °C (425 K). Since this is well above room temperature, butane can be easily liquefied at normal temperatures just by applying moderate pressure. This is why butane is a preferred fuel for portable lighters and camping stoves—it stays liquid under pressure in a thin plastic or metal container and turns back into gas the moment the pressure is released.

Property	Methane (CH₄)	Butane (C₄H₁₀)
Carbon Atoms	1	4
Critical Temp (T꜀)	-82.6 °C (Very Low)	152 °C (Above Room Temp)
Liquefaction Ease	Hard; requires extreme cooling	Easy; requires only pressure

Sources: Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64

6. Unconventional Gas Resources (exam-level)

To understand unconventional gas resources, we must first look at the fundamental chemistry of natural gas. While many people assume natural gas is a single substance, it is actually a mixture of gaseous hydrocarbons. The heavyweight champion of this mixture is Methane (CH₄), which typically accounts for 80% to 90% of the total volume Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15. Other components include ethane, propane, and butane, which are often referred to as 'Natural Gas Liquids' (NGLs) because they are much easier to condense into a liquid state.

The ease of liquefying these gases depends on a physical property called the Critical Temperature. This is the temperature above which a gas cannot be liquefied, no matter how much pressure you apply. Butane has a critical temperature of approximately 152 °C (425 K), which is well above room temperature. This explains why the butane in a common cigarette lighter or a camping stove cylinder stays liquid under modest pressure. In sharp contrast, Methane has a very low critical temperature of -82.6 °C. To store methane as Liquefied Natural Gas (LNG), we must use expensive cryogenic technology to cool it far below sub-zero temperatures.

In the Indian context, these gases are often found in specific geological settings. Conventional gas is usually found trapped in the upper parts of oil reservoirs, while unconventional gas like Coal Bed Methane (CBM) is adsorbed within coal seams. India’s coal reserves are divided into the Gondwana Period (making up over 98% of reserves, including high-quality Bituminous and Anthracite coal) and the Tertiary Era (about 2% of production, often called 'brown coal' or lignite) Geography of India, Majid Husain, Energy Resources, p.1. The extraction and production of these resources in India are heavily dominated by Public Sector Undertakings (PSUs), which account for more than 80% of national production Indian Economy, Nitin Singhania, Infrastructure, p.447.

Feature	Methane (CH₄)	Butane (C₄H₁₀)
Abundance in Natural Gas	Dominant (80-90%)	Minor constituent
Critical Temperature	Very Low (-82.6 °C)	High (152 °C)
Ease of Liquefaction	Difficult (Requires cryogenics)	Easy (Liquefies under pressure at room temp)

Sources: Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15; Geography of India, Majid Husain, Energy Resources, p.1; Indian Economy, Nitin Singhania, Infrastructure, p.447

7. Comparative Analysis: Methane (CH₄) vs. Butane (C₄H₁₀) (exam-level)

To understand why different gases are used for different household and industrial purposes, we must look at the structural differences between Methane (CH₄) and Butane (C₄H₁₀). Both are hydrocarbons belonging to the alkane series, where carbon atoms are linked by single bonds Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. However, the length of their carbon 'chain' drastically changes their physical behavior. Methane is the simplest hydrocarbon with just one carbon atom, while butane has a chain of four carbon atoms. As the number of carbon atoms increases, the interparticle forces of attraction become stronger, leading to higher melting and boiling points Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

This difference in molecular weight and attraction forces dictates how easily these gases can be turned into liquids (liquefaction). Methane has an extremely low boiling point (111 K) and a very low critical temperature, meaning it cannot be liquefied at room temperature regardless of how much pressure you apply; it must be cooled to cryogenic temperatures to become Liquefied Natural Gas (LNG). In contrast, Butane has a much higher boiling point (approx. 272 K) and a critical temperature well above room temperature (around 152 °C). This allows butane to be easily liquefied under modest pressure at ambient temperatures, which is why it is a primary component of Liquefied Petroleum Gas (LPG) used in cylinders and pocket lighters.

In terms of natural occurrence, Methane is the undisputed king of Natural Gas, typically making up 80% to 90% of its volume. While butane is found in natural gas deposits, it is considered a minor constituent or a 'Natural Gas Liquid' (NGL) that is often separated during processing. Therefore, while butane's physical properties make it easier to store as a liquid, it is methane that fuels the vast majority of our natural gas infrastructure.

Feature	Methane (CH₄)	Butane (C₄H₁₀)
Carbon Chain	Short (1 Carbon)	Longer (4 Carbons)
Boiling Point	Very Low (111 K)	Higher (Approx. 272 K)
Primary Source	Main component of Natural Gas	Component of LPG / NGL
Liquefaction	Requires cryogenic cooling	Liquefies under pressure at room temp

Sources: Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59, 64

8. Solving the Original PYQ (exam-level)

In your recent study of Hydrocarbons and Energy Resources, you explored the chemical compositions of various fuels and the physical principles of gas behavior. This question brings those building blocks together by testing your ability to distinguish between Natural Gas and Liquefied Petroleum Gas (LPG). While both are fossil fuels, their primary constituents differ significantly. You learned that Methane (CH4) is the simplest hydrocarbon and the dominant component of natural gas, whereas heavier hydrocarbons like Butane are the primary components of LPG. This fundamental distinction is the key to deconstructing the logic of this Assertion-Reasoning pair.

Let’s walk through the reasoning like a seasoned aspirant. First, evaluate Assertion (A): Is butane the main component of natural gas? Based on Environment and Ecology by Majid Hussain, Methane typically constitutes 80% to 90% of natural gas, making Assertion (A) objectively false. Next, evaluate Reason (R): Is butane easily liquefied? Yes, because it has a critical temperature well above room temperature, allowing it to transition to a liquid state under moderate pressure. Since the assertion is false but the reason is a scientifically accurate statement, the correct answer is (D).

UPSC frequently uses "conceptual overlap" traps to catch students off guard. The most common pitfall here is confusing the ease of liquefaction (which applies to butane in LPG) with the primary composition of natural gas. Students often see the word "liquefied" and instinctively think of LNG (Liquefied Natural Gas), leading them to choose Option (A) or (B). Remember, while methane can be liquefied, it requires extreme cryogenic cooling, unlike butane. By spotting that Assertion (A) is false immediately, you can bypass the complex task of determining if the reason explains the assertion, leading you straight to the correct choice.