To weld metals together, high temperature is required. Such a high temperature is obtained by burning:

1. Classification of Hydrocarbons: Alkanes, Alkenes, and Alkynes (basic)

Welcome to our journey into the world of chemistry! To understand how the world around us works—from the fuel in our cars to the oil in our kitchens—we must start with the most basic building blocks of organic chemistry: Hydrocarbons. As the name suggests, these are compounds made up exclusively of Hydrogen and Carbon Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65. Carbon is unique because it can bond with itself to form long chains or rings, and how these atoms are linked determines the properties of the substance.

We classify these hydrocarbons into three primary families based on the type of bonds between the carbon atoms:

• Alkanes: These are saturated hydrocarbons. "Saturated" means every carbon atom is joined to others by single bonds only, effectively "filling up" the carbon's capacity with the maximum possible number of hydrogen atoms. A common example is Methane (CH₄).
• Alkenes: These are unsaturated hydrocarbons that contain one or more double bonds (C=C) between carbon atoms. Because of this double bond, they have fewer hydrogen atoms than alkanes.
• Alkynes: These are also unsaturated but contain one or more triple bonds (C≡C) Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65. The triple bond makes them highly reactive and useful in industrial applications.

This distinction between "saturated" and "unsaturated" isn't just academic; it has massive real-world implications. For instance, most of the fuels we use are hydrocarbons because they release a large amount of energy during combustion Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69. Furthermore, in nutrition, you might have heard that vegetable oils are "healthier" than animal fats. This is because vegetable oils generally contain long unsaturated carbon chains, whereas animal fats are typically saturated Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

Type	Bonding	Saturation Status	General Formula
Alkanes	Single (C-C)	Saturated	CₙH₂ₙ₊₂
Alkenes	Double (C=C)	Unsaturated	CₙH₂ₙ
Alkynes	Triple (C≡C)	Unsaturated	CₙH₂ₙ₋₂

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

2. Chemistry of Combustion and Exothermic Reactions (basic)

At its core, combustion is a chemical process where a substance reacts with oxygen to release energy in the form of heat and light. Any substance that can undergo this reaction is known as a combustible substance, such as wood, kerosene, or LPG Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62. Because these reactions release energy into the surroundings, they are classified as exothermic reactions. In contrast, reactions that absorb energy from the surroundings are called endothermic Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14.

For combustion to occur, two things are non-negotiable: a fuel and an oxidant (usually oxygen). If you limit the oxygen supply, the reaction becomes "incomplete." For example, while saturated hydrocarbons typically burn with a clean blue flame, unsaturated carbon compounds (like acetylene) or even saturated ones in a limited air supply will produce a yellow, sooty flame due to unburnt carbon particles Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69.

In high-stakes applications like welding, the intensity of this exothermic reaction is critical. While air contains oxygen, it is roughly 78% Nitrogen (N₂). During combustion in air, this nitrogen does not contribute to the heat; instead, it absorbs a significant portion of the energy produced, acting like a "thermal sponge" that lowers the overall flame temperature. To achieve the 3000°C+ temperatures needed to melt steel, we must use pure oxygen. By removing nitrogen from the equation, the entire energy of the exothermic reaction is concentrated, allowing the fuel to reach its maximum possible flame temperature.

Feature	Combustion in Air	Combustion in Pure Oxygen
Flame Temperature	Lower (Nitrogen absorbs heat)	Significantly Higher
Efficiency	Often incomplete/sooty	Highly efficient/complete
Application	Cooking, heating, campfires	Metal cutting, welding, rocketry

Sources: Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3, 14; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69

3. LPG, CNG, and Biogas: Composition and Uses (intermediate)

In our daily lives, we interact with various gaseous fuels, each tailored for specific tasks based on their chemical energy and combustion properties. These fuels are primarily composed of hydrocarbons—compounds made of carbon and hydrogen. At the simplest level, we have Methane (CH₄), which is the primary component of both Biogas and Compressed Natural Gas (CNG). Methane is a single-carbon alkane that burns cleanly, making it an ideal choice for the local city gas distribution (CGD) networks and transport sectors Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. In India, these natural gas reserves are strategically significant, found in regions like the Mumbai High and the Krishna-Godavari basin India People and Economy, Class XII (NCERT 2025 ed.), Mineral and Energy Resources, p.61.

When we move to Liquefied Petroleum Gas (LPG), the chemistry becomes slightly more complex. LPG is primarily a mixture of Butane (C₄H₁₀) and Propane (C₃H₈). These are molecules with longer carbon chains compared to methane Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. Because these gases can be liquefied under relatively low pressure, they are easily stored in cylinders for domestic cooking. While they are excellent for heating, their flame temperature is limited. For instance, burning propane with oxygen reaches about 2800°C, which is sufficient for many tasks but not ideal for high-precision industrial metal fusion.

For heavy-duty industrial applications like welding, a different chemistry is required. While LPG and CNG are used for heating, Acetylene (Ethyne, C₂H₂) is the gold standard for welding. This is because acetylene, an unsaturated hydrocarbon with a triple bond, produces an exceptionally high flame temperature (3100°C to 3316°C) when burned with pure oxygen. Pure oxygen is used instead of air because the nitrogen in the air (about 78%) does not participate in combustion; instead, it absorbs heat and lowers the flame temperature below the melting point of most metals. This distinction explains why your kitchen stove (LPG) can boil water efficiently but cannot easily melt a steel beam.

Comparison of Common Fuel Gases

Fuel Type	Primary Components	Common Use Case
CNG	Methane (CH₄)	Transport (Buses/Autos), Industrial fuel
LPG	Butane (C₄H₁₀) & Propane (C₃H₈)	Domestic cooking (Cylinders), Heating
Biogas	Methane (CH₄) & CO₂	Rural cooking, Small-scale power

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64; India People and Economy, Class XII (NCERT 2025 ed.), Mineral and Energy Resources, p.61

4. Role of Oxygen and Nitrogen in Industrial Processes (intermediate)

To understand industrial chemistry, we must first look at the 'balancing act' between the two primary gases in our atmosphere: Nitrogen and Oxygen. Air is a uniform mixture where Nitrogen makes up about 78% and Oxygen about 21% Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.118. While Oxygen is the 'active' partner that supports life and combustion, Nitrogen serves as the essential 'moderator.'

In industrial settings, Nitrogen is prized for being relatively inert (non-reactive). Its main function in nature is to dilute Oxygen, preventing spontaneous and uncontrollable combustion in our atmosphere Physical Geography by PMF IAS, Earths Atmosphere, p.272. We harness this property in several ways:

• Food Preservation: Nitrogen is pumped into chip packets to displace Oxygen, preventing rancidity (the oxidation and spoiling of fats) Physical Geography by PMF IAS, Earths Atmosphere, p.272.
• Lighting: It is used in electric bulbs to protect the tungsten filament. If Oxygen were present, the white-hot filament would burn up instantly; Nitrogen provides a safe, inactive environment Physical Geography by PMF IAS, Earths Atmosphere, p.272.

Conversely, when we need intense energy, we turn to Oxygen. While normal combustion requires Oxygen Science, Class VII NCERT, Changes Around Us: Physical and Chemical, p.62, atmospheric air is often not enough for heavy industry. In oxy-acetylene welding, we use pure Oxygen instead of air to burn acetylene gas. This is because the Nitrogen in the air does not participate in the fire; it simply 'steals' heat, lowering the flame temperature. By removing Nitrogen and using pure Oxygen, we can reach staggering temperatures of 3100°C to 3316°C, which is necessary to melt and fuse metals effectively.
Oxygen vs. Nitrogen in Industry

Gas	Primary Characteristic	Industrial Application
Oxygen (O₂)	Supports Combustion	High-temperature welding, steel manufacturing.
Nitrogen (N₂)	Inert/Non-reactive	Food packaging, preventing filament oxidation in bulbs.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.118; Physical Geography by PMF IAS, Earths Atmosphere, p.272; Science, Class VII NCERT, Changes Around Us: Physical and Chemical, p.62

5. Properties of Acetylene (Ethyne) (intermediate)

At its core, Acetylene (scientifically known as Ethyne) is the simplest member of the alkyne family, with the chemical formula C₂H₂. Unlike the saturated hydrocarbons we find in cooking gas, ethyne contains a triple bond between its two carbon atoms. This triple bond is a concentrated storehouse of energy. Because it has these multiple bonds, it is classified as an unsaturated carbon compound, which makes it significantly more reactive than saturated compounds like methane or ethane Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. This high reactivity and energy density are what make it indispensable in industrial applications, particularly welding.

The most famous application of this gas is oxy-acetylene welding. To join metals like steel, you need temperatures high enough to melt the base metal and achieve fusion. When acetylene is burned in pure oxygen, it produces a flame temperature of approximately 3100°C to 3316°C—the highest of any common fuel gas. While other gases like propane (LPG) or methane can be used as fuels Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.78, they cannot match this thermal intensity. Furthermore, the oxy-acetylene flame features a unique reducing zone, which prevents the metal from reacting with oxygen (oxidizing) while it is molten, ensuring a high-quality, strong weld.

A common question is why we must use pure oxygen cylinders instead of just blowing air into the torch. The answer lies in the composition of our atmosphere. Air is roughly 78% Nitrogen. Nitrogen does not contribute to the combustion process; instead, it acts as a 'heat sink,' absorbing a massive amount of the thermal energy produced and significantly lowering the flame temperature. By using pure oxygen, we eliminate the nitrogen 'ballast,' allowing the flame to reach the extreme temperatures necessary for the fusion point of most structural metals.

Feature	Acetylene (Ethyne)	Propane (LPG) / Methane
Bond Type	Triple Bond (Unsaturated)	Single Bond (Saturated)
Flame Temp (in O₂)	~3300°C	~2800°C
Primary Use	High-temperature welding	Heating and cooking

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.78

6. Industrial Welding: Oxy-Acetylene vs Other Fuel Gases (exam-level)

To join two pieces of metal through welding, we must reach a temperature high enough to melt the base metals and facilitate fusion. While a standard gas stove uses LPG (mostly propane and butane) to cook food, industrial welding requires something much more intense. This is where Oxy-Acetylene welding becomes the gold standard. Acetylene (C₂H₂) is an unsaturated hydrocarbon. While unsaturated compounds typically produce a sooty, yellow flame in limited air due to incomplete combustion Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.69, they release a tremendous amount of energy when burned in a controlled, oxygen-rich environment.

The choice of Acetylene over other fuel gases like Propane (LPG) or Methane boils down to the flame temperature and chemistry. When mixed with pure oxygen, Acetylene produces a concentrated flame reaching 3100°C to 3300°C, which is significantly higher than the ~2800°C produced by Propane. Furthermore, the oxy-acetylene flame has a unique reducing zone. In chemical terms, reduction involves the loss of oxygen Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13. This specific part of the flame prevents the molten metal from reacting with atmospheric oxygen (oxidation), ensuring a high-quality, strong weld without brittle oxide inclusions.

A common question is why we use pure oxygen instead of simply blowing compressed air. Air consists of roughly 78% Nitrogen. In a combustion reaction, Nitrogen is an "inert passenger"—it does not contribute to the fire but instead absorbs a massive amount of the heat produced to warm itself up. This "thermal tax" prevents the flame from ever reaching the fusion point of most industrial metals. By using pure oxygen, we eliminate the cooling effect of Nitrogen, allowing the heat to be concentrated solely on the welding task.

Comparison of Fuel Gases in Welding

Feature	Oxy-Acetylene	Oxy-Propane (LPG)
Flame Temp	~3100°C - 3300°C	~2800°C
Primary Use	Welding, Cutting, & Brazing	Cutting & Heating (not ideal for welding steel)
Chemical Nature	Unsaturated (High C-C bond energy)	Saturated (Lower energy density)

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.69; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13

7. Solving the Original PYQ (exam-level)

This question bridges your understanding of hydrocarbon combustion and exothermic reactions. You have previously learned that the energy released during combustion depends on the molecular structure of the fuel. In industrial welding, the goal is to reach the fusion point of metals like steel, which requires a highly concentrated heat source. As noted in ScienceDirect, achieving this fusion requires a flame that is significantly hotter than the melting point of the metal itself. This is where the concept of flame temperature becomes the deciding factor in your reasoning.

To arrive at the correct answer, (A) Acetylene in oxygen, you must identify the fuel gas with the highest energy density. Acetylene (C2H2) contains a triple bond which, when broken, releases an immense amount of thermal energy. When mixed with pure oxygen, it produces a flame temperature of approximately 3100°C to 3316°C. As explained in Virtual Labs (DEI), pure oxygen is essential because the nitrogen in regular air does not contribute to combustion and instead acts as a "heat sink," absorbing energy and lowering the temperature. Therefore, burning acetylene in oxygen provides the most efficient propagation rate for joining metals.

UPSC often includes distractors like LPG and Methane (Options B and C) because they are common fuels; however, their flame temperatures (around 2800°C) are insufficient for high-quality steel welding. The most significant trap is Option D (Acetylene in nitrogen). This option tests your fundamental chemistry knowledge: nitrogen is an inert gas that does not support combustion. By recognizing that an oxidizer like oxygen is mandatory for high-heat reactions and that acetylene is the superior fuel for thermal intensity, you can easily eliminate the incorrect alternatives.