Consider the following statements about acetylene : I. It is used in welding industry. II. It is raw material for preparing plastics. III. It is easily obtained by mixing silicon carbide and water. Of these statements :

1. Introduction to Hydrocarbons: Alkanes, Alkenes, and Alkynes (basic)

At the heart of organic chemistry lies a family of compounds called hydrocarbons, which, as the name suggests, are composed entirely of carbon and hydrogen atoms Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65. Carbon is unique because it can form stable chains of varying lengths, from a single carbon atom in Methane (CH₄) to long chains found in plastics and fuels. We classify these into two primary groups based on how the carbon atoms are bonded together: saturated and unsaturated hydrocarbons. Saturated hydrocarbons, also known as Alkanes, contain only single bonds between carbon atoms. Because every available bond is 'filled' with an atom, they are relatively stable and do not easily react with other substances. Common examples include Propane (used in stoves) and Butane (found in lighters) Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64. On the other hand, Unsaturated hydrocarbons contain at least one double bond (Alkenes) or a triple bond (Alkynes). These 'extra' bonds make them more reactive, allowing them to undergo addition reactions where new atoms, like hydrogen, are added to the chain.

Type	Bonding	Key Example	Common Use
Alkanes	Single Bond	Methane (CH₄)	Natural Gas (Fuel)
Alkenes	Double Bond	Ethene (C₂H₄)	Plant ripening / Plastic production
Alkynes	Triple Bond	Ethyne/Acetylene (C₂H₂)	Welding and Industrial feedstock

This chemical distinction has massive real-world implications. For instance, in the food industry, liquid vegetable oils (which are unsaturated) are often converted into solid fats through a process called hydrogenation, using a nickel catalyst Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71. In heavy industry, the alkyne Acetylene is prized because it burns with an incredibly hot flame suitable for cutting steel. Interestingly, industrial acetylene is produced by the reaction of water with calcium carbide (CaC₂), a process that yields the gas needed for both welding and as a raw material for plastics like PVC.

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

2. Physical and Chemical Properties of Alkynes (basic)

When we talk about alkynes, we are entering the world of high-energy chemistry. Alkynes are a group of unsaturated hydrocarbons characterized by at least one triple bond between two carbon atoms. The simplest and most famous member of this family is ethyne (commonly known as acetylene), with the chemical formula C₂H₂. Because of this triple bond, carbon atoms share three pairs of electrons, making the molecule much more reactive than saturated compounds like methane or ethane Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64.

In terms of physical properties, simple alkynes like ethyne are colorless gases. However, their real magic lies in their chemical behavior. When ethyne burns in the presence of pure oxygen, it undergoes complete combustion to produce a flame so hot (over 3000°C) that it can melt steel. This is why oxy-acetylene welding is a staple in construction and repair. Beyond fuel, alkynes serve as the building blocks for the modern world; for instance, ethyne is a vital chemical feedstock used to create vinyl chloride, which is the precursor for PVC (Polyvinyl Chloride) plastics Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62.

One critical detail for your exams is how we obtain this gas. In industrial settings, acetylene is typically produced by the hydrolysis of calcium carbide (CaC₂). When you add water to calcium carbide, a vigorous reaction occurs: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. It is important not to confuse this with silicon carbide (an abrasive) or calcium carbonate (limestone). Only calcium carbide gives us the ethyne needed for industrial applications.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64

3. Polymers and Plastics in Daily Life (intermediate)

In the world of Applied Chemistry, polymers are the giants. These large molecules are built from small, repeating building blocks called monomers. One of the most versatile plastics we use today is Polyvinyl Chloride (PVC), found in everything from construction pipes to medical tubing. To understand how these plastics are born, we have to look at their chemical ancestry, which often begins with a simple but powerful gas: Acetylene (C₂H₂). Acetylene is a highly reactive hydrocarbon that serves two primary roles in industry. First, it is an essential chemical feedstock; it is the starting material for synthesizing vinyl chloride, which then polymerizes to form PVC. Second, it is famous for its use in oxy-acetylene welding. When acetylene is burned with pure oxygen, it produces a flame temperature exceeding 3000°C—the highest of any common fuel gas—allowing it to melt and cut through heavy metals with ease. This reaction is intensely exothermic, meaning it releases a massive amount of energy, a concept related to the thermal energy changes discussed when substances change state Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. But where does this gas come from? In industrial settings, acetylene is typically generated through a specific chemical reaction: the hydrolysis of Calcium Carbide (CaC₂). When water is added to calcium carbide, it produces acetylene gas and calcium hydroxide (CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂). It is a common misconception to confuse this with Silicon Carbide, which is an extremely hard abrasive (carborundum) and is not used for gas production. Mastering these balanced chemical equations is a fundamental skill in understanding how raw materials are transformed into daily-use products Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6.

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6

4. Industrial Carbides: Silicon Carbide vs. Calcium Carbide (intermediate)

In industrial chemistry, carbides are compounds composed of carbon and a less electronegative element. Two of the most significant industrial carbides are Silicon Carbide (SiC) and Calcium Carbide (CaC₂), which serve entirely different purposes based on their physical and chemical properties. Silicon Carbide, often known as carborundum, is prized for its extreme hardness—approaching that of a diamond. This makes it an ideal material for abrasives and cutting tools. In geological terms, we see the power of hardness in processes like abrasion, where particles grind against surfaces to wear them away Certificate Physical and Human Geography, Arid or Desert Landforms, p.69. Because SiC is chemically inert and has a very high melting point, it is also used in high-temperature electronics and kiln linings.
In contrast, Calcium Carbide (CaC₂) is used primarily for its chemical reactivity rather than its structural strength. When Calcium Carbide reacts with water, it undergoes a vigorous hydrolysis reaction to produce acetylene gas (C₂H₂) and calcium hydroxide (slaked lime). The reaction is represented as: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. This is reminiscent of how calcium oxide reacts with water to form slaked lime, a common combination reaction taught in basic chemistry Science Class X, Chemical Reactions and Equations, p.6. The resulting acetylene gas is a crucial industrial tool, producing a high-temperature flame for oxy-acetylene welding and serving as a foundational 'building block' or feedstock for manufacturing plastics like PVC (Polyvinyl Chloride).

Feature	Silicon Carbide (SiC)	Calcium Carbide (CaC₂)
Primary Property	Extreme hardness and thermal stability.	High chemical reactivity with water.
Major Use	Abrasives, sandblasting, and semiconductors.	Production of Acetylene gas.
Industrial Role	Mechanical/Structural material.	Chemical feedstock (Precursor to plastics).

Sources: Certificate Physical and Human Geography, Arid or Desert Landforms, p.69; Science Class X (NCERT 2025), Chemical Reactions and Equations, p.6

5. Applications of High-Temperature Flames (intermediate)

To understand why certain flames are used in heavy industry, we must first look at the nature of combustion. A standard flame, like that of a candle or a gas stove, relies on the oxygen in the surrounding air. However, for tasks like welding and cutting metals, we need temperatures far exceeding 1000°C. This is achieved by using specific fuels and providing a concentrated supply of oxygen, as normal combustion is limited by the 21% oxygen concentration in our atmosphere Physical Geography by PMF IAS, Earths Atmosphere, p.272. In a laboratory or kitchen setting, we see that saturated hydrocarbons (like methane) typically burn with a clean blue flame when air supply is sufficient, whereas unsaturated compounds tend to produce yellow, sooty flames due to incomplete combustion Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69.

The gold standard for high-temperature industrial flames is the oxy-acetylene flame. Acetylene (C₂H₂) is an unsaturated hydrocarbon that, when mixed with pure oxygen in a specialized torch, produces a flame temperature of approximately 3200°C. This intense heat is necessary because different metals have high melting points and varying reactivities; for instance, while iron doesn't burn easily in its solid form, it reacts vigorously when heat and oxygen are concentrated Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.42. This allows the torch to effectively melt and join (weld) or even slice through thick steel plates.

Beyond its use as a fuel, Acetylene is a vital chemical feedstock. In industrial chemistry, a feedstock is a raw material used to manufacture other products. Acetylene is the starting point for producing vinyl chloride, which is then polymerized to create Polyvinyl Chloride (PVC)—the plastic used in everything from pipes to medical tubing. To produce acetylene industrially, we use a simple yet powerful reaction: the hydrolysis of Calcium Carbide (CaC₂). When water is added to calcium carbide, it releases acetylene gas and leaves behind calcium hydroxide (slaked lime). It is a common misconception to confuse this with silicon carbide (an abrasive); for industrial gas production, calcium is the key element.

Application	Process/Fuel Used	Key Outcome
Welding/Cutting	Oxy-acetylene Flame	Melts metals at 3000°C+
Plastics Industry	Acetylene as Feedstock	Production of PVC and polymers
Industrial Production	Calcium Carbide + Water	Generates high-purity Acetylene gas

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.272; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.69; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.42

6. Industrial Synthesis and Uses of Acetylene (exam-level)

Acetylene, scientifically known as Ethyne (C₂H₂), is a highly reactive gas and the simplest member of the alkyne family. Its industrial importance stems from its carbon-carbon triple bond, which stores a significant amount of energy. While many organic compounds were historically thought to require a "vital force" for synthesis, modern industrial chemistry allows us to produce ethyne from inorganic precursors like carbides Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.63.

The primary industrial method for synthesizing acetylene is the hydrolysis of Calcium Carbide (CaC₂). When calcium carbide reacts with water, it undergoes a vigorous chemical reaction to produce acetylene gas and calcium hydroxide (Ca(OH)₂) as a byproduct. This reaction is represented by the following equation:

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

It is important to distinguish this from other carbides like silicon carbide, which is an abrasive and not used for gas production. The byproduct, calcium hydroxide, is a common base used in various chemical tests, such as identifying carbon dioxide Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20.

In terms of application, acetylene is indispensable in two major fields:

• Metalworking: When burned with pure oxygen (oxy-acetylene), it produces a flame temperature exceeding 3,000°C. This intense heat is sufficient to melt and join metals, making it the gold standard for welding and cutting.
• Chemical Feedstock: It serves as a vital building block in the plastics industry. Acetylene is used to manufacture organic intermediates like vinyl chloride, which is the monomer polymerized to create Polyvinyl Chloride (PVC)—the material used in everything from pipes to medical tubing.

Use Case	Role of Acetylene	Benefit
Welding	Fuel gas	High-temperature flame for melting steel
Plastics	Chemical precursor	Synthesis of Vinyl Chloride and PVC

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.63; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.15

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your recent study of alkynes and their industrial applications. By understanding the chemical nature of the triple bond in ethyne (acetylene), you can see why it is such a high-energy molecule. In your modules on thermochemistry and organic synthesis, we discussed how the combustion of acetylene is extremely exothermic, providing the intense heat necessary for oxy-acetylene welding (Statement I). Furthermore, the reactivity of this triple bond makes it an ideal building block for polymerization, allowing it to serve as a raw material for plastics like PVC (Statement II). These two statements reflect the fundamental functional utility of acetylene in modern chemistry.

To arrive at the correct answer, (A) I and II are correct, you must navigate a classic UPSC trap found in Statement III. The examiners often use distractor compounds that sound similar to the correct ones. While it is true that acetylene is produced by the reaction of a carbide with water (hydrolysis), the specific reactant is calcium carbide (CaC2), not silicon carbide (SiC). Silicon carbide, as you may recall from our lessons on refractory materials, is an abrasive known as carborundum and does not react with water to produce fuel gas. Recognizing this subtle substitution is key to eliminating options B, C, and D. This highlights a critical UPSC strategy: always double-check the specific chemical identity in statements that describe a process accurately but use the wrong reagent.