Which of the following statements regarding carbon dioxide is/are correct? 1. It is prepared on large scale by the action of water on lime. 2. In the solid form it can be used as a refrigerant. Select the correct answer using the code given below :

1. Carbon Dioxide: General Properties and Atmospheric Role (basic)

To master the chemistry of our environment, we must first understand Carbon Dioxide (CO₂) — a colorless, odorless gas that is often misunderstood as merely a pollutant. In reality, it is the backbone of the Earth's carbon cycle, acting as a bridge between the atmosphere, oceans, and living organisms. Naturally, CO₂ is released through respiration by animals and the decomposition of organic matter, while green plants 'inhale' it during photosynthesis to create energy Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19. However, human activities like burning fossil fuels and industrial processes (such as the calcination of limestone) have significantly increased its concentration, making it the primary greenhouse gas (GHG) emitted by humans Environment, Shankar IAS Academy, Climate Change, p.255. From a chemical perspective, CO₂ exhibits fascinating properties that we apply in everyday life. One such property is sublimation — the ability of solid CO₂ (commonly known as Dry Ice) to turn directly into a gas without passing through a liquid phase. This makes it an exceptional refrigerant for transporting frozen foods because it leaves no messy liquid residue. Chemically, CO₂ is produced when metal carbonates or hydrogencarbonates react with acids, a reaction that yields a salt, water, and the characteristic effervescence of CO₂ gas Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21. In the context of climate change, CO₂ serves as the global baseline. We measure the potency of all other greenhouse gases against it using a metric called Global Warming Potential (GWP). By definition, CO₂ has a GWP of 1 Environment, Shankar IAS Academy, Climate Change, p.260. While other gases like methane (CH₄) are much more powerful at trapping heat pound-for-pound, the sheer volume and long atmospheric residence time of CO₂ make it the most critical factor in global warming discussions.

Property	Description / Role
State Change	Sublimes from solid (Dry Ice) directly to gas.
GWP Value	Assigned a value of 1 (The reference standard).
Natural Sink	Absorbed by oceans and used by plants in photosynthesis.
Acid Reaction	Produced when acids react with carbonates (e.g., antacids in the stomach).

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Environment, Shankar IAS Academy, Climate Change, p.255, 260; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21

2. The Calcium Cycle: Limestone, Quicklime, and Slaked Lime (basic)

In chemistry, the transformation of calcium compounds is a beautiful, circular journey known as the Calcium Cycle. It starts with Limestone (Calcium Carbonate, CaCO₃), a naturally occurring mineral found in rocks, marble, and even the shells of sea creatures. When we heat limestone intensely, it undergoes a process called calcination. This thermal decomposition breaks the limestone down into Quicklime (Calcium Oxide, CaO) and releases Carbon Dioxide (CO₂) as a byproduct. Understanding this cycle is vital because these materials are the backbone of the construction industry and traditional building techniques.

The next stage of the cycle is the fascinating reaction between Quicklime and water, often called slaking. When you add water to CaO, a vigorous combination reaction occurs: CaO + H₂O → Ca(OH)₂. This reaction is highly exothermic, meaning it releases a significant amount of heat Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6. The resulting white powder is Slaked Lime (Calcium Hydroxide). If you dissolve this powder in water and filter it, you get Lime Water, which is a clear solution commonly used in laboratories to test for the presence of CO₂ Science-Class VII, NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.8.

The cycle completes itself through Whitewashing. When a solution of slaked lime is applied to walls, it reacts slowly with the CO₂ present in the atmosphere. This reaction creates a thin, hard layer of Calcium Carbonate (CaCO₃) on the surface, which is why walls develop a shiny, marble-like finish after two or three days Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. Effectively, the chemistry has returned the substance to its original "limestone" state, right on your living room wall!

Common Name	Chemical Name	Chemical Formula
Limestone / Marble	Calcium Carbonate	CaCO₃
Quicklime	Calcium Oxide	CaO
Slaked Lime / Lime Water	Calcium Hydroxide	Ca(OH)₂

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7; Science-Class VII, NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.8

3. Industrial Chemistry: Thermal Decomposition and Calcination (intermediate)

In the world of industrial chemistry, thermal decomposition is a fundamental process where a single chemical compound breaks down into two or more simpler substances when subjected to heat. This is an endothermic process, meaning it requires a continuous input of energy to break the chemical bonds within the reactant. A classic example encountered in textbooks is the heating of ferrous sulphate crystals, which changes color as it decomposes and releases the characteristic smell of burning sulphur Science, Class X (NCERT 2025 ed.), Chapter 1, p.8.

One of the most economically significant applications of this concept is calcination. In industrial terms, calcination refers to heating a substance (often an ore or a carbonate) to high temperatures, typically in a limited supply of air or oxygen, to drive off volatile impurities or cause chemical change. The most prominent example is the decomposition of calcium carbonate (limestone or marble) into calcium oxide (quick lime) and carbon dioxide gas. This reaction is the backbone of the cement industry, where quick lime is a primary raw material Science, Class X (NCERT 2025 ed.), Chapter 1, p.8.

Common Name	Chemical Name	Chemical Formula	Industrial Context
Limestone / Marble	Calcium Carbonate	CaCO₃	Raw material for calcination; naturally found in corals/shells Environment, Shankar IAS Academy (ed 10th), p.263.
Quick Lime	Calcium Oxide	CaO	Product of calcination; used in cement and steel manufacturing.
Slaked Lime	Calcium Hydroxide	Ca(OH)₂	Produced by adding water to quick lime; used for whitewashing Science, Class X (NCERT 2025 ed.), Chapter 1, p.7.

It is crucial to distinguish between the production of CO₂ via industrial decomposition and its production through biological means. While factories might produce CO₂ by heating rocks (calcination), living organisms produce it through respiration, where glucose is broken down in the presence of oxygen Science, Class VII, NCERT (Revised ed 2025), p.149. Understanding these different pathways is essential for mastering the carbon cycle and industrial sustainability.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.8; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.7; Environment, Shankar IAS Academy (ed 10th), Ocean Acidification, p.263; Science, Class VII, NCERT (Revised ed 2025), Life Processes in Plants, p.149

4. Environmental Impact: Greenhouse Gases and Carbon Sequestration (intermediate)

To understand the chemistry of our changing climate, we must first look at the Greenhouse Effect. Think of the Earth’s atmosphere as a glass ceiling in a nursery. The glass is transparent to incoming short-wave solar radiation (sunlight), but it is opaque to the long-wave infrared radiation (heat) that the Earth tries to radiate back into space Fundamentals of Physical Geography, World Climate and Climate Change, p.96. Greenhouse Gases (GHGs) like CO₂, CH₄, and N₂O act as this 'glass,' trapping heat and warming the planet. However, not all GHGs are created equal. We measure their impact using Global Warming Potential (GWP), which compares the energy a gas will absorb over a specific period (usually 100 years) relative to Carbon Dioxide. While CO₂ is the baseline (GWP = 1), other gases are far more potent even in smaller quantities Environment, Shankar IAS Academy, Climate Change, p.260:

Greenhouse Gas	GWP (100-year)	Atmospheric Lifetime
Carbon Dioxide (CO₂)	1	Variable (~100 years)
Methane (CH₄)	21	12 years
Nitrous Oxide (N₂O)	310	120 years
HFCs / PFCs	1,400 – 10,000+	Up to 5,000 years

To tackle the rising levels of these gases, we use Carbon Capture, Utilization, and Storage (CCUS). This involves 'sequestering' or capturing CO₂ from industrial sources like power plants or the calcination of limestone. Once captured, the CO₂ can be stored permanently underground in geological formations or 'utilized' to create valuable products like synthetic fuels, construction materials, or even dry ice (solid CO₂), which is a common refrigerant that sublimes directly into gas without leaving a liquid residue Environment, Shankar IAS Academy, Mitigation Strategies, p.281-282.

Sources: Fundamentals of Physical Geography, World Climate and Climate Change, p.96; Environment, Shankar IAS Academy, Climate Change, p.260; Environment, Shankar IAS Academy, Mitigation Strategies, p.281-282

5. Physics of Cooling: Refrigerants and Phase Changes (intermediate)

Concept: Physics of Cooling: Refrigerants and Phase Changes

6. Solid Carbon Dioxide: The Science of Dry Ice (exam-level)

Solid Carbon Dioxide, popularly known as dry ice, is a fascinating substance that defies our everyday experience of 'melting.' Unlike water ice, which turns into a puddle, dry ice undergoes sublimation—a process where a solid transitions directly into a gas without ever becoming a liquid Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.329. This happens because, at standard atmospheric pressure, CO₂ cannot exist as a liquid; it requires significantly higher pressure to do so. This 'dry' property makes it an elite refrigerant for shipping perishables like vaccines or frozen foods, as it leaves no messy residue and provides nearly double the cooling energy of water ice per unit of weight.

Understanding where this CO₂ comes from is crucial for your chemistry foundations. Industrially, carbon dioxide is often captured as a byproduct during the calcination (thermal decomposition) of limestone (CaCO₃). When limestone is heated intensely, it breaks down into lime (CaO) and releases CO₂ gas. A common confusion in exams involves the reverse or related reactions: for instance, when you add water to lime (quicklime), it produces calcium hydroxide (slaked lime) and significant heat, but it does not produce carbon dioxide. In fact, large-scale Carbon Capture and Storage (CCS) technologies now focus on trapping this CO₂ from industrial exhausts to prevent it from entering the atmosphere Environment, Shankar IAS Academy, Mitigation Strategies, p.281.

Physically, dry ice is quite different from the ice we put in our drinks. While water ice is unique because it expands and becomes less dense when it freezes (allowing it to float), solid CO₂ is much denser and will sink in water Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148. Below is a quick comparison to help you distinguish between these two 'ices':

Feature	Water Ice (H₂O)	Dry Ice (Solid CO₂)
Phase Change	Melting (Solid to Liquid)	Sublimation (Solid to Gas)
Temperature	0 °C (32 °F)	-78.5 °C (-109.3 °F)
Density	Floats (Less dense than liquid water)	Sinks (Denser than liquid water)
Residue	Leaves liquid water	Leaves no residue (Dry)

Sources: Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Hydrological Cycle (Water Cycle), p.329; Environment, Shankar IAS Academy (ed 10th), Mitigation Strategies, p.281; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148

7. Solving the Original PYQ (exam-level)

This question perfectly bridges your recent study of chemical reactions and the states of matter. To solve Statement 1, you must recall the specific interaction between water and Quicklime (Calcium Oxide). As highlighted in Science, Class X (NCERT), this is a classic combination reaction that produces Slaked Lime (Calcium Hydroxide) and significant heat, but it does not release Carbon Dioxide. In fact, the industrial production of CO₂ occurs during the reverse logic: the thermal decomposition (calcination) of limestone to create lime. Recognizing this distinction is key to identifying that the first statement is a conceptual reversal of industrial facts.

Moving to Statement 2, the reasoning shifts to the physical properties of Dry Ice. You've learned that certain substances undergo sublimation, transitioning directly from solid to gas. Because solid CO₂ does this at very low temperatures without leaving a liquid residue, it acts as an ideal refrigerant for transporting perishables. This makes Statement 2 factually correct. Therefore, by methodically verifying the chemical products in the first instance and the physical applications in the second, we arrive at the Correct Answer: (B) 2 only.

The common trap UPSC sets here is the "related terminology" snare. In Statement 1, the examiner uses familiar terms like "lime," "water," and "large scale" to mimic the description of an industrial process, hoping you will conflate slaking with calcination. Option (C) is often chosen by students who remember that CO₂ is involved in the lime cycle but forget the direction of the chemical equation. Always double-check if the reaction described actually yields the gas mentioned before committing to the answer.