The gas which turns limewater milky is

1. Understanding Chemical Changes and Precipitates (basic)

To begin our journey into chemistry, we must first understand how substances transform. In nature, we categorize changes into two types: physical and chemical. A physical change is like freezing water into ice—the substance is still water, just in a different state Science-Class VII, Chapter 5, p.59. However, a chemical change is a more profound transformation where entirely new substances are formed through a chemical reaction Science-Class VII, Chapter 5, p.68. One of the most fascinating ways to 'see' a chemical change in action is through the formation of a precipitate—an insoluble solid that emerges from a liquid solution during a reaction Science, Class X, Chapter 1, p.12.

A classic example used by scientists to identify gases is the Limewater Test. Limewater is a clear solution of Calcium Hydroxide [Ca(OH)₂]. When we bubble Carbon Dioxide (CO₂) through it, a chemical reaction occurs. The CO₂ reacts with the calcium hydroxide to produce Calcium Carbonate (CaCO₃) and water. Because Calcium Carbonate is insoluble in water, it doesn't dissolve; instead, it hangs in the liquid as tiny white particles, making the solution look 'milky' or cloudy Science, Class X, Chapter 2, p.22. This 'milkiness' is the visual proof that a new substance—the precipitate—has been created.

Feature	Physical Change	Chemical Change
New Substance	No new substance formed.	One or more new substances formed.
Reversibility	Often reversible (e.g., melting ice).	Usually irreversible (e.g., burning wood).
Example	Chopping vegetables Science-Class VII, Chapter 5, p.70.	Formation of a precipitate.

It is important to note that if you continue to bubble Carbon Dioxide into the milky solution for a long time, the milkiness eventually disappears. This happens because the insoluble calcium carbonate reacts further to form calcium bicarbonate, which is soluble in water. However, the initial milky appearance remains the definitive diagnostic test for the presence of Carbon Dioxide gas in a laboratory setting.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 68, 70; Science , class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.12; Science , class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.22

2. Bases and Alkalis: Focus on Calcium Hydroxide (basic)

In our journey through chemistry, few compounds are as versatile as Calcium Hydroxide, commonly known as slaked lime. It is produced through a vigorous combination reaction where water is added to calcium oxide (quick lime). This process is highly exothermic, meaning it releases a significant amount of heat Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6. As a strong base, it plays a critical role in identifying other substances and understanding the nature of chemical oxides.

One of the most famous applications of calcium hydroxide is the limewater test for carbon dioxide (CO₂). When CO₂ gas is bubbled through a clear aqueous solution of calcium hydroxide (limewater), a chemical reaction occurs: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The product, calcium carbonate (CaCO₃), is an insoluble white precipitate that remains suspended in the liquid, giving it a characteristic 'milky' or cloudy appearance Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. This reaction is not just a lab trick; it is why whitewashed walls develop a hard, shiny finish after a few days as the slaked lime reacts slowly with the CO₂ in the air to form a layer of marble-like calcium carbonate.

Beyond identification, this reaction teaches us a fundamental principle about chemical nature. Since calcium hydroxide is a base and it reacts with carbon dioxide to produce a salt (calcium carbonate) and water, we can conclude that non-metallic oxides like CO₂ are acidic in nature Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22. This parallels the classic neutralization reaction where an acid and a base combine to form salt and water.

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 X (NCERT 2025 ed.), Acids, Bases and Salts, p.22

3. Reaction of Non-metallic Oxides with Bases (intermediate)

In our journey through chemical principles, we often look for patterns to simplify how we understand the world. One of the most significant patterns is the reaction of non-metallic oxides with bases. When a non-metallic oxide (like Carbon Dioxide) reacts with a base, it produces a salt and water. If this sounds familiar, it is because this is the exact same outcome as a neutralization reaction between an acid and a base. From this observation, we can logically conclude that non-metallic oxides are acidic in nature Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22.

A classic laboratory example of this is the reaction between Calcium Hydroxide (Ca(OH)₂, also known as limewater) and Carbon Dioxide (CO₂). Calcium hydroxide is a base. When CO₂ gas is bubbled through it, they react to form Calcium Carbonate (CaCO₃) and water. Because calcium carbonate is an insoluble white solid, it remains suspended in the liquid, giving the solution a characteristic 'milky' or cloudy appearance Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. This specific reaction is the standard chemical test used to confirm the presence of Carbon Dioxide.

To help you categorize these substances in your mind, remember that the periodic table gives us a bit of a roadmap. While most metals produce basic oxides, most non-metals produce acidic oxides when they react with oxygen and subsequently dissolve in water Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40. This property is why non-metal oxides like Sulphur Dioxide (SO₂) or Nitrogen Dioxide (NO₂) lead to the formation of acid rain when they interact with atmospheric moisture.

Type of Oxide	Nature	Product with Water/Base
Metal Oxide	Basic	Base / Salt + Water
Non-Metal Oxide	Acidic	Acid / Salt + Water

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40

4. Properties and Risks of Carbon Monoxide and Nitrogen Oxides (intermediate)

To understand atmospheric pollutants, we must look closely at Carbon Monoxide (CO) and Nitrogen Oxides (NOx). Unlike Carbon Dioxide (CO₂), which is a necessary part of the carbon cycle, these gases are primarily byproducts of high-temperature or incomplete combustion and pose immediate risks to both human health and the environment Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.119. Carbon Monoxide (CO) is often called the 'silent killer' because it is colorless, odorless, and tasteless. Its primary danger lies in its chemical affinity for hemoglobin in our blood. CO binds to hemoglobin roughly 200 to 300 times more strongly than oxygen does, forming carboxyhemoglobin. This effectively 'locks' the hemoglobin, preventing it from carrying oxygen to vital organs. While CO₂ is easily identified in a lab because it turns limewater milky (forming calcium carbonate), CO does not react this way, making it harder to detect without specialized sensors Science, Class X NCERT (2025 ed.), Acids, Bases and Salts, p.22. Nitrogen Oxides (NOx), specifically Nitric Oxide (NO) and Nitrogen Dioxide (NO₂), are critical in environmental chemistry. While CO is a health hazard, NOx gases are catalytic destroyers of the environment. In the upper atmosphere, Nitric Oxide reacts with ozone (O₃) to form Nitrogen Dioxide and Oxygen, effectively thinning the ozone layer that protects us from UV radiation Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.269. Furthermore, NO₂ is responsible for the reddish-brown haze often seen over polluted cities and contributes significantly to acid rain.

Gas	Key Property	Major Risk/Effect
Carbon Monoxide (CO)	Colorless, Odorless	High affinity for hemoglobin; causes hypoxia.
Nitrogen Dioxide (NO₂)	Reddish-brown color	Respiratory irritant; major component of smog.
Nitric Oxide (NO)	Highly reactive	Catalytically destroys the ozone layer.
Nitrous Oxide (N₂O)	Potent GHG	GWP is ~300 times higher than CO₂ over 100 years.

Sources: Science, Class VIII NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.119; Science, Class X NCERT (2025 ed.), Acids, Bases and Salts, p.22; Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.269

5. Ammonia and Other Common Industrial Gases (intermediate)

In the industrial world, gases like Ammonia (NH₃) and Carbon Dioxide (CO₂) are not just laboratory subjects; they are the backbones of our agricultural and energy sectors. To master these, we must understand how to identify them and why they behave the way they do. Ammonia is a molecule formed by one nitrogen atom sharing electrons with three hydrogen atoms to achieve a stable configuration Science, class X (NCERT 2025 ed.), Chapter 4, p.60. Chemically, Ammonia is basic in nature. In contrast, gases like CO₂ are non-metallic oxides which generally exhibit acidic properties when dissolved in water.

One of the most critical diagnostic tests in chemistry is the Limewater Test used to identify Carbon Dioxide. When CO₂ is bubbled through limewater—which is a solution of Calcium Hydroxide [Ca(OH)₂]—a chemical reaction occurs that produces Calcium Carbonate (CaCO₃). Because Calcium Carbonate is insoluble in water, it forms a white precipitate that makes the solution appear 'milky' or cloudy Science-Class VII, NCERT (Revised ed 2025), Chapter 5, p.61. It is important to note that if you continue to pass CO₂ through this milky solution, it eventually becomes clear again as soluble calcium bicarbonate forms Science, class X (NCERT 2025 ed.), Chapter 2, p.22. Other industrial gases like Methane (CH₄), the primary component of CNG, or Nitrogen Dioxide do not produce this specific 'milky' reaction with limewater.

From a policy and economic perspective, Ammonia is the primary feedstock for Urea, the most widely used nitrogenous fertilizer in India. The production of Urea is so vital that the government implements specific policies, such as the New Urea Policy 2015, which involves pooling domestic natural gas with imported Liquified Natural Gas (LNG) to ensure a uniform price for urea manufacturing plants Indian Economy, Vivek Singh (7th ed. 2023-24), Subsidies, p.288. Understanding the chemical purity and the reactions of these gases is therefore essential for both industrial efficiency and agricultural stability.

Gas	Chemical Nature	Common Identification Test
Carbon Dioxide (CO₂)	Weakly Acidic	Turns limewater milky (forms CaCO₃)
Ammonia (NH₃)	Basic	Turns moist red litmus paper blue
Methane (CH₄)	Neutral	Combustible; burns with a blue flame

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.61; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22; Indian Economy, Vivek Singh (7th ed. 2023-24), Subsidies, p.288

6. The Lime-Water Test and Calcium Carbonate Formation (exam-level)

In chemistry, identifying gases is a fundamental skill, and the lime-water test is the gold standard for detecting carbon dioxide (CO₂). Limewater is not just water with lime juice; it is a clear, colorless aqueous solution of calcium hydroxide [Ca(OH)₂]. When CO₂ is bubbled through this solution, a chemical reaction occurs that serves as a visual indicator of the gas's presence. Science - Class VII, Changes Around Us, p. 61

The science behind the "milkiness" is straightforward: the carbon dioxide reacts with the calcium hydroxide to produce calcium carbonate (CaCO₃) and water. Calcium carbonate is an insoluble white precipitate. Because it cannot dissolve in water, it forms tiny suspended particles that scatter light, giving the solution its characteristic cloudy or milky appearance. This specific reaction is unique to CO₂; other common gases like oxygen or nitrogen do not produce this insoluble carbonate with limewater. Science, Class VIII, Nature of Matter, p. 119

A critical detail for advanced learners is what happens when you continue to add CO₂. If excess carbon dioxide is passed through the now-milky solution, the milkiness eventually disappears, and the liquid becomes clear again. This happens because the insoluble calcium carbonate reacts further with the additional CO₂ and water to form calcium bicarbonate [Ca(HCO₃)₂]. Unlike the carbonate, calcium bicarbonate is soluble in water, causing the suspended particles to dissolve away. Science, Class X, Acids, Bases and Salts, p. 21

In nature, calcium carbonate is incredibly versatile. It is the primary component of limestone, chalk, and marble. In the environment, it exists in different mineral forms like calcite (found in algae and oyster shells) and aragonite (found in most corals). Understanding this transition between soluble and insoluble states is key to understanding everything from how stalactites form in caves to the impact of ocean acidification on marine life. Environment, Shankar IAS Academy, Ocean Acidification, p. 263

Sources: Science - Class VII, NCERT, Changes Around Us: Physical and Chemical, p.61; Science, Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.119; Science, Class X, NCERT, Acids, Bases and Salts, p.21; Environment, Shankar IAS Academy, Ocean Acidification, p.263

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your recently learned concepts regarding chemical reactions and the properties of non-metallic oxides. You have studied that non-metallic oxides are acidic in nature. In this scenario, carbon dioxide (CO2) acts as the acid reacting with a base, limewater (calcium hydroxide). As detailed in Science, class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts, this specific interaction is a classic example of a salt-forming reaction where the product is calcium carbonate (CaCO3).

To arrive at the correct answer, (A) carbon dioxide, you must visualize the physical state of the products. While many reactions produce salts, the milky appearance occurs specifically because calcium carbonate is insoluble in water, creating a white precipitate that remains suspended. As a coach, I want you to remember a crucial nuance: if CO2 is passed in excess, the milkiness eventually disappears because the precipitate converts into soluble calcium bicarbonate. This secondary step is a favorite topic for UPSC to test your depth of understanding.

UPSC often uses common industrial or atmospheric gases like carbon monoxide, ammonia, and nitrogen dioxide as traps. While nitrogen dioxide is an acidic oxide, it produces a brownish color and doesn't yield an insoluble carbonate. Ammonia is basic, and carbon monoxide is neutral; neither can trigger the specific precipitation reaction required to turn limewater milky. Always look for the unique chemical signature—in this case, the formation of an insoluble carbonate—to distinguish between similar-looking options.