A student heated some sulphur in a spatula and collected the gas ‘X’. Which one among the following is correct about ‘X’ ?

1. Chemical Properties of Metals and Non-Metals (basic)

To understand the chemistry of the world around us, we must start with elements—the fundamental building blocks of matter that cannot be broken down into simpler substances Science-Class VII NCERT (2025), The World of Metals and Non-metals, p.53. While there are 118 known elements, we group them into two primary categories based on their behavior: Metals and Non-metals. While physical traits like shine (lustre) or conductivity are helpful, their chemical properties—how they react with other substances—provide a much deeper classification Science, Class X NCERT (2025), Metals and Non-metals, p.40.

One of the most defining chemical tests is how an element reacts with Oxygen. When elements burn or oxidize, they form "oxides." The nature of these oxides acts as a chemical signature. Metals typically react with oxygen to produce metal oxides, which are basic in nature. Conversely, non-metals react with oxygen to produce oxides that are acidic in nature Science-Class VII NCERT (2025), The World of Metals and Non-metals, p.54. This means if you dissolve a non-metal oxide in water and test it with litmus paper, it will behave like an acid.

Feature	Metals	Non-Metals
Reaction with Oxygen	Form Basic Oxides	Form Acidic (or Neutral) Oxides
Reaction with Water	Often react (releasing H₂)	Generally do not react with water
Litmus Test (of oxide)	Turns Red Litmus Blue	Turns Blue Litmus Red

Another key distinction lies in their interaction with water. While many metals react with water (sometimes vigorously, like Sodium), most non-metals do not react with water at all Science-Class VII NCERT (2025), The World of Metals and Non-metals, p.54. This is why some very reactive non-metals, like Phosphorus, are actually stored in water to prevent them from contacting the oxygen in the air!

Sources: Science-Class VII NCERT (Revised ed 2025), The World of Metals and Non-metals, p.53; Science-Class VII NCERT (Revised ed 2025), The World of Metals and Non-metals, p.54; Science, Class X NCERT (2025 ed.), Metals and Non-metals, p.40

2. Nature of Oxides: Acidic, Basic, and Amphoteric (basic)

When we look at the chemistry of the world around us, one of the most fundamental interactions is between an element and oxygen. This process, known as oxidation, results in the formation of oxides. However, not all oxides behave the same way. Their chemical nature—whether they act as an acid, a base, or both—depends largely on whether the parent element is a metal or a non-metal.

As a general rule of thumb, metals produce basic oxides. For instance, when you heat copper in the air, it reacts with oxygen to form Copper(II) oxide (2Cu + O₂ → 2CuO), which is basic in nature Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41. On the flip side, non-metals typically produce acidic oxides when they dissolve in water Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40. A classic example is Sulphur; when burnt, it forms Sulphur dioxide (SO₂). When this gas meets moisture, it creates sulphurous acid, which would turn blue litmus paper red.

Interestingly, some oxides refuse to pick a side. These are known as amphoteric oxides. These unique substances, such as Aluminum oxide (Al₂O₃) and Zinc oxide (ZnO), exhibit both acidic and basic properties. They are capable of reacting with both acids and bases to produce salt and water Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41. This versatility is a key concept in understanding chemical reactivity.

Type of Oxide	Parent Element	Nature/Reaction	Examples
Basic Oxide	Metals	Reacts with acids to form salt and water.	Na₂O, CuO, MgO
Acidic Oxide	Non-metals	Reacts with water to form acids.	SO₂, CO₂, NO₂
Amphoteric Oxide	Certain Metals	Reacts with both acids and bases.	Al₂O₃, ZnO

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.55

3. Chemical Indicators and the pH Scale (basic)

In chemistry, we cannot always rely on taste or touch to identify a substance—doing so could be dangerous! Instead, we use indicators. These are special substances that change their color when added to an acidic or basic solution. Think of them as chemical 'signposts' that tell us the nature of the environment they are in. These indicators can be naturally occurring, such as litmus (extracted from lichens), turmeric, or red cabbage leaves, or they can be synthesized in a lab, like phenolphthalein and methyl orange Science Class X, Acids, Bases and Salts, p.17.

The most commonly used indicator is litmus. In its neutral state, litmus solution is purple. However, it reacts distinctly to the presence of hydrogen ions (acids) or hydroxide ions (bases): acids turn blue litmus red, while bases turn red litmus blue Science Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19. Another fascinating indicator is phenolphthalein; it remains colorless in acidic solutions but turns a vibrant pink in basic solutions. Interestingly, this change is reversible—if you add enough acid to a pink basic solution to neutralize it, the pink color will disappear Science Class X, Acids, Bases and Salts, p.21.

A critical nuance to remember for your exams is that moisture is essential for most indicators to work. For example, a dry acidic gas will not change the color of dry litmus paper. This is because the acidic properties only manifest when the gas dissolves in water to release ions. Without that thin film of moisture, the chemical 'handshake' between the indicator and the substance cannot happen.

Indicator	Color in Acid	Color in Base
Blue Litmus	Red	No Change (remains Blue)
Red Litmus	No Change (remains Red)	Blue
Phenolphthalein	Colorless	Pink
Turmeric	No Change (Yellow)	Reddish-Brown

Sources: Science Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.17; Science Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21; Science Class VII (NCERT 2025 ed.), Exploring Substances: Acidic, Basic, and Neutral, p.19

4. Environmental Impact: Acid Rain and Atmospheric Chemistry (intermediate)

To understand the chemistry of acid rain, we must start with the fundamental nature of non-metallic oxides. When fossil fuels containing sulphur or nitrogen are burned, these elements react with atmospheric oxygen to form gases like Sulphur Dioxide (SO₂) and Nitrogen Oxides (NOₓ). These are the primary precursors to acid rain Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.8.

In the atmosphere, these gases don't remain isolated. They undergo a transformation driven by photo-oxidants like ozone (O₃) and sunlight, which facilitate their oxidation into more potent forms like Sulphuric acid (H₂SO₄) and Nitric acid (HNO₃) Environment, Shankar IAS Academy, Environmental Pollution, p.103. The transition from a gas to an acid occurs through the following basic chemical principles:

• Combustion: S + O₂ → SO₂
• Acid Formation: SO₂ + H₂O → H₂SO₃ (Sulphurous acid)
• Oxidation: Further reaction with oxygen/oxidants leads to H₂SO₄ (Sulphuric acid).

A crucial detail for any aspiring civil servant to remember is the role of moisture. Acids only exhibit their acidic properties (like turning blue litmus red) when they are in an aqueous state. Without water, the SO₂ gas cannot dissociate into Hydrogen ions (H⁺). This is why dry litmus paper remains unchanged when exposed to dry SO₂ gas; the chemical "activation" requires moisture to bridge the gap between a gas and an acid.

Pollutant Oxide	Resulting Acid	Primary Source
Sulphur Dioxide (SO₂)	Sulphurous/Sulphuric Acid	Coal combustion, Smelting
Nitrogen Oxides (NOₓ)	Nitric Acid	Vehicle exhaust, Fertilizers

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.8; Environment, Shankar IAS Academy, Environmental Pollution, p.103

5. Industrial Applications and Pollutants (intermediate)

In both industrial processes and environmental science, understanding how elements react with oxygen is fundamental. When we burn elements, they form oxides, but the nature of these oxides depends entirely on whether the element is a metal or a non-metal. Generally, metal oxides (like Magnesium oxide) are basic, while non-metal oxides (like Sulphur dioxide) are acidic. This is why burning sulphur powder produces fumes that, when dissolved in water, turn blue litmus paper red, indicating the formation of an acid Science, Class X, Metals and Non-metals, p.40.

These chemical principles have direct industrial applications. For instance, the soda-acid fire extinguisher utilizes the reaction between an acid (sulphuric acid) and a metal hydrogencarbonate (sodium hydrogencarbonate). This reaction vigorously produces Carbon Dioxide (CO₂), which is used to extinguish fires because it is non-combustible and smothers the flames by cutting off the oxygen supply Science, Class X, Acids, Bases and Salts, p.36.

From an environmental perspective, these substances are classified as pollutants based on how they persist in nature. Primary pollutants are those emitted directly from a source in the form they were created (like SO₂ or CO₂ from factories). Secondary pollutants, however, are formed through chemical reactions between primary pollutants and atmospheric components. For example, when sulphur dioxide (SO₂) reacts with moisture in the air, it forms Sulphurous acid (H₂SO₃), a key component of acid rain Environment, Shankar IAS Academy, Environmental Pollution, p.63.

Type of Oxide	Example	Nature (pH)	Effect on Litmus
Metal Oxide	Magnesium Oxide (MgO)	Basic	Red to Blue
Non-Metal Oxide	Sulphur Dioxide (SO₂)	Acidic	Blue to Red (Moist)

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.36; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.63

6. The Necessity of Moisture for Ionization (exam-level)

In chemistry, the properties we associate with acids and bases—like changing the color of litmus paper or conducting electricity—are not inherent to the molecules in their dry, isolated state. Instead, these properties emerge only when the substance interacts with moisture or water. This is because the acidic behavior of a substance depends on the release of Hydrogen ions (H⁺), and basic behavior depends on the release of Hydroxide ions (OH⁻). In a solid or gaseous state, these ions are often bound within the molecular structure and are not "free" to react with indicators like litmus.

When an acidic gas (like HCl) or a non-metallic oxide (like SO₂) comes into contact with water, a process called ionization or dissociation occurs. Water molecules, being polar, pull the acid molecules apart, allowing them to release H⁺ ions. Since H⁺ ions cannot exist alone, they combine with water molecules to form hydronium ions (H₃O⁺). As noted in Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23, it is these hydronium ions that actually interact with the chemical dyes in litmus paper to trigger a color change. Without water, no H₃O⁺ is formed, and the litmus remains unchanged even if the gas is present in high concentrations.

State	Presence of Ions	Effect on Dry Litmus
Dry Gas (e.g., HCl)	No free H⁺ ions	No color change
Moist/Aqueous Gas	Free H₃O⁺ ions formed	Blue litmus turns Red

This principle is equally true for bases. A solid crystal of Sodium Hydroxide (NaOH) will not show basic properties until it is dissolved in water to generate free-moving OH⁻ ions Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25. This is why, in laboratory experiments, if the weather is very humid, scientists use a "drying tube" containing calcium chloride to remove moisture from gases; otherwise, the gas might react prematurely with the moisture in the air and change the results of the experiment.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25

7. Chemistry of Sulphur Dioxide (SO₂) (exam-level)

When sulphur—a non-metal—is heated in the presence of atmospheric oxygen, it undergoes a combustion reaction to form Sulphur Dioxide (SO₂) gas. This is a classic example of an oxidation reaction because the sulphur atom gains oxygen to form a new compound Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12. The chemical equation for this process is: S + O₂ → SO₂.

To understand the chemical nature of SO₂, we must look at how it interacts with water. In chemistry, most non-metallic oxides are acidic in nature, whereas metallic oxides (like Magnesium Oxide) tend to be basic Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40. When SO₂ gas comes into contact with moisture or water, it reacts to form Sulphurous acid (H₂SO₃):

SO₂ + H₂O → H₂SO₃

This explains why SO₂ turns moist blue litmus paper red. The presence of water is absolutely critical; dry SO₂ gas will not change the color of dry litmus paper because the gas must first dissolve and react to release the hydrogen ions (H⁺) that characterize an acid. In a broader environmental context, this same principle explains acid rain, where atmospheric SO₂ dissolves in rainwater and is carried back to the earth as a weak acid, impacting soil and aquatic ecosystems Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.21.

Oxide Type	Example	Litmus Test Result	Nature
Metallic Oxide	MgO	Red turns Blue	Basic
Non-Metallic Oxide	SO₂	Blue turns Red (Moist)	Acidic

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Environment, Shankar IAS Academy (10th ed.), Functions of an Ecosystem, p.21

8. Solving the Original PYQ (exam-level)

This question perfectly synthesizes three core concepts you have just mastered: the combustion of non-metals, the chemical nature of non-metallic oxides, and the mechanics of pH indicators. When sulfur is heated, it reacts with atmospheric oxygen to produce sulfur dioxide (SO₂). As a non-metallic oxide, SO₂ is inherently acidic. By applying the coaching rule that non-metal oxides form acids when dissolved in water, you can deduce that the gas will react with moisture to form sulphurous acid. This logic is a direct application of the fundamentals found in NCERT Class 8 Science (Materials: Metals and Non-Metals) and NCERT Class 10 Science (Acids, Bases and Salts).

To arrive at the correct answer, Option (C), you must navigate the "moisture trap" that UPSC frequently sets. A dry gas will not change the color of dry litmus because water is essential to liberate the ions that trigger the indicator's color change. Therefore, the gas must be SO₂ and the paper must be moist to turn red. This step-by-step reasoning eliminates Option (D) immediately, as dry litmus remains unchanged even in the presence of an acidic gas.

Finally, sharpen your eye for common UPSC distractor techniques. Option (A) is scientifically incorrect because acidic substances never turn litmus blue—that is a property of bases. Option (B) is a "typographical distractor" (SO,,) designed to test your attention to detail under exam pressure. By sticking to the fundamental principle that non-metal oxides are acidic and require moisture for detection, you can confidently conclude that X is SO₂ and it turns moist litmus to red.