In which one of the following situations a chemical reaction does not occur?

1. Physical vs. Chemical Changes (basic)

In the study of chemistry, matter is constantly undergoing transformations. To understand these, we categorize changes into two primary types: physical and chemical. A physical change occurs when a substance alters its physical properties—such as its shape, size, or state (solid, liquid, gas)—without changing its underlying identity. Crucially, in a physical change, no new substance is formed Science-Class VII, Changes Around Us: Physical and Chemical, p.59. For instance, when you crush a piece of chalk or melt an ice cube (H₂O), the chemical identity remains the same; it is still calcium carbonate or water, just in a different form.

Conversely, a chemical change (or a chemical reaction) involves the transformation of one or more substances into entirely new substances with different chemical properties. This process happens because atoms break old bonds and form new ones Science, class X, Chemical Reactions and Equations, p.6. Common examples include combustion (burning), rusting, and cooking Science-Class VII, Changes Around Us: Physical and Chemical, p.68. For example, when coal burns in air, carbon (C) reacts with oxygen (O₂) to form carbon dioxide (CO₂), a gas with properties completely different from the original solid coal.

It is important to distinguish between a reaction and a simple interaction with the environment. For instance, common salt (NaCl) is a highly stable compound. When exposed to air, it does not undergo a chemical reaction; if it appears damp, it is usually because of a physical process called deliquescence (absorbing moisture from the air), but the salt remains NaCl Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.131.

Feature	Physical Change	Chemical Change
New Substance	None formed.	One or more new substances created.
Nature of Change	Usually reversible (e.g., melting ice).	Usually irreversible (e.g., burning wood).
Properties	Chemical properties remain the same.	New substances have different properties.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59, 68; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6; Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.131

2. Characteristics of Chemical Reactions (basic)

Concept: Characteristics of Chemical Reactions

3. The Reactivity Series of Metals (intermediate)

In the world of chemistry, not all metals are created equal. Some are incredibly energetic, while others are remarkably calm. The Reactivity Series is essentially a "leaderboard" that ranks metals based on how easily they react with other substances, primarily by losing electrons to form positive ions. Metals at the top of this list are highly unstable in their pure form because they are so eager to react, whereas those at the bottom are chemically "noble" and prefer to stay as they are. Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.45

A fundamental way we determine this ranking is through displacement reactions. Imagine a "chemical tug-of-war": if you place a more reactive metal into a solution containing a less reactive metal's salt, the more reactive metal will "kick out" (displace) the weaker one. For example, if you put an iron nail in a blue copper sulfate solution, the iron will displace the copper, turning the solution green as iron sulfate forms. This logic allows scientists to arrange metals in a definitive sequence. Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

Reactivity Level	Metals	Characteristics
High	Potassium (K), Sodium (Na)	React violently with water; must be stored in kerosene to prevent reaction with air.
Medium	Zinc (Zn), Iron (Fe), Lead (Pb)	React with acids; iron reacts slowly with moist air (rusting).
Low	Gold (Au), Silver (Ag), Platinum (Pt)	Do not react easily; often found in nature in their "free state" (pure form).

Understanding this series is vital for metallurgy (the extraction of metals). Metals like Gold, being at the bottom, can be found as pure nuggets in the earth's crust. However, metals at the top, like Sodium or Potassium, are so reactive that they are never found alone in nature; they are always bound in compounds and require high-energy processes like electrolysis to be separated. Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.49

4. Oxidation, Combustion, and Corrosion (intermediate)

At the heart of many chemical transformations is the process of oxidation—a reaction where a substance combines with oxygen or loses electrons. While oxidation is a broad chemical concept, we encounter it most frequently in two distinct forms: the rapid, energy-releasing process of combustion and the slow, destructive process of corrosion. Understanding these allows us to distinguish between stable substances that resist change and reactive ones that transform when exposed to the environment.

Combustion is a high-temperature oxidation reaction where a combustible substance (like coal, wood, or magnesium) reacts with oxygen to release heat and light. For instance, when coal burns, carbon (C) combines with oxygen (O₂) to form carbon dioxide (CO₂). Similarly, burning magnesium in air is a classic chemical reaction that produces magnesium oxide Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3. For combustion to occur, oxygen is essential; without it, the flame is extinguished Science-Class VII (NCERT 2025), Changes Around Us, p.62.

Corrosion, on the other hand, is the gradual deterioration of metals due to their interaction with air and moisture. The most common example is the rusting of iron, where iron reacts with oxygen and water to form a new brown substance called iron oxide (rust) Science-Class VII (NCERT 2025), Changes Around Us, p.62. Other metals also corrode: copper develops a green coating, and silver turns black over time Science-Class VII (NCERT 2025), The World of Metals and Non-metals, p.50. To protect against this, we use methods like galvanisation—coating iron with a thin layer of zinc—to prevent the underlying metal from reacting with the atmosphere Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Interestingly, not everything reacts when exposed to air. Common salt (sodium chloride) is a highly stable ionic compound. Unlike iron or sodium, NaCl does not undergo a chemical reaction when left open. If it appears to become wet or "sticky," it is usually a physical change called deliquescence (absorbing moisture due to impurities), but the chemical identity of the salt remains unchanged.

Feature	Combustion	Corrosion (Rusting)
Speed	Very Rapid	Slow and Gradual
Energy Release	Significant heat and light	Negligible heat (hardly detectable)
Conditions	Requires ignition temperature	Requires exposure to air and moisture

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3; Science-Class VII (NCERT 2025), Changes Around Us: Physical and Chemical, p.62; Science-Class VII (NCERT 2025), The World of Metals and Non-metals, p.50; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54

5. Ionic Compounds and Chemical Stability (intermediate)

To understand why certain substances are incredibly stable while others react violently, we must look at the ionic bond. In nature, atoms strive for stability, which they often achieve by having a completely filled outermost shell. Take Sodium Chloride (NaCl) as the perfect example: a sodium atom loses one electron to become a positive cation (Na⁺), while a chlorine atom gains that electron to become a negative anion (Cl⁻). As explained in Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.47, these oppositely charged ions don't just sit near each other; they are held together by strong electrostatic forces of attraction. This bond is so powerful that sodium chloride doesn't exist as simple individual molecules, but as massive, organized aggregates of ions. This internal strength translates into remarkable physical and chemical stability. Because the inter-ionic attraction is so robust, it requires a massive amount of thermal energy to break them apart. This is why ionic compounds typically have high melting and boiling points—for instance, NaCl melts at a staggering 1074 K Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.48-49. While they are hard and brittle, their chemical identity is "locked in." Comparing chemical behaviors helps illustrate this stability. While coal (carbon) reacts with oxygen to form CO₂ or iron reacts with moist air to form rust, common salt remains chemically unchanged when exposed to air. Even though the constituent elements—sodium (a reactive metal) and chlorine (a toxic gas)—are dangerous on their own, their combination results in a harmless, stable compound essential for life Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.124. Any change you see in salt when it's humid, such as it becoming "sticky," is usually just a physical process called deliquescence caused by impurities, not a chemical reaction of the NaCl itself.

Common Properties of Ionic Compounds:

Property	Description	Reason
Physical State	Solid and hard	Strong force of attraction between ions.
Melting Point	Very High	Requires significant energy to break ionic bonds.
Solubility	Soluble in Water	Water molecules can interact with and pull ions apart.
Conductivity	High (Molten/Solution)	Charged particles (ions) are free to move.

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.47-49; Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.124

6. Hygroscopy and Deliquescence in Air (exam-level)

In the study of chemistry and environmental science, we often encounter substances that have a unique "affinity" for water. Hygroscopy is the physical property of a substance that allows it to absorb or adsorb water molecules from the surrounding environment. While many materials (like wood or silica gel) are hygroscopic because they take in moisture, they generally maintain their solid form. However, when this process goes a step further, we call it deliquescence. A deliquescent substance is so hungry for moisture that it absorbs enough water vapor from the air to eventually dissolve completely and form a liquid solution.

A classic example that aspirants often find confusing is common salt (sodium chloride, NaCl). In its chemically pure form, sodium chloride is a highly stable ionic compound that does not react with the oxygen or nitrogen in the air, nor is it naturally deliquescent Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29. However, the salt we use in our kitchens often becomes sticky or "wet" during the monsoon. This is not because of a chemical reaction, but due to the presence of impurities like Magnesium Chloride (MgCl₂) and Calcium Chloride (CaCl₂). These impurities, which are naturally found in sea-derived salts, are highly deliquescent and pull water from the atmosphere Physical Geography by PMF IAS, Ocean temperature and salinity, p.518.

Feature	Hygroscopy	Deliquescence
Definition	Absorbs moisture without necessarily changing state.	Absorbs so much moisture that it dissolves into a solution.
Physical State	Remains a solid (may feel damp).	Changes from solid to liquid solution.
Common Example	Silica gel, Honey.	Magnesium Chloride, Sodium Hydroxide.

It is crucial to distinguish these physical changes from chemical reactions. When coal burns, it reacts with oxygen to form CO₂; when iron rusts, it reacts with oxygen and moisture to form iron oxide Science - Class VII, Chapter 4: The World of Metals and Non-metals, p.52. In contrast, when salt absorbs water, no new chemical bond is formed between the NaCl and the air; it is simply a physical phase change driven by the presence of thirsty impurities.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Physical Geography by PMF IAS, Ocean temperature and salinity, p.518; Science - Class VII, Chapter 4: The World of Metals and Non-metals, p.52

7. Solving the Original PYQ (exam-level)

This question brings together your understanding of chemical stability, the reactivity series, and the fundamental distinction between physical and chemical changes. To solve this, you must recall that a chemical reaction necessitates the formation of a new substance with a different molecular structure. As you have learned, Sodium Chloride (NaCl) is a highly stable ionic compound with a high lattice energy. When Common salt is exposed to air, it does not react with oxygen or nitrogen; at most, it may undergo deliquescence—a physical process where it absorbs water vapor due to impurities—but the NaCl molecules remain chemically identical. This is why it stands out as the only scenario where no chemical change occurs, as supported by Science, Class VIII, NCERT (Revised ed 2025).

To navigate the other options, you must apply the logic of reactivity and oxidation, which are common themes in UPSC science questions. In option (B), the burning of coal is a classic combustion reaction where carbon combines with oxygen to form Carbon Dioxide. Option (C) tests your knowledge of the reactivity series; as detailed in Science, Class X, NCERT (2025 ed.), sodium is so reactive that it undergoes a violent exothermic reaction with water to produce Sodium Hydroxide. Finally, option (D) refers to rusting, a redox reaction where iron is oxidized in the presence of moisture to form iron oxide. UPSC often uses "exposure to air" as a distractor because while it causes chemical reactions in metals (like iron or sodium), it only causes a physical change in stable salts like Common salt. By identifying the stability of the compound, you can systematically eliminate the reactive traps.