Which one among the following statements is correct ?

1. States of Matter and Phase Transitions (basic)

Welcome to your first step in mastering the chemical principles that govern our world! To understand matter, we must first look at its particulate nature. Everything around us is made of tiny particles held together by attractive forces. The physical state of a substance—whether it is a solid, liquid, or gas—is a tug-of-war between two factors: the strength of these attractive forces and the thermal energy of the particles Science, Class VIII, Particulate Nature of Matter, p.112. In a solid like ice, thermal energy is low, so particles stay close in a rigid structure. As we add heat, particles vibrate faster until they eventually break free from their fixed positions, turning the solid into a liquid.

One of the most fascinating aspects of this process is what happens to the temperature. You might expect that if you keep heating a pot of melting ice, the temperature would steadily rise. However, during a phase transition (like melting or boiling), the temperature remains constant Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. This happens because the added heat is being used as Latent Heat—energy spent entirely on overcoming the attractive forces between particles rather than raising the temperature Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. For instance, while ice is melting into water, both the ice and the resulting liquid stay at 0°C until the transition is complete.

Finally, we must distinguish between physical and chemical changes. A physical change, such as the melting of ice or the grinding of chalk, does not create a new substance; the chemical identity (H₂O) remains the same and the process is often reversible Science, Class VII, Changes Around Us, p.65, 68. In contrast, a chemical change involves a reaction that produces entirely new substances. A classic example is a burning candle: while the wax melting is a physical change, the actual burning (combustion) of the wax vapor reacts with oxygen to produce CO₂ and water vapor, making it a chemical change Science, Class VII, Changes Around Us, p.62.

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.112; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294-295; Science, Class VII (NCERT), Changes Around Us: Physical and Chemical, p.62-68

2. Characteristics of Physical Changes (basic)

In our journey to understand matter, the first distinction we must master is the physical change. Think of a physical change as a change in the personality of a substance without changing its identity. When a substance undergoes a physical change, its physical properties—such as shape, size, color, or state (solid, liquid, gas)—might transform, but the internal chemical structure remains exactly the same Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.59.

The golden rule for identifying a physical change is this: no new substance is formed. Consider water as an example. Whether it is ice (solid), water (liquid), or steam (gas), the molecules remain H₂O. Melting ice or boiling water are classic physical changes because they only involve a change of state Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68. Similarly, natural processes like the weathering of rocks or soil erosion caused by wind and water are physical changes because they break down larger structures into smaller ones without altering the basic minerals involved Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68.

A common point of confusion is reversibility. While many physical changes are reversible—like freezing melted ice back into a solid—some are not. For instance, if you chop vegetables or peel a potato, the shape and size have changed (a physical change), but you cannot "un-chop" them to get the original whole vegetable back Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.66, 70. However, because the vegetable material itself hasn't turned into a different chemical substance, it remains a physical change.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 66, 68, 70

3. Fundamentals of Chemical Changes (basic)

4. Combustion and Oxidation Processes (intermediate)

To understand combustion, we must first view it as a specific type of chemical reaction called oxidation. At its core, combustion is a process where a substance (the fuel) reacts with oxygen to release energy in the form of heat and, usually, light. This is fundamentally a chemical change because the original substances are transformed into entirely new chemical entities with different properties Science-Class VII, Chapter 5, p. 65. For instance, when we burn a candle, the wax (a hydrocarbon) doesn't just disappear; it reacts with oxygen to produce carbon dioxide (CO₂) and water vapor (H₂O). While the melting of wax is a physical change (reversible), the actual burning of the wick and vaporized wax is an irreversible chemical process Science-Class VII, Chapter 5, p. 65.

For combustion to occur, three specific factors must align, often referred to as the "Fire Triangle":

• Combustible Substance (Fuel): Materials like wood, kerosene, or carbon-based compounds Science-Class VII, Chapter 5, p. 62.
• Oxidizer: Usually oxygen from the air. Without a continuous supply of oxygen, the reaction ceases Science-Class VII, Chapter 5, p. 62.
• Ignition Temperature: The minimum temperature at which a substance catches fire. A piece of paper won't spontaneously ignite in air unless heated to this threshold Science-Class VII, Chapter 5, p. 63.

However, oxidation is a much broader category than just combustion. While all combustion is oxidation, not all oxidation involves fire. For example, some metals react with oxygen at room temperature. Sodium and Potassium react so vigorously that they must be stored under kerosene to prevent them from catching fire Science-Class X, Chapter 3, p. 42. Other metals, like Magnesium or Aluminium, undergo a "quiet" oxidation where a thin, protective layer of oxide forms on their surface, actually shielding the metal from further reaction Science-Class X, Chapter 3, p. 42. Understanding this spectrum—from the slow tarnishing of metal to the explosive burning of fuel—is essential for mastering chemical principles.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.62, 63, 65; Science , class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.69; Science , class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.42

5. Chemical Changes in Environment: Rusting and Corrosion (intermediate)

In our study of chemical changes, corrosion stands out as one of the most visible and economically significant processes. At its heart, corrosion is a chemical reaction where a metal is attacked by substances in its environment—primarily oxygen, moisture, and sometimes acids. This process results in the gradual deterioration of the metal surface Science-Class VII, Chapter 4, p.50.

While "corrosion" is the general term for this deterioration, rusting refers specifically to the corrosion of iron. When iron is exposed to both oxygen and water (moisture), it reacts to form a new brown-colored substance called iron oxide (Fe₂O₃·xH₂O). Because a new substance with entirely different properties is formed, rusting is classified as a chemical change Science-Class VII, Chapter 5, p.62. Interestingly, not all metals react the same way. For example, silver develops a black coating due to sulfur in the air, and copper develops a characteristic green coating when exposed to moist air Science-Class VII, Chapter 4, p.50.

One of the most fascinating aspects of corrosion is how some metals use it for self-protection. Take aluminium, for instance. When exposed to air, it naturally develops a thin, tough layer of aluminium oxide. Unlike rust on iron, which is porous and allows the metal underneath to keep rotting, this aluminium oxide layer acts as a shield, preventing further corrosion. We can even artificially thicken this protective layer through a process called anodising Science, Class X, Chapter 3, p.42.

Sources: Science-Class VII . NCERT(Revised ed 2025), The World of Metals and Non-metals, p.50; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.62; Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.54

6. Simultaneous Physical and Chemical Changes (intermediate)

In our study of matter, we often categorize changes as either physical or chemical. However, nature rarely operates in such isolated compartments. In many complex processes, both changes occur simultaneously. A physical change typically involves a change in the state or appearance without altering the molecular identity—like ice melting into water—whereas a chemical change results in the formation of entirely new substances with different properties Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p. 65.

The most classic example of this duality is the burning of a candle. When you light a candle, the heat of the flame causes the solid wax to melt into a liquid. This melting is a physical change because the wax is still wax; it has merely changed its state from solid to liquid. Some of this molten wax then travels up the wick and evaporates into a vapor—another physical change. However, once that wax vapor reacts with the oxygen in the air, a combustion reaction occurs. This reaction produces carbon dioxide (CO₂) and water vapor (H₂O), while releasing energy as heat and light Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p. 62-63. Because new chemical substances are formed, this part of the process is a chemical change.

To verify that a chemical change has occurred during combustion, we can test for the presence of the new substances. For instance, if you capture the gas from a burning candle and pass it through lime water, the water turns milky, confirming the presence of CO₂ Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p. 63. Understanding that these changes happen together is crucial for a deeper grasp of thermodynamics and material science.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.62-65

7. Solving the Original PYQ (exam-level)

You have just mastered the building blocks of matter, specifically the distinction between physical and chemical changes. This UPSC question is a classic application of those concepts. A physical change, like the melting of ice, involves a change in state or appearance without altering the substance's chemical identity, whereas a chemical change, such as burning, results in the formation of entirely new substances with different properties. By recognizing these characteristics, you can see how the fundamental definitions you learned directly dictate the behavior of everyday objects.

To arrive at the correct answer, think like a scientist: ask if the process is reversible and if a new substance is born. When ice melts, it remains water (H₂O) and can be easily refrozen; thus, it is a physical change. However, as highlighted in Science-Class VII . NCERT(Revised ed 2025), the burning of a candle involves combustion, where wax vapor reacts with oxygen to produce carbon dioxide and water vapor. Because these are new chemical products that cannot be reverted to wax, (B) Melting of ice is a physical change and burning of candle is a chemical change is the only logically sound choice.

UPSC often includes traps like Options (A) and (D) to see if you will oversimplify the processes. They hope you will focus only on the melting aspect of the candle wax and ignore the actual burning of the wick and vapor. Option (C) is a simple reversal trap intended to catch candidates who are rushing. The key to avoiding these traps is to always check for the "new substance" criteria—ice stays water, but a burnt candle produces heat, light, and gases that represent a permanent chemical transformation.