Which one among the following is not a chemical change?

1. Matter and its Physical States (basic)

Welcome to your first step in understanding the material world! To master chemistry, we must first look at Matter—which is simply anything that has mass and occupies space. Rather than being a continuous block, matter is made of tiny particles that are constantly in motion. The way these particles are arranged determines whether a substance is a solid, a liquid, or a gas.

The state of matter is a balance between interparticle forces (which pull particles together) and kinetic energy (which makes them move apart). In solids, the forces are so strong that particles are closely packed in fixed positions, only vibrating in place. This explains why solids have a definite shape and volume Science Class VIII, Particulate Nature of Matter, p.113. When you heat a solid, you provide energy to these particles. If they gain enough energy to overcome some of these attractive forces, the solid melts into a liquid.

In a liquid, particles are still close but have enough energy to slide past one another. This is why liquids can flow and take the shape of their container while maintaining a fixed volume. If you continue adding heat, the particles gain even more energy until they break free entirely, entering the gaseous state. Here, interparticle spaces are very large, and particles move freely in all directions Science Class VIII, Particulate Nature of Matter, p.112.

Feature	Solid	Liquid	Gas
Interparticle Space	Very small	Small (but larger than solids)	Very large
Movement	Fixed (vibrate only)	Move past each other	Free and rapid
Forces of Attraction	Very strong	Moderate	Negligible

It is important to remember that changing between these states—like evaporation or melting—is a physical change. The particles themselves do not change; H₂O is still H₂O whether it is ice, liquid water, or steam Science Class VII, Changes Around Us, p.68. We are only changing the physical arrangement and energy of those particles.

Sources: Science Class VIII, Particulate Nature of Matter, p.113; Science Class VIII, Particulate Nature of Matter, p.112; Science Class VII, Changes Around Us, p.68

2. Understanding Physical Changes (basic)

In our daily lives, we observe matter changing all the time—water boils, ice melts, and wood is chopped. To master chemistry, the first step is distinguishing between a physical change and a chemical change. A physical change is a process where a substance undergoes a transformation in its physical properties such as shape, size, color, or state (solid, liquid, or gas), but crucially, no new substance is formed Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.59.

Think of it this way: the identity of the molecules remains exactly the same. For example, when water (H₂O) evaporates into steam, it is still H₂O; the molecules are just moving faster and are further apart. Because the chemical identity is preserved, many physical changes are reversible. You can freeze liquid water into ice and then melt that ice back into water Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.66. However, keep in mind that not all physical changes are easily reversed—tearing a piece of paper is a physical change because it is still paper, but you cannot easily "un-tear" it.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are created.
Properties Affected	Changes in shape, size, or state.	Changes in chemical composition and identity.
Examples	Melting ice, breaking glass, evaporation.	Burning coal, curdling milk, rusting iron.

In the context of the environment, these changes happen on a massive scale. For instance, the weathering of rocks involves both physical and chemical processes. When flowing water or wind causes erosion, it is primarily a physical change as the rock is broken into smaller pieces to eventually form soil Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68.

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

3. Core Principles of Chemical Changes (basic)

At its heart, a chemical change is a process where the fundamental identity of a substance is transformed. Unlike a physical change—where a substance might change its shape or state (like ice melting into water)—a chemical change results in the formation of one or more new substances with entirely different properties than the original Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 68. This process is driven by a chemical reaction, where atoms are rearranged to create new molecular structures.

How do we identify a chemical change in the real world? Often, these changes are accompanied by observable signs: the evolution of a gas (bubbles), a distinct change in color, or the production of heat and light. For instance, when magnesium burns in air, it reacts with oxygen to produce a white powder called magnesium oxide (MgO). This is a chemical change because the powder is a completely new substance compared to the original metal Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 70. Similarly, rusting—the formation of brown deposits on iron—occurs when iron reacts with oxygen and moisture to form iron oxide, a change that cannot be easily reversed to get the original iron back.

One of the most common types of chemical changes is combustion. This is a reaction where a substance reacts with oxygen to release energy in the form of heat and light Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 68. Whether it is coal burning in a power plant or the metabolic "burning" of glucose in our cells (respiration), the essence remains the same: the starting materials are chemically converted into new products like CO₂ and H₂O.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Nature of Change	Changes in physical properties (size, shape, state).	Changes in chemical composition and identity.
Reversibility	Often reversible (e.g., freezing water).	Usually irreversible (e.g., cooking an egg).
Examples	Evaporation, tearing paper, melting wax.	Burning wood, curdling of milk, rusting.

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

4. Thermal Phenomena: Latent Heat and Evaporation (intermediate)

When we heat a substance, we typically expect its temperature to rise. However, during a phase change—such as ice melting into water or water turning into steam—the temperature remains stubbornly constant despite the continuous addition of heat. This "hidden" energy is known as Latent Heat. It is the energy absorbed or released by a substance to change its physical state without changing its temperature Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. For instance, as long as ice is melting, the thermometer will stay at 0 °C because the energy is being used to break the bonds between molecules rather than increasing their kinetic energy.

There are two primary types of latent heat we encounter in the physical world:

• Latent Heat of Fusion: The heat required to change a solid to a liquid (or released when a liquid freezes).
• Latent Heat of Vaporization: The heat absorbed when a liquid turns into a gas. This energy is carried away by the escaping vapor molecules Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

It is crucial to distinguish between boiling and evaporation. While boiling is a rapid process occurring throughout the bulk of the liquid at a specific temperature, evaporation is a slower, surface-level process that occurs at all temperatures Science, Class VIII NCERT, Particulate Nature of Matter, p.105. In the atmosphere, when water vapor condenses back into raindrops, it releases this stored energy as Latent Heat of Condensation. This release of heat is a vital driver of weather patterns; for example, it causes rising saturated air parcels to cool more slowly than dry air, fueling the energy of storms and monsoons Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299.

Furthermore, the state of matter is not just dependent on temperature but also on ambient pressure. We often think of water boiling only at 100 °C, but if you reduce the pressure (like on a high mountain or in a vacuum), water can boil at much lower temperatures—even room temperature. Conversely, under high pressure, water can remain liquid even at 230 °C, as was the case in Earth's early history Physical Geography by PMF IAS, Geological Time Scale The Evolution of The Earths Surface, p.43.

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.299; Science, Class VIII NCERT, Particulate Nature of Matter, p.105; Physical Geography by PMF IAS, Geological Time Scale The Evolution of The Earths Surface, p.43

5. Oxidation, Combustion, and Energy Changes (intermediate)

In our study of chemical principles, understanding how substances interact with oxygen and how energy flows during these interactions is crucial. At the heart of many daily phenomena is Oxidation. In its simplest form, oxidation occurs when a substance gains oxygen or loses hydrogen during a reaction Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14. When one substance is oxidized, another is typically reduced (loses oxygen or gains hydrogen), creating a Redox reaction. A classic example is the reaction of copper(II) oxide with hydrogen, where copper loses oxygen and hydrogen gains it Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12.

Combustion is a specific, rapid type of oxidation reaction. For combustion to occur, a combustible substance (like wood, kerosene, or coal) must react with oxygen, which acts as a supporter of combustion. This process is a definitive chemical change because it results in the formation of entirely new substances, such as carbon dioxide and water vapor, while releasing significant energy Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62. Unlike physical changes—such as the evaporation of water where the H₂O molecule remains unchanged—combustion permanently alters the chemical identity of the fuel.

This brings us to the concept of energy changes: Exothermic and Endothermic reactions. In an exothermic reaction, heat is released into the surroundings along with the formation of products Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7. A vital biological example of this is Respiration. During digestion, carbohydrates are broken down into glucose; this glucose then combines with oxygen in our cells to provide the energy we need to stay alive. Because energy is released during this process, respiration is classified as an exothermic reaction. Conversely, endothermic reactions are those that absorb energy from the surroundings to proceed.

Reaction Type	Energy Flow	Common Examples
Exothermic	Heat is released	Combustion of coal, Respiration, Burning of natural gas
Endothermic	Energy is absorbed	Decomposition of calcium carbonate, Photosynthesis

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7

6. Biochemical Changes in Food and Nature (exam-level)

To understand the world around us, we must distinguish between changes that are merely 'skin-deep' and those that alter the very soul of a substance. A physical change involves a transition in physical properties—like shape, size, or state (liquid to gas)—without creating a new chemical substance. For instance, the evaporation of water or the transpiration of moisture through the leathery leaves of Mediterranean citrus trees are physical changes because the water remains H₂O throughout. In contrast, a chemical change (or biochemical change when occurring in living systems) results in the formation of entirely new substances with different properties. Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p. 68 Take the example of milk turning into curd. This is a classic biochemical transformation. The bacterium Lactobacillus feeds on the milk sugar (lactose) and ferments it into lactic acid. This acid not only changes the taste (making it sour) but also alters the milk's structure. Because fresh milk has a pH of approximately 6, the production of lactic acid lowers the pH, making it more acidic. This explains why a milkman might add a pinch of alkaline baking soda to fresh milk—it shifts the pH to be slightly more basic, thereby delaying the time it takes for the milk to sour and set as curd. Science, Class VIII, The Invisible Living World, p. 22 and Science, class X, Acids, Bases and Salts, p. 35 In nature, these chemical shifts are often driven by hormones or environmental factors. Fruit ripening is a complex biochemical process where the ovary of a flower matures, changing its chemical composition, flavor, and texture. Science, class X, How do Organisms Reproduce?, p. 121 However, external pollutants can interfere with these natural 'chemistries.' For example, high levels of Sulphur dioxide (SO₂) can cause metabolic inhibition and chlorosis (loss of chlorophyll), while hydrocarbons like ethylene can trigger premature leaf fall or fruit drop. These are not just physical damages; they are disruptions of the plant's internal chemical balance. Environment, Shankar IAS Academy, Environmental Pollution, p. 69

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 coal).
Examples	Evaporation, cutting paper, stretching a rubber band.	Curdling of milk, fruit ripening, digestion of food.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.68; Science ,Class VIII . NCERT(Revised ed 2025), The Invisible Living World: Beyond Our Naked Eye, p.22; Science , class X (NCERT 2025 ed.), Acids, Bases and Salts, p.35; Science , class X (NCERT 2025 ed.), How do Organisms Reproduce?, p.121; Environment, Shankar IAS Academy .(ed 10th), Environmental Pollution, p.69

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental differences between matter and its transformations, this question tests your ability to apply the "New Substance" rule. As you learned, a Physical Change alters the form or state of a substance without changing its molecular identity, while a Chemical Change results in the formation of entirely new substances with different properties. According to Science-Class VII . NCERT(Revised ed 2025), identifying whether a process is reversible or results in a new chemical composition is the key to solving such UPSC Prelims questions.

Let's walk through the logic: in the case of Evaporation of water, molecules simply transition from a liquid phase to a gaseous phase. The chemical formula remains H2O throughout the process, and the change can be reversed through condensation. Because the identity of the molecules remains unchanged, it is a physical process. Therefore, (C) Evaporation of water is the correct answer because it is the only option that is not a chemical change. This demonstrates how a change in the state of matter is a primary indicator of a physical transformation.

UPSC often uses biological and combustion processes as distractors to test your conceptual clarity. Curdling of milk and Ripening of fruit are common traps; these are complex biochemical reactions where enzymes and bacteria create new compounds (like lactic acid or complex sugars) that cannot be easily reversed. Similarly, Burning of coal is a combustion reaction—a classic chemical change where carbon reacts with oxygen to produce carbon dioxide and ash. Always ask yourself: "Is a new substance being formed?" If the answer is yes, you are looking at a chemical change.