Which one among the following is an example of chemical change ?

1. Matter and its Physical Properties (basic)

Welcome to your first step in mastering chemistry! To understand the world around us, we begin with Matter — which is essentially anything that has mass and occupies space. Every piece of matter possesses unique characteristics that allow us to identify and categorize it. These are divided into two main categories: physical properties and chemical properties.

Physical properties are traits we can observe or measure without changing the substance's fundamental identity. Think of characteristics like shape, size, color, and state (whether it is a solid, liquid, or gas). When a substance undergoes a physical change, its appearance might transform, but its chemical makeup remains identical. For example, when an ice cube (solid H₂O) melts into liquid water (liquid H₂O), it is still the same chemical compound; only its physical state has changed. Similarly, attracting an iron nail with a magnet is a physical process because the iron remains iron throughout Science-Class VII, Changes Around Us: Physical and Chemical, p.59.

In contrast, a chemical change occurs when substances react to form entirely new substances with different properties. This usually involves the rearrangement of atoms. A classic example is cooking or frying. When you fry an egg, the heat causes the proteins to undergo denaturation — a process that permanently alters their structure to create a new texture and composition that cannot be reversed. Unlike physical changes like boiling or melting, chemical changes are often difficult to reverse because the original substance has been fundamentally altered Science-Class VII, Changes Around Us: Physical and Chemical, p.68.

It is also interesting to note that as the complexity or molecular mass of a substance increases, its physical properties often change in a predictable way. For instance, heavier molecules typically have higher melting and boiling points, even if their chemical behavior remains similar due to their functional groups Science, class X, Carbon and its Compounds, p.67.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Reversibility	Usually easy to reverse (e.g., freezing water).	Usually irreversible (e.g., burning wood).
Examples	Melting, shredding, dissolving sugar.	Rusting, cooking, fermentation.

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.59; Science-Class VII, Changes Around Us: Physical and Chemical, p.68; Science, class X, Carbon and its Compounds, p.67

2. Phase Transitions and States of Matter (basic)

At its most fundamental level, all matter is composed of tiny constituent particles. What determines whether something is a solid, a liquid, or a gas is not what the particles are, but how they interact. These particles are held together by interparticle attractions. The strength of these forces is highly sensitive to distance; even a tiny increase in the space between particles causes the attractive force to drop drastically Science, Class VIII. NCERT (Revised ed 2025), Particulate Nature of Matter, p.101. In a solid, particles are packed tightly with strong attractions; in a gas, they are far apart with negligible attraction.

A phase transition occurs when we add or remove energy (usually heat) to overcome or strengthen these interparticle bonds. It is crucial to remember that phase transitions are physical changes. This means the chemical identity of the substance remains identical throughout. Whether water is ice, liquid, or steam, it remains H₂O. No new substance is formed; only the arrangement and energy of the molecules have changed Science, Class VII. NCERT (Revised ed 2025), Changes Around Us, p.68.

One of the most fascinating aspects of these transitions is Latent Heat—the "hidden" heat. When a substance changes state (e.g., ice melting into water), its temperature stays constant even though you are adding heat Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. This energy isn't increasing the kinetic energy (temperature) of the molecules; instead, it is being consumed entirely to break the attractive forces holding the particles in their current state. This is why a pot of boiling water stays at exactly 100 °C until the very last drop has evaporated Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295.

Phase Change	Process	Energy Action
Melting (Fusion)	Solid to Liquid	Absorbs Latent Heat
Vaporisation	Liquid to Gas	Absorbs Latent Heat
Condensation	Gas to Liquid	Releases Latent Heat
Solidification	Liquid to Solid	Releases Latent Heat

Sources: Science, Class VIII. NCERT (Revised ed 2025), Particulate Nature of Matter, p.101; Science, Class VII. NCERT (Revised ed 2025), Changes Around Us, p.68; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

3. Elements, Compounds, and Mixtures (intermediate)

In the study of matter, we categorize substances based on their composition and how their particles behave. At the most fundamental level, we distinguish between pure substances and mixtures. A pure substance is unique because every single particle within it is identical, and it cannot be separated into other kinds of matter by physical means like filtration or evaporation Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121.

Pure substances are further divided into Elements and Compounds.

• Elements: These are the simplest form of matter and the basic building blocks of everything around us. They cannot be broken down into simpler substances by chemical reactions. Examples include Iron (Fe), Sulfur (S), and Oxygen (O₂) Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130.
• Compounds: When two or more elements combine chemically in a fixed ratio, they form a compound. The most critical characteristic of a compound is that its properties are entirely different from the elements that formed it. For instance, water (H₂O) is a liquid, even though it is made of two gases, hydrogen and oxygen Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130.

In contrast, a Mixture consists of two or more substances that are physically blended but not chemically bonded. In a mixture, each component retains its individual properties. For example, if you mix iron filings and sulfur powder, the iron remains magnetic and the sulfur remains yellow. You can easily separate them using a magnet. However, if you heat that same mixture, a chemical reaction occurs to form the compound Iron Sulfide (FeS), which is non-magnetic and has a completely new set of properties Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.128, 132.

Feature	Element	Compound	Mixture
Composition	Single type of atom	Two+ elements in fixed ratio	Two+ substances in any ratio
Properties	Unique to the element	Different from its components	Retains properties of components
Separation	Cannot be broken down	Only by chemical methods	Physical methods (e.g., magnet)

Sources: Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.128; Science Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.132

4. Redox Reactions: Corrosion and Rancidity (exam-level)

In our daily lives, chemistry isn't just confined to laboratory beakers; it is actively happening on our balconies and in our kitchens. Two of the most common and impactful chemical changes driven by Redox reactions are Corrosion and Rancidity. To understand these, we must remember that Oxidation occurs when a substance gains oxygen, while Reduction occurs when it loses oxygen Science, Class X, Chemical Reactions and Equations, p.12. In the real world, these reactions often occur simultaneously as Redox processes.

Corrosion is the slow deterioration of metals when they react with substances in their environment, such as oxygen, moisture, or acids. The most common example is the rusting of iron. When iron (Fe) is exposed to moist air, it undergoes oxidation to form a flaky, reddish-brown substance called hydrated iron oxide (Fe₂O₃·xH₂O). Unlike some metals like aluminum, which form a protective oxide layer, iron rust continuously falls off, exposing fresh metal to further damage. This is why we use Galvanisation—a process of coating iron or steel with a thin layer of Zinc. Interestingly, galvanised articles remain protected even if the zinc coating is broken because zinc is more reactive and continues to undergo oxidation in place of the iron Science, Class X, Metals and Non-metals, p.54.

Rancidity is the chemical equivalent of "food spoilage" for fats and oils. When the fats and oils present in food items are exposed to air, they undergo oxidation. This results in the formation of volatile compounds that give the food an unpleasant smell and taste. To combat this, we use antioxidants or packaging techniques. For instance, potato chip manufacturers flush bags with Nitrogen gas. Since Nitrogen is relatively inert, it prevents the oil in the chips from coming into contact with oxygen, thereby stopping the redox reaction that causes spoilage Science, Class VII, The World of Metals and Non-metals, p.54.

Feature	Corrosion (Rusting)	Rancidity
Target	Metals (e.g., Iron, Copper, Silver)	Fats and Oils in food
Chemical Cause	Oxidation by moisture/oxygen	Oxidation by atmospheric oxygen
Prevention	Painting, Galvanisation, Alloying	Adding antioxidants, Nitrogen flushing

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Science, Class VII (NCERT 2025 ed.), The World of Metals and Non-metals, p.54

5. Energy Changes: Exothermic and Endothermic Processes (intermediate)

Every chemical reaction involves a change in energy, typically in the form of heat. This happens because breaking chemical bonds in reactants requires energy, while forming new bonds in products releases energy. When the energy released during bond formation is greater than the energy required to break the initial bonds, the reaction is exothermic (literally 'heat out'). Conversely, if the reaction requires more energy to break bonds than what is released, it is endothermic ('heat in'). As noted in Science Class X, Chemical Reactions and Equations, p.7, reactions where heat is released along with the formation of products are exothermic. A vital example for your UPSC preparation is respiration. During digestion, carbohydrates are broken down into glucose, which then reacts with oxygen in our cells to provide the energy we need to stay alive.

On the other hand, many reactions cannot proceed without a constant supply of energy. These are known as endothermic reactions, where energy is absorbed from the surroundings Science Class X, Chemical Reactions and Equations, p.14. Most decomposition reactions fall into this category. For instance, breaking down a single compound into simpler substances often requires energy in the form of heat, light, or electricity to overcome the strong internal bonds of the reactant.

Feature	Exothermic Reactions	Endothermic Reactions
Energy Flow	Released to the surroundings.	Absorbed from the surroundings.
Temperature Change	Surroundings typically feel warmer.	Surroundings typically feel cooler.
Common Examples	Combustion (burning), Respiration, Neutralization.	Photosynthesis, Decomposition reactions, Evaporation.

Sources: Science Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7; Science Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14

6. Identifying Physical vs. Chemical Changes (intermediate)

To understand the world around us, we must distinguish between changes that merely alter a substance's appearance and those that transform its very identity. A Physical Change is a process where a substance undergoes a change in its physical properties—such as shape, size, state, or color—without forming a new chemical substance Science-Class VII, NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.68. For instance, when water freezes into ice or boils into steam, the molecules remain H₂O; only the physical state has shifted from liquid to solid or gas Science-Class VII, NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.66. Many, though not all, physical changes are reversible, meaning you can return to the original form, such as melting an ice cube and then refreezing it. In contrast, a Chemical Change occurs when one or more new substances are formed through a chemical reaction. This involves the rearrangement of atoms to create materials with entirely different properties. Classic indicators of a chemical change include the evolution of gas, a permanent change in color, the production of heat or light (combustion), or a change in odor Science-Class VII, NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.68. For example, when you fry an egg, the heat causes the proteins to denature and cross-link, creating a new structure that cannot be turned back into a raw egg. Similarly, the rusting of iron (forming iron oxide) or the burning of magnesium in air (forming magnesium oxide) are chemical changes because the starting material has been chemically transformed into something else Science-Class VII, NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.70.

To help you distinguish between the two, consider this comparison table:

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Nature of Change	Usually temporary and reversible.	Usually permanent and irreversible.
Energy	Minimal energy change involved.	Significant energy (heat/light) often absorbed or released.
Examples	Melting wax, tearing paper, magnetizing a nail.	Cooking food, rusting, burning wood, digestion.

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

7. The Chemistry of Cooking and Denaturation (exam-level)

In our daily lives, the kitchen serves as a fascinating laboratory of chemical transformations. To master this concept, we must distinguish between physical changes—where only properties like shape, size, or state change—and chemical changes—where entirely new substances are formed through chemical reactions Science-Class VII, Changes Around Us: Physical and Chemical, p.68. For instance, chopping vegetables, peeling potatoes, or slicing fruits are merely physical changes because the chemical identity of the food remains the same; you have simply changed its form Science-Class VII, Changes Around Us: Physical and Chemical, p.70.

However, when we apply heat through cooking, frying, or baking, we initiate fundamental changes in the molecular structure of the food. A pivotal concept here is denaturation, particularly in proteins. Think about frying an egg: the clear, liquid egg white (albumen) consists of proteins folded into specific shapes. Heat energy causes these protein chains to vibrate, uncoil, and then cross-link with one another to form a solid, opaque structure. Because this process creates a substance with entirely different properties and cannot be reversed to return to a raw egg, it is a definitive chemical change Science-Class VII, Changes Around Us: Physical and Chemical, p.68.

It is important to contrast these with phase changes. For example, boiling water to create steam or melting butter are physical changes. In these cases, the molecules (like H₂O) remain chemically identical; they have simply gained enough energy to change their physical state from liquid to gas or solid to liquid. In contrast, processes like the curdling of milk involve the formation of new substances with different chemical compositions, making them chemical changes Science-Class VII, Changes Around Us: Physical and Chemical, p.70.

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., cooking an egg)
Example	Chopping onions, Boiling water	Baking bread, Curdling milk

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.68; Science-Class VII, Changes Around Us: Physical and Chemical, p.70

8. Solving the Original PYQ (exam-level)

You have just mastered the distinction between physical and chemical changes, and this question is a classic application of those principles. To solve this, you must look beyond the surface and identify whether a new substance is formed. While physical changes only alter the form or state of matter, a chemical change involves a fundamental rearrangement of atoms. As you learned, the primary indicator for a chemical change is often irreversibility and the creation of matter with entirely different chemical properties from the original ingredients.

When we examine the options, The frying of an egg stands out because the application of heat triggers a complex chemical process called protein denaturation. This is not merely a change in texture; the chemical bonds within the egg proteins are permanently altered, creating a new solid structure that cannot be reverted to its original liquid state. This aligns perfectly with the core concepts found in Science-Class VII . NCERT(Revised ed 2025). In your UPSC preparation, always look for processes like cooking, rusting, or burning, as these are hallmark indicators of chemical transformations where the identity of the substance is fundamentally changed.

UPSC frequently uses phase changes as traps to catch students who confuse a change in appearance with a change in chemistry. Both the melting of ice and the boiling of gasoline are merely transitions between states of matter (solid to liquid or liquid to gas); the molecular identity remains identical ($H_2O$ is still water regardless of its state). Similarly, the magnetic attraction of an iron nail is a physical property and does not result in any chemical reaction. By eliminating these reversible or state-based changes, you can clearly identify the correct answer as (C) The frying of an egg.