Which one of the following is not a chemical change ?

1. Properties of Matter: Physical vs. Chemical (basic)

Welcome to your first step in mastering chemistry! To understand the world around us, we must first distinguish between how matter looks and what matter is. Every substance has a unique identity, and when that identity remains intact despite a change in appearance, we call it a physical change. However, when the substance transforms into something entirely new, we call it a chemical change.

Physical properties include characteristics like shape, size, color, and state (solid, liquid, or gas). In a physical change, only these outward traits are altered, and no new substance is formed Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68. For example, when seawater evaporates to leave behind crystals of salt, the chemical identity of the sodium chloride remains the same. Similarly, melting ice at 0 °C is a physical change because it is still H₂O, just in a different state Science-Class VIII, Chapter 7: Particulate Nature of Matter, p.103. Many physical changes, like dissolving salt or melting wax, can be reversed.

Chemical properties, on the other hand, describe how a substance reacts with others. A chemical change occurs when a chemical reaction takes place, resulting in the formation of one or more new substances with entirely different properties Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68. Think of burning coal: the solid carbon reacts with oxygen to form a new gas, CO₂. Once this happens, you cannot simply "un-burn" the gas to get the coal back. Other common examples include the fermentation of juice into alcohol, the curdling of milk, or the rusting of iron.

Feature	Physical Change	Chemical Change
New Substance	None formed.	One or more new substances formed.
Identity	Chemical composition remains the same.	Chemical composition changes.
Reversibility	Usually easy to reverse (e.g., melting).	Usually difficult to reverse (e.g., cooking).
Example	Crystallization, Tearing paper.	Combustion, Digestion.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 68; Science-Class VIII . NCERT(Revised ed 2025), Chapter 7: Particulate Nature of Matter, p.103

2. Mechanics of Physical Changes (basic)

At its core, a physical change is a process that alters the form, state, or appearance of a substance without modifying its underlying chemical identity. Think of it as a change in the 'packaging' rather than the 'content.' As noted in Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68, the defining characteristic is that no new substance is formed. Common physical properties that might change include shape, size, color, or state (solid, liquid, or gas). For instance, when rocks undergo weathering through the physical action of wind or water, they break into smaller pieces to form soil, but the minerals within them remain chemically the same Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68.

The mechanics of these changes often involve energy and interparticle forces. When a substance changes state—such as ice melting into water—the temperature remains constant during the transition because the energy added is used as latent heat to overcome the forces holding the molecules together, rather than increasing the kinetic energy of the particles Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294. At the boiling point, particles move so vigorously that they escape the liquid state to become vapor, yet the molecules (like H₂O) remain intact Science, Class VIII, Particulate Nature of Matter, p.105. This explains why physical changes are often reversible; for example, salt crystallized from seawater via evaporation can be returned to its original dissolved state simply by adding water Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68.

In the context of the UPSC, it is vital to distinguish these from chemical changes, where atoms are rearranged to create entirely new substances with different properties—such as the conversion of milk to curd or the combustion of fuel. While physical changes might seem 'surface-level,' they are the drivers of massive natural processes, from the global water cycle to the geological shaping of our planet.

Feature	Physical Change	Chemical Change
Composition	Remains unchanged	New substances formed
Reversibility	Usually reversible (e.g., melting)	Often irreversible (e.g., burning)
Energy	Involves phase change energy (Latent Heat)	Involves breaking/forming chemical bonds

Sources: Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Science, Class VIII, Particulate Nature of Matter, p.105

3. Mechanics 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 only the appearance or state (like liquid to gas) changes—a chemical change involves the rearrangement of atoms to form entirely new substances with different properties Science-Class VII, Changes Around Us: Physical and Chemical, p.61. For example, when you burn coal, the carbon reacts with oxygen to produce carbon dioxide (CO₂). You cannot turn that gas back into coal simply by cooling it down; a permanent transformation has occurred because new chemical bonds were formed.

We often identify chemical changes through specific indicators. These include the evolution of a gas (seen as bubbling or effervescence), a change in temperature, the production of light, or a change in color. In the laboratory, we represent these changes using chemical equations, where 'reactants' (the starting materials) are shown turning into 'products' (the new substances) Science, Class X, Chemical Reactions and Equations, p.14. For instance, the biochemical process of fermentation turns sugarcane juice into alcohol and CO₂, a complex change driven by yeast that fundamentally alters the molecular structure of the sugar.

To master this concept, it is helpful to contrast chemical changes with physical ones, like crystallisation. When salt is obtained from seawater, the water evaporates, leaving salt crystals behind. Although the salt looks different (from dissolved to solid), its chemical formula, NaCl, remains exactly the same. This is a physical change because no new substance is created and it is generally reversible by redissolving the salt in water Science-Class VII, Changes Around Us: Physical and Chemical, p.68.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Nature	Usually reversible.	Usually irreversible.
Examples	Melting ice, Boiling water, Crystallisation.	Burning wood, Fermentation, Rusting of iron.

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.61; Science-Class VII, Changes Around Us: Physical and Chemical, p.68; Science, Class X, Chemical Reactions and Equations, p.14

4. Separation Techniques for Mixtures (intermediate)

When we encounter matter in nature, it is rarely in a pure form. Most things around us are mixtures, which can be either uniform (homogeneous), where components are spread evenly and invisible to the naked eye like salt in water, or non-uniform (heterogeneous), where you can clearly see the different parts, such as a mixture of iron filings and sulfur Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117, 128. The goal of separation techniques in chemistry is to isolate pure substances from these mixtures so they can be studied or used effectively Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.120.

The most critical concept to master here is that standard separation techniques are physical processes. This means they exploit the physical properties of the components—like their size, boiling point, or magnetism—without altering their chemical identity. For instance, the crystallisation of table salt (NaCl) from seawater is a physical change. By using heat to drive evaporation, we transform liquid water into vapor, leaving the salt crystals behind Fundamentals of Physical Geography, Class XI, Water in the Atmosphere, p.86. Throughout this process, the sodium chloride remains sodium chloride; no new substance is formed, and the process can be reversed by simply re-dissolving the salt in water Science, Class VII, Changes Around Us, p.68.

To truly understand separation, we must distinguish it from processes that actually change the chemical nature of a substance. While separating salt from water is physical, cracking petroleum (breaking long-chain hydrocarbons into smaller ones) or the fermentation of sugarcane juice into alcohol are chemical changes because they involve chemical reactions that produce entirely new substances Science, Class VII, Changes Around Us, p.68. Understanding this distinction is vital for any competitive exam, as it helps you identify whether a process is merely "sorting" molecules or "remaking" them.

Process	Type of Change	Core Mechanism
Crystallisation	Physical	Removal of solvent (evaporation) to leave pure solid crystals.
Magnetic Separation	Physical	Exploiting magnetic properties (e.g., pulling iron out of sulfur) Science, Class VIII, p.128.
Petroleum Cracking	Chemical	Breaking chemical bonds to create smaller, different molecules.
Fermentation	Chemical	Biochemical conversion of sugars into alcohol and CO₂.

Sources: Science, Class VIII (NCERT 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117, 120, 128; Science, Class VII (NCERT 2025), Changes Around Us: Physical and Chemical, p.68; Fundamentals of Physical Geography, Class XI (NCERT 2025), Water in the Atmosphere, p.86

5. Biochemical Processes: Fermentation and Digestion (intermediate)

To understand the chemistry of life, we must look at biochemical processes—chemical reactions that take place within living organisms or are mediated by their enzymes. Unlike simple physical mixing, these processes involve the breaking of old atomic bonds and the formation of new ones, resulting in entirely different substances. A primary example is fermentation, a process where microorganisms like yeast convert organic nutrients (like sugars) into simpler compounds such as alcohol and carbon dioxide (CO₂) Science-Class VII, Chapter 5, p.72.

In a laboratory or kitchen setting, adding yeast to a sugar solution demonstrates this perfectly. As the yeast 'eats' the sugar, it releases CO₂ gas. This gas is what causes bread dough to rise and become fluffy. If this gas is captured and passed through lime water, the water turns milky—a classic chemical test confirming the production of carbon dioxide Science, Class X, Chapter: Life Processes, p.87. Because the sugar has been chemically transformed into ethanol and gas, fermentation is strictly classified as a chemical change.

These reactions are part of a broader biological system called respiration. Cells break down organic compounds like glucose to generate ATP (Adenosine Triphosphate), which acts as the 'energy currency' for the body Science, Class X, Chapter: Life Processes, p.99. While humans mostly use aerobic respiration (with oxygen), fermentation is a form of anaerobic respiration (without oxygen), used by various bacteria and fungi to survive in oxygen-poor environments.

Feature	Aerobic Respiration	Anaerobic Respiration (Fermentation)
Oxygen Requirement	Requires O₂	Occurs without O₂
End Products	CO₂, Water, High ATP	Ethanol/Lactic Acid, CO₂, Low ATP
Efficiency	Very high energy yield	Lower energy yield

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.72; Science, Class X (NCERT 2025 ed.), Life Processes, p.87, 99

6. Industrial Chemistry: Petroleum and Combustion (exam-level)

At its core, petroleum (or crude oil) is a complex mixture of hydrocarbons in liquid and gaseous states, varying in chemical composition and density INDIA PEOPLE AND ECONOMY, TEXTBOOK IN GEOGRAPHY FOR CLASS XII, Mineral and Energy Resources, p.59. In industrial chemistry, we treat petroleum not just as a fuel, but as a primary raw material. The transformation of crude oil into usable products involves two distinct types of changes. First, fractional distillation physically separates the mixture based on boiling points. Second, cracking is a chemical process where complex, long-chain hydrocarbons are broken down into smaller, more useful molecules like petrol or diesel NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p. 68. This makes petroleum refineries a nodal industry, providing the chemical building blocks for synthetic fibers, fertilizers, and plastics NCERT (Revised ed 2022), Contemporary India II, p.115.

The utility of petroleum primarily comes from combustion, a high-temperature chemical reaction between a fuel and an oxidant (usually oxygen). When hydrocarbons burn completely, they produce a clean blue flame, releasing carbon dioxide (CO₂), water vapor (H₂O), and significant energy. However, if the oxygen supply is insufficient, incomplete combustion occurs, leading to the formation of carbon monoxide (CO) and unburnt carbon particles (soot), which blacken the bottoms of cooking vessels Science, class X, Carbon and its Compounds, p.70.

Process	Type of Change	Chemical Outcome
Fractional Distillation	Physical	Separates components without changing molecular structure.
Cracking	Chemical	Breaks large molecules into new, smaller substances.
Combustion	Chemical	Reacts with O₂ to form CO₂, H₂O, and heat.
Crystallisation	Physical	Solute separates from solution; identity remains unchanged.

Environmental concerns in industrial chemistry stem from the impurities found in fossil fuels. Natural petroleum often contains traces of nitrogen and sulphur. When these fuels undergo combustion, they don't just produce CO₂, but also oxides of nitrogen (NOₓ) and sulphur (SO₂), which are the primary contributors to acid rain and atmospheric pollution Science, class X, Carbon and its Compounds, p.70.

Sources: INDIA PEOPLE AND ECONOMY, TEXTBOOK IN GEOGRAPHY FOR CLASS XII, Mineral and Energy Resources, p.59; NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.68; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.70; NCERT (2022), Contemporary India II, Print Culture and the Modern World, p.115

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental distinction between physical changes (where only state or appearance changes) and chemical changes (where new substances are formed), this PYQ tests your ability to look past complex terminology. The core concept here is the conservation of molecular identity. In a physical change, the molecules remain the same even if they rearrange; in a chemical change, new chemical bonds are broken or formed to create something entirely different. When you see Crystallisation of table salt from sea water, you should recognize it as a separation technique. As emphasized in Science-Class VII . NCERT, the sodium chloride (NaCl) remains NaCl throughout the process—it simply transitions from a dissolved state to a solid state as water evaporates, making it a classic physical change.

To arrive at the correct answer, (C), use the coach's "reversibility and identity" test. If you can revert the change by simple physical means (like re-dissolving the salt) and no new substance was created, it is not chemical. The other options are common UPSC traps designed to sound complex. Burning of coal is a combustion reaction that produces carbon dioxide, while Fermentation is a biochemical process that transforms sugar into alcohol—both involve creating new molecules. Similarly, Cracking of petroleum might sound like a physical "breaking," but it is actually a chemical decomposition where long-chain hydrocarbons are broken into smaller, different molecules. Always ask yourself: Is the molecule at the end the same as the molecule at the start? if yes, it is physical.