Which one of the following is a physical change?

1. Nature of Matter: Elements, Compounds, and Mixtures (basic)

To understand chemistry, we must first look at what the world is made of. Everything you see—the air you breathe, the water in your bottle, and even the clothes you wear—is matter. As you may recall from earlier studies, all matter is composed of tiny particles that are constantly in motion Science, Class VIII, NCERT, Particulate Nature of Matter, p.112. However, in science, we categorize this matter based on how these particles are organized. While a juice box might be labeled "100% pure" in a grocery store, a scientist would call it a mixture because it contains water, sugars, and acids. To a scientist, a pure substance is one that consists of only one type of particle throughout Science, Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.121.

Pure substances are divided into two categories: elements and compounds. Elements are the "building blocks" of the universe; they are the simplest substances and cannot be broken down into anything simpler by chemical means. Compounds, on the other hand, are formed when two or more elements chemically combine in a fixed ratio. A fascinating aspect of compounds is that they have entirely different properties from the elements that make them. For example, liquid water (H₂O) is very different from the explosive hydrogen gas and life-sustaining oxygen gas that form it Science, Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.130.

In contrast, a mixture occurs when substances are put together without reacting chemically. In a mixture, like iron filings mixed with sulfur powder, each component retains its own identity—the iron remains magnetic, and the sulfur remains yellow Science, Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.128. Because there is no chemical bond, you can usually separate mixtures using physical methods like filtration or using a magnet.

Feature	Element	Compound	Mixture
Composition	Only one type of atom.	Two or more elements chemically bonded.	Two or more substances physically blended.
Properties	Unique to that element.	Different from its components.	Components retain their own properties.
Separation	Cannot be broken down further.	Only by chemical reactions.	By physical methods (e.g., filtration).

Sources: Science, Class VIII, NCERT (Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.112, 117, 121, 128, 130

2. Physical Changes: Characteristics and Reversibility (basic)

In our journey to understand how matter behaves, the first step is distinguishing between changes that alter a substance's appearance and those that change its very soul. A physical change occurs when a substance undergoes a shift in its physical properties—such as its shape, size, color, or state (solid, liquid, or gas)—but its chemical identity remains exactly the same Science-Class VII, Chapter 5, p.59. Think of it like a person changing their clothes; the person (the substance) is the same, even if they look different on the outside.

The defining characteristic of a physical change is that no new substance is formed Science-Class VII, Chapter 5, p.68. For example, when you melt ice, it turns into liquid water. While the "state" has changed from solid to liquid, the molecules are still H₂O. Similarly, when water evaporates into steam, it is still H₂O. These are examples of physical changes because the fundamental nature of the material hasn't shifted.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Nature of Change	Only physical properties (size, state, shape) change.	Chemical properties and composition change.
Common Example	Melting ice, heating a metal until it glows.	Burning wood, rusting of iron.

A common point of confusion is reversibility. Many physical changes are reversible, meaning you can get the original substance back—like freezing water back into ice Science-Class VII, Chapter 5, p.66. However, some physical changes are irreversible. If you break a glass tumbler or shred a piece of paper, it is still glass or paper (a physical change), but you cannot easily "un-break" or "un-shred" them. A fascinating example is heating a platinum crucible. Platinum is a noble metal that does not react with oxygen even at high temperatures. It might glow red-hot or even melt, but it remains pure platinum throughout the process. Because no new chemical compound is formed, heating platinum is a classic physical change.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 66, 68, 69

3. Chemical Changes: Indicators and Energy Exchange (basic)

In our journey through chemistry, we must distinguish between a temporary change in appearance and a fundamental transformation of identity. A chemical change occurs when one or more new substances are formed, possessing entirely different chemical properties and molecular structures than the starting materials Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > p. 68. This process is often irreversible and is represented scientifically as a chemical equation. For instance, when vinegar reacts with baking soda, the production of carbon dioxide gas bubbles is a clear signal that the original ingredients have been rearranged into something new Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > p. 61. To identify a chemical reaction, we look for specific indicators. These include the evolution of a gas, a distinct change in color, or the formation of a precipitate (a solid that settles out of a liquid solution). However, one of the most significant indicators is the exchange of energy. Many reactions, such as combustion (burning), 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. While a physical change like boiling water also involves heat, the water molecules (H₂O) remain water. In a chemical change, like heating potassium chlorate, the heat actually breaks chemical bonds to create new products like oxygen gas and potassium chloride Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > p. 8. It is important to note that heating does not always guarantee a chemical change. Consider a platinum crucible used in a lab: because platinum is a noble metal with high chemical stability, heating it causes it to glow or increase in temperature (a physical change), but it does not react with oxygen or change its identity. In contrast, burning wood or coal results in ash and smoke, which are entirely new substances, marking a definitive chemical change. Understanding these nuances helps us predict how matter will behave under different environmental stresses.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.61; Science , class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.1-2; Science , class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.8

4. The Chemistry of Combustion (Burning) (intermediate)

At its heart, combustion is a chemical process where a substance reacts with oxygen to release energy in the form of heat and light. It is a classic example of a chemical change because the starting materials (like wood or coal) are transformed into entirely new substances, such as carbon dioxide, water vapor, and ash Science-Class VII, Changes Around Us: Physical and Chemical, p.68. To understand the chemistry better, consider the Burning of Coal: it is a combination reaction where solid carbon (C) reacts with oxygen (O₂) to form carbon dioxide (CO₂). This reaction is exothermic, meaning it evolves a significant amount of heat Science, class X, Chemical Reactions and Equations, p.7.

A common point of confusion is whether every instance of "heating" is combustion. It is not. For example, when you heat a platinum crucible in a lab, it may glow bright red, but it does not react with the oxygen in the air or turn into a new substance. Once it cools, it is still the same platinum. This is merely a physical change. In contrast, burning wood is irreversible and creates new molecular structures Science-Class VII, Changes Around Us: Physical and Chemical, p.68. For combustion to occur, you generally need three things: a combustible substance (like kerosene or paper), a supporter of combustion (usually oxygen), and the reach of an ignition temperature Science-Class VII, Changes Around Us: Physical and Chemical, p.62.

Have you ever wondered why a candle has a flame, but charcoal in an angithi just glows? The secret lies in the state of matter. A flame is only produced when gaseous substances burn. When you light wood, the heat causes volatile substances within it to vaporize; these gases then burn to create the flame we see. Once these volatiles are exhausted, the remaining carbon (charcoal) simply glows red as it reacts at its surface without a flame Science, class X, Carbon and its Compounds, p.70. From an environmental perspective, "complete" combustion is ideal, but "impure" or incomplete combustion leads to the formation of black carbon (soot) and brown carbon, which are significant contributors to global warming Environment, Shankar IAS Academy, Climate Change, p.258.

Sources: Science-Class VII, NCERT, Changes Around Us: Physical and Chemical, p.62, 68; Science, class X, NCERT, Chemical Reactions and Equations, p.7; Science, class X, NCERT, Carbon and its Compounds, p.70; Environment, Shankar IAS Academy, Climate Change, p.258

5. Thermal Decomposition of Compounds (intermediate)

In our journey through chemical principles, we now encounter Thermal Decomposition—a fascinating process where heat acts as a chemical "sledgehammer," breaking down a single complex substance into two or more simpler products. Unlike a physical change where a substance might just get hot or melt (like heating a platinum crucible), thermal decomposition is a chemical change because the original molecular structure is permanently altered to form entirely new substances Science, Class VII (NCERT 2025 ed.), Chapter 5, p. 68.

Consider the industrial production of cement. It relies on the thermal decomposition of Calcium carbonate (limestone). When heated, it breaks down into Calcium oxide (quick lime) and Carbon dioxide gas. This is a staple in manufacturing and demonstrates how heat can drive essential chemical transformations Science, Class X (NCERT 2025 ed.), Chapter 1, p. 8. Another striking example is the heating of Lead nitrate crystals. As the reaction progresses, you would observe the emission of characteristic brown fumes, which are actually Nitrogen dioxide (NO₂) gas, alongside the formation of Lead oxide and Oxygen Science, Class X (NCERT 2025 ed.), Chapter 1, p. 9.

A key hallmark of these reactions is their endothermic nature. Because energy is required to break the chemical bonds of the reactant, heat must be constantly supplied. This is why decomposition reactions are often considered the opposite of combination reactions, where energy is typically released as new bonds form Science, Class X (NCERT 2025 ed.), Chapter 1, p. 15. Whether it is the color change of green Ferrous sulphate crystals into reddish-brown Ferric oxide or the release of oxygen from Potassium chlorate, thermal decomposition always involves a fundamental change in the identity of the matter involved.

Sources: Science, Class VII (NCERT 2025 ed.), Chapter 5: Changes Around Us: Physical and Chemical, p.68; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.8-9, 15

6. Noble Metals and Chemical Inertness (intermediate)

In the world of chemistry, Noble Metals—such as Gold (Au), Silver (Ag), and Platinum (Pt)—occupy a prestigious position due to their chemical inertness. Unlike reactive metals like Sodium or Potassium, which must be stored under oil to prevent violent reactions with air or water Science Class X, Metals and Non-metals, p.57, noble metals do not easily react with environmental reagents. This resistance to oxidation and corrosion is the primary reason why gold and silver are prized for jewellery; they maintain their luster for centuries without tarnishing Science Class X, Metals and Non-metals, p.57.

To understand why these metals are "noble," we must look at their electronic configuration. Elements generally react to achieve a stable, completely filled valence shell, similar to the noble gases (like Argon or Neon) Science Class X, Metals and Non-metals, p.46. While noble metals are not as completely non-reactive as noble gases, they are at the very bottom of the reactivity series. For example, while heating copper in air results in a black coating of copper(II) oxide, gold and silver do not react with oxygen even at extremely high temperatures Science Class X, Metals and Non-metals, p.42. This stability is why heating a platinum crucible in a laboratory is considered a physical change: the metal may glow or expand due to heat, but its chemical identity remains identical before and after the process.

Feature	Reactive Metals (e.g., Na, Mg)	Noble Metals (e.g., Pt, Au)
Reaction with Oxygen	Form oxides easily; some burn vigorously.	Do not react with oxygen even at high heat.
Heating Outcome	Usually results in a chemical change (oxidation).	Results in a physical change (glowing/expansion).
Malleability	Varies, but often reactive.	Highly malleable; can be beaten into thin foils Science Class VII, The World of Metals and Non-metals, p.43.

This chemical stability makes platinum indispensable in scientific apparatus. When a chemist heats a substance inside a platinum crucible, they can be confident that the container itself will not interfere with the reaction or decompose. This illustrates a fundamental principle: a chemical change requires the formation of new molecular structures, whereas the mere application of heat to a noble metal only alters its physical state (temperature or glow) without breaking or forming chemical bonds Science Class VII, Changes Around Us, p.68.

Sources: Science Class X (NCERT 2025 ed.), Metals and Non-metals, p.57; Science Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science Class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science Class VII (NCERT 2025 ed.), The World of Metals and Non-metals, p.43; Science Class VII (NCERT 2025 ed.), Changes Around Us: Physical and Chemical, p.68

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental definitions of matter, this question serves as the perfect application of the "Identity Test." As you learned in Science-Class VII . NCERT(Revised ed 2025), the hallmark of a physical change is that no new substance is formed; the molecules remain identical even if the appearance or temperature shifts. To solve this, you must ask yourself: "After the process is over, do I still have the exact same chemical species I started with?" This logical filter is your most reliable tool for tackling UPSC General Science questions.

Applying this logic, we arrive at the correct answer: (C) Heating of a platinum crucible. Platinum is a noble metal known for its extreme chemical stability and high melting point. When heated in a laboratory setting, it may glow or increase in temperature, but it does not react with atmospheric oxygen to form a new compound. Because the platinum remains chemically identical before and after the process, it is a physical change. In contrast, the UPSC often uses the term "heating" to mask chemical reactions. For instance, heating potassium chlorate leads to thermal decomposition, breaking it down into potassium chloride and oxygen—a clear chemical transformation according to Science , class X (NCERT 2025 ed.).

The other options, burning of coal and burning of wood, are classic traps designed to test your understanding of combustion. As highlighted in National Geographic Education, burning always results in the formation of new substances like carbon dioxide, water vapor, and ash. Since these products have entirely different molecular structures than the original fuel, these are chemical changes. Remember, while "heating" can be a physical change (like boiling water or heating metal), "burning" is a definitive signal of a chemical reaction in the UPSC syllabus.