Which among the following is a chemical change?

1. Foundations: Physical Changes and Phase Transitions (basic)

In our study of chemistry and geography, we first distinguish between how matter behaves and how it transforms. A physical change is a process where a substance undergoes a change in its physical properties—such as shape, size, color, or state—without forming any new chemical substance Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.59. For example, if you chop vegetables or if a rock breaks down into smaller particles due to wind erosion, the material itself remains the same; only its physical form has been altered Science-Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.68. Unlike chemical reactions, physical changes are often (though not always) reversible. One of the most important types of physical changes is a phase transition, where a substance moves between solid, liquid, and gaseous states. A classic example is the movement of water in our atmosphere. When a wet towel dries, water molecules absorb heat and transform from a liquid to a gas—a process called evaporation FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Water in the Atmosphere, p.86. Crucially, whether it is liquid water in a bucket or water vapor in the air, the chemical identity remains H₂O. Similarly, when air cools and water vapor turns back into liquid droplets (forming dew or clouds), this is condensation, another physical transition driven by temperature changes FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Water in the Atmosphere, p.87. Beyond simple state changes, physical processes also include extraction and convection. When you brew coffee by passing steam or hot water through grounds, you are physically dissolving soluble compounds into the water; no new molecules are being created through a reaction. Likewise, when hot air rises over a radiator, it is a physical movement driven by density differences. In the UPSC context, always ask yourself: "Has a new substance been created?" If the answer is no, you are likely looking at a physical change.

Sources: Science-Class VII (NCERT 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.59, 68; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025), Water in the Atmosphere, p.86, 87

2. Defining Chemical Changes: Characteristics and Evidence (basic)

At its heart, a chemical change is a process where the identity of a substance is fundamentally altered. Unlike physical changes—where a substance might change its shape or state (like ice melting) but remains the same chemically—a chemical change results in the formation of one or more new substances. This happens because the atoms involved do not just move around; the chemical bonds between atoms are broken and new bonds are formed to create entirely different molecular structures Science, Class X, Chemical Reactions and Equations, p.6. Because new substances are created, these changes are usually difficult to reverse.

How do we know a chemical change has taken place? Scientists look for specific signs of a chemical reaction. These include a change in color, the evolution of a gas (often seen as bubbling), or a change in temperature (releasing or absorbing heat). For instance, when baking soda is mixed with vinegar, the immediate fizzing and bubbling is due to the production of a new substance: carbon dioxide gas Science, Class VII, Changes Around Us: Physical and Chemical, p.61. Similarly, the rusting of iron is a chemical change because the iron reacts with oxygen and moisture to form a new brown-colored compound called iron oxide Science, Class VII, Changes Around Us: Physical and Chemical, p.62.

It is important to distinguish these from complex physical processes. For example, brewing coffee or tea involves extraction—where hot water dissolves flavor compounds—but the compounds themselves largely retain their identity. However, if you add lemon juice to tea and the color shifts, you are witnessing a chemical reaction between the citric acid and the tea's polyphenols, resulting in a new chemical arrangement. Here is a quick way to differentiate them:

Feature	Physical Change	Chemical Change
New Substance	No new substance formed.	One or more new substances created.
Nature	Usually reversible (e.g., melting).	Usually irreversible (e.g., burning).
Example	Water evaporating from a towel.	Iron reacting with air to form rust.

Sources: Science, Class X, Chemical Reactions and Equations, p.6; Science, Class VII, Changes Around Us: Physical and Chemical, p.61; Science, Class VII, Changes Around Us: Physical and Chemical, p.62

3. Adjacent Concept: Acids, Bases, and Natural Indicators (intermediate)

In chemistry, substances are often classified based on their chemical nature as acidic, basic, or neutral. Acids are generally characterized by a sour taste and the ability to turn blue litmus paper red. On the other hand, bases feel soapy to the touch, taste bitter, and turn red litmus paper blue Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19. When an acid and a base react in the right proportions, they undergo a neutralization reaction, forming salt and water (Base + Acid → Salt + Water). For example, the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) produces common salt (NaCl) and water (H₂O) Science, class X, Acids, Bases and Salts, p.21.

Since it is unsafe to taste or touch every substance to determine its nature, we use indicators. These are special substances that change their color when added to a solution containing an acidic or a basic substance. Many of these are natural indicators derived from plants. The most common is litmus, a purple dye extracted from lichens. Other examples include turmeric, red cabbage leaves, and the petals of flowers like Hibiscus (China rose), Hydrangea, and Petunia Science, class X, Acids, Bases and Salts, p.17.

The behavior of these natural indicators is a key concept in identifying chemical changes and substance properties. For instance, a yellow turmeric stain on a white cloth turns reddish-brown when washed with soap because soap is basic in nature Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19. Understanding these shifts helps us interpret the chemical environment around us.

Indicator	Color in Acidic Solution	Color in Basic Solution
Litmus	Red	Blue
Turmeric	Yellow (No change)	Red/Reddish-brown
China Rose	Dark Pink (Magenta)	Green

Sources: Science-Class VII . NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.19; Science , class X (NCERT 2025 ed.), Acids, Bases and Salts, p.17; Science , class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21

4. Mixtures and Separation: Extraction vs Reaction (intermediate)

To understand the world around us, we must distinguish between substances that are simply sitting next to each other and those that have fundamentally bonded. A mixture consists of two or more substances mixed together where each component retains its individual properties and no chemical reaction occurs Science, Class VIII, Nature of Matter, p.130. For example, when you make a sprout salad or dissolve sugar in water, you are creating a mixture because the components do not change their identity Science, Class VIII, Nature of Matter, p.117. These components can often be separated through physical extraction—a process that pulls a substance out of a mixture without changing its molecular structure, such as extracting sugar from sugarcane or caffeine from coffee beans Environment, Shankar IAS, Agriculture, p.353.

In contrast, a chemical reaction involves the formation of entirely new substances with different properties. When substances react, their atoms rearrange to form compounds, which have a fixed ratio and unique characteristics compared to their original elements Science, Class VIII, Nature of Matter, p.130. A classic example is heating a mixture of iron filings and sulfur; while they start as a simple mixture, heat triggers a reaction to form iron sulfide (FeS), a compound with properties unlike either iron or sulfur Science, Class VIII, Nature of Matter, p.132. Even everyday actions, like adding lemon juice to tea, can involve reactions where citric acid interacts with tea polyphenols (tannins), resulting in a visible chemical change in color Science, Class VII, Changes Around Us, p.68.

Feature	Physical Extraction / Mixture	Chemical Reaction / Compound
Identity	Substances keep their original identity.	New substances are formed.
Properties	Shows properties of all components.	Properties differ from original elements.
Separation	Separated by physical means (filtration, evaporation).	Requires chemical methods to break bonds.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132; Environment, Shankar IAS Academy .(ed 10th), Agriculture, p.353; Science, Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68

5. Chemistry in Daily Life: Polyphenols and Organic Reactions (exam-level)

Understanding the chemistry of our daily beverages reveals the fascinating interplay between organic compounds. At the heart of tea's chemistry are polyphenols, specifically a group known as tannins. These are complex organic molecules found abundantly in tea leaves, especially in black tea where they contribute to the deep color and astringent taste INDIA PEOPLE AND ECONOMY, Land Resources and Agriculture, p.34. Tannins are versatile substances; industrially, they are used to coagulate proteins in hides to create leather, and they serve as natural dyes because of their intense pigmentation Geography of India, Natural Vegetation and National Parks, p.27. When you brew tea, you are performing a physical extraction—dissolving these soluble compounds into hot water.

The real magic happens when you squeeze a lemon into your cup. Lemon juice contains citric acid Science class X, Acids, Bases and Salts, p.28. The addition of this acid triggers a chemical change. Unlike a physical change—such as water evaporating from a wet towel or steam passing through coffee grounds—a chemical change results in the formation of new substances or structural alterations. The citric acid reacts with the polyphenol molecules, changing their chemical structure and their ability to absorb light. This is why the tea visibly lightens in color; the polyphenols are essentially acting as natural pH indicators.

To distinguish these processes clearly, consider the following comparison:

Feature	Physical Change	Chemical Change
Definition	Change in form/state without forming new substances.	Formation of one or more new substances through a reaction.
Examples	Steam brewing coffee, drying a towel, convection of hot air.	Adding lemon to tea, fermenting black tea leaves, neutralization.
Key Indicator	Process is often easily reversible (e.g., vapor to liquid).	Visible color change or temperature shift indicating molecular rearrangement.

It is also important to note the difference in how tea is processed. Black tea undergoes a fermentation process which increases its tannin complexity, whereas green tea remains unfermented Certificate Physical and Human Geography, Agriculture, p.254. This difference in chemical processing is why lemon often has a more dramatic visual impact on black tea than on green tea.

Sources: INDIA PEOPLE AND ECONOMY, Land Resources and Agriculture, p.34; Geography of India, Natural Vegetation and National Parks, p.27; Science class X, Acids, Bases and Salts, p.28; Certificate Physical and Human Geography, Agriculture, p.254

6. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental differences between physical and chemical changes, let’s apply that logic to this question. Recall that a chemical change is defined by the formation of new substances through the breaking and forming of chemical bonds, often signaled by irreversible indicators like color shifts, gas evolution, or temperature changes. In this specific case, the correct answer is (B). When lemon juice is added to tea, the citric acid reacts with the tea's polyphenols (tannins). This isn't just a simple mixing of ingredients; it is a chemical reaction where the change in pH alters the molecular structure of the tea's pigments, resulting in a visible color change that signifies the creation of new chemical identities.

UPSC often uses common daily phenomena to test your ability to distinguish between phase changes and chemical reactions. For instance, option (A) describes a physical change because the drying of a towel is merely evaporation—water moving from a liquid state to a gaseous state without changing its molecular formula. Similarly, option (C) involves convection, a physical process where air moves due to density differences. The most common trap is option (D); while brewing coffee might seem complex, it is primarily a process of extraction and dissolution. The steam acts as a solvent to pull existing compounds out of the coffee grounds into the water. As explained in Science-Class VII . NCERT(Revised ed 2025), the ultimate litmus test for a chemical change is whether the identity of the substance itself has been transformed into something fundamentally new.