Which of the following represent a chemical change? 1. Magnetization of iron 2. Condensation of liquid 3. Burning of fuel 4. Rusting of iron Select the correct answer using the code given below : Code :

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

To understand the world around us, we must first look at Matter—anything that has mass and occupies space. From the staircase you climb to the air you breathe, everything is composed of tiny particles Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.117. In chemistry, we classify matter based on its chemical composition into two broad categories: Pure Substances and Mixtures.

A Pure Substance is matter that consists of only one type of particle and cannot be separated into other kinds of matter by physical processes like filtration or hand-picking Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.121. These are further divided into:

• Elements: The simplest form of matter consisting of only one kind of atom (e.g., Iron, Oxygen, Gold).
• Compounds: Substances formed when two or more elements combine chemically in a fixed proportion. For example, Water (H₂O) is a compound because hydrogen and oxygen atoms are chemically bonded together and cannot be separated by simple physical means.

In contrast, a Mixture contains two or more substances that are physically blended but not chemically bonded. Most things we encounter, like snacks, detergents, or alloys like Brass and Stainless Steel, are mixtures Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.133. In a mixture, the individual components retain their original properties and can usually be separated using physical methods.

Feature	Pure Substance	Mixture
Composition	Fixed and uniform throughout.	Variable; substances are just mixed.
Separation	Requires chemical or electrochemical reactions.	Can be separated by physical methods (e.g., evaporation).
Properties	Entirely different from constituent elements.	Constituents retain their individual properties.

Sources: Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.117; Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.121; Science Class VIII, NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.133

2. States of Matter and Phase Transitions (basic)

Matter is everything around us, and it primarily exists in three states: solid, liquid, and gas. These states are determined by how the particles within the substance move and interact. In a solid, particles are closely packed and vibrate in fixed positions; in a liquid, they have more freedom to slide past one another; and in a gas, they move rapidly and are far apart Science Class VIII, Particulate Nature of Matter, p.109. A phase transition occurs when matter moves from one of these states to another, typically due to changes in temperature or pressure.

A fascinating aspect of these transitions is the concept of Latent Heat. This is the energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature. For example, if you heat a pot of boiling water, the temperature stays at exactly 100°C until the very last drop has turned into steam. The heat you continue to add isn't making the water "hotter"; instead, it is being used as latent heat of vaporization to break the molecular bonds and turn the liquid into gas Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294.

Phase transitions are categorized by whether they absorb or release energy:

• Energy Absorbed (Cooling effect on surroundings): Melting (solid to liquid) and Vaporization (liquid to gas).
• Energy Released (Warming effect on surroundings): Condensation (gas to liquid) and Freezing/Solidification (liquid to solid).

In the atmosphere, when water vapor condenses into clouds or raindrops, it releases latent heat of condensation, which is a major driver of weather patterns and energy transfer in our environment Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295.

Sources: Science Class VIII, Particulate Nature of Matter, p.109; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.294; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295

3. Properties of Metals and Magnetism (intermediate)

To understand metals, we must distinguish between their inherent physical properties and the chemical changes they undergo. A primary example is iron (Fe). In its pure form, iron is attracted to magnets, a physical property. When we magnetize a piece of iron, we are performing a physical change. This process is reversible and does not alter the chemical identity of the iron atoms; it merely aligns their internal magnetic domains Science-Class VII, Changes Around Us, p. 68. Similarly, physical transitions like the condensation of a metal vapor back into a liquid do not change the substance's molecular structure, maintaining its chemical essence.

However, the properties of metals can be significantly modified through alloying or chemical reactions. An alloy is a 1homogeneous mixture of a metal with other elements. For instance, mixing iron with carbon creates steel, while adding chromium and nickel produces stainless steel, which is resistant to rusting Science, Class X, Metals and Non-metals, p. 54. While alloys are mixtures, the formation of a compound is a chemical change that fundamentally alters properties. For example, if iron filings and sulfur are heated, they react to form iron sulfide (FeS). Unlike the original mixture, iron sulfide is not attracted to a magnet because the iron has lost its individual metallic characteristics to form a new substance Science, Class VIII, Nature of Matter, p. 128.

In the industrial context, we strategically use these principles to 'tune' a metal's utility. By adding specific minerals, we can enhance certain traits:

Added Element	Resulting Property Change in Iron
Manganese	Improves magnetic qualities and toughness for cutting tools.
Chromium	Retards rusting (corrosion resistance).
Vanadium	Increases resilience, making it excellent for springs.
Tungsten	Raises the melting point.

Certificate Physical and Human Geography, Manufacturing Industry, p. 284

Sources: Science-Class VII, Changes Around Us, p.68; Science, Class X, Metals and Non-metals, p.54; Science, Class VIII, Nature of Matter, p.128; Certificate Physical and Human Geography, Manufacturing Industry, p.284

4. Oxidation, Reduction, and Corrosion (intermediate)

At its core, chemistry is often about the transfer of elements and energy. One of the most fundamental ways this happens is through Oxidation and Reduction. Historically, these terms were defined by the movement of oxygen. If a substance gains oxygen during a reaction, we say it has been oxidised. Conversely, if a substance loses oxygen, it is reduced. Because these two processes almost always occur together—where one substance gives up oxygen and another takes it—we call them Redox reactions (Reduction-Oxidation) Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12. For instance, when hydrogen gas is passed over heated copper(II) oxide, the copper oxide loses oxygen (is reduced) to become copper metal, while the hydrogen gains oxygen (is oxidised) to become water (H₂O).

While some oxidation happens rapidly, like the burning of magnesium ribbon in air to form magnesium oxide Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2, other processes are slow and destructive. This is known as Corrosion. Corrosion is the gradual deterioration of a metal surface caused by its reaction with environmental factors like air, moisture, or chemicals. The most famous example is the rusting of iron. When iron is exposed to oxygen and water, it undergoes a chemical change to form a flaky, brown substance called iron oxide (rust) Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62.

It is important to note that corrosion isn't unique to iron. Other metals show different signs of atmospheric reaction: silver develops a black coating, and copper develops a greenish layer over time Science-Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.50. From an administrative and economic perspective, understanding corrosion is vital because it causes massive damage to infrastructure like bridges, ships, and railway tracks, requiring significant resources for prevention and repair.

Process	Change in Oxygen	Example
Oxidation	Gain of Oxygen	Magnesium burning (2Mg + O₂ → 2MgO)
Reduction	Loss of Oxygen	Extraction of metals from oxides
Corrosion	Slow oxidation by environment	Rusting of iron (Fe → Iron Oxide)

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2; Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.62; Science-Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.50

5. Combustion and Fuel Chemistry (intermediate)

At its heart, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat and light. Unlike physical changes—such as the magnetization of iron or the condensation of steam, where the molecular identity remains the same—combustion is a chemical change because it fundamentally rearranges atoms to create entirely new substances like Carbon Dioxide (CO₂) and Water (H₂O) Science-Class VII, Changes Around Us: Physical and Chemical, p. 62. For combustion to occur, three things are essential: a combustible substance (fuel), a supply of oxygen, and reaching the ignition temperature. If you limit the oxygen supply, the reaction becomes 'incomplete,' leading to the formation of soot, which is simply unburnt carbon particles Science, class X, Carbon and its Compounds, p. 70.

The nature of the fuel also dictates how it burns. Saturated hydrocarbons (like the methane in your PNG or LPG) typically undergo complete combustion and give a clean, blue flame. However, unsaturated carbon compounds or even saturated ones burned in a limited air supply will produce a yellow, sooty flame Science, class X, Carbon and its Compounds, p. 69. This yellow color is actually caused by those tiny unburnt carbon atoms heating up until they glow—a process known as incandescence. This is why a gas stove has air inlets; if these holes get blocked, the oxygen supply drops, the flame turns yellow, and your utensils get blackened by soot.

It is helpful to compare combustion with rusting. Both are chemical changes involving oxygen, but they differ in speed and energy release. Rusting is a slow oxidation of iron in the presence of moisture to form iron oxide, whereas combustion is a rapid oxidation that releases significant energy Science-Class VII, Changes Around Us: Physical and Chemical, p. 68. Understanding these distinctions is crucial for identifying how matter transforms in our environment.

Type of Change	Example	Key Characteristic
Physical Change	Condensation, Magnetization	No new substance formed; often reversible.
Chemical Change	Combustion, Rusting	New substances formed; involves breaking/making bonds.

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.62, 68; Science , class X (NCERT 2025 ed.), Carbon and its Compounds, p.69, 70

6. Criteria for Physical vs. Chemical Changes (exam-level)

To master chemistry, you must first distinguish between changes that merely alter the 'look' of a substance and those that transform its 'soul' or chemical identity. A physical change is one where a substance undergoes a change in its physical properties—such as shape, size, or state—but no new substance is formed Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59. Think of melting ice or condensing steam; while the form changes from solid to liquid or gas, the molecules remain H₂O throughout. Processes like the magnetization of iron or the weathering of rocks by wind and water are also physical, as the underlying chemical composition remains intact Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68.

In contrast, a chemical change occurs when one or more new substances are created through a chemical reaction. This process involves the breaking and forming of chemical bonds, resulting in products with entirely different properties than the original materials Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68. For instance, in combustion (burning), a fuel reacts with oxygen to produce heat, light, and new gases like CO₂. Similarly, rusting transforms shiny iron into flaky iron oxide (rust) when exposed to moisture and air. While many physical changes can be reversed (like refreezing melted water), most chemical changes, such as cooking an egg or curdling milk, are difficult or impossible to undo Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.66.

Here is a quick reference to help you categorize these changes during your revision:

Feature	Physical Change	Chemical Change
New Substance	None formed	One or more new substances formed
Properties	Only physical properties change	Chemical properties and composition change
Reversibility	Often reversible (e.g., melting)	Usually irreversible (e.g., burning)
Examples	Boiling, chopping, magnetization	Rusting, digestion, photosynthesis

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

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental distinctions between physical and chemical changes, this question serves as the perfect application of those principles. You have learned that the litmus test for any change is the formation of a new substance. While a physical change only alters the appearance or state of matter—like shape or phase—a chemical change involves a chemical reaction that fundamentally rearranges atoms into new molecular structures. This question tests your ability to look past the surface and identify whether the chemical identity of the material has truly transformed.

Walking through the options, we apply your building blocks: Magnetization is a physical change because the iron atoms simply align their magnetic moments without changing into something else, and condensation is merely a phase transition where gas molecules slow down to become liquid. In contrast, burning of fuel (combustion) and rusting of iron are irreversible reactions that produce entirely new substances like carbon dioxide and iron oxide, respectively. Therefore, by identifying that only statements 3 and 4 involve the creation of new chemical species, we arrive at the correct answer: (C) 3 and 4.

UPSC frequently uses phase changes (like condensation) and mechanical/magnetic alterations (like magnetization) as traps because they can appear complex, but they remain physical changes. A common mistake is to assume that because a process seems "permanent" or "drastic," it must be chemical. Always lean on the core rule from Science-Class VII . NCERT(Revised ed 2025): if the molecular composition remains the same, it is physical; if a new substance is born, it is chemical. Master this distinction, and you will navigate these types of questions with ease.