Which of the following are not chemical changes ? 1. Tempering of iron 2. Conversion of iron piece into an electro-magnet by passing the current around the iron 3. Melting of iron 4. Rusting of iron Select the correct answer using the code given below :

1. Foundations: Understanding Physical Changes (basic)

Welcome to your first step in mastering chemical principles! To understand how the world works, we must first distinguish between changes that merely alter the appearance of a substance and those that change its very essence. 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. Essentially, the molecules or atoms remain exactly the same before and after the change Science-Class VII, Chapter 5, p. 68.

Think of physical properties as the "outer clothing" of a substance. If you melt a block of ice into liquid water, you have changed its state from solid to liquid, but the chemical formula remains H₂O. Similarly, if you take a piece of iron and heat it until it glows or melts, it is still iron (Fe). Even sophisticated industrial processes like tempering—where metal is heated and cooled to change its hardness or ductility—are physical changes because they only rearrange the internal crystal structure (microstructure) without altering the chemical identity of the metal itself Science-Class X, Chapter 3, p. 54.

Another fascinating example is the creation of an electromagnet. When an electric current passes through a wire coiled around an iron nail, the nail becomes magnetic. This is a physical change because the magnetism is a temporary property gained due to the flow of electrons; no chemical reaction occurs, and the iron does not turn into a different material Science-Class VIII, Chapter 4, p. 58. While many physical changes are reversible (like freezing water), some are irreversible (like chopping wood into sawdust), but the golden rule remains: no new substance is born Science-Class VII, Chapter 5, p. 66, 69.

Feature	Physical Change	Chemical Change
New Substance	None formed	One or more new substances created
Properties Affected	Shape, size, state, magnetism	Chemical composition, reactivity
Examples	Melting ice, shredding paper, tempering steel	Rusting iron, burning wood, curdling milk

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.59, 66, 68, 69; Science-Class VIII, Electricity: Magnetic and Heating Effects, p.58; Science-Class X, Metals and Non-metals, p.54

2. Fundamentals of Chemical Reactions (basic)

At the heart of chemistry lies the distinction between how matter changes. A physical change involves a shift in the physical properties of a substance—such as its shape, size, or state (solid, liquid, gas)—without altering its fundamental chemical identity. For example, when iron is heated until it melts, it remains iron; only its state has changed from solid to liquid Science-Class VII, Chapter 5, p.68. Similarly, magnetizing a piece of iron or tempering it to adjust its hardness are physical processes because the arrangement of iron atoms might change slightly, but no new chemical bonds are formed.

In contrast, a chemical change (or chemical reaction) occurs when substances react to form entirely new products with different chemical properties. The most common example is the rusting of iron. Here, iron reacts with oxygen and moisture in the air to form a new substance: iron oxide (Fe₂O₃). Unlike physical changes, chemical changes usually involve the breaking and making of chemical bonds, often accompanied by observable signs like gas bubbles, color changes, or temperature shifts Science-Class VII, Chapter 5, p.60-62.

To describe these changes precisely, scientists use chemical equations. These represent the reactants (the starting materials) and the products (the resulting substances) symbolically Science, Class X, Chapter 1, p.14. A fundamental rule in these reactions is the Law of Conservation of Mass: atoms are neither created nor destroyed. Therefore, an equation must be balanced, ensuring the number of atoms of each element is the same on both sides of the arrow.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Reversibility	Often easily reversible (e.g., freezing water).	Usually irreversible by simple physical means.
Example	Melting of wax, stretching a rubber band.	Burning of paper, digestion of food.

Beyond identity, reactions are also classified by their energy flow. Exothermic reactions (like respiration) release heat into the surroundings, while endothermic reactions (like photosynthesis) absorb energy to proceed Science, Class X, Chapter 1, p.15.

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.60, 62, 68; Science, Class X, Chemical Reactions and Equations, p.14, 15

3. Atmospheric Corrosion and Oxidation (intermediate)

At its core, corrosion is a process of chemical deterioration. It occurs when a metal is attacked by substances in its environment—primarily oxygen, moisture (water vapor), and gases like carbon dioxide or sulphur dioxide. Unlike physical changes like melting or tempering, where the metal's internal chemistry remains the same, corrosion is a chemical change because it results in the formation of entirely new substances, such as metal oxides or sulphides Science-Class VII, NCERT (Revised ed 2025), Chapter 5, p. 62. For example, when iron reacts with oxygen and moisture, it forms a reddish-brown flaky substance called rust (hydrated iron oxide), which lacks the strength and lustre of the original metal Science, class X (NCERT 2025 ed.), Chapter 1, p. 13. While iron is the most common victim of atmospheric oxidation, other metals also undergo similar transformations, often referred to as tarnishing. This process creates distinct visual markers depending on the metal involved:

Metal	Reactant in Atmosphere	Corrosion Product (New Substance)	Visible Appearance
Iron	Oxygen + Moisture	Hydrated Iron Oxide	Reddish-brown powder (Rust)
Silver	Sulphur compounds	Silver Sulphide (Ag₂S)	Black coating
Copper	Moist Carbon Dioxide	Basic Copper Carbonate	Green coating

Science, class X (NCERT 2025 ed.), Chapter 3, p. 53 To combat this, we use prevention techniques that either block the metal's contact with air or change its chemical properties. Galvanisation is a popular industrial method where iron or steel is coated with a thin layer of zinc; interestingly, even if the zinc layer is scratched, it continues to protect the iron underneath by reacting in its place. Alloying is another powerful tool—pure iron is too soft for construction, but by mixing it with small amounts of carbon, chromium, or nickel (to create stainless steel), we create a material that is both strong and highly resistant to rusting Science, class X (NCERT 2025 ed.), Chapter 3, p. 54.

Sources: Science-Class VII, NCERT (Revised ed 2025), Chapter 5: Changes Around Us: Physical and Chemical, p.62; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.13; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.53-54

4. Electromagnetism: Physical vs. Chemical Effects (intermediate)

To understand the world of matter, we first distinguish between physical and chemical changes. A physical change involves a shift in the material's properties—like its shape, state, or magnetic character—without altering its fundamental chemical identity. In contrast, a chemical change results in the formation of entirely new substances with different properties. For instance, when iron reacts with oxygen and moisture to form iron oxide (rust), it undergoes a chemical change because a new compound is created Science Class VII, Chapter 5, p. 62.

When we talk about electromagnetism, we are looking at the magnetic effect of electric current. When an electric current flows through a conductor, it generates a magnetic field around it Science Class VIII, Chapter 4, p. 48. If we wind this wire into a coil (a solenoid) and place a soft iron core inside, the iron becomes a powerful magnet called an electromagnet Science Class X, Chapter 12, p. 201. This is a classic physical change. Why? Because the iron (Fe) remains iron. Its atoms have simply aligned their magnetic domains in response to the external field. No chemical bonds are broken, and no new molecules are formed. Furthermore, this effect is often temporary; the magnetism typically disappears when the current is switched off Science Class VIII, Chapter 4, p. 58.

It is helpful to compare these processes side-by-side to see why electromagnetism sits firmly in the 'physical' category:

Process	Type of Change	Reasoning
Magnetizing Iron	Physical	Changes magnetic properties; no new substance formed; identity remains Fe.
Melting Iron	Physical	Change of state (solid to liquid); chemical identity unchanged.
Rusting of Iron	Chemical	Reaction with O₂ and H₂O; forms new substance (Iron Oxide).
Battery Operation	Chemical	Current is produced by chemical reactions inside the cell Science Class VIII, Chapter 4, p. 58.

Sources: Science Class VII, Chapter 5: Changes Around Us: Physical and Chemical, p.62; Science Class VIII, Chapter 4: Electricity: Magnetic and Heating Effects, p.48; Science Class VIII, Chapter 4: Electricity: Magnetic and Heating Effects, p.58; Science Class X, Chapter 12: Magnetic Effects of Electric Current, p.201

5. Metallurgy: Heat Treatment and Microstructure (exam-level)

To understand metallurgy, we must first look at the hidden world of microstructure. Metals are not uniform blocks; they are composed of microscopic crystals called grains. The size, shape, and arrangement of these grains determine how hard, brittle, or ductile a metal is. Heat treatment is the process of using controlled heating and cooling to rearrange these grains without changing the chemical identity of the metal itself. Because the metal remains chemically the same (e.g., iron remains iron), these treatments are classified as physical changes, unlike rusting, which creates a entirely new substance like iron oxide Science-Class VII, Changes Around Us: Physical and Chemical, p. 62.

One of the most vital heat treatments is tempering. When iron or steel is quenched (cooled rapidly), it often becomes very hard but extremely brittle, meaning it might shatter like glass under stress. Tempering involves reheating the metal to a specific temperature below its melting point and then cooling it slowly. This allows the internal stresses to relax and the microstructure to reorganize, striking a perfect balance between toughness and hardness. In industrial settings, high-quality steel is often refined further in electric furnaces or through oxygen processes to ensure the purity of this microstructure Certificate Physical and Human Geography, Manufacturing Industry and The Iron and Steel Industry, p.286.

It is helpful to compare these processes to understand the boundaries of physical and chemical science. While melting iron is a physical change of state (solid to liquid) and tempering is a physical change of structure, rusting is a chemical change because it involves a reaction with oxygen and moisture Science, class X, Chemical Reactions and Equations, p. 13. Even creating an electromagnet by passing a current through an iron core is a physical change, as the magnetism is a temporary physical property that doesn't alter the iron's atoms Science, Class VIII, Electricity: Magnetic and Heating Effects, p. 58.

Process	Type of Change	Result
Tempering	Physical	Altered grain structure (toughness)
Melting	Physical	Change of state (solid to liquid)
Rusting	Chemical	Formation of iron oxide (corrosion)

Sources: Science-Class VII, Changes Around Us: Physical and Chemical, p.62; Certificate Physical and Human Geography, Manufacturing Industry and The Iron and Steel Industry, p.286; Science, class X, Chemical Reactions and Equations, p.13; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58

6. Distinguishing Iron's Physical and Chemical Transformations (exam-level)

To master the chemistry of iron, we must distinguish between changes that simply alter its physical state or properties and those that fundamentally transform its chemical identity. A physical change involves a transition in form—such as size, shape, or state of matter—without the formation of any new chemical substance. For instance, when iron is heated to its melting point of 1538 °C Science, Class VIII, Particulate Nature of Matter, p.103, the thermal energy simply overcomes the attractive forces holding the iron atoms in a rigid lattice. While it turns from a solid to a liquid, the particles remain iron (Fe) atoms; no new substance is created Science, Class VIII, Particulate Nature of Matter, p.112.

Similarly, certain industrial and magnetic processes are purely physical. Magnetization, such as turning an iron core into an electromagnet using an electric current, is a temporary physical change where the iron gains magnetic properties without any internal chemical reaction Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58. Even tempering—a heat treatment used to adjust hardness and ductility—is a physical transformation. While tempering changes the "microstructure" (the arrangement of iron crystals), the chemical composition remains 100% iron. It is a modification of the metal's physical characteristics, not its molecular essence.

In sharp contrast, a chemical change results in the formation of entirely new substances with different properties. The most common example is rusting. When iron is exposed to oxygen and moisture, it undergoes a chemical reaction to form iron oxide (Fe₂O₃·nH₂O), a brown, flaky substance that is chemically distinct from the original metal Science, Class VII, Changes Around Us: Physical and Chemical, p.62. Other chemical changes include the reaction of iron with acids, such as adding dilute hydrochloric acid to iron filings, which produces hydrogen gas and iron chloride Science, Class X, Chemical Reactions and Equations, p.15.

Feature	Physical Change (e.g., Melting, Tempering)	Chemical Change (e.g., Rusting, Acid Reaction)
New Substance	No new substance formed.	New substance (e.g., Iron Oxide) created.
Reversibility	Often easily reversible (e.g., freezing molten iron).	Usually irreversible by simple physical means.
Internal Identity	Atomic identity (Fe) remains unchanged.	Atoms bond with others to form compounds.

Sources: Science-Class VII, NCERT (2025), Changes Around Us: Physical and Chemical, p.62; Science, Class VIII, NCERT (2025), Particulate Nature of Matter, p.103, 112; Science, Class VIII, NCERT (2025), Electricity: Magnetic and Heating Effects, p.58; Science, Class X, NCERT (2025), Chemical Reactions and Equations, p.15

7. Solving the Original PYQ (exam-level)

This question tests your ability to apply the fundamental distinction between physical and chemical changes to real-world industrial and scientific processes. In your previous lessons, you learned that a physical change alters the form or state of a substance without changing its molecular identity, while a chemical change results in the formation of a new substance. To solve this, you must analyze each process: melting of iron is a simple change of state (solid to liquid), and creating an electro-magnet merely induces temporary magnetic properties; neither creates a new chemical species. Tempering is often a trap for students—it is a heat-treatment process that modifies the microstructure to adjust hardness, but the substance remains iron. Conversely, rusting involves a chemical reaction with oxygen and moisture to form iron oxide, as detailed in Science-Class VII . NCERT(Revised ed 2025).

To arrive at the correct answer, you must pay close attention to the negative constraint in the question: "not chemical changes." This means you are searching for physical changes. Since tempering (1), electromagnetism (2), and melting (3) do not alter the chemical composition of iron, they are physical. Rusting (4) is a definitive chemical change and must be excluded. Therefore, the combination of 1, 2, and 3 leads us to Correct Answer: (C). In the UPSC Civil Services Examination, the examiners frequently include technical-sounding terms like "tempering" to intimidate candidates, but if you stick to the core principle—is a new substance being formed?—you can navigate these options confidently.

Common traps in this type of question include the misinterpretation of the word "not" or confusing microstructural changes (like tempering) with chemical reactions. Students often select Option D because they instinctively associate heat-related processes with chemical changes. However, as noted in Science-Class VIII . NCERT(Revised ed 2025), effects like magnetism and state changes are classic physical phenomena. Always double-check if the process is reversible in principle or if the chemical formula of the material stays the same to ensure you haven't fallen for a distractor.