Why this chapter matters for UPSC: Prelims general science keeps returning to one question: is this change physical or chemical? Rusting and its prevention, the energy given out or taken in by a reaction, and oxidation in everyday life (rancid oil, bleaching, the thermit welding of rails) all grow from the simple sorting this chapter begins. This chapter was dropped from NCERT's rationalised 2023-24 edition; the topic is now taught in NCERT Class VII, Chapter 5, "Physical and Chemical Changes". BharatNotes keeps it because the ideas run through the whole general-science syllabus.

🧠 First Principles — Read This First

Changes happen around us all the time. Crops change in the fields, leaves change colour and dry, flowers bloom and wither, and in our own bodies nails and hair grow and we grow taller. NCERT asks how such changes can be grouped, and offers two ideas.

Can the change be reversed? Blow up a balloon and let the air out; fold paper into a toy aeroplane and unfold it; roll dough into a roti and make it a ball again. In each case we get back the material we started with: these changes can be reversed. Now burst the balloon, cut the aeroplane out of the paper, bake the roti on a tawa: these changes cannot be reversed.
How was the change brought about? NCERT names two ways: by heating a substance (wax melts, water boils away, an iron ring expands) and by mixing it with another substance (salt dissolves in water, warm milk with a little curd sets into curd).

In higher classes these ideas become physical and chemical changes: a physical change alters only properties such as shape, size, colour and state, and forms no new substance; a chemical change forms one or more new substances.

PART 1 — Quick Reference

Changes in NCERT Class VI: Can They Be Reversed?

Change	Can it be reversed?	Why
Blowing a balloon, then letting the air out	Yes	Shape and size come back
Folding paper into an aeroplane	Yes	Unfold it
Rolling dough into a roti	Yes	Roll it back into a ball
Bursting the balloon; cutting the paper; baking the roti	No	The original cannot be recovered
Shaping a lump of clay into a pot	Yes, while the clay is wet
Baking the clay pot in an oven	No
Heating wax in a pan; melting ice	Yes	Cooling makes them solid again
Burning a candle, an incense stick or a matchstick	No	Burning produces new material: ash and gases
Dissolving salt or sugar in water	Yes	Recover the salt by evaporation (Chapter 5)
Setting curd from warm milk	No
Sawing wood; cooking food; ripening of a mango; souring of milk	No	NCERT exercise
Plaster of Paris setting hard on a bandage; cement wetted by rain	No	NCERT exercise

Physical and Chemical Changes (NCERT Class VII)

	Physical change	Chemical change
Definition	A change in physical properties: shape, size, colour, state	A change in which one or more new substances are formed; also called a chemical reaction
New substance?	No	Yes
Reversible?	Generally reversible	New substances cannot be turned back easily
NCERT examples	Cutting paper; chalk dust rolled back into chalk; ice melting and refreezing; steam condensing; a hack-saw blade turning colour in a flame and back on cooling; crystallisation	Rusting; burning of magnesium, coal, wood or leaves; copper sulphate + iron; vinegar + baking soda; digestion, ripening, fermentation; fireworks; an apple slice turning brown

PART 2 — Concepts & Narrative

Change by Heating: Expansion and Contraction

Explainer

NCERT's two village examples:

Fixing a handle: the iron blade of a digging tool has a ring slightly smaller than its wooden handle. The ring is heated, becomes slightly larger (expands), and the handle slips in. As it cools the ring contracts and grips the handle tightly.
Fixing a cart wheel: the metal rim is made slightly smaller than the wooden wheel. Heated, it expands and fits over the wheel; cold water poured over it makes it contract and hold tight.

Other changes by heating in the chapter: a candle gets shorter as it burns (cannot be reversed), but wax heated in a pan melts and sets again (can be reversed); water boils into vapour, and vapour cooled on a plate turns back to water; a blacksmith heats iron till it is red-hot and soft, then beats it into shape.

NCERT's lemon-juice ink: write a message on paper with lemon juice and let it dry; the letters vanish. Warm the paper with a hot iron or above a candle flame and the letters reappear in dark brown. The drying can be reversed; the browning cannot.

From Reversible to Physical and Chemical

NCERT Class VII makes the idea precise. In a physical change no new substance is formed: paper cut into pieces is still paper, and chalk dust made into a paste and dried is chalk again. In a chemical change a new substance appears. NCERT's demonstrations:

Burning magnesium ribbon gives a brilliant white light and leaves a powdery ash, magnesium oxide. Mixed with water, the ash forms magnesium hydroxide, a base that turns red litmus blue.
Iron in copper sulphate solution: the blue solution turns green (iron sulphate) and a brown deposit of copper forms on the nail.
Vinegar and baking soda (acetic acid and sodium hydrogencarbonate) fizz and give carbon dioxide. Passed through lime water, it forms calcium carbonate, which turns lime water milky: the standard test for carbon dioxide.

Key Term

What may accompany a chemical change (NCERT Class VII): besides new products,

heat, light or other radiation (ultraviolet, for example) may be given off or absorbed;
sound may be produced;
a change in smell, or a new smell, may occur;
a colour change may take place;
a gas may be formed.

Burning of any substance is a chemical change and is always accompanied by heat. A firework's explosion gives heat, light, sound and unpleasant, polluting gases. Spoiled food smells foul; a cut apple, potato or brinjal turns brown as new substances form. The deciding test is always whether a new substance has formed: a hack-saw blade changes colour in a flame but gets its colour back on cooling, which is a physical change.

Both at once: when a candle burns, the wax melting is a physical change and the wax burning is a chemical change.

Rusting and Corrosion

Iron left in the open gets a film of a brownish substance, rust. Rust is not iron; it is a different substance deposited on it, so rusting is a chemical change:

Iron (Fe) + Oxygen (O₂, from the air) + Water (H₂O) → Rust (iron oxide, Fe₂O₃)

Both oxygen and water (or water vapour) are essential. The more humid the air, the faster iron rusts. That is why iron rusts faster in coastal areas than in deserts.
Salt water speeds rusting. Ships, partly under water and wet above it, suffer so much damage that a fraction of a ship's iron has to be replaced every year.
Prevention: keep oxygen, water or both away from the iron: a coat of paint or grease, applied regularly; or a layer of chromium or zinc. Depositing zinc is galvanisation; household water pipes are galvanised.
Stainless steel does not rust. NCERT Class VII: iron mixed with carbon and metals like chromium, nickel and manganese. NCERT Class X: iron mixed with nickel and chromium; it calls stainless steel an alloy, a homogeneous mixture of two or more metals, or of a metal and a non-metal.

NCERT Class X widens rusting into corrosion: a metal attacked by substances around it, such as moisture and acids, is said to corrode. The black coating on silver and the green coating on copper are corrosion too. Corrosion damages car bodies, bridges, iron railings and ships, and every year an enormous amount of money is spent replacing damaged iron.

UPSC Connect

The Iron Pillar near the Qutub Minar, Delhi. NCERT presents it as a wonder of ancient Indian metallurgy: built more than 1,600 years ago, it has not rusted, and scientists from all parts of the world have examined it for its rust resistance. The two NCERT books give slightly different dimensions:

NCERT Class VII: "more than 7 metres high", weighing "more than 6000 kg".
NCERT Class X: "8 m high", weighing "6 tonnes (6000 kg)".

NCERT's Class VI history book dates it differently again, to "about 1,500 years ago", and notes that its inscription mentions a ruler named Chandra, who probably belonged to the Gupta dynasty; see Buildings, Paintings and Books.

Crystallisation: A Physical Change That Purifies

Salt obtained by evaporating sea water is not pure, and its crystals are not clearly shaped. Large crystals of pure substances can be formed from their solutions; the process is called crystallisation, and it is a physical change. NCERT's activity: dissolve copper sulphate powder in boiling water (with a few drops of dilute sulphuric acid) until no more dissolves, filter, and let the solution cool undisturbed: blue crystals of copper sulphate appear. NCERT Class VII also suggests making crystals of alum. For why crystallisation purifies better than plain evaporation, see Separation of Substances.

Energy in Chemical Reactions (NCERT Class X)

Key Term

Exothermic reactions release heat along with forming products. NCERT's examples:

Quicklime and water (calcium oxide + water gives slaked lime): a large amount of heat is evolved. The slaked-lime solution is used for whitewashing; it reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls.
Burning of natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O.
Respiration: glucose + oxygen → carbon dioxide + water + energy.
Decomposition of vegetable matter into compost.

Endothermic reactions absorb energy. NCERT's examples:

Decomposition reactions need energy as heat, light or electricity to break down the reactants. Silver bromide breaks down in sunlight into silver and bromine, the reaction once used in black-and-white photography.
Barium hydroxide + ammonium chloride: mix them in a test tube and the bottom feels cold.

Do not mix up reactions and physical changes. Melting ice and evaporating water also take in heat, but they are physical changes, not reactions, so they are not "endothermic reactions". In the same way, dissolving a solid in water forms no new substance, so it is not a reaction either.

Oxidation and Reduction (NCERT Class X)

A substance that gains oxygen or loses hydrogen is oxidised.
A substance that loses oxygen or gains hydrogen is reduced.
One reactant is oxidised while the other is reduced: a redox reaction. In CuO + H₂ → Cu + H₂O, copper oxide is reduced and hydrogen is oxidised; in ZnO + C → Zn + CO, carbon is oxidised and zinc oxide is reduced.

Rancidity: fats and oils left for a long time are oxidised; they become rancid and their smell and taste change. Antioxidants are added to foods containing fats and oil; airtight containers slow oxidation; chips makers flush their bags with nitrogen.

UPSC Connect

Chemistry at work in India (NCERT Class X):

Thermit reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat. Aluminium, more reactive than iron, displaces it from its oxide. The heat is so large that the iron is produced molten, and the reaction is "used to join railway tracks or cracked machine parts". Iron oxide is reduced; aluminium is oxidised.
Bleaching powder: made by the action of chlorine on dry slaked lime; represented as Ca(ClO)₂, "though the actual composition is quite complex". It bleaches cotton and linen, wood pulp and washed clothes, works as an oxidising agent in chemical industries, and is used to make drinking water free from germs.
Biogas: anaerobic bacteria digest animal waste to produce biogas (a chemical change), and burning the biogas as fuel is a second chemical change (NCERT Class VII exercise).
The ozone shield: ozone in the atmosphere absorbs harmful ultraviolet radiation from the sun and breaks down to oxygen, a different substance; NCERT Class VII asks students to recognise this as a chemical change.

PART 3 — UPSC Integration

The habit this chapter builds, asking whether a new substance has formed, is the key to a large share of general-science questions. Corrosion explains why bridges, ships and pipelines need paint, galvanising and stainless steel, and why coastal infrastructure costs more to maintain. Exothermic reactions supply the energy in fuels, respiration and composting; the thermit reaction welds rail joints in the field. Oxidation explains rancid oil and the use of antioxidants and nitrogen-filled packs in food processing, and bleaching powder's oxidising power disinfects drinking water. Environmental questions use the same vocabulary: the burning of fuels and fireworks releases polluting gases, biogas and compost turn waste into useful products through chemical change, and the ozone layer protects life by a chemical change of its own.

Exam Strategy

Prelims traps:

Rusting needs both oxygen and water. Iron under ordinary water still rusts; humid and salty conditions make it faster.
Crystallisation is a physical change; rusting, burning, curd setting, ripening, digestion and photosynthesis are chemical changes.
Dissolving salt or sugar can be reversed by evaporation; it forms no new substance.
Galvanisation = zinc coating. Chromium plating is a different coating.
Lime water turns milky with carbon dioxide because calcium carbonate forms.
Oxidation: gain of oxygen or loss of hydrogen. Reduction: loss of oxygen or gain of hydrogen.
Thermit: iron oxide + aluminium; used to join railway tracks.
LPG turning from liquid to gas is physical; its burning is chemical.
Iron pillar: NCERT VII says "more than 7 m"; NCERT X says "8 m". Both science books say it is more than 1,600 years old; the Class VI history book says about 1,500.

Mains angle: Explain why rusting is faster in coastal regions and how galvanisation, painting and alloying prevent it; or show how one familiar process (a burning candle, cooking, biogas) combines physical and chemical changes.

Practice Questions

Practice (UPSC-pattern, not past papers).

Prelims:

LPG leaves the cylinder as a gas (Change A) and then burns (Change B). Which of the following is correct?
(a) Change A is a chemical change
(b) Change B is a chemical change
(c) Both A and B are chemical changes
(d) Neither A nor B is a chemical change
Anaerobic bacteria digest animal waste to produce biogas (Change A), and the biogas is burnt as fuel (Change B). Which of the following is correct?
(a) Only A is a chemical change
(b) Only B is a chemical change
(c) Both A and B are chemical changes
(d) Neither is a chemical change
Which of the following is a physical change?
(a) Rusting of iron
(b) Setting of curd
(c) Formation of copper sulphate crystals from its solution
(d) Burning of magnesium ribbon
Consider the following statements:
1. A substance that gains oxygen in a reaction is oxidised.
2. A substance that loses hydrogen in a reaction is oxidised.
3. A substance that gains hydrogen in a reaction is oxidised.
  Which of the statements given above are correct?
  (a) 1 and 2 only
  (b) 2 and 3 only
  (c) 1 and 3 only
  (d) 1, 2 and 3
The thermit reaction, used to join railway tracks, is between:
(a) Copper oxide and hydrogen
(b) Iron(III) oxide and aluminium
(c) Zinc oxide and carbon
(d) Calcium oxide and water
Iron articles rust faster in coastal areas than in deserts mainly because:
(a) Coastal air has more oxygen
(b) Coastal air is more humid and carries salt, both of which speed rusting
(c) Deserts are hotter, and heat prevents rusting
(d) Sea breezes remove the protective rust layer
Which of the following reactions absorbs heat?
(a) Calcium oxide reacting with water
(b) Burning of natural gas
(c) Barium hydroxide reacting with ammonium chloride
(d) Respiration

Mains (UPSC-pattern):

"Rusting is a chemical change that costs nations dearly." Explain the conditions for rusting and the methods used to prevent it, with examples from Indian infrastructure. (150 words)
Distinguish between physical and chemical changes. Show how a burning candle involves both. (150 words)

📦 Revision Capsule

Revision Capsule

Hard Facts

Some changes can be reversed (folding paper, melting wax, dissolving salt); some cannot (burning, baking a roti or a clay pot, curd setting, ripening, souring)
Changes can be brought about by heating or by mixing
Heating expands, cooling contracts: iron ring on a tool handle; metal rim on a cart wheel
Physical change: no new substance; chemical change (reaction): new substance(s)
Rust: iron + oxygen + water; needs both oxygen and water; faster when humid or salty
Prevention: paint, grease, galvanisation (zinc), chromium; stainless steel = iron + carbon + chromium, nickel, manganese (NCERT VII)
Lime water turns milky with CO₂ (calcium carbonate)
Thermit: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat; joins railway tracks
Bleaching powder: chlorine + dry slaked lime; Ca(ClO)₂; disinfects drinking water

Core Concepts

The test of a chemical change is a new substance, not just heat, colour or gas
Exothermic reactions release heat (burning, respiration, quicklime + water, composting); endothermic reactions absorb it (decomposition by heat, light or electricity)
Oxidation = gain of oxygen or loss of hydrogen; rancidity is oxidation of fats and oils

Confused Pairs

Reversible (wax melting) vs irreversible (candle burning)
Physical (crystallisation, melting) vs chemical (rusting, burning)
Galvanisation (zinc) vs stainless steel (alloy)
Endothermic reaction vs a physical change that absorbs heat (melting, evaporation)
Corrosion of iron (reddish-brown) vs silver (black) vs copper (green)

PYQ Pattern

No question in BharatNotes' PYQ bank tests this chapter directly. Expect Prelims general science to ask physical vs chemical classification, conditions for rusting, galvanisation, redox and exothermic examples.

Sources

Sources

NCERT, Science (Class VI), ch. 6 "Changes Around Us" (2020-21 print; dropped from the Rationalised 2023-24 edition): NCERT's book archives as captured by the Wayback Machine, 2020 copy and 2023-24 copy.
NCERT, Science (Class VII), ch. 5 "Physical and Chemical Changes", Reprint 2024-25: book archive.
NCERT, Science (Class X), ch. 1 "Chemical Reactions and Equations", ch. 2 "Acids, Bases and Salts" and ch. 3 "Metals and Non-metals", Reprint 2025-26: book archive.