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← Science — all chapters

Class 10ScienceScienceChapter 1

Chemical Reactions and Equations

≈ 40 min read · 4 Prelims and 0 Mains questions from this chapter

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Start here: why this chapter matters

In simple wordsCooking an egg, a nail going rusty, milk turning sour, a matchstick burning — in each, the stuff you started with turns into new stuff that was not there before. Chemistry has a shorthand for writing these changes down, like a recipe: what goes in, what comes out, and in what amounts. This chapter teaches that shorthand and sorts reactions into a few families, so that thousands of reactions start to look like variations on five patterns.

This is the first chapter of NCERT's Class 10 Science textbook (pages 1–16 of the 2026-27 reprint). It has three parts — chemical equations (1.1, with writing and balancing), types of chemical reactions (1.2: combination, decomposition, displacement, double displacement, oxidation and reduction) and oxidation in everyday life (1.3: corrosion and rancidity) — with 11 activities, Figures 1.1–1.10, one ‘Do You Know?’ box, three in-text question boxes, a summary and 20 exercise questions plus a group activity. It is the first chemistry chapter on this site: every equation below is checked atom by atom in the page's source, and the same conventions will carry into the next chapters.

Neighbouring chapters: the next two chapters of this book — Acids, Bases and Salts (neutralisation, pH, bleaching powder, baking soda, plaster of Paris) and Metals and Non-metals (the reactivity series, corrosion and its prevention, extraction of metals) — build directly on these reaction types, and Life Processes returns to respiration and photosynthesis. Live geography chapters give the industrial setting: Minerals and Energy Resources and Manufacturing Industries (Class 10, Contemporary India II — limestone, the cement industry and industrial pollution) and Composition and Structure of Atmosphere (Class 11) for the gases these reactions use and release.
Prelims — science: UPSC has asked which chemical is used in photography (silver bromide, 1982), whether photography is a chemical change (1990), which of three changes is chemical (souring of milk, 2014) and how green hydrogen is made (electrolysis of water with renewable power, 2026). Expect statement questions on types of reaction, exothermic and endothermic examples, oxidation and reduction, and everyday chemistry (whitewash, rust, rancid food, nitrogen in chip packets).
GS3 — science and technology, environment: corrosion of bridges, railways and ships as an infrastructure cost; food preservation and packaging; lime and cement; the acid-forming gases SO₂ and NO₂ that appear in this chapter's decomposition equations; electrolysis of water as the route to green hydrogen.

Word	What it means here
Reactant / product	A substance that changes in the reaction (left of the arrow) / a new substance formed (right of the arrow)
Chemical equation	The reaction written with formulae: reactants → products
Skeletal equation	Formulae written correctly but atoms not yet equal on both sides — e.g. Mg + O₂ → MgO
Balanced equation	Same number of atoms of each element on both sides — e.g. 2Mg + O₂ → 2MgO
Coefficient	The number written in front of a formula (the 4 in 4H₂O); the only thing you may change while balancing
State symbols	(s) solid, (l) liquid, (g) gas, (aq) aqueous — dissolved in water
Exothermic / endothermic	Gives out heat / takes in energy (heat, light or electricity)
Precipitate	An insoluble solid that forms when two solutions are mixed
Oxidation / reduction	Gain of oxygen or loss of hydrogen / loss of oxygen or gain of hydrogen (this chapter's definitions)
Corrosion / rancidity	A metal being eaten away by its surroundings / fats and oils going stale by oxidation

Reading NCERT's chapter opening (page 1)The opening page carries a one-line epigraph by Martin H. Fischer comparing a heap of facts to a dictionary — not yet science, as a dictionary is not literature — a small drawing of a stoppered conical flask with crystals at the bottom, and a QR code. It then lists six everyday changes — milk left out in summer, an iron tawa or nail in humid air, grapes fermenting, cooking, digestion and respiration — and says that in each the identity of the starting substance has changed, so a chemical reaction has happened. The point: start from what you see at home; the chapter turns those observations into equations.

Common confusions

Physical vs chemical change: melting ice, dissolving salt and crystallising it again make no new substance; souring milk, rusting and burning do. UPSC 2014 tested exactly this.
A chemical reaction rearranges atoms; it never turns one element into another (that would be a nuclear change).
NCERT's ‘chemical change’ and ‘chemical reaction’ are used as the same idea.

At a glance — the six ideas to carry away

A chemical reaction makes new substances; you can tell from a change in state, change in colour, evolution of a gas or change in temperature.
Word equation → skeletal equation → balanced equation. Balance because mass is conserved: change only the coefficients, never the formulae. Add (s), (l), (g), (aq) and conditions such as heat or 340 atm over the arrow.
Combination: two or more → one product (CaO + H₂O → Ca(OH)₂, with much heat). Exothermic = heat given out — burning, respiration, composting.
Decomposition: one reactant → simpler products, needing heat (FeSO₄, CaCO₃, Pb(NO₃)₂), light (AgCl, AgBr — black-and-white photography) or electricity (water: H₂ : O₂ = 2 : 1). Endothermic = energy absorbed.
Displacement: a more reactive metal pushes a less reactive one out (Fe + CuSO₄). Double displacement: ions are exchanged; a precipitate such as white BaSO₄ often forms.
Oxidation = gain of oxygen or loss of hydrogen; reduction = loss of oxygen or gain of hydrogen; they happen together (redox). In daily life: corrosion (rust) and rancidity (stale fats) — slowed by paint, antioxidants, air-tight packs and nitrogen flushing.

1. What a chemical reaction is — and how to recognise one

In simple wordsYou cannot see atoms swapping partners, so you look for clues, like a detective. A solid strip crumbling into powder, a clear liquid suddenly turning yellow, fizzing bubbles, a test tube getting hot or cold in your hand — each is a sign that something new has been made.

NCERT opens with three quick activities. In Activity 1.1 a cleaned magnesium ribbon about 3–4 cm long, held in tongs, burns with a dazzling white flame and leaves a white powder — magnesium oxide, formed with oxygen from the air. In Activity 1.2 lead nitrate solution is mixed with potassium iodide solution. In Activity 1.3 dilute hydrochloric or sulphuric acid is added to zinc granules: bubbles of gas appear and the flask warms. From these, NCERT lists four observations that tell us a chemical reaction has taken place: a change in state, a change in colour, the evolution of a gas and a change in temperature.

The four signs of a chemical reaction, each tied to an NCERT activity — our schematic (page 2).

Reading NCERT's Activity 1.1 and Figure 1.1 (page 1)A caution box asks for the teacher's help and suitable eyeglasses. The drawing shows a hand holding a strip of magnesium in tongs over a burner, with the white ash falling into a watch-glass on a wooden board; the labels name tong, magnesium ribbon, burner, watch-glass and magnesium oxide. The point: the ribbon is cleaned with sandpaper first and burnt away from the eyes — the flame is intensely bright — and the product is collected so you can see that a new, white substance has formed.

Reading NCERT's Activity 1.2 (page 2)A short box: take lead nitrate solution, add potassium iodide solution, observe. No result is given here; the book returns to it on page 12 (‘Recall Activity 1.2’) and asks for the colour and name of the precipitate. The point: it is the chapter's example of a change in colour (and, later, of double displacement) — a yellow solid appears from two colourless solutions (outline in the in-text section).

Reading NCERT's Activity 1.3 and Figure 1.2 (page 2)Zinc granules in a conical flask with dilute acid. The figure shows a conical flask with a cork and a glass tube through it, zinc granules at the bottom under dilute sulphuric acid, and an arrow marking H₂ gas rising up the tube. The point: two signs at once — gas given off and the flask becoming warm. The equation is NCERT's (1.3): Zn + H₂SO₄ → ZnSO₄ + H₂.

Common confusions

One sign alone is a clue, not proof: boiling water gives off gas and changes state without any reaction. Ask whether a new substance formed.
In Activity 1.1 magnesium reacts with oxygen of the air; the white powder is magnesium oxide, not ash in the everyday sense.
The ribbon is cleaned because its surface carries a layer of magnesium oxide (and dirt) that slows burning — NCERT's first in-text question.

Why this matters for PrelimsUPSC asks the physical-versus-chemical distinction directly (2014: crystallisation, melting, souring of milk) and indirectly (1990: photographic film is used once because exposure is a chemical change, while magnetic video tape records by a reversible physical change). The four signs are the quickest way to classify any everyday example.

2. Writing chemical equations: word, skeletal, balanced

In simple wordsA chemical equation is a recipe card. Words take too long, so we use formulae; and a recipe must account for every ingredient — if 8 hydrogen atoms go into the pot, 8 must come out. Balancing is simply adjusting how many of each ingredient you use, never what the ingredient is.

Activity 1.1 in a sentence is long, so NCERT first writes a word equation: magnesium + oxygen → magnesium oxide (1.1). Reactants go on the left-hand side (LHS), joined by +; products on the right-hand side (RHS); the arrowhead points to the products and shows the direction. Using formulae gives a chemical equation: Mg + O₂ → MgO (1.2). Count the atoms: one oxygen atom on the right but two on the left. Such an equation, with correct formulae but unequal atoms, is a skeletal equation.

By the law of conservation of mass (Class 9), mass is neither created nor destroyed in a chemical reaction, so the number of atoms of each element must be the same before and after. Zinc and sulphuric acid already satisfy this: Zn + H₂SO₄ → ZnSO₄ + H₂ (1.3) has 1 Zn, 2 H, 1 S and 4 O on each side — it is balanced. Iron and steam are not: Fe + H₂O → Fe₃O₄ + H₂ (1.4/1.5).

⤵ Balancing by hit-and-trial — NCERT's steps for iron + steam

Step I: box each formulanothing inside a box may change: Fe + H₂O → Fe₃O₄ + H₂

Step II: list the atomsFe 1 : 3, H 2 : 2, O 1 : 4 (LHS : RHS)

Step III: start with the biggest formulaFe₃O₄, and its commonest element, oxygen: write 4H₂O (not H₂O₄)

Step IV: next elementhydrogen is now 8 : 2, so write 4H₂

Step V: the element leftiron 1 : 3, so write 3Fe

Step VI: check every element3Fe + 4H₂O → Fe₃O₄ + 4H₂ — balanced (1.9)

Step VII: add state symbols3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) (1.10)

Atom tallies at each stage of balancing iron + steam — computed from the equations themselves (pages 3–5).

Reading NCERT's balancing boxes and atom-count tables (pages 3–5)The book works the example in coloured panels: a brown table counting atoms for the zinc equation (1.3) and again for the skeletal iron equation; a green table for oxygen (initial 1 vs 4; ‘to balance’ 1 × 4); a pink table for hydrogen (8 vs 2; 2 × 4); and a final table for iron (1 vs 3; 1 × 3). Each partly balanced equation (1.6, 1.7) is drawn with the formulae in boxes. The point: one element at a time, coefficients only, then count everything again. (On page 4 the printed text runs Step I, Step II, the oxygen table, then the hydrogen table, with the ‘Step III’ paragraph placed after Step IV in the layout — read them in number order.)

State symbols and conditions. To make an equation more informative, write (g) gas, (l) liquid, (aq) aqueous (dissolved in water) and (s) solid after each formula. In (1.10) water is written H₂O(g) because it reacts as steam. States are usually left out unless they matter. Conditions — temperature, pressure, catalyst — go above or below the arrow: CO(g) + 2H₂(g) —[340 atm]→ CH₃OH(l) (1.11, methanol); 6CO₂(aq) + 12H₂O(l) —[sunlight, chlorophyll]→ C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l) (1.12, glucose by photosynthesis).

Equation stage	Example from NCERT	What it tells you
Word equation (1.1)	Magnesium + Oxygen → Magnesium oxide	names only
Skeletal (1.2)	Mg + O₂ → MgO	formulae, atoms not yet equal
Balanced	2Mg + O₂ → 2MgO	atoms equal — NCERT asks you to do this one yourself
With states (1.10)	3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)	physical state of each substance
With conditions (1.11)	CO(g) + 2H₂(g) —[340 atm]→ CH₃OH(l)	what the reaction needs

Common confusions

Never change a subscript to balance: H₂O₄ or (H₂O)₄ would be a different substance. Change only the coefficient.
Use the smallest whole numbers (that is why it is hit-and-trial).
(aq) ≠ (l): aqueous means dissolved in water; liquid means the pure liquid itself.
A balanced equation conserves atoms and mass, not the number of molecules: 3 + 4 = 7 molecules on the left of (1.9), 1 + 4 = 5 on the right.
NCERT slip: on page 5 the photosynthesis equation (1.12) writes CO₂ and O₂ as (aq) — they are gases taken in and given out by the leaf; write them (g) in your own answers unless you are quoting the book.

Beyond NCERT

Methanol from CO and H₂ (1.11) is made industrially over a catalyst; the CO + H₂ mixture is called synthesis gas (syngas), made from coal, natural gas or biomass.
The law of conservation of mass is credited to Antoine Lavoisier (18th century).

3. Combination reactions and exothermic reactions

In simple wordsCombination is joining: two people shake hands and walk off as one pair. Many such joinings release energy — the way a ball rolling downhill gives up energy. That released energy is the heat you feel when quick lime meets water, or when coal burns.

Section 1.2 of the book sorts reactions into five types. Four of them are patterns of how atoms regroup — drawn below with lettered circles — and the fifth, oxidation and reduction, is about oxygen or hydrogen moving between substances, so it can overlap with the others (Section 7). Sections 3 to 7 take them one by one.

Four patterns of reaction, with lettered circles as atoms and an NCERT example under each — our schematic (Section 1.2).

Chemical reactions break and make bonds between atoms; atoms are never created, destroyed or changed into another element (bonds come in Chapters 3 and 4). In Activity 1.4, water added slowly to calcium oxide (quick lime) in a beaker reacts vigorously and the beaker becomes hot: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat (1.13). Calcium oxide and water form a single product, calcium hydroxide (slaked lime). A reaction in which a single product forms from two or more reactants is a combination reaction. Two more: burning of coal, C(s) + O₂(g) → CO₂(g) (1.15), and the formation of water, 2H₂(g) + O₂(g) → 2H₂O(l) (1.16).

Reading NCERT's Activity 1.4 and Figure 1.3 (page 6)A hand touches the outside of a beaker holding water with lumps of calcium oxide at the bottom and small bubbles rising. The point: the reaction is felt through the glass — a large amount of heat is released — so it is both a combination and an exothermic reaction. Add water slowly: the reaction is vigorous.

Reading NCERT's ‘Do You Know?’ box on whitewashing (page 7)A solution of slaked lime from (1.13) is used to whitewash walls. Calcium hydroxide reacts slowly with carbon dioxide in the air and, after two to three days, leaves a thin layer of calcium carbonate that gives the wall a shiny finish: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (1.14). The box notes that marble has the same formula, CaCO₃. The point: the same chemical can be a rock, a building stone and a wall finish.

The lime cycle: limestone → quick lime → slaked lime → calcium carbonate on the wall (equations 1.20, 1.13 and 1.14).

⤵ Whitewashing a wall, step by step

Quick lime + waterCaO(s) + H₂O(l) → Ca(OH)₂(aq) — combination, much heat

Slaked lime solutionbrushed on the wall as whitewash

Slow reaction with airCa(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

After 2–3 daysa thin, shiny layer of calcium carbonate — the same formula as marble

Exothermic reactions. Reactions in which heat is released along with the products are exothermic. NCERT's examples: burning of natural gas, CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) (1.17); respiration — carbohydrates in rice, potatoes and bread are broken down to glucose during digestion, and glucose combines with oxygen in our cells to give energy, C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy (1.18); and the decomposition of vegetable matter into compost. Activity 1.1 is also exothermic and a combination: 2Mg + O₂ → 2MgO.

Exothermic and endothermic reactions as energy levels — a standard aid, not drawn in NCERT (pages 7 and 10).

Common confusions

Combination is about the number of products (one); exothermic is about energy. A reaction can be both (CaO + water, burning magnesium) — the two labels answer different questions.
Quick lime = CaO (calcium oxide); slaked lime = Ca(OH)₂ (calcium hydroxide); limestone and marble = CaCO₃. The whitewash is slaked lime; the shine is calcium carbonate.
Respiration is exothermic; photosynthesis (1.12) takes in light energy — it is endothermic.
Composting is an exothermic decomposition of vegetable matter — so ‘decomposition’ is not always endothermic (see Section 4's trap).
NCERT inconsistency: page 7 says respiration is studied in ‘Chapter 6’; in this rationalised edition Life Processes is Chapter 5 (the old numbering survived the deletion of a chapter). Equation (1.18) also writes O₂ and CO₂ as (aq) — dissolved gases inside cells — which is defensible here.

Beyond NCERT — lime, cement and carbon dioxide

Turning limestone into quick lime (1.20) releases CO₂ from the rock itself, not only from the fuel burnt in the kiln. Because cement making starts this way, the industry is counted among the large industrial sources of CO₂ — a point used in GS3 answers on industrial decarbonisation (the live Manufacturing Industries chapter covers the cement industry's location and raw materials).
Slaked lime is only sparingly soluble in water; the whitewash is really a milky suspension (‘milk of lime’), with the clear part called lime water. NCERT's (aq) in (1.13) is a simplification.

Why this matters for Prelims and GS3Lime chemistry runs through cement, steel fluxes, water treatment and soil liming (acidic soils are limed to raise pH — covered in the next chapter). Exothermic examples — burning fuels, respiration, compost heaps — are favourite statement material; so is ‘marble and limestone are calcium carbonate’.

4. Decomposition reactions and endothermic reactions

In simple wordsDecomposition is taking apart — one thing breaking into simpler pieces. Taking apart needs effort: you must pay energy to break the bonds, as heat from a flame, as light from the sun, or as electricity from a battery.

In Activity 1.5, about 2 g of green ferrous sulphate crystals are heated in a dry boiling tube; the colour changes and there is a smell of burning sulphur: 2FeSO₄(s) —[heat]→ Fe₂O₃(s) + SO₂(g) + SO₃(g) (1.19). The crystals (FeSO₄·7H₂O) first lose water and change colour, then decompose to ferric oxide (a solid) and the gases SO₂ and SO₃. A single reactant breaking down into simpler products is a decomposition reaction; when heating does it, it is thermal decomposition. Heating calcium carbonate is an important industrial case: CaCO₃(s) —[heat]→ CaO(s) + CO₂(g) (1.20) — calcium oxide (lime or quick lime) is used, among other things, in making cement. In Activity 1.6, about 2 g of lead nitrate heated in a boiling tube gives off brown fumes of nitrogen dioxide: 2Pb(NO₃)₂(s) —[heat]→ 2PbO(s) + 4NO₂(g) + O₂(g) (1.21).

⤵ Heating ferrous sulphate crystals (Activity 1.5)

Green crystalsFeSO₄·7H₂O — ferrous sulphate with water of crystallisation

Gentle heatingFeSO₄·7H₂O(s) —[heat]→ FeSO₄(s) + 7H₂O(g) — the green colour goes (Beyond NCERT: the book says only that water is lost)

Strong heating: decomposition2FeSO₄(s) —[heat]→ Fe₂O₃(s) + SO₂(g) + SO₃(g)

What you noticea reddish-brown solid (ferric oxide) and the choking smell of burning sulphur (SO₂)

Reading NCERT's Activity 1.5 and Figure 1.4 (page 8)A student in a lab coat holds a boiling tube of ferrous sulphate crystals in a holder over a burner, and uses her other hand to waft the gas gently towards her nose; a label warns: do not point the mouth of the tube at neighbours or yourself. The point: the safe way to heat a tube and to smell a gas — never sniff directly; SO₂ and SO₃ are harmful.

Reading NCERT's Activity 1.6 and Figure 1.5 (page 8)A hand holds a boiling tube of lead nitrate with a pair of tongs over a burner flame; wavy lines show fumes leaving the mouth. The point: brown fumes = nitrogen dioxide; one solid gives a solid (lead oxide) and two gases (NO₂ and O₂) — thermal decomposition.

Electricity and light. In Activity 1.7, carbon electrodes fixed through rubber stoppers in the base of a plastic mug are connected to a 6 volt battery; the mug holds water with a few drops of dilute sulphuric acid, and water-filled test tubes are inverted over the electrodes. Bubbles form at both electrodes and push water out of the tubes; the volumes are not equal, and the gases are tested with a burning candle. In Activity 1.8, about 2 g of white silver chloride in a china dish turns grey in sunlight: 2AgCl(s) —[sunlight]→ 2Ag(s) + Cl₂(g) (1.22). Silver bromide behaves the same way, 2AgBr(s) —[sunlight]→ 2Ag(s) + Br₂(g) (1.23); these reactions are used in black-and-white photography.

Electrolysis of water — our schematic for Activity 1.7 and Figure 1.6 (page 9). Gas volumes follow the equation's 2 : 1.

⤵ Electrolysis of water (Activity 1.7)

Set upcarbon electrodes in water with a few drops of dilute sulphuric acid; inverted water-filled test tubes over them

Switch on the 6 V batterybubbles form at both electrodes and push water out of the tubes

Cathode (−)hydrogen — twice the volume

Burning candle testhydrogen burns with a pop (Beyond NCERT wording)

Anode (+)oxygen — half the volume

Burning candle testthe candle burns more brightly

Equation2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g) — decomposition by electricity

Reading NCERT's Activity 1.7 and Figure 1.6 (page 9)A cut-away plastic mug of water with two graphite rods rising through rubber stoppers in its base; an inverted test tube over each rod, the left one labelled oxygen above the anode and the right one hydrogen above the cathode, with the gas visibly filling more of the hydrogen tube; wires run through a switch to a 6 V battery. The point: electricity splits water into two gases in a fixed ratio — hydrogen double oxygen by volume, as the equation's coefficients (2 : 1) predict. NCERT's text says ‘carbon electrodes’ and the figure ‘graphite rod’ — graphite is carbon.

Reading NCERT's Activity 1.8 and Figure 1.7 (page 9)A china dish of silver chloride sits on a sill in a beam of sunlight coming through an open window. The point: light alone drives this decomposition (a photochemical reaction) — white silver chloride turns grey as fine silver forms.

Endothermic reactions. Decomposition reactions need energy — heat, light or electricity — to break down the reactants. Reactions in which energy is absorbed are endothermic. NCERT's ‘carry out’ activity makes the point by touch: mixing about 2 g of barium hydroxide with 1 g of ammonium chloride makes the test tube feel cold — Ba(OH)₂ + 2NH₄Cl → BaCl₂ + 2NH₃ + 2H₂O (equation Beyond NCERT; the book gives none).

Energy supplied	NCERT example	What you see
Heat (thermal decomposition)	2FeSO₄(s) —[heat]→ Fe₂O₃(s) + SO₂(g) + SO₃(g)	green crystals change colour; smell of burning sulphur
Heat	CaCO₃(s) —[heat]→ CaO(s) + CO₂(g)	limestone → quick lime + carbon dioxide (cement)
Heat	2Pb(NO₃)₂(s) —[heat]→ 2PbO(s) + 4NO₂(g) + O₂(g)	brown fumes of NO₂
Electricity (electrolysis)	2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g)	hydrogen and oxygen, 2 : 1 by volume
Light (photolysis)	2AgCl(s) —[sunlight]→ 2Ag(s) + Cl₂(g)	white silver chloride turns grey
Light	2AgBr(s) —[sunlight]→ 2Ag(s) + Br₂(g)	the basis of black-and-white photography

Reading NCERT's ‘Carry out the following Activity’ box (page 10)A blue band asks students to mix about 2 g barium hydroxide and 1 g ammonium chloride with a glass rod, touch the bottom of the tube and decide whether the reaction is exothermic or endothermic. The point: the tube feels cold — the mixture takes heat from your hand — so it is endothermic, and it is not a decomposition: endothermic is a wider family.

Common confusions

Decomposition ≠ always endothermic, and endothermic ≠ always decomposition. Most decompositions need energy (NCERT's examples), but composting is an exothermic decomposition (page 7), and Ba(OH)₂ + NH₄Cl is endothermic without being a decomposition.
In electrolysis of water, hydrogen collects at the cathode (−) in double volume; oxygen at the anode (+). The acid only helps current flow.
Ferrous sulphate (FeSO₄, green) → ferric oxide (Fe₂O₃). ‘Ferrous’ = iron(II), ‘ferric’ = iron(III).
Lead nitrate gives brown NO₂; do not confuse with the colourless SO₂/SO₃ of ferrous sulphate.
NCERT state-symbol slip: (1.23) writes bromine as Br₂(g); at room temperature bromine is a red-brown liquid. Quote the book's equation if asked, but know the fact.

Beyond NCERT — photography, acid gases and green hydrogen

Photography: light falling on silver bromide in the film starts its breakdown to silver, forming an invisible image; developing builds up the dark silver, and a fixer (sodium thiosulphate, ‘hypo’) removes the unexposed silver halide — UPSC asked about the fixer in 1995.
SO₂, SO₃ and NO₂ — the gases in (1.19) and (1.21) — are the same gases that, released by burning fossil fuels and smelting, form sulphuric and nitric acids in the air: acid rain (next chapter and the Our Environment chapter).
Green hydrogen is hydrogen made by electrolysis of water using renewable electricity; hydrogen from natural gas with the CO₂ captured is usually called ‘blue’. India's National Green Hydrogen Mission was approved by the Union Cabinet in January 2023 with a target of about 5 million tonnes a year of green hydrogen production capacity by 2030 (UPSC 2026 asked about the mission's emission-abatement aim).

Why this matters for Prelims and GS3Silver bromide in photography (1982) and photography as a chemical change (1990) come straight from (1.23). Electrolysis of water is the science under green hydrogen (2026). The decomposition gases are acid-rain gases; the limestone reaction is the chemistry of cement.

5. Displacement reactions

In simple wordsThink of a stronger player taking a seat from a weaker one. Iron is ‘stronger’ (more reactive) than copper, so when an iron nail sits in copper sulphate solution, iron takes copper's place in the compound and copper is pushed out as a brown coating.

In Activity 1.9, three iron nails are cleaned with sandpaper. Two, tied with a thread, are dipped for about 20 minutes in about 10 mL of copper sulphate solution in test tube B; tube A keeps the same solution untouched, and one nail is kept aside. Afterwards the dipped nails have turned brownish and the blue colour of the solution in B has faded: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (1.24). Iron has displaced copper from copper sulphate — a displacement reaction. Other examples: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) (1.25) and Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s) (1.26). Zinc and lead are more reactive than copper, so they displace copper from its compounds.

⤵ Iron nails in copper sulphate (Activity 1.9)

Clean three iron nailssandpaper removes rust and dirt so the metal surface can react

Two nails into tube Babout 10 mL blue copper sulphate solution, about 20 minutes; tube A is the comparison

Iron displaces copperFe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

The nailscoated with brownish copper

The solutionblue fades as copper sulphate is used up (iron sulphate solution is pale green — Beyond NCERT)

Reading NCERT's Activity 1.9 and Figure 1.8 (pages 10–11)Figure 1.8(a): a test tube clamped to a stand, holding blue copper sulphate solution with two iron nails hanging in it from a thread. Figure 1.8(b): on the left, a clean grey nail beside a brown nail taken out of the solution; on the right, a test-tube stand with tube A still deep blue and tube B — the reaction mixture — almost colourless. The point: a control (the spare nail and tube A) lets you see that both the nail and the solution have changed.

Who displaces whom: every displacement in this chapter follows one ladder (the ladder itself is in Chapter 3).

Common confusions

Displacement (single): an element replaces another element in a compound — one element in, one element out. Double displacement: two compounds swap partners (Section 6).
It only works downhill: copper cannot displace iron, zinc or lead from their salts.
NCERT calls FeSO₄ ‘iron sulphate’ in (1.24) and ‘ferrous sulphate’ in (1.19) — the same compound.
Exercise 2's Fe₂O₃ + 2Al is a displacement (aluminium takes iron's place) — and also a redox reaction. If a question asks for one type from NCERT's options, the answer is displacement.

Beyond NCERT

The aluminium–iron oxide reaction (exercise 2) is the thermite reaction; it releases so much heat that the iron comes out molten, and it is used to weld railway tracks and cracked machine parts (Chapter 3 of this book).
Displacing copper with scrap iron from copper-bearing solutions (‘cementation’) is an old way of recovering copper; exercise 14's copper-for-silver displacement is the same idea used in refining silver.

Why this matters for PrelimsQuestions on why galvanised iron resists rust, why some metals corrode faster and how metals are recovered all rest on the reactivity order that displacement reveals. UPSC's galvanising questions (1982, 1983, 1992, 1994) are mapped to Chapter 3, where the series and zinc coating are taught.

6. Double displacement and precipitation reactions

In simple wordsTwo couples at a dance swap partners. In solution, compounds exist as charged pieces called ions; when two solutions are mixed, the ions swap. If one new pair cannot stay dissolved, it drops out as a solid — a precipitate, like sugar that will not dissolve settling at the bottom of a glass.

In Activity 1.10, about 3 mL of sodium sulphate solution is mixed with about 3 mL of barium chloride solution. A white, insoluble substance forms — a precipitate; any reaction that produces a precipitate is a precipitation reaction: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (1.27). The white BaSO₄ forms from the SO₄²⁻ and Ba²⁺ ions; sodium chloride stays in solution. Reactions in which there is an exchange of ions between the reactants are double displacement reactions. Activity 1.2 was one too: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) — the yellow precipitate is lead iodide.

⤵ What happens when the two solutions meet (Activity 1.10)

Two clear solutionssodium sulphate (Na⁺ and SO₄²⁻ ions) and barium chloride (Ba²⁺ and Cl⁻ ions)

Ions exchange partnersBa²⁺ meets SO₄²⁻; Na⁺ meets Cl⁻

Barium sulphateinsoluble — white precipitate

Sodium chloridesoluble — stays in solution

EquationNa₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Reading NCERT's Activity 1.10 and Figure 1.9 (page 11)One hand pours sodium sulphate solution from a tilted test tube into another test tube held upright, containing barium chloride solution; small white specks gather at the bottom of the lower tube. The point: the solid appears the moment the solutions mix — the visible sign of ions exchanging partners.

Reading NCERT's ‘Recall Activity 1.2’ box (page 12)The book returns to lead(II) nitrate and potassium iodide and asks: the colour of the precipitate and its name; the balanced equation; and whether this is also double displacement. The point: Activity 1.2's colour change was a precipitate — yellow lead iodide — formed by ion exchange, so yes, it is double displacement (outline in the in-text section).

	Displacement	Double displacement
Reactants	an element + a compound	two compounds (usually in solution)
What moves	a more reactive element takes another's place	ions of the two compounds exchange
NCERT example	Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)	Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Typical sign	metal coating; colour of solution fades	a precipitate (white BaSO₄, yellow PbI₂)

Common confusions

Precipitation describes the result (an insoluble solid); double displacement describes the mechanism (ion exchange). Most precipitation reactions in this chapter are double displacements, but neutralisation (exercise 6a, 6b; in-text Q 3ii) is a double displacement without a precipitate.
Ion charges: Ba²⁺ and SO₄²⁻ — the 2 is a charge, not a count of atoms.
NCERT error to handle: exercise 8(a) writes potassium bromide + barium iodide → potassium iodide + barium bromide(s). Barium bromide is in fact soluble in water, and all four salts stay dissolved, so no precipitate forms and no visible reaction occurs. For the exam, balance it as asked (2KBr + BaI₂ → 2KI + BaBr₂) and call it double displacement — that is the expected answer.

Beyond NCERT

Barium sulphate is so insoluble that it can be swallowed safely as a ‘barium meal’ to make the gut show up on X-rays, even though soluble barium salts are poisonous.
Precipitation is used to treat water — for example, removing phosphate or heavy metals as insoluble salts in effluent treatment.

Why this matters for PrelimsPrecipitates and their colours are a frequent ‘which statement is correct’ hook; ion-exchange thinking also underlies water softening and effluent treatment in GS3 environment answers.

7. Oxidation and reduction

In simple wordsPicture oxygen as a ball being passed. Whoever catches it is oxidised; whoever lets go is reduced. A pass always needs two players, so oxidation and reduction always happen together — that pair is a redox reaction.

In Activity 1.11, about 1 g of copper powder is heated in a china dish; its surface becomes coated with black copper(II) oxide because oxygen has been added: 2Cu + O₂ —[heat]→ 2CuO (1.28). If hydrogen is passed over the heated black material, the coating turns brown again as copper is recovered: CuO + H₂ —[heat]→ Cu + H₂O (1.29). A substance that gains oxygen is oxidised; one that loses oxygen is reduced. In (1.29) copper(II) oxide loses oxygen and is reduced, while hydrogen gains oxygen and is oxidised — one reactant is oxidised as the other is reduced: an oxidation-reduction or redox reaction. More examples: ZnO + C → Zn + CO (1.31), where carbon is oxidised to CO and ZnO is reduced to Zn; and MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (1.32), where HCl is oxidised to Cl₂ (it loses hydrogen) and MnO₂ is reduced to MnCl₂. So NCERT widens the definitions: oxidation = gain of oxygen or loss of hydrogen; reduction = loss of oxygen or gain of hydrogen.

One equation, two processes — our version of NCERT's bracket scheme (1.30), with the book's two definitions (pages 12–13).

⤵ Copper there and back (Activity 1.11 and equation 1.29)

Heat brown copper powder in airabout 1 g in a china dish on a wire gauze

Black coating forms2Cu + O₂ —[heat]→ 2CuO — copper is oxidised

Pass hydrogen over the hot black solidCuO + H₂ —[heat]→ Cu + H₂O

Brown copper returnsCuO is reduced; H₂ is oxidised to water — a redox reaction

Reading NCERT's Activity 1.11 and Figure 1.10 (page 12)A china dish of copper powder rests on a wire gauze on a tripod stand over a burner. The point: heating in open air supplies oxygen, and the brown metal turns black — oxidation you can see. (The figure's label reads ‘copper power’ — a printing slip for powder.)

Reading NCERT's equation (1.30) and ‘Recall Activity 1.1’ (pages 12–13)Equation (1.30) repeats CuO + H₂ → Cu + H₂O with a bracket labelled Oxidation joining H₂ to H₂O above, and one labelled Reduction joining CuO to Cu below. The ‘Recall Activity 1.1’ box then asks whether magnesium burning in air is oxidised or reduced. The point: always trace each reactant to its product and ask what it gained or lost — magnesium gains oxygen, so it is oxidised.

Reaction (NCERT)	Oxidised	Reduced
CuO + H₂ —[heat]→ Cu + H₂O	H₂ (gains O)	CuO (loses O)
ZnO + C → Zn + CO	C (gains O)	ZnO (loses O)
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂	HCl (loses H)	MnO₂ (loses O)
4Na(s) + O₂(g) → 2Na₂O(s)	Na (gains O)	O₂ (Beyond NCERT: oxygen is reduced)
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)	C (gains O)	PbO (loses O)

Common confusions

Name the substance, not an atom inside it: in 2PbO + C → 2Pb + CO₂, lead oxide is reduced (not ‘lead’), and carbon is oxidised (not ‘carbon dioxide’ — CO₂ is a product). That is exactly NCERT's exercise 1.
Oxidation and reduction always occur together. The substance that is oxidised (H₂, C) is the one that removes oxygen from the other — in later classes it is called the reducing agent.
Loss of hydrogen = oxidation (HCl → Cl₂); gain of hydrogen = reduction. Do not reverse them.
Burning, rusting and respiration are all oxidations; photosynthesis reduces CO₂.

Beyond NCERT — the electron definition

In Class 11 the definitions widen again: oxidation = loss of electrons, reduction = gain of electrons (‘OIL RIG’), and oxidation numbers track them. NCERT's oxygen/hydrogen rule is the special case. (UPSC 2002 asked oxidation numbers — Class 11 material.)
In a blast furnace, coke (carbon) reduces iron ore and also burns as fuel — the industrial form of (1.31); UPSC asked this in 1998 (mapped to Chapter 3's metallurgy).

Why this matters for Prelims and GS3Metal extraction (reduction of ores), batteries and fuel cells, bleaching, rusting and the body's antioxidants are all redox stories. A clean grasp of ‘who gains oxygen, who loses it’ settles most Prelims statements on these topics.

8. Oxidation in everyday life: corrosion and rancidity

In simple wordsOxygen is everywhere, so slow oxidation is always nibbling at things around us. On iron it shows up as rust; in a forgotten packet of fried snacks it shows up as a stale, bitter smell. Most ways of fighting it work by keeping oxygen (and moisture) away.

Corrosion. New iron articles are shiny but in time get coated with a reddish-brown powder — rusting. Other metals tarnish too: silver gets a black coating and copper a green one. When a metal is attacked by substances around it such as moisture and acids, it corrodes, and the process is corrosion. It damages car bodies, bridges, iron railings, ships and all metal objects, especially iron ones; every year a great deal of money is spent replacing damaged iron (more in Chapter 3). Painting iron (exercise 18) keeps air and moisture off its surface.

Rancidity. When fats and oils are oxidised, they become rancid — their smell and taste change. To slow this, antioxidants (substances that prevent oxidation) are usually added to foods containing fats and oils; air-tight containers help; and chips manufacturers usually flush packets with nitrogen so the chips are not oxidised.

Corrosion and rancidity side by side — oxidation in daily life and how it is slowed (page 13).

	Corrosion	Rancidity
What is oxidised	metals — iron most of all	fats and oils in food
What you notice	reddish-brown rust on iron; black on silver; green on copper	changed smell and taste
Slowed by (NCERT)	paint (exercise 18); more methods in Chapter 3	antioxidants; air-tight containers; nitrogen flushing
Common thread	keep oxygen and moisture away	keep oxygen away

Common confusions

Rusting is corrosion of iron only; corrosion is the wider term (silver, copper too).
Chip packets are flushed with nitrogen — an unreactive gas — not with oxygen or air, and not to keep them crisp by pressure alone: the aim is to prevent oxidation (exercise 19).
Antioxidants in food slow oxidation of the food; the 2011 UPSC question on antioxidants in fruit and vegetables was about the body (neutralising free radicals) — a biology topic.
NCERT lists ‘moisture, acids, etc.’ as attackers; air (oxygen) is the partner in rusting — Chapter 3's experiment shows iron needs both air and water to rust.

Beyond NCERT — names, numbers and prevention

Rust is mainly hydrated iron(III) oxide, Fe₂O₃·xH₂O; silver's black tarnish is silver sulphide; copper's green coat is basic copper carbonate (Chapter 3 names the last two).
Prevention methods taught in Chapter 3: painting, oiling and greasing, galvanising (a zinc coat), chrome plating, anodising and alloying (stainless steel).
A global study by NACE International (the IMPACT study, 2016) estimated the cost of corrosion at about US$2.5 trillion a year, roughly 3.4% of world GDP — use with attribution.
Common food antioxidants include BHA and BHT (synthetic) and vitamins C and E (natural); vacuum and modified-atmosphere packaging work on the same principle as nitrogen flushing.

Why this matters for GS3Corrosion is a cost and safety issue for bridges, railways, pipelines, ships and power lines — an infrastructure-maintenance point in GS3. Rancidity, antioxidants and modified-atmosphere packaging belong to food processing and food security (shelf life, post-harvest loss). Both are everyday oxidation — the same idea as Section 7.

NCERT's in-text questions and activities — short answers

The chapter sets questions inside activities, in three ‘Questions’ boxes and in ‘Recall’ boxes. Short outlines follow, in page order.

Reading NCERT's three ‘Questions’ boxes (pages 6, 10 and 13)Lilac boxes with a large question mark close each stretch of text: page 6 (cleaning magnesium; three balancing tasks; two equations with state symbols), page 10 (whitewashing substance ‘X’; why one electrolysis tube holds double the gas) and page 13 (iron nail in copper sulphate; another double displacement; what is oxidised and reduced). The point: they test exactly the skills of each part — balancing, naming types, tracing oxygen.

Activity 1.1 (page 1): What do you observe when a cleaned magnesium ribbon is burnt?

It burns with a dazzling white flame and turns into a white powder, magnesium oxide: 2Mg + O₂ → 2MgO. A combination reaction that also releases heat and light.

Activity 1.3 (page 2): What happens around the zinc granules? Is there any change in temperature?

Bubbles of hydrogen gas form on the granules and the flask becomes warm: Zn + H₂SO₄ → ZnSO₄ + H₂ (with HCl: Zn + 2HCl → ZnCl₂ + H₂).

Page 5: Using these steps, can you balance equation 1.2 (Mg + O₂ → MgO)?

Oxygen is 2 : 1, so write 2MgO; magnesium is then 1 : 2, so write 2Mg: 2Mg + O₂ → 2MgO.

Page 6, Q1: Why should a magnesium ribbon be cleaned before burning in air?

Its surface is covered by a layer of magnesium oxide (formed slowly in air) and dirt, which prevents or slows burning. Rubbing with sandpaper exposes fresh metal, so it burns readily.

Page 6, Q2: Balance (i) hydrogen + chlorine → hydrogen chloride; (ii) barium chloride + aluminium sulphate → barium sulphate + aluminium chloride; (iii) sodium + water → sodium hydroxide + hydrogen.

(i) H₂(g) + Cl₂(g) → 2HCl(g)
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Page 6, Q3: Write balanced equations with state symbols: (i) barium chloride + sodium sulphate solutions → insoluble barium sulphate + sodium chloride solution; (ii) sodium hydroxide solution + hydrochloric acid solution → sodium chloride solution + water.

(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Page 7: Identify the type of reaction in Activity 1.1, where heat is given out along with a single product.

A combination reaction that is also exothermic (and, by Section 7, an oxidation of magnesium).

Activity 1.5 (page 8): What colour change do you see when ferrous sulphate crystals are heated?

The green crystals lose their water of crystallisation and the colour fades; on stronger heating a reddish-brown solid, ferric oxide, remains, with a smell of burning sulphur: 2FeSO₄(s) —[heat]→ Fe₂O₃(s) + SO₂(g) + SO₃(g).

Activity 1.7 (page 9): Is the volume of gas the same in both tubes? What happens with a burning candle, and which gas is in each tube?

No. The tube over the cathode collects hydrogen, twice the volume of the oxygen over the anode. Hydrogen burns with a pop sound; oxygen makes the candle burn more brightly. 2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g)

Page 10: What form of energy is causing the decomposition of silver chloride and silver bromide?

Light (sunlight). Equations (1.22) and (1.23) — photochemical decomposition.

Page 10 (carry-out activity): Mix about 2 g barium hydroxide with 1 g ammonium chloride and touch the tube. What do you feel? Exothermic or endothermic?

The tube feels cold: the reaction absorbs heat from the surroundings, so it is endothermic. Ba(OH)₂ + 2NH₄Cl → BaCl₂ + 2NH₃ + 2H₂O (equation Beyond NCERT; ammonia smell may be noticed).

Page 10, Q1: A solution of substance X is used for whitewashing. (i) Name X and give its formula. (ii) Write its reaction with water.

(i) X is calcium oxide (quick lime), CaO — its reaction with water gives slaked lime, Ca(OH)₂, whose solution is the whitewash. (ii) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. (The question's wording is loose: strictly, the solution used is of slaked lime; the standard answer names quick lime as X because it is ‘the substance that reacts with water’.)

Page 10, Q2: Why is the gas collected in one test tube in Activity 1.7 double that in the other? Name this gas.

Water contains hydrogen and oxygen atoms in the ratio 2 : 1, so its decomposition gives two molecules of H₂ for every one of O₂ — and equal numbers of gas molecules occupy equal volumes. The gas with double volume is hydrogen. 2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g)

Activity 1.9 (pages 10–11): Why does the iron nail become brownish and the blue colour of copper sulphate fade?

Iron, being more reactive, displaces copper: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Brown copper deposits on the nail; blue copper sulphate is used up and replaced by (pale green) iron sulphate.

Activity 1.10 (page 11): What do you observe on mixing sodium sulphate and barium chloride solutions?

A white precipitate of barium sulphate: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) — a double displacement and precipitation reaction.

Page 12 (Recall Activity 1.2): (i) Colour and name of the precipitate? (ii) Balanced equation? (iii) Is this double displacement?

(i) Yellow precipitate of lead(II) iodide. (ii) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). (iii) Yes — the ions are exchanged between the two reactants.

Activity 1.11 (page 12): What do you observe when copper powder is heated in a china dish?

The brown powder becomes coated with black copper(II) oxide: 2Cu + O₂ —[heat]→ 2CuO — copper is oxidised.

Page 13 (Recall Activity 1.1): Is magnesium oxidised or reduced when it burns in air?

Oxidised — it gains oxygen to form magnesium oxide (2Mg + O₂ → 2MgO).

Page 13, Q1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron displaces copper from copper sulphate (Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)); blue CuSO₄ is replaced by pale-green FeSO₄, so the blue fades.

Page 13, Q2: Give an example of a double displacement reaction other than Activity 1.10.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) (yellow precipitate), or AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) (white precipitate of silver chloride), or the neutralisation NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).

Page 13, Q3: Identify the substances oxidised and reduced: (i) 4Na(s) + O₂(g) → 2Na₂O(s); (ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l).

(i) 4Na(s) + O₂(g) → 2Na₂O(s): sodium is oxidised (gains oxygen); oxygen is reduced (Beyond NCERT's oxygen rule, by the electron definition). (ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l): CuO is reduced (loses oxygen); H₂ is oxidised (gains oxygen).

NCERT exercise — answer outlines

Reading NCERT's ‘What you have learnt’ and Exercises (pages 14–16)Ten summary points restate the chapter: equations show reactants, products and states; equations must be balanced; combination, decomposition (the opposite of combination), exothermic, endothermic, displacement, double displacement and precipitation; and oxidation/reduction as gain or loss of oxygen or hydrogen. Twenty exercise questions follow — three multiple-choice, the rest balancing and short answers — with a Group Activity on temperature changes placed between questions 11 and 12. Typo to ignore: exercise 5(b) prints ‘sulpur dioxide’.

Try each question first, then open the outline. Write equations with correct formulae, balance them, and add state symbols when the question gives them.

1. Which statements about 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect? (a) Lead is getting reduced (b) Carbon dioxide is getting oxidised (c) Carbon is getting oxidised (d) Lead oxide is getting reduced. Options: (i) (a) and (b) (ii) (a) and (c) (iii) (a), (b) and (c) (iv) all.

(i) (a) and (b). 2PbO(s) + C(s) → 2Pb(s) + CO₂(g). Lead oxide is reduced (it loses oxygen) and carbon is oxidised (it gains oxygen), so (c) and (d) are correct. (a) names lead instead of lead oxide; (b) names a product, CO₂, which is not oxidised further.

2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of a (a) combination (b) double displacement (c) decomposition (d) displacement reaction.

(d) displacement. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe — aluminium, more reactive, takes iron's place. It is also a redox reaction (Al oxidised, Fe₂O₃ reduced); the thermite reaction (Beyond NCERT).

3. What happens when dilute hydrochloric acid is added to iron filings? (a) Hydrogen gas and iron chloride are produced (b) Chlorine gas and iron hydroxide are produced (c) No reaction takes place (d) Iron salt and water are produced.

(a). Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) — iron is above hydrogen in reactivity and displaces it from the acid.

4. What is a balanced chemical equation? Why should chemical equations be balanced?

One in which the number of atoms of each element is the same on both sides. It must be balanced because of the law of conservation of mass: atoms are neither created nor destroyed in a reaction, so total mass of products equals total mass of reactants. Example: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

5. Translate and balance: (a) hydrogen + nitrogen → ammonia; (b) hydrogen sulphide burns in air → water + sulphur dioxide; (c) barium chloride + aluminium sulphate → aluminium chloride + barium sulphate precipitate; (d) potassium + water → potassium hydroxide + hydrogen.

(a) N₂(g) + 3H₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

6. Balance: (a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O (b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O (c) NaCl + AgNO₃ → AgCl + NaNO₃ (d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃ (already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

7. Write balanced equations: (a) calcium hydroxide + carbon dioxide → calcium carbonate + water (b) zinc + silver nitrate → zinc nitrate + silver (c) aluminium + copper chloride → aluminium chloride + copper (d) barium chloride + potassium sulphate → barium sulphate + potassium chloride.

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

8. Balance and name the type: (a) potassium bromide(aq) + barium iodide(aq) → potassium iodide(aq) + barium bromide(s) (b) zinc carbonate(s) → zinc oxide(s) + carbon dioxide(g) (c) hydrogen(g) + chlorine(g) → hydrogen chloride(g) (d) magnesium(s) + hydrochloric acid(aq) → magnesium chloride(aq) + hydrogen(g).

(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — double displacement (as the book intends; see the trap in Section 6: barium bromide is actually soluble).
(b) ZnCO₃(s) —[heat]→ ZnO(s) + CO₂(g) — decomposition (thermal).
(c) H₂(g) + Cl₂(g) → 2HCl(g) — combination.
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — displacement.

9. What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic: heat is released with the products — CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat; burning of natural gas CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g); respiration. Endothermic: energy is absorbed — CaCO₃(s) —[heat]→ CaO(s) + CO₂(g); 2AgCl(s) —[sunlight]→ 2Ag(s) + Cl₂(g); Ba(OH)₂ + NH₄Cl.

10. Why is respiration considered an exothermic reaction? Explain.

Food carbohydrates are broken down to glucose, which combines with oxygen in our cells and releases energy: C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy. Because energy is given out, it is exothermic; that energy keeps us alive and warm.

11. Why are decomposition reactions called the opposite of combination reactions? Write equations.

In combination, two or more reactants form one product; in decomposition, one reactant breaks into two or more products. Combination: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. Decomposition: CaCO₃(s) —[heat]→ CaO(s) + CO₂(g). Water shows both: 2H₂(g) + O₂(g) → 2H₂O(l) and 2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g).

Group Activity (page 16): Beakers A–C hold 25 mL water, D holds copper sulphate solution; add potassium sulphate, ammonium nitrate, anhydrous copper sulphate and iron filings to A–D. Which are exothermic, which endothermic?

Expected (Beyond NCERT — check against your readings): A potassium sulphate — a slight fall in temperature (dissolving is mildly endothermic); B ammonium nitrate — a marked fall (endothermic; used in instant cold packs); C anhydrous copper sulphate — a rise, and the white powder turns blue as it takes up water (exothermic); D iron filings in copper sulphate solution — a rise as iron displaces copper (exothermic). Note: A–C are dissolving/hydration, D is a true displacement reaction.

12. Write one equation each for decomposition reactions where energy is supplied as heat, light or electricity.

Heat: CaCO₃(s) —[heat]→ CaO(s) + CO₂(g). Light: 2AgCl(s) —[sunlight]→ 2Ag(s) + Cl₂(g). Electricity: 2H₂O(l) —[electricity]→ 2H₂(g) + O₂(g).

13. What is the difference between displacement and double displacement reactions? Write equations.

Displacement: a more reactive element replaces another in its compound — Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Double displacement: two compounds exchange ions — Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).

14. In the refining of silver, silver is recovered from silver nitrate solution by displacement with copper. Write the reaction.

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s) — copper is more reactive than silver.

15. What do you mean by a precipitation reaction? Explain with examples.

A reaction that produces an insoluble solid (precipitate) from solutions. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (white BaSO₄); Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) (yellow PbI₂); AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) (white AgCl).

16. Explain oxidation and reduction in terms of gain or loss of oxygen, with two examples each.

Oxidation = gain of oxygen: 2Cu + O₂ —[heat]→ 2CuO (copper); H₂ → H₂O in CuO + H₂ —[heat]→ Cu + H₂O; also C(s) + O₂(g) → CO₂(g). Reduction = loss of oxygen: CuO → Cu in (1.29); ZnO → Zn in ZnO + C → Zn + CO.

17. A shiny brown element X becomes black on heating in air. Name X and the black compound.

X is copper (Cu); the black compound is copper(II) oxide (CuO): 2Cu + O₂ —[heat]→ 2CuO.

18. Why do we apply paint on iron articles?

Paint keeps air and moisture away from the iron surface, so it does not rust (corrode). It also improves appearance.

19. Oil and fat containing food items are flushed with nitrogen. Why?

Nitrogen is unreactive; flushing pushes out oxygen, so the fats and oils are not oxidised and do not turn rancid — the food keeps its smell and taste longer.

20. Explain with one example each: (a) corrosion (b) rancidity.

(a) Corrosion: a metal is slowly attacked by substances around it (moisture, air, acids) — e.g. rusting of iron, a reddish-brown coating; black coating on silver; green on copper. (b) Rancidity: oxidation of fats and oils in food that changes smell and taste — e.g. chips or butter left open for long go rancid; prevented by antioxidants, air-tight packing and nitrogen flushing.

What to remember

Numbers

Signs of a chemical reaction4 — change in state, colour or temperature; gas given off

Magnesium ribbon in Activity 1.1about 3–4 cm, cleaned with sandpaper

State symbols4 — (s) solid, (l) liquid, (g) gas, (aq) aqueous

Iron + steam, balanced3Fe + 4H₂O → Fe₃O₄ + 4H₂

Methanol from CO + 2H₂ (1.11)340 atm written over the arrow

Photosynthesis (1.12)6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Whitewash turns to CaCO₃after 2–3 days (marble is CaCO₃ too)

Ferrous sulphate crystalsFeSO₄·7H₂O, green; about 2 g heated

Lead nitrate on heatingabout 2 g; brown NO₂ fumes

Electrolysis of water6 V battery; H₂ : O₂ = 2 : 1 by volume

Silver chloride in sunlightabout 2 g; white turns grey

Carry-out activity (endothermic)about 2 g Ba(OH)₂ + 1 g NH₄Cl — feels cold

Activity 1.9 (displacement)about 10 mL CuSO₄; nails in for about 20 minutes

Activity 1.10 (precipitation)about 3 mL each of Na₂SO₄ and BaCl₂

Activity 1.11 (oxidation)about 1 g copper powder

Group activity25 mL water in beakers A, B, C

Reaction types in 1.25 — combination, decomposition, displacement, double displacement, redox

Chapter size11 activities, Figures 1.1–1.10, 20 exercise questions

One-liners

A balanced equation has the same number of atoms of each element on both sides — mass is conserved.
Balance with coefficients only; never change a formula's subscripts.
Combination: many → one. Decomposition: one → many.
Exothermic gives out heat (CaO + water, burning, respiration); endothermic takes in energy (most decompositions, Ba(OH)₂ + NH₄Cl).
Decomposition needs heat (thermal), light (photolysis) or electricity (electrolysis).
Quick lime CaO; slaked lime Ca(OH)₂; limestone and marble CaCO₃.
Silver chloride and silver bromide decompose in light — black-and-white photography.
Electrolysis of water: hydrogen at the cathode, twice the volume of oxygen at the anode.
A more reactive metal displaces a less reactive one from its salt solution (Fe + CuSO₄).
Double displacement = exchange of ions; a precipitate often forms (white BaSO₄, yellow PbI₂).
Oxidation = gain of O or loss of H; reduction = loss of O or gain of H; together = redox.
Corrosion attacks metals (rust on iron); rancidity spoils fats and oils — both are oxidation.
Chip packets are flushed with nitrogen to prevent oxidation.

Traps to avoid

In 2PbO + C → 2Pb + CO₂, lead OXIDE is reduced and carbon is oxidised — ‘lead is reduced’ and ‘CO₂ is oxidised’ are the incorrect statements (exercise 1).
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is a displacement (and redox), not a double displacement.
Not every decomposition is endothermic (composting gives out heat), and not every endothermic reaction is a decomposition (Ba(OH)₂ + NH₄Cl).
Respiration is exothermic; photosynthesis is endothermic.
Hydrogen, not oxygen, is the gas collected in double volume in electrolysis of water.
Ferrous sulphate (green, FeSO₄) gives ferric oxide (Fe₂O₃) — ferrous is iron(II), ferric is iron(III).
Precipitation names the result; double displacement names the ion exchange. Neutralisation is double displacement without a precipitate.
Rusting is corrosion of iron only; corrosion also covers silver (black) and copper (green).
NCERT slips: Life Processes is Chapter 5 (not ‘Chapter 6’); Br₂ is a liquid at room temperature; BaBr₂ in exercise 8(a) is actually soluble.

How to strategise

Prelims — what UPSC asks

Photography and chemical change: silver bromide as the chemical of photography (1982) and photographic film as a one-use chemical change against reusable magnetic tape (1990) both come from equation (1.23). Know the light-decomposition pair AgCl/AgBr and, Beyond NCERT, the fixer (sodium thiosulphate, asked in 1995).
Physical vs chemical change: 2014 asked which of crystallisation of salt, melting of ice and souring of milk is a chemical change — only souring. Use the four signs plus ‘is there a new substance?’.
Electrolysis to green hydrogen: 2026 asked that green hydrogen is made by electrolysis of water with renewable electricity (and not by gas reforming with carbon capture). Activity 1.7 is the science; learn the colour labels and the National Green Hydrogen Mission's headline targets alongside.
Likely next: exothermic versus endothermic examples; quick lime, slaked lime and whitewash; why chip packets hold nitrogen; antioxidants and rancidity; corrosion and its prevention (galvanising is asked often but is taught in Chapter 3); identifying what is oxidised and reduced; balancing-by-inspection statements.

Mains — where it appears and how to answer

No GS Mains question in the bank so far is built on this chapter's core content. Its use in GS3 is as the science behind applied answers: corrosion of bridges, railways, pipelines and ships (maintenance cost, safety, coatings); food processing and shelf life (rancidity, antioxidants, modified-atmosphere packaging); cement and lime (CO₂ released from limestone itself); acid-rain gases (SO₂, NO₂); and green hydrogen by electrolysis.
Answer frame for a science-and-technology question: one-line science (with a balanced equation) → application in India → benefits → concerns or limits stated with their source → way forward. A correct, balanced equation is a quick credibility marker in a 150-word answer.
A small labelled sketch — electrolysis cell, redox bracket, or the lime cycle — takes a minute and lifts a science answer.

Self-check

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

1. In the reaction 2PbO(s) + C(s) → 2Pb(s) + CO₂(g), which substance is oxidised?

2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of a:

3. Which of these is an endothermic reaction?

4. In the electrolysis of water (Activity 1.7), the gas collected in double volume is:

5. Mixing sodium sulphate and barium chloride solutions gives a white solid. The reaction is best described as:

6. Chips manufacturers flush packets with nitrogen mainly to:

7. White silver chloride turns grey in sunlight because:

UPSC questions from this chapter

Open any question to attempt it in the Question bank — your attempt is saved to your progress.

Prelims

4 questions · newest first

2026Prelims GSWhich of the following statements with regard to Green Hydrogen is/are correct ? 1. It is decarbonized hydrogen obtained from natural gas reforming combined with carbon capture and storage (CCS). 2. It is produced using electrolysis of water with electricity generated by renewable energy. 3. National Green Hydrogen Mission of India aims for abatement of nearly 50 MMT of annual greenhouse gas emissions by 2030. Select the answer using the code given below :Attempt →2014Prelims GSWhich of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Souring of milk Select the correct answer using the code given below.Attempt →1990Prelims GSAssertion (A) : Unlike photographic film, which can be used only once, video tapes can be reused several times. Reason (R) : Photography involves chemical change but video recording is a physical change.Attempt →1982Prelims GSWhich of the following is useful in the chemistry of photographyAttempt →

Mains

0 questions

4 Prelims and 0 Mains UPSC questions are linked to this chapter. More from Science.