BharatNotes Why this chapter matters for UPSC: This is Chapter 1 of NCERT's Class X Science (Reprint 2026-27). It teaches the grammar of chemistry: how to write a reaction as a balanced equation, and how to sort reactions into five types. Prelims general science tests exactly these skills (which substance is oxidised, which reaction is a displacement, why a chip packet holds nitrogen), and two of the chapter's textbook reactions sit under current GS3 policy: the electrolysis of water is how green hydrogen is made, and the heating of limestone is why cement emits carbon dioxide even when its fuel is clean.
Contemporary hook: The Union Cabinet approved the National Green Hydrogen Mission on 4 January 2023 with an initial outlay of ₹19,744 crore; its "likely outcomes by 2030" include "green hydrogen production capacity of at least 5 MMT (Million Metric Tonne) per annum" (PIB, 4 January 2023). The core reaction is NCERT's Activity 1.7, the electrolysis of water, run on renewable electricity.
🧠 First Principles — Read This First
A chemical change makes new substances. When milk sours, iron rusts or food is digested, "the nature and the identity of the initial substance have somewhat changed". NCERT's test for whether a reaction has happened is observation: a change in state, a change in colour, the evolution of a gas, or a change in temperature.
Atoms are only rearranged. "Chemical reactions involve the breaking and making of bonds between atoms to produce new substances." Atoms of one element do not turn into another, nor do atoms disappear. So mass is conserved, and every equation must carry the same number of atoms of each element on both sides. Balancing an equation is the law of conservation of mass written in symbols.
Five patterns cover the chapter. Two substances join (combination); one splits (decomposition, which needs heat, light or electricity); a more reactive element pushes a less reactive one out of its compound (displacement); two compounds swap ions (double displacement); and oxygen or hydrogen moves from one substance to another (oxidation and reduction, which always happen together).
Energy decides the direction of use. Reactions that give out heat (burning, respiration, compost) are exothermic; reactions that absorb energy are endothermic, and every decomposition in the chapter is one. Oxidation also works slowly around us: it corrodes metals and turns fats rancid.
PART 1 — Quick Reference
Table 1: The five reaction types, with NCERT's own examples
| Type | What happens | NCERT's equations |
|---|---|---|
| Combination | Two or more reactants form a single product | CaO + H₂O → Ca(OH)₂ + heat; C + O₂ → CO₂; 2H₂ + O₂ → 2H₂O |
| Decomposition (thermal) | One reactant breaks down on heating | 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃; CaCO₃ → CaO + CO₂; 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ |
| Decomposition (electricity) | Current splits a compound | Electrolysis of water, Activity 1.7 (NCERT prints no equation; balanced, it is 2H₂O → 2H₂ + O₂) |
| Decomposition (light) | Sunlight splits a compound | 2AgCl → 2Ag + Cl₂; 2AgBr → 2Ag + Br₂ |
| Displacement | A more reactive element displaces a less reactive one | Fe + CuSO₄ → FeSO₄ + Cu; Zn + CuSO₄ → ZnSO₄ + Cu; Pb + CuCl₂ → PbCl₂ + Cu |
| Double displacement | Two compounds exchange ions | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl (a precipitation reaction) |
| Oxidation and reduction (redox) | One reactant gains oxygen or loses hydrogen, the other loses oxygen or gains hydrogen | CuO + H₂ → Cu + H₂O; ZnO + C → Zn + CO; MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ |
Source: NCERT, Science Class X, ch. 1, Reprint 2026-27, equations 1.13-1.32 (pp. 6-13).
Table 2: What you see in each NCERT activity
| Activity | What is done | What you observe | Product |
|---|---|---|---|
| 1.1 | Magnesium ribbon, cleaned with sandpaper, burnt in air | Dazzling white flame; white powder | Magnesium oxide, MgO |
| 1.2 | Lead nitrate solution + potassium iodide solution | A coloured precipitate (NCERT asks you to name it) | Lead iodide, PbI₂ (yellow) |
| 1.3 | Zinc granules + dilute HCl or H₂SO₄ | Gas bubbles around the zinc; flask temperature changes | Hydrogen |
| 1.4 | Water added slowly to quick lime | Beaker becomes hot | Slaked lime, Ca(OH)₂ |
| 1.5 | Green ferrous sulphate crystals heated | Colour changes; smell of burning sulphur | Fe₂O₃, SO₂, SO₃ |
| 1.6 | Lead nitrate heated | Brown fumes | Nitrogen dioxide, NO₂ (with PbO and O₂) |
| 1.7 | Water (with a few drops of dilute H₂SO₄) electrolysed with carbon electrodes, 6 V | Bubbles at both electrodes; one tube collects twice the gas of the other | Hydrogen (double volume) and oxygen |
| 1.8 | White silver chloride kept in sunlight | Turns grey | Silver and chlorine |
| (box) | Barium hydroxide + ammonium chloride | Test tube feels cold | An endothermic reaction |
| 1.9 | Iron nails in blue copper sulphate solution for 20 minutes | Nail turns brownish; blue colour fades | Iron sulphate and copper |
| 1.10 | Sodium sulphate + barium chloride solutions | White insoluble substance | Barium sulphate precipitate |
| 1.11 | Copper powder heated in a china dish | Surface turns black; turns brown again when hydrogen is passed over it | Copper(II) oxide, then copper |
Source: NCERT, Science Class X, ch. 1, Reprint 2026-27, Activities 1.1-1.11 (pp. 1-12). The colour of lead iodide is the expected answer to NCERT's own question on p. 12.
Table 3: Lime, limestone and marble
| Name | Formula | How it appears in the chapter |
|---|---|---|
| Limestone | CaCO₃ | Heated to give quick lime and CO₂, "an important decomposition reaction used in various industries" |
| Quick lime (lime) | CaO | Used "in the manufacture of cement"; reacts vigorously with water |
| Slaked lime | Ca(OH)₂ | Its solution is used for whitewashing |
| Calcium carbonate on a whitewashed wall | CaCO₃ | Forms in two to three days from Ca(OH)₂ and CO₂ in air; gives a shiny finish. "The chemical formula for marble is also CaCO₃" |
Source: NCERT, Science Class X, ch. 1, Reprint 2026-27, pp. 6-8 (equations 1.13, 1.14 and 1.20, and the "Do You Know?" box).
Table 4: The exercise equations, balanced
| NCERT question | Balanced equation | Type |
|---|---|---|
| In-text 2(i), Ex. 8(c) | H₂ + Cl₂ → 2HCl | Combination |
| In-text 2(ii), Ex. 5(c) | 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ | Double displacement |
| In-text 2(iii) | 2Na + 2H₂O → 2NaOH + H₂ | Displacement |
| In-text 3(ii) | NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) | Double displacement |
| Ex. 5(a) | N₂ + 3H₂ → 2NH₃ | Combination |
| Ex. 5(b) | 2H₂S + 3O₂ → 2H₂O + 2SO₂ | Redox (H₂S is oxidised) |
| Ex. 5(d) | 2K + 2H₂O → 2KOH + H₂ | Displacement |
| Ex. 6(a) | 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O | Double displacement |
| Ex. 6(b) | 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O | Double displacement |
| Ex. 6(c) | NaCl + AgNO₃ → AgCl + NaNO₃ | Double displacement (AgCl precipitates) |
| Ex. 6(d) | BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl | Double displacement |
| Ex. 7(a) | Ca(OH)₂ + CO₂ → CaCO₃ + H₂O | The whitewash reaction |
| Ex. 7(b) | Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag | Displacement |
| Ex. 7(c) | 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu | Displacement |
| Ex. 7(d) | BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl | Double displacement |
| Ex. 8(a) | 2KBr + BaI₂ → 2KI + BaBr₂ | Double displacement |
| Ex. 8(b) | ZnCO₃ → ZnO + CO₂ | Decomposition |
| Ex. 8(d) | Mg + 2HCl → MgCl₂ + H₂ | Displacement |
| Ex. 14 | Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag | Displacement |
Source: NCERT, Science Class X, ch. 1, Reprint 2026-27, in-text questions (pp. 6, 13) and Exercises 5-8 and 14 (pp. 15-16); balanced on this page by atom count.
Table 5: NCERT lines to read with care
| NCERT says | The record |
|---|---|
| Respiration is a reaction "the process of which you will study in Chapter 6" (p. 7) | In Reprint 2026-27, Life Processes is Chapter 5. The cross-reference was written for the earlier 16-chapter edition, where Life Processes was Chapter 6, and has not been updated. |
| Exercise 5(b): "water and sulpur dioxide" | A spelling slip for sulphur dioxide. |
Source: NCERT, Science Class X, Reprint 2026-27, ch. 1 pp. 7 and 15, and the book's Contents page.
PART 2 — Concepts & Narrative
Chemical equations
Burning magnesium can be written as a sentence, then as a word-equation: Magnesium + Oxygen → Magnesium oxide. The substances that change, magnesium and oxygen, are the reactants; magnesium oxide is the product. Reactants go on the left-hand side and products on the right, each joined by plus signs, and the arrowhead "shows the direction of the reaction".
Writing formulae instead of words gives Mg + O₂ → MgO. Count the atoms: two oxygen atoms on the left, one on the right. An equation in which the atoms of some element do not match is a skeletal chemical equation. Zn + H₂SO₄ → ZnSO₄ + H₂, by contrast, has one zinc, two hydrogen, one sulphur and four oxygen atoms on each side, so it is already balanced.
Balanced chemical equation. An equation in which "the number of atoms of each element remains the same, before and after a chemical reaction", as the law of conservation of mass requires: "mass can neither be created nor destroyed in a chemical reaction". An unbalanced (skeletal) equation is unbalanced "because the mass is not the same on both sides".
Balancing by hit-and-trial
NCERT balances Fe + H₂O → Fe₃O₄ + H₂ (iron reacting with steam) in a fixed order of steps.
The steps, worked.
- Draw a box around each formula. Nothing inside a box may change: to get four oxygen atoms you write 4H₂O, never H₂O₄ or (H₂O)₄.
- List the atoms on each side: Fe 1 and 3, H 2 and 2, O 1 and 4.
- Start with the compound that has the most atoms (Fe₃O₄) and, within it, the element with the most atoms (oxygen). Put 4 before H₂O: Fe + 4H₂O → Fe₃O₄ + H₂.
- Hydrogen is now 8 on the left and 2 on the right. Put 4 before H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂.
- Only iron is left: 1 on the left, 3 on the right. Put 3 before Fe: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
- Check every element: Fe 3 = 3, H 8 = 8, O 4 = 4. Balanced.
- Add state symbols: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). The (g) on water says the reaction uses steam.
The method is called hit-and-trial "as we make trials to balance the equation by using the smallest whole number coefficient". Applied to magnesium, it gives 2Mg + O₂ → 2MgO.
State symbols and conditions. The notations (g), (l), (aq) and (s) stand for gaseous, liquid, aqueous and solid; "aqueous" means present as a solution in water. States are usually written only when they need to be specified. Conditions go above or below the arrow: CO(g) + 2H₂(g) → CH₃OH(l) at 340 atm; and photosynthesis, 6CO₂(aq) + 12H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l), needs sunlight and chlorophyll.
Combination reactions, and heat given out
Quick lime (calcium oxide) "reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat": CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. A reaction in which "a single product is formed from two or more reactants" is a combination reaction. Burning coal (C + O₂ → CO₂) and forming water from hydrogen and oxygen (2H₂ + O₂ → 2H₂O) are others, and so is the burning of magnesium in Activity 1.1.
Because heat is released, slaking lime is also exothermic. NCERT's other exothermic examples are not all combinations: burning natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O), respiration (glucose combining with oxygen in our cells, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy), and the decomposition of vegetable matter into compost.
Whitewash. The slaked lime solution from the quick-lime reaction is used to whitewash walls. "Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls": Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The carbonate forms "after two to three days of whitewashing and gives a shiny finish". Marble has the same formula, CaCO₃.
Decomposition reactions, and energy taken in
Heat. Green ferrous sulphate crystals (FeSO₄·7H₂O) lose water when heated and change colour, and the tube gives off the smell of burning sulphur: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. Ferric oxide is a solid; the two sulphur oxides are gases. Heating limestone gives quick lime: CaCO₃ → CaO + CO₂. Heating lead nitrate releases brown fumes of nitrogen dioxide: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. A decomposition carried out by heating is a thermal decomposition.
Electricity. In Activity 1.7, a current passed through water (with a few drops of dilute sulphuric acid) produces bubbles at both carbon electrodes, and one test tube fills with twice as much gas as the other: 2H₂O → 2H₂ + O₂.
Why one tube holds double the gas (NCERT's in-text question). The equation gives two molecules of hydrogen for every molecule of oxygen, and under the same conditions a gas's volume follows the number of molecules. So the tube with twice the volume holds hydrogen. Hydrogen collects at the negative electrode (cathode) and oxygen at the positive electrode (anode). Brought near a burning candle, hydrogen burns with a pop; oxygen makes the flame burn brighter.
Light. White silver chloride turns grey in sunlight as it decomposes into silver and chlorine (2AgCl → 2Ag + Cl₂); silver bromide behaves the same way. "The above reactions are used in black and white photography."
All three kinds of decomposition need energy, as heat, light or electricity, to break the reactants down. "Reactions in which energy is absorbed are known as endothermic reactions." NCERT's suggested demonstration: mix about 2 g of barium hydroxide with 1 g of ammonium chloride; the bottom of the test tube feels cold.
Exothermic and endothermic. Exothermic: "heat is released along with the formation of products" (respiration, burning, slaking lime). Endothermic: energy is absorbed (every decomposition in the chapter, and the barium hydroxide and ammonium chloride mixture). Decomposition is "opposite to combination": one substance gives two or more.
Displacement reactions
Iron nails left in blue copper sulphate solution for about 20 minutes turn brownish, and the blue colour fades: Fe + CuSO₄ → FeSO₄ + Cu. Iron "has displaced or removed another element, copper", from its compound. Zinc and lead do the same (Zn + CuSO₄ → ZnSO₄ + Cu; Pb + CuCl₂ → PbCl₂ + Cu) because they "are more reactive elements than copper". Chapter 3 arranges metals in this order as the reactivity series.
The colour change answers NCERT's in-text question: copper sulphate solution is blue and iron sulphate solution is pale green, so as iron replaces copper in solution the blue fades, while the displaced copper coats the nail brown.
Double displacement and precipitation
Mixing sodium sulphate and barium chloride solutions forms a white substance that is insoluble in water, a precipitate: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. The white BaSO₄ forms from Ba²⁺ and SO₄²⁻ ions; sodium chloride stays in solution. "Such reactions in which there is an exchange of ions between the reactants are called double displacement reactions", and "any reaction that produces a precipitate can be called a precipitation reaction". Activity 1.2 is another: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃, with yellow lead iodide precipitating. The neutralisation in the in-text question (NaOH + HCl → NaCl + H₂O) is also a double displacement, one with no precipitate.
Displacement vs double displacement. Displacement: an element replaces another element in a compound (A + BC → AC + B). Double displacement: two compounds exchange ions (AB + CD → AD + CB). Thermite, Fe₂O₃ + 2Al → Al₂O₃ + 2Fe (NCERT Exercise 2), is a displacement: aluminium displaces iron from its oxide.
Oxidation and reduction
Heated copper powder becomes coated with black copper(II) oxide because oxygen is added to it: 2Cu + O₂ → 2CuO. Pass hydrogen over the hot black material and it turns brown again: CuO + H₂ → Cu + H₂O. Copper oxide loses oxygen and is reduced; hydrogen gains oxygen and is oxidised. One reactant is oxidised while the other is reduced, so these are oxidation-reduction (redox) reactions.
In ZnO + C → Zn + CO, carbon is oxidised to CO and ZnO is reduced to zinc. In MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, HCl is oxidised to chlorine (it loses hydrogen) and MnO₂ is reduced to MnCl₂ (it loses oxygen). Hence NCERT's full definitions: oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen.
Oxidising and reducing agents. The substance that supplies the oxygen (or removes the hydrogen) is the oxidising agent and is itself reduced; the substance that takes the oxygen is the reducing agent and is itself oxidised. In CuO + H₂ → Cu + H₂O, CuO is the oxidising agent and H₂ the reducing agent. Class XI chemistry widens the same idea to electron transfer: oxidation is loss of electrons, reduction is gain.
Oxidation in everyday life: corrosion
New iron is shiny but in time gains "a reddish brown powder", rust. Silver gains a black coating and copper a green one. "When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion." It damages "car bodies, bridges, iron railings, ships", and NCERT notes that "an enormous amount of money is spent to replace damaged iron".
Chapter 3 completes the picture. Silver turns black because it reacts with sulphur in the air to form silver sulphide; copper reacts with moist carbon dioxide to form a green coat of basic copper carbonate. In Activity 3.14, iron nails rust only in the tube where they meet both air and water, not in boiled water sealed under oil (no air) or over anhydrous calcium chloride (dry air). Rusting is prevented by "painting, oiling, greasing, galvanising, chrome plating, anodising or making alloys"; galvanised iron, coated with zinc, "is protected against rusting even if the zinc coating is broken".
Oxidation in everyday life: rancidity
"When fats and oils are oxidised, they become rancid and their smell and taste change." Three defences follow from the cause: antioxidants (substances that prevent oxidation) are added to fatty foods; airtight containers slow oxidation; and chip makers "flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised".
Did the 2020-21 edition differ?
No. The chapter was Chapter 1 in the 2020-21 edition with the same text, activities and exercises. Only its cross-reference changed in meaning: "Chapter 6" was Life Processes in the 16-chapter book; the 2026-27 book has 13 chapters and Life Processes is Chapter 5 (Table 5).
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.
Beyond the textbook: two reactions in current policy
Electrolysis and green hydrogen. The Union Cabinet approved the National Green Hydrogen Mission on 4 January 2023. Its initial outlay is ₹19,744 crore, "including an outlay of Rs.17,490 crore for the SIGHT programme, Rs.1,466 crore for pilot projects, Rs.400 crore for R&D, and Rs. 388 crore towards other Mission components". Its likely outcomes by 2030 include green hydrogen production capacity of "at least 5 MMT (Million Metric Tonne) per annum with an associated renewable energy capacity addition of about 125 GW". MNRE frames the guidelines. The chemistry is Activity 1.7: water split by current, here from renewable power.
Limestone and cement emissions. CaCO₃ → CaO + CO₂ releases carbon dioxide from the rock itself, whatever fuel heats the kiln. By formula mass (CaCO₃ 100, CO₂ 44), every 100 g of limestone decomposed gives off 44 g of CO₂ (arithmetic from the equation, not a published plant figure). Such process emissions are why carbon capture matters for cement. The Union Budget speech of 1 February 2026 (para 38) said that "CCUS technologies at scale will achieve higher readiness levels" in end-use applications "across five industrial sectors, including, power, steel, cement, refineries and chemicals", and that "An outlay of ₹20,000 crore is proposed over the next 5 years". That is a Budget proposal, not yet a notified scheme.
Source: PIB (Ministry of New and Renewable Energy), "Cabinet approves National Green Hydrogen Mission", 4 January 2023; Union Budget 2026-27, Budget Speech, 1 February 2026, para 38.
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — identifying reaction types, oxidised and reduced substances, observations (colours, gases, precipitates), corrosion and its prevention, rancidity.
- GS3 (Science and technology, energy) — electrolysis of water as the basis of green hydrogen; the National Green Hydrogen Mission.
- GS3 (Environment and climate) — process emissions from limestone in cement; carbon capture, utilisation and storage.
- GS3 (Infrastructure) — corrosion of bridges, ships and railings; galvanising and alloying.
Past questions on these themes: Prelims 2026 (green hydrogen; question ID in the Revision Capsule).
Frames for Mains Answers
1. Green hydrogen from a Class X reaction. Electrolysis (2H₂O → 2H₂ + O₂) needs electricity; the hydrogen is "green" only when that electricity is renewable. Policy: the National Green Hydrogen Mission (4 January 2023, ₹19,744 crore, at least 5 MMT a year of production capacity as a likely 2030 outcome, about 125 GW of associated renewable capacity).
2. Why cement is hard to decarbonise. Two sources of CO₂: fuel burnt in the kiln, and the decomposition of limestone itself (44 g of CO₂ per 100 g of CaCO₃ by the equation). Clean fuel removes only the first; capture handles the second, hence the CCUS outlay proposed in the 2026-27 Budget for cement among five sectors.
3. Corrosion as a public cost. NCERT's own framing: damage to bridges, railings, ships and car bodies, and "an enormous amount of money" spent on replacement. Prevention menu from Chapter 3: paint, oil, grease, galvanise, chrome-plate, anodise, alloy.
Exam Strategy
Prelims fact-traps:
- In 2PbO + C → 2Pb + CO₂ (NCERT Exercise 1), lead oxide is reduced and carbon is oxidised. "Lead is getting reduced" and "Carbon dioxide is getting oxidised" are the incorrect statements: lead is a product, and CO₂ is a product, not a reactant.
- The thermite reaction (Fe₂O₃ + 2Al → Al₂O₃ + 2Fe) is a displacement reaction (and a redox reaction), not a combination.
- Silver darkening on exposure to air is corrosion (silver sulphide). Silver chloride turning grey in sunlight is photo-decomposition into silver metal. Different reactions, different causes.
- Decomposition reactions are endothermic; respiration is exothermic.
- Quick lime is CaO, slaked lime is Ca(OH)₂, limestone and marble are CaCO₃.
- In electrolysis of water, the hydrogen volume is double the oxygen volume.
- Iron rusts only with both air and water. Galvanised iron stays protected even where the zinc coat is broken.
Mains: use NCERT reactions as the science core of energy and climate answers (electrolysis for hydrogen, calcination for cement), and date every policy figure.
Practice Questions
Questions 1-3 are the NCERT exercise MCQs. Practice (UPSC-pattern, not past papers): questions 4-7.
1. Which of the statements about the reaction below are incorrect? 2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Answer: (i). Lead oxide is reduced and carbon is oxidised.
2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of a
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Answer: (d). Aluminium displaces iron from iron oxide.
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.
Answer: (a). Fe + 2HCl → FeCl₂ + H₂.
4. Consider the following statements:
1. Food packets of chips are flushed with nitrogen to slow the oxidation of fats.
2. The black coating on old silver articles is silver oxide formed by sunlight.
3. A thin coat of calcium carbonate forms on a whitewashed wall over two to three days.
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
Answer: (c). The black coating on silver is silver sulphide, from sulphur in the air.
5. Which of the following reactions are endothermic?
1. Decomposition of calcium carbonate into quick lime and carbon dioxide
2. Reaction of quick lime with water
3. Decomposition of silver chloride in sunlight
Select the correct answer:
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (c). Slaking quick lime releases a large amount of heat.
6. With reference to green hydrogen, consider the following statements:
1. It is produced by electrolysis of water using electricity from renewable sources.
2. Electrolysis of water gives hydrogen and oxygen in the volume ratio 1:2.
3. The National Green Hydrogen Mission was approved by the Union Cabinet in January 2023.
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
Answer: (c). Hydrogen to oxygen is 2:1 by volume.
7. Process emissions make cement one of the hardest industries to decarbonise. Explain with reference to the thermal decomposition of limestone, and examine the role of carbon capture. (150 words)
NCERT exercises (where the answers are on this page): 4 and 11 the "Balanced chemical equation" box and decomposition section; 5-8 and 14 Table 4; 9 and 10 the exothermic and endothermic box; 12 Table 1; 13 the displacement key-term box; 15 the precipitation section; 16 the redox section; 17 copper and black copper(II) oxide (Activity 1.11); 18 corrosion; 19 and 20 rancidity and corrosion.
📦 Revision Capsule
Hard Facts
- Four signs of a reaction (NCERT): change in state, change in colour, evolution of a gas, change in temperature.
- State symbols: (g), (l), (aq), (s); (aq) = solution in water.
- 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (iron and steam, the hit-and-trial example).
- FeSO₄ crystals: green; heating gives Fe₂O₃ + SO₂ + SO₃ and a burning-sulphur smell. Lead nitrate: brown NO₂ fumes.
- Electrolysis of water: hydrogen and oxygen, 2:1 by volume.
- AgCl and AgBr decompose in light: black and white photography.
- Na₂SO₄ + BaCl₂: white BaSO₄ precipitate. Pb(NO₃)₂ + KI: yellow PbI₂.
- Silver tarnish: silver sulphide. Copper: basic copper carbonate (green). Rust: reddish-brown.
Core Concepts
- Balancing = conservation of mass; change coefficients, never formulae.
- Decomposition needs energy (heat, light, electricity), so it is endothermic.
- Displacement follows reactivity: zinc, lead and iron displace copper.
- Redox: oxidation = gain of oxygen or loss of hydrogen; reduction = the reverse; the two always occur together.
- Corrosion and rancidity are slow, unwanted oxidations.
Confused Pairs
- Quick lime CaO vs slaked lime Ca(OH)₂ vs limestone/marble CaCO₃.
- Displacement (element replaces element) vs double displacement (ions exchanged).
- Silver tarnish (sulphide, corrosion) vs silver chloride greying (photo-decomposition).
- Exothermic (slaking lime, respiration, compost) vs endothermic (decompositions, Ba(OH)₂ + NH₄Cl).
- Oxidising agent (gets reduced) vs reducing agent (gets oxidised).
Data Points
- Activity 1.7: 6 V battery, carbon electrodes, a few drops of dilute H₂SO₄.
- CO + 2H₂ → CH₃OH at 340 atm (NCERT's conditions example).
- 100 g CaCO₃ → 56 g CaO + 44 g CO₂ (formula masses).
- National Green Hydrogen Mission: Cabinet 4 January 2023; ₹19,744 crore; at least 5 MMT a year by 2030 (likely outcome).
- CCUS: ₹20,000 crore proposed over 5 years (Budget speech, 1 February 2026).
PYQ Pattern
- Prelims: prelims-2026-gs1-070 (green hydrogen: what it is and how it is produced).
- No Mains question in the bank is set directly on this chapter; its reactions feed GS3 energy and climate answers.
Sources
- NCERT, Science, Textbook for Class X, ch. 1 "Chemical Reactions and Equations", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Textbook for Class X, ch. 3 "Metals and Non-metals", Reprint 2026-27 (corrosion, Activity 3.14) — ncert.nic.in PDF.
- NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021 — Wayback Machine.
- Press Information Bureau (Ministry of New and Renewable Energy), "Cabinet approves National Green Hydrogen Mission", 4 January 2023 — pib.gov.in.
- Government of India, Budget 2026-2027, Speech of Nirmala Sitharaman, Minister of Finance, 1 February 2026 (para 38) — indiabudget.gov.in PDF.
