Chemical Reactions and Equations PYQs – Chemistry, SSC & Railway

Correct Answer: Option 2 — Only a and b
In classical terms, reduction is the removal of oxygen from a substance or the addition of hydrogen to it. Oxidation is the exact opposite — addition of oxygen or removal of hydrogen. In electronic terms, reduction is the gain of electrons and oxidation is the loss of electrons.
So statements (a) Loss of oxygen and (b) Gain of hydrogen describe reduction correctly.
⚗️ The Reaction
CuO + H₂ → (heat) Cu + H₂O
• CuO loses oxygen → it is reduced to Cu; H₂ here acts as the reducing agent.
• H₂ gains oxygen → it is oxidised to H₂O; CuO acts as the oxidising agent.
• Oxidation and reduction always occur together, so such a reaction is called a redox reaction.
❌ Why the other options are incorrect
✗ Only a and d — (a) is right but (d) loss of hydrogen is oxidation, not reduction.
✗ Only b and c — (b) is right but (c) gain of oxygen is oxidation; the two halves have been swapped.
✗ Only d — loss of hydrogen is oxidation, so this option is entirely wrong.
📌 SSC & Railway Exam Facts
• Reduction = loss of O₂ / gain of H₂ / gain of electrons.
• Oxidation = gain of O₂ / loss of H₂ / loss of electrons. Remember OIL RIG.
• In a redox reaction the oxidising agent is itself reduced and the reducing agent is itself oxidised.
• Corrosion and rancidity are everyday examples of oxidation.
• Rancidity is delayed by adding antioxidants and by packing food in nitrogen gas.

Correct Answer: Option 2 — Thermal decomposition
When a single compound breaks down into two or more simpler substances on being heated, the change is called thermal decomposition. The heat supplies the energy needed to break the bonds, so such reactions are endothermic.
⚗️ The Reaction
CaCO₃(s) → (heat) CaO(s) + CO₂(g)
• Limestone (calcium carbonate, CaCO₃) is heated to about 1000 °C in a lime kiln — the process is called calcination.
• Quick lime (calcium oxide, CaO) is left behind and carbon dioxide escapes as a gas.
• CaO with water gives slaked lime, Ca(OH)₂, an exothermic reaction used for whitewashing.
❌ Why the other options are incorrect
✗ Reduction reaction — nothing here gains hydrogen or loses oxygen; CaCO₃ simply splits into two products.
✗ Photochemical decomposition — that requires light as the energy source, as in 2AgCl → 2Ag + Cl₂ in sunlight; here the energy is heat.
✗ Oxidation reaction — no substance gains oxygen or loses hydrogen; in fact oxygen leaves the compound as part of CO₂.
📌 SSC & Railway Exam Facts
• CaCO₃ → CaO + CO₂ is the classic thermal decomposition; industrially it is calcination.
• CaO = quick lime, Ca(OH)₂ = slaked lime, CaCO₃ = limestone / marble / chalk.
• Whitewash shines because Ca(OH)₂ + CO₂ → CaCO₃ + H₂O forms a hard CaCO₃ layer.
• 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ gives brown fumes of NO₂ on heating.
• Three kinds of decomposition: thermal (heat), photolytic (light) and electrolytic (electricity).

Correct Answer: Option 1 — Barium sulphate
This is a classic double displacement (precipitation) reaction. Both reactants are soluble salts taken in aqueous solution, and of the two products, sodium chloride remains dissolved (aq) while barium sulphate is insoluble in water and separates out immediately as a thick white precipitate — that is, as a solid.
⚗️ The Reaction
Na₂SO₄(aq) + BaCl₂(aq) → 2NaCl(aq) + BaSO₄(s)
• The two compounds exchange their ions — Ba²⁺ pairs with SO₄²⁻, and Na⁺ pairs with Cl⁻.
• BaSO₄ is insoluble, so it is written with (s) and appears as a white precipitate.
• The coefficient 2 before NaCl is needed to balance the Na and Cl atoms.
Because an insoluble solid is formed, the reaction is also called a precipitation reaction.
❌ Why the other options are incorrect
✗ Sodium chloride — common salt is highly soluble in water, so it stays in solution as NaCl(aq); it is a product, but not the solid one.
✗ Barium chloride — this is a reactant, and it is taken as a soluble aqueous solution, so it is neither a product nor a solid here.
✗ Sodium sulphate — also a reactant and soluble in water; it is used as Na₂SO₄(aq).
📌 SSC & Railway Exam Facts
• BaSO₄ is an insoluble white precipitate; it is given as a barium meal for X-ray imaging of the stomach and intestines.
• Common white precipitates: BaSO₄, AgCl, PbSO₄, CaCO₃; yellow ones: PbI₂, AgBr.
• AgNO₃ + NaCl → AgCl↓ (white) + NaNO₃ is the standard precipitation PYQ.
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution; a ↓ also marks a precipitate.
• All sodium and potassium salts are soluble in water; most sulphates are soluble except BaSO₄, PbSO₄ and CaSO₄.

Answer: Option 2 — CaO + H₂O → Ca(OH)₂
Two reactants, calcium oxide and water, join to give a single product, calcium hydroxide — that is a combination reaction.
Revision note — Types of chemical reactions
• Combination: A + B → AB, a single product; e.g. CaO + H₂O → Ca(OH)₂ + heat, 2Mg + O₂ → 2MgO, C + O₂ → CO₂.
• CaO is quicklime; Ca(OH)₂ is slaked lime — its solution is used for whitewashing.
• Decomposition: AB → A + B, the reverse of combination; e.g. CaCO₃ → CaO + CO₂ (thermal), 2KClO₃ → 2KCl + 3O₂.
• Double displacement: ions are exchanged; e.g. NaCl + AgNO₃ → AgCl + NaNO₃ — the white AgCl is a precipitate.
• Decomposition may be thermal (heat), electrolytic (electricity) or photolytic (light: 2AgCl → 2Ag + Cl₂).
• Most combination reactions are exothermic; most decomposition reactions are endothermic.
Also asked from this topic: the reaction type of CaO + H₂O → Ca(OH)₂ + heat (exothermic combination reaction); the observation that confirms a combination reaction (only one new substance is formed).

Answer: Option 1 — CuO
CuO loses oxygen and becomes copper, so CuO is reduced.
Hydrogen gains oxygen and becomes water, so H_(2) is oxidised.
Revision note — Oxidation and reduction (Class 10)
• Oxidation: gain of oxygen or loss of hydrogen.
• Reduction: loss of oxygen or gain of hydrogen.
• A redox reaction has both at once — one substance is oxidised while the other is reduced.
• Oxidising agent = the substance that gives oxygen (and itself gets reduced) — here CuO.
• Reducing agent = the substance that takes oxygen (and itself gets oxidised) — here H_(2).
• Copper heated in air turns black (2Cu + O_(2) → 2CuO); passing hydrogen over hot CuO gives back reddish-brown copper.
Do not mix these up
• Substance reduced vs reducing agent — the substance reduced is CuO; the reducing agent is H_(2).
Also asked from this topic: the substance reduced when MnO_(2) reacts with HCl (MnO_(2)) and the one oxidised (HCl); the reducing agent that extracts zinc from zinc oxide (carbon).

Answer: Option 2 — nitrogen dioxide (NO_(2))
On heating, lead nitrate gives brown fumes of nitrogen dioxide (NO_(2)).
2Pb(NO_(3))_(2) → 2PbO + 4NO_(2) + O_(2)
PbO is a yellow solid and O_(2) is colourless.
Revision note — Thermal decomposition (NCERT Class 10, Ch 1)
• Decomposition: one compound breaks into simpler substances — by heat, light or electricity.
• CaCO_(3) → CaO + CO_(2) on heating — quicklime for cement; a thermal decomposition.
• 2FeSO_(4) → Fe_(2)O_(3) + SO_(2) + SO_(3) — green crystals turn reddish-brown, with a smell of burning sulphur.
• 2AgCl → 2Ag + Cl_(2) in sunlight — white turns grey; used in black-and-white photography (photolytic).
• Electrolysis of water: H_(2) and O_(2) in the ratio 2 : 1 by volume (electrolytic).
• Decomposition reactions are mostly endothermic — they need energy.
Also asked from this topic: the energy forms that drive decomposition (heat, light or electricity); the type of reaction when a single compound breaks into simpler substances (decomposition).

Answer: Option 4 — NaOH + FeCl_(3)
The ions exchange partners and iron(III) hydroxide, which is insoluble, separates out.
FeCl_(3) + 3NaOH → Fe(OH)_(3)↓ (reddish-brown) + 3NaCl
In the other mixtures every possible product is a sodium or potassium salt, which stays dissolved.
Revision note — Precipitation (double displacement) reactions
• Double displacement: two compounds exchange ions — AB + CD → AD + CB.
• Precipitate: the insoluble solid formed in such a reaction; shown by ↓ or (s).
• Na_(2)SO_(4) + BaCl_(2) → BaSO_(4)↓ (white) + 2NaCl — the NCERT example.
• Pb(NO_(3))_(2) + 2KI → PbI_(2)↓ (yellow) + 2KNO_(3).
• With NaOH: Cu^(2+) gives a blue ppt, Fe^(2+) a green ppt, Fe^(3+) a reddish-brown ppt.
• Solubility rule: all sodium, potassium, ammonium salts and all nitrates are soluble in water.
Also asked from this topic: the white product when barium chloride is mixed with sodium sulphate (barium sulphate, a precipitate); the state symbol of BaSO_(4) in that reaction ((s)).

Answer: Option 1 — NaCl + AgNO_(3) → NaNO_(3) + AgCl
The two salts exchange ions, and silver chloride (AgCl) forms as an insoluble white solid.
An insoluble solid that separates out is a precipitate, so this is a precipitation reaction.
Revision note — Types of chemical reactions
• Precipitation: two solutions give an insoluble solid; e.g. Na_(2)SO_(4) + BaCl_(2) → BaSO_(4)↓ (white) + 2NaCl.
• Every precipitation reaction is a double displacement reaction (ions exchanged).
• Combination: 2H_(2) + O_(2) → 2H_(2)O — two substances form one.
• Decomposition: CaCO_(3) → CaO + CO_(2) on heating (thermal decomposition).
• Neutralisation: HCl + NaOH → NaCl + H_(2)O — acid + base → salt + water.
• AgCl turns grey in sunlight (2AgCl → 2Ag + Cl_(2)) — photolytic decomposition, used in black-and-white photography.
Also asked from this topic: the product formed when barium chloride and sodium sulphate solutions are mixed (a precipitate of barium sulphate).
HCl + NaOH is also a double displacement, but water is not a precipitate — look for the insoluble solid.

Answer: Option 1 — combination
When two or more substances join to form a single new product, it is a combination reaction (A + B → AB).
Two elements forming a compound is the simplest case, e.g. C + O_(2) → CO_(2).
Revision note — Types of chemical reactions
• Combination: A + B → AB; e.g. CaO + H_(2)O → Ca(OH)_(2) + heat (quicklime to slaked lime).
• Decomposition: AB → A + B; needs heat, light or electricity.
• Displacement: a more reactive element pushes out a less reactive one; e.g. Fe + CuSO_(4) → FeSO_(4) + Cu.
• Double displacement: ions are exchanged; e.g. Na_(2)SO_(4) + BaCl_(2) → BaSO_(4) + 2NaCl.
• Most combination reactions are exothermic (release heat).
• Burning of coal and formation of water from H_(2) and O_(2) are combination reactions.
Do not mix these up
• Combination vs decomposition — combination makes one product from many; decomposition breaks one reactant into many.
Also asked from this topic: the type of reaction CaO + H_(2)O → Ca(OH)_(2) + heat (combination reaction); the observation that confirms a combination reaction (a single product forms from two or more reactants).

Answer: Option 3 — A + B → AB
In a combination reaction, two or more reactants combine to form a single product.
So its general form is A + B → AB.
Revision note — Types of chemical reactions
• Combination: A + B → AB — e.g. CaO + H_(2)O → Ca(OH)_(2) (quicklime to slaked lime, exothermic).
• Decomposition: AB → A + B — by heat (CaCO_(3)), electricity (water) or light (AgCl, AgBr).
• Displacement: A + BC → AC + B — e.g. Fe + CuSO_(4) → FeSO_(4) + Cu (blue colour fades).
• Double displacement: AB + CD → AD + CB — e.g. Na_(2)SO_(4) + BaCl_(2) gives a white BaSO_(4) precipitate.
• Burning of coal and formation of water from H_(2) and O_(2) are also combination reactions.
Do not mix these up
• Combination vs decomposition — combination makes one product from many; decomposition breaks one reactant into many. Their general forms are exact reverses.
Also asked from this topic: the type of reaction CaO + H_(2)O → Ca(OH)_(2) + heat (combination reaction); the observation that confirms a combination reaction (only one new substance is produced).
Look at the right-hand side — a single product means combination.

Answer: Option 4 — barium sulphate
The ions exchange partners: Ba²⁺ joins SO₄²⁻ and Na⁺ joins Cl⁻.
Barium sulphate is insoluble, so it separates as a white precipitate; sodium chloride stays dissolved.
Revision note — Double displacement and precipitation reactions
• Double displacement: two compounds exchange ions (AB + CD → AD + CB).
• A reaction that gives an insoluble solid (precipitate) is a precipitation reaction.
• BaSO_(4) — white precipitate; PbI_(2) — yellow precipitate (lead nitrate + potassium iodide).
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
• In this reaction no oxidation or reduction takes place — oxidation states stay the same.
Do not mix these up
• Displacement vs double displacement — displacement: a metal pushes out another metal (Fe + CuSO_(4)); double displacement: two compounds swap ions.
Also asked from this topic: the state symbol of BaSO_(4) in this reaction ((s)); the type of reaction shown by the white precipitate (double displacement / precipitation).

Answer: Option 4 — Quick lime dissolved in water
Quick lime combines with water to form one product, slaked lime, and gives out a lot of heat:
CaO + H_(2)O → Ca(OH)_(2) + heat
So it is both a combination and an exothermic reaction.
Revision note — Combination and exothermic reactions
• Combination reaction: two or more reactants form a single product (A + B → AB).
• Quick lime = calcium oxide (CaO); slaked lime = calcium hydroxide (Ca(OH)_(2)), used for whitewashing.
• Whitewash turns shiny in 2–3 days as Ca(OH)_(2) reacts with CO_(2) to form calcium carbonate (CaCO_(3)).
• Other exothermic combinations: burning of coal (C + O_(2) → CO_(2)); 2H_(2) + O_(2) → 2H_(2)O.
• Exothermic = heat released; endothermic = heat absorbed (e.g. photosynthesis, most decomposition reactions).
• Dissolving ammonium chloride in water absorbs heat — the solution turns cold.
Do not mix these up
• Respiration — it is exothermic, but glucose breaks down into CO_(2) and water, so it is not a combination reaction.
Also asked from this topic: the type of reaction CaO + H_(2)O → Ca(OH)_(2) (combination); an example of an exothermic reaction (combustion of petrol).

Answer: Option 4 — reduces to form brownish copper metal
Hot copper(II) oxide loses oxygen to hydrogen, so it is reduced to reddish-brown copper:
CuO (black) + H_(2) → Cu (brown) + H_(2)O
At the same time hydrogen gains oxygen and is oxidised to water.
Revision note — Oxidation and reduction
• Oxidation: gain of oxygen or loss of hydrogen; reduction: loss of oxygen or gain of hydrogen.
• Both happen together — a redox reaction.
• Here CuO is the oxidising agent and H_(2) the reducing agent.
• Reverse change: heating copper in air, 2Cu + O_(2) → 2CuO — brown surface turns black.
• Another example: ZnO + C → Zn + CO (ZnO reduced, carbon oxidised).
• Blue colour belongs to copper sulphate solution, not to copper metal.
Do not mix these up
• What is reduced vs what is oxidised — CuO is reduced (to Cu); hydrogen is oxidised (to H_(2)O).
Also asked from this topic: what is oxidised and what is reduced in CuO + H_(2) → Cu + H_(2)O (hydrogen oxidised, CuO reduced).

Answer: Option 1 — 2H_(2)O_(2) → 2H_(2)O + O_(2)
Hydrogen peroxide breaks down into water and oxygen.
Atoms check: left 4 H, 4 O; right 4 H, 2 + 2 = 4 O — balanced.
Revision note — Decomposition reactions
• One compound breaks into two or more simpler substances (AB → A + B).
• Thermal: CaCO_(3) → CaO + CO_(2) on heating; 2Pb(NO_(3))_(2) → 2PbO + 4NO_(2) + O_(2) (brown fumes).
• Electrolytic: 2H_(2)O → 2H_(2) + O_(2) — hydrogen and oxygen in the ratio 2 : 1 by volume.
• Photolytic: 2AgCl → 2Ag + Cl_(2) in sunlight — used in black-and-white photography.
• H_(2)O_(2) decomposes slowly on its own; MnO_(2) acts as a catalyst to speed it up.
• A balanced equation has equal numbers of each atom on both sides — the law of conservation of mass.
Do not mix these up
• Option 2 keeps 2 O on the left but 3 O on the right; option 3 puts 2 H on the left and 4 H on the right.
Also asked from this topic: the H_(2) : O_(2) ratio in electrolysis of water (2 : 1); silver salts used in black-and-white photography (AgCl, AgBr).

Answer: Option 4 — The total number of atoms of each element is the same on both sides.
Atoms are neither created nor destroyed in a reaction (law of conservation of mass).
So a balanced equation has the same number of atoms of each element on both sides.
Revision note — Balancing chemical equations
• Example: 2H_(2) + O_(2) → 2H_(2)O — 3 molecules become 2 molecules, yet H (4 = 4) and O (2 = 2) match.
• So the number of molecules or moles need not be equal on both sides.
• The total mass of reactants equals the total mass of products.
• Balance by changing only the coefficients, never the subscripts of a formula.
• Method taught in NCERT: hit-and-trial, starting with the compound that has the most atoms.
• Law of conservation of mass — given by Antoine Lavoisier (1789).
Also asked from this topic: who gave the law of conservation of mass (Antoine Lavoisier); the reaction that violates it (2Mg + O_(2) → MgO); what stoichiometric calculations need (a balanced equation).

Answer: Option 1 — Sodium chloride and barium sulfate
In double displacement, the two compounds swap ions.
Na_(2)SO_(4)(aq) + BaCl_(2)(aq) → BaSO_(4)(s) + 2NaCl(aq)
Revision note — Double displacement reactions
• Ba^(2+) pairs with SO_(4)^(2−); Na^(+) pairs with Cl^(−).
• Barium sulphate is insoluble — it separates as a white precipitate.
• A reaction that gives an insoluble solid is called a precipitation reaction.
• Another NCERT example: lead nitrate + potassium iodide → yellow precipitate of lead iodide.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
• In single displacement, a more reactive metal pushes out a less reactive one — e.g. Fe + CuSO_(4) → FeSO_(4) + Cu.
Also asked from this topic: the physical state of BaSO_(4) in this reaction ((s), a solid precipitate); the evidence that the reaction has occurred (formation of a precipitate).

Answer: Option 3 — The total number of atoms for each element is the same on both sides of the equation.
Atoms are neither created nor destroyed in a reaction, so each element must have the same number of atoms on both sides.
This follows the law of conservation of mass.
Revision note — Balancing chemical equations
• A balanced equation obeys the law of conservation of mass (Lavoisier).
• Total mass of reactants = total mass of products.
• Balance by changing only the coefficients — never the subscripts in a formula.
• The number of molecules need not match: 2H_(2) + O_(2) → 2H_(2)O has 3 molecules on the left and 2 on the right.
• Method: hit and trial — start with the compound having the most atoms.
• Example: 3Fe + 4H_(2)O → Fe_(3)O_(4) + 4H_(2).
• State symbols: (s), (l), (g), (aq).
Do not mix these up
• Atoms vs molecules — atoms of each element balance; molecules on each side can differ.
Also asked from this topic: the reaction that violates conservation of mass (2Mg + O_(2) → MgO, which is unbalanced); what may not be changed while balancing (the formula of a substance).

Answer: Option 4 — ions
In a double displacement reaction two compounds exchange ions to form two new compounds: AB + CD → AD + CB.
Revision note — Double displacement reactions
• Example: Na_(2)SO_(4) + BaCl_(2) → BaSO_(4) + 2NaCl — white precipitate of barium sulphate.
• When an insoluble solid forms, it is also called a precipitation reaction.
• Pb(NO_(3))_(2) + 2KI → PbI_(2) + 2KNO_(3) — yellow precipitate of lead iodide.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
• Compare: displacement — a more reactive element pushes out a less reactive one (Fe + CuSO_(4) → FeSO_(4) + Cu).
Do not mix these up
• Displacement vs double displacement — one element replaces another vs two compounds swap ions.
Also asked from this topic: the main characteristic of a double displacement reaction (exchange of ions between reactants); the precipitate when barium chloride and sodium sulphate solutions mix (barium sulphate).

Answer: Option 3 — Ions in two compounds switch places to form two new compounds.
In a double displacement reaction two compounds exchange ions: AB + CD → AD + CB.
Example: Na_(2)SO_(4) + BaCl_(2) → BaSO_(4) (white precipitate) + 2NaCl.
Revision note — Types of chemical reactions (Class 10)
• Combination: two or more substances form one product — CaO + H_(2)O → Ca(OH)_(2) (slaked lime).
• Decomposition: one compound breaks into simpler substances — by heat (CaCO_(3)), electricity (water) or light (AgCl).
• Displacement: a more reactive element pushes out a less reactive one — Fe + CuSO_(4) → FeSO_(4) + Cu.
• Double displacement: exchange of ions; when an insoluble solid forms it is a precipitation reaction.
• Redox: gain of oxygen / loss of hydrogen = oxidation; the reverse = reduction; electron transfer is the redox idea.
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
Do not mix these up
• Displacement vs double displacement — displacement: an element replaces another (one swap); double displacement: two compounds swap partners (two swaps).
Also asked from this topic: why Na_(2)SO_(4) + BaCl_(2) is a double displacement reaction (exchange of ions); two salt solutions giving an insoluble solid (precipitation reaction).

Answer: Option 1 — Formation of a precipitate
NaCl(aq) + AgNO_(3)(aq) → AgCl(s) + NaNO_(3)(aq)
The ions swap partners and silver chloride, which is insoluble, separates as a white curdy precipitate.
Revision note — Precipitation reactions
• A precipitate is an insoluble solid formed when two solutions are mixed; the reaction is a double displacement (precipitation) reaction.
• AgNO_(3) + NaCl → AgCl↓ (white) + NaNO_(3) — the standard test for chloride ions.
• Na_(2)SO_(4) + BaCl_(2) → BaSO_(4)↓ (white) + 2NaCl.
• Pb(NO_(3))_(2) + 2KI → PbI_(2)↓ (yellow) + 2KNO_(3).
• AgCl turns grey in sunlight: 2AgCl → 2Ag + Cl_(2) — a photolytic decomposition, used in black-and-white photography.
Do not mix these up
• Precipitate vs salt — AgCl and NaNO_(3) are both salts, but the observable result that marks this reaction is the insoluble precipitate.
Also asked from this topic: the name of the white insoluble solid formed in a double displacement reaction (precipitate); which compound shows a visible change in sunlight (silver chloride).

Answer: Option 3 — Electrolysis of water
Passing an electric current through water breaks it into hydrogen and oxygen:
2H_(2)O → 2H_(2) + O_(2) — an electrolytic decomposition reaction.
The other three options are all decompositions caused by heat.
Revision note — Types of decomposition reactions
• Decomposition — one compound breaks into two or more simpler substances (AB → A + B); it needs energy, so most are endothermic.
• Thermal (heat): CaCO_(3) → CaO + CO_(2) (quicklime for cement); 2FeSO_(4) → Fe_(2)O_(3) + SO_(2) + SO_(3); 2Pb(NO_(3))_(2) → 2PbO + 4NO_(2) + O_(2) (brown fumes).
• Electrolytic (electricity): water → H_(2) at the cathode and O_(2) at the anode in the ratio 2 : 1 by volume; also molten NaCl and Al_(2)O_(3).
• Photolytic (light): 2AgCl → 2Ag + Cl_(2), 2AgBr → 2Ag + Br_(2) — used in black-and-white photography.
• Heating baking soda: 2NaHCO_(3) → Na_(2)CO_(3) + H_(2)O + CO_(2); heating KMnO_(4) releases O_(2).
Also asked from this topic: H_(2) : O_(2) by volume in electrolysis of water (2 : 1); heating CaCO_(3) is which reaction (decomposition); silver salts used in black-and-white photography (AgCl, AgBr).

Answer: Option 1 — Writing the skeleton equation
You can balance only what is written, so the first step is the skeletal (unbalanced) equation with correct formulae of all reactants and products.
Atoms are then counted and balanced one element at a time.
Revision note — Balancing a chemical equation (hit-and-trial method)
• A balanced equation obeys the law of conservation of mass — atoms are neither created nor destroyed.
• Step 1: write the skeletal equation; step 2: draw boxes around each formula and count atoms of each element on both sides.
• Start with the compound having the most atoms and, in it, the element with the most atoms (e.g. oxygen in Fe_(3)O_(4)).
• Change only the coefficients, never the formula or subscripts (writing H_(2)O_(2) for H_(2)O changes the substance).
• Example: 3Fe + 4H_(2)O → Fe_(3)O_(4) + 4H_(2).
• Add physical states (s), (l), (g), (aq) and conditions (heat, catalyst) over the arrow at the end.
Also asked from this topic: a reaction that violates conservation of mass (2Mg + O_(2) → MgO); why H_(2)O cannot become H_(2)O_(4) while balancing (it changes the substance).

Answer: Option 2 — Lead iodide (precipitate)
Pb(NO_(3))_(2)(aq) + 2KI(aq) → PbI_(2)(s) + 2KNO_(3)(aq)
The ions exchange partners; lead iodide is insoluble and separates as a yellow precipitate. Potassium nitrate stays in solution.
Revision note — Double displacement and precipitation (Class 10)
• Double displacement: two compounds exchange ions — AB + CD → AD + CB.
• A reaction that forms an insoluble solid is a precipitation reaction.
• Pb(NO_(3))_(2) + 2KI → PbI_(2)↓ yellow + 2KNO_(3).
• Na_(2)SO_(4) + BaCl_(2) → BaSO_(4)↓ white + 2NaCl.
• AgNO_(3) + NaCl → AgCl↓ white + NaNO_(3).
• On heating, lead nitrate decomposes: 2Pb(NO_(3))_(2) → 2PbO + 4NO_(2) (brown fumes) + O_(2).
Do not mix these up
• Lead iodide vs lead nitrate — lead nitrate is the soluble reactant; lead iodide is the yellow insoluble product.
Also asked from this topic: which compound needs light, not heat, to decompose (silver chloride, unlike lead nitrate).

Answer: Option 4 — corrosion
When a metal is attacked by substances around it such as moisture, acids or air, it is said to corrode.
This slow oxidation of metals is corrosion.
Revision note — Corrosion and rancidity
• Rusting is the corrosion of iron only — a reddish-brown coat of hydrated iron(III) oxide, Fe_(2)O_(3)·xH_(2)O.
• Iron rusts only when both oxygen and water are present.
• Silver turns black (silver sulphide, Ag_(2)S); copper gains a green coat (basic copper carbonate).
• Prevention: painting, oiling, greasing, galvanising (zinc coat), chrome plating, alloying.
• Rancidity — oxidation of fats and oils in food, giving a bad smell and taste.
• Rancidity is slowed by antioxidants, airtight containers and flushing chip packets with nitrogen.
Do not mix these up
• Corrosion vs rusting — corrosion is the general term for all metals; rusting is corrosion of iron alone.
Also asked from this topic: the reddish-brown coating on iron in moist air (hydrated iron(III) oxide); what leads to rancidity (oxidation).

Answer: Option 2 — Zinc is oxidised.
Zn + 2HCl → ZnCl_(2) + H_(2)↑
Zinc loses 2 electrons to become Zn²⁺ in ZnCl_(2), so zinc is oxidised.
The H⁺ ions of the acid gain those electrons and become H_(2) gas — hydrogen is reduced.
Revision note — Oxidation and reduction
• Oxidation = gain of oxygen / loss of hydrogen / loss of electrons.
• Reduction = loss of oxygen / gain of hydrogen / gain of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain).
• A redox reaction has oxidation and reduction happening together.
• The substance that is oxidised is the reducing agent (here Zn); the one reduced is the oxidising agent (here HCl / H⁺).
• This is also a displacement reaction — the more reactive zinc displaces hydrogen from the acid.
• Metals above hydrogen in the reactivity series give H_(2) with dilute acids; the gas burns with a pop sound.
Do not mix these up
• Oxidised vs oxidising agent — zinc is oxidised, so it is the reducing agent, not the oxidising agent.
Also asked from this topic: the type of reaction when zinc reacts with dilute hydrochloric acid (displacement reaction); in CuO + H_(2) → Cu + H_(2)O, the substance oxidised (hydrogen).

Answer: Option 4 — AB + C → ABC
In a combination reaction two or more substances join to form a single product.
AB and C combine to give one product, ABC.
Revision note — Types of chemical reactions
• Combination: A + B → AB — e.g. CaO + H_(2)O → Ca(OH)_(2) (quicklime to slaked lime, exothermic).
• Decomposition: AB → A + B — the reverse of combination; needs heat, light or electricity (e.g. CaCO_(3) → CaO + CO_(2)).
• Displacement: A + BC → AC + B — a more reactive element pushes out a less reactive one (e.g. Fe + CuSO_(4) → FeSO_(4) + Cu).
• Double displacement: AB + CD → AD + CB — ions are exchanged, often forming a precipitate (e.g. BaCl_(2) + Na_(2)SO_(4)).
• Burning of coal (C + O_(2) → CO_(2)) and forming water from H_(2) and O_(2) are also combination reactions.
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• Combination vs decomposition — combination ends in one product; decomposition starts from one reactant.
Also asked from this topic: the type of reaction CaO + H_(2)O → Ca(OH)_(2) + heat (combination reaction); the observation that confirms a combination reaction (only one new substance is produced).

Answer: Option 2 — addition of hydrogen
Reduction is the gain of hydrogen or the loss of oxygen by a substance.
Addition of oxygen, addition of an electronegative element and loss of electrons all describe oxidation.
Revision note — Oxidation and reduction
• Oxidation: gain of oxygen, loss of hydrogen, loss of electrons.
• Reduction: loss of oxygen, gain of hydrogen, gain of electrons.
• Redox: oxidation and reduction happen together in the same reaction.
• Example: CuO + H_(2) → Cu + H_(2)O — CuO is reduced, H_(2) is oxidised.
• Oxidising agent gets reduced itself; reducing agent gets oxidised itself.
• Everyday oxidation: corrosion (rusting) and rancidity of fats and oils.
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• Electron view — memory aid OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Also asked from this topic: what oxidation involves in a redox reaction (loss of electrons); in CuO + H_(2) → Cu + H_(2)O, what is reduced (CuO, while hydrogen is oxidised).

Answer: Option 3 — Two elements combine to form a compound
In a combination reaction two or more reactants join to give a single product, e.g. C + O_(2) → CO_(2).
Revision note — Types of chemical reactions
• Combination: A + B → AB — CaO + H_(2)O → Ca(OH)_(2) (quicklime + water → slaked lime, with heat).
• Decomposition: AB → A + B — the reverse of combination; needs heat, light or electricity.
• Displacement: a more reactive metal pushes out a less reactive one — Fe + CuSO_(4) → FeSO_(4) + Cu.
• Double displacement: ions are exchanged, often giving a precipitate — Na_(2)SO_(4) + BaCl_(2) → BaSO_(4)↓ + 2NaCl.
• Burning of coal and formation of water from H_(2) and O_(2) are combination reactions.
• Most combination reactions are exothermic (release heat).
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• Combination vs decomposition — combination makes one product from many; decomposition breaks one reactant into many.
Also asked from this topic: what identifies a combination reaction (only one new substance is produced); the type of CaO + H_(2)O → Ca(OH)_(2) + heat (combination).

Answer: Option 4 — 2H₂ + O₂ → 2H₂O
A balanced equation has the same number of atoms of each element on both sides.
Left: 4 H, 2 O; right: 2H_(2)O = 4 H, 2 O — both match.
Revision note — Balancing chemical equations
• Balancing follows the law of conservation of mass (Lavoisier): atoms are neither created nor destroyed.
• Change only the coefficients in front of formulae — never the subscripts (H_(2)O cannot become H_(2)O_(2)).
• Method: balance the element in the largest formula first, leave H and O for the end (hit-and-trial method).
• An equation before balancing is a skeletal equation.
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
• Burning hydrogen in oxygen is a combination reaction and is exothermic.
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• Balanced vs correct — balancing only matches atom counts; the formulae themselves must be the right reactants and products, and they are never altered to force a balance.
Also asked from this topic: a reaction that breaks the law of conservation of mass as written (2Mg + O_(2) → MgO, correct form 2Mg + O_(2) → 2MgO).

Answer: Option 2 — Manganese (Mn)
MnO_(2) loses its oxygen and becomes MnCl_(2), so it is reduced.
In oxidation-number terms, Mn falls from +4 in MnO_(2) to +2 in MnCl_(2) — a gain of electrons.
Revision note — Oxidation and reduction (NCERT Class 10)
• Oxidation = gain of oxygen or loss of hydrogen (or loss of electrons).
• Reduction = loss of oxygen or gain of hydrogen (or gain of electrons).
• In this reaction HCl is oxidised to Cl_(2) (it loses hydrogen; Cl goes from −1 to 0).
• The substance that is reduced is the oxidising agent — here MnO_(2); the substance oxidised is the reducing agent — here HCl.
• A reaction in which one substance is oxidised and another reduced is a redox reaction.
• Another NCERT example: CuO + H_(2) → Cu + H_(2)O — CuO is reduced, H_(2) is oxidised.
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• Reduced vs reducing agent — MnO_(2) is reduced but is the oxidising agent; HCl is oxidised but is the reducing agent.
Also asked from this topic: the substance oxidised in MnO_(2) + 4HCl → MnCl_(2) + 2H_(2)O + Cl_(2) (HCl); why MnO_(2) is said to be reduced (it loses oxygen).

Answer: Option 2 — aqueous solution
A double displacement reaction is an exchange of ions between two compounds.
Ions move freely only when the salts are dissolved in water, so these reactions usually take place in aqueous solution.
Revision note — Double displacement reactions
• General form: AB + CD → AD + CB — the partners are swapped.
• Standard example: Na_(2)SO_(4)(aq) + BaCl_(2)(aq) → BaSO_(4)(s) + 2NaCl(aq).
• The white precipitate of barium sulphate marks it as a precipitation reaction.
• Other example: AgNO_(3) + NaCl → AgCl (white precipitate) + NaNO_(3).
• Neutralisation (acid + base → salt + water) is also a double displacement reaction.
• State symbols: (aq) dissolved in water, (s) solid, (l) liquid, (g) gas.
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• Displacement vs double displacement — in displacement a more reactive element pushes out a less reactive one (Fe + CuSO_(4)); in double displacement two compounds exchange ions.
Also asked from this topic: the white product of barium chloride + sodium sulphate (barium sulphate precipitate); the main feature of a double displacement reaction (exchange of ions between reactants).