Exercise — Chemical Reactions and Equations

1. 11 markNCERT Cl-10 Science, Exercise Q1

   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

   Hint. Notice that (a) and (d) look like they're saying the same thing — one names the element, the other names the compound. Only one of those two phrasings is chemically precise here.

   Step 1 — Track what happens to lead and carbon. Lead goes from PbO to Pb — it loses oxygen, i.e. lead oxide is reduced to lead. Carbon goes from C to CO₂ — it gains oxygen, i.e. carbon is oxidised.

   Step 2 — Check each statement against this. (a) 'Lead is getting reduced' — imprecise, since it is the compound lead oxide that loses oxygen and is reduced, not the element 'lead' (which isn't a separate reactant here to be reduced from anything) — INCORRECT as worded. (b) 'Carbon dioxide is getting oxidised' — CO₂ is the product already formed by carbon's oxidation; it is not itself undergoing any further oxidation — INCORRECT. (c) 'Carbon is getting oxidised' — matches Step 1 exactly — CORRECT. (d) 'Lead oxide is getting reduced' — matches Step 1 exactly — CORRECT.

   Step 3 — Match to the options. The incorrect statements are (a) and (b), which is option (i).

   ✦ Answer: (i) (a) and (b).

   Where students slip. Picking (ii) or (iii) by assuming (c) must also be wrong because it looks similar to (b) — but (c) talks about carbon, the element actually being oxidised, while (b) talks about carbon dioxide, the already-oxidised product. That mismatch is the deliberate trap in this question.

2. 21 markNCERT Cl-10 Science, Exercise Q2

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

   Hint. One of the reactants here is a free element, not a compound — that's the signature of one specific reaction type.

   Step 1 — Check the pattern against each option. Combination needs two or more reactants forming one product — not this. Decomposition needs one reactant breaking into many — not this either, since there are two reactants and two products. Double displacement needs two compounds exchanging ions, with no free element appearing in the reaction — but here aluminium is a free element.

   Step 2 — Confirm displacement. Aluminium is more reactive than iron, so it displaces iron from its oxide: aluminium takes iron's place in the compound, and iron is released as the free metal. (This specific reaction is the thermite reaction, used to weld railway tracks because of the heat it releases.)

   ✦ Answer: (d) displacement reaction.

   Where students slip. Calling this double displacement just because it 'looks like a swap' — double displacement needs two compounds trading ions with each other; here one reactant (Al) is a free element, which marks it as displacement, not double displacement.

3. 31 markNCERT Cl-10 Science, Exercise Q3

   What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. (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.

   Hint. Recall Activity 1.3 — a similarly reactive metal was dropped into a dilute acid earlier in this same chapter.

   Step 1 — Recall the pattern: metal + acid → salt + hydrogen gas. Iron is reactive enough to displace hydrogen from dilute hydrochloric acid, exactly like the zinc-and-acid reaction seen earlier in the chapter (Activity 1.3).

   Step 2 — Write the reaction to check. Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g), since iron replaces hydrogen in the acid. This produces iron(II) chloride, a salt, plus hydrogen gas — matching option (a).

   ✦ Answer: (a) Hydrogen gas and iron chloride are produced.

   Where students slip. Picking (d) because 'iron salt and water' sounds plausible — but a metal reacting with an acid releases hydrogen GAS, not water; water comes from a base neutralising an acid, not a metal displacing hydrogen.

4. 42 marksNCERT Cl-10 Science, Exercise Q4

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

   Hint. Connect the definition directly to the Law of Conservation of Mass.

   Step 1 — Definition. A balanced chemical equation has equal numbers of atoms of every element on both the reactant (LHS) and product (RHS) sides.

   Step 2 — Why this matters. The Law of Conservation of Mass says mass can neither be created nor destroyed in a chemical reaction — the atoms present in the reactants only rearrange into products; none appear from nowhere and none vanish. Since mass and atom-count are directly linked, an unbalanced equation would imply atoms were created or destroyed, which is impossible. Balancing an equation is simply making it obey this law.

   ✦ Answer: A balanced chemical equation has the same number of atoms of each element on both sides. Equations must be balanced because mass can neither be created nor destroyed in a reaction (Law of Conservation of Mass), so every atom going in must be accounted for in what comes out.

   Where students slip. Explaining 'balanced' in terms of the arrow's direction rather than atom counts — the direction of the arrow has nothing to do with balancing.

5. 54 marksNCERT Cl-10 Science, Exercise Q5

   Translate the following statements into chemical equations and then balance them. (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

   Hint. In (b), balancing oxygen will first give you a fractional coefficient — clear it by doubling the whole equation, don't leave it as a fraction.

   Step 1 — (a) Hydrogen + Nitrogen → Ammonia. Skeletal: H₂ + N₂ → NH₃. Balancing nitrogen first (2 N needs 2 NH₃, giving 6 H) and then matching hydrogen: 3H₂ + N₂ → 2NH₃.

   Step 2 — (b) Hydrogen sulphide + Oxygen → Water + Sulphur dioxide. Skeletal: H₂S + O₂ → H₂O + SO₂. Hydrogen and sulphur already match 1-for-1, so only oxygen needs balancing: the right side needs 1(H₂O) + 2(SO₂) = 3 O, which needs 3⁄2 O₂ — doubling the whole equation clears the fraction: 2H₂S + 3O₂ → 2H₂O + 2SO₂.

   Step 3 — (c) Barium chloride + Aluminium sulphate → Aluminium chloride + Barium sulphate. The same balance worked out earlier for this pair, with the products in this order: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄.

   Step 4 — (d) Potassium + Water → Potassium hydroxide + Hydrogen. Skeletal: K + H₂O → KOH + H₂, balanced the same way as sodium earlier in the chapter: 2K + 2H₂O → 2KOH + H₂.

   ✦ Answer: (a) 3H₂ + N₂ → 2NH₃ (b) 2H₂S + 3O₂ → 2H₂O + 2SO₂ (c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄ (d) 2K + 2H₂O → 2KOH + H₂

   Where students slip. In (b), forgetting to clear the fractional coefficient (3⁄2 O₂) by doubling the whole equation — a properly balanced equation uses whole-number coefficients.

6. 64 marksNCERT Cl-10 Science, Exercise Q6

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

   Hint. Not every equation in a 'balance these' list actually needs new coefficients — check each one on its own merits.

   Step 1 — (a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O. Ca(NO₃)₂ needs 2 NO₃⁻, so 2 HNO₃ are needed; those 2 HNO₃ bring 2 H, which pair with the 2 OH from Ca(OH)₂ to give 2 H₂O: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O.

   Step 2 — (b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O. Na₂SO₄ needs 2 Na, so 2 NaOH are needed, contributing 2 OH; H₂SO₄ has 2 H, which pair with those 2 OH to give 2 H₂O: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.

   Step 3 — (c) NaCl + AgNO₃ → AgCl + NaNO₃. Every element already appears once on each side — already balanced as written, no coefficients needed.

   Step 4 — (d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl. Ba, S and O already match 1-for-1, but the 2 Cl on the left need 2 HCl on the right: BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl.

   ✦ Answer: (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

   Where students slip. Assuming every equation in the list must need new coefficients, and forcing extra ones onto (c) — it's already balanced as given.

7. 74 marksNCERT Cl-10 Science, Exercise Q7

   Write the balanced chemical equations for the following reactions. (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

   Hint. In (c), balance chlorine first — its count forces both the aluminium and copper coefficients to fall into place afterwards.

   Step 1 — (a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Every element already matches 1-for-1 — balanced as written. (This is the same reaction that gives whitewashed walls their shiny finish, mentioned in the chapter's 'Do You Know?' box.)

   Step 2 — (b) Zn + AgNO₃ → Zn(NO₃)₂ + Ag. Zinc is more reactive than silver, so it displaces it. Zn(NO₃)₂ needs 2 NO₃⁻, so 2 AgNO₃ are needed, releasing 2 Ag: Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag.

   Step 3 — (c) Al + CuCl₂ → AlCl₃ + Cu. Aluminium is more reactive than copper. Chlorine needs a count that's a common multiple of 2 (from AlCl₃) and the CuCl₂ supply: using 2 AlCl₃ (6 Cl) matched by 3 CuCl₂ (6 Cl) also fixes 2 Al and 3 Cu: 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu.

   Step 4 — (d) BaCl₂ + K₂SO₄ → BaSO₄ + KCl. Ba and S already match 1-for-1, but the 2 Cl (left) and 2 K (left, in K₂SO₄) both need the KCl coefficient raised to 2: BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl.

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

   Where students slip. In (c), balancing chlorine on only one side and not checking that Al and Cu also come out equal afterwards — all three counts have to be rechecked together, not fixed one at a time and left alone.

8. 84 marksNCERT Cl-10 Science, Exercise Q8

   Write the balanced chemical equation for the following and identify the type of reaction in each case. (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)

   Hint. Match each equation's shape (one reactant vs two, one product vs two, any free element present) to the four reaction types before naming it.

   Step 1 — (a) KBr(aq) + BaI₂(aq) → KI(aq) + BaBr₂(s). BaI₂ supplies 2 I, so 2 KI are needed; BaBr₂ needs 2 Br, so 2 KBr are needed: 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s). Two compounds exchange ions (K↔Ba, Br↔I) with no free element involved — DOUBLE DISPLACEMENT.

   Step 2 — (b) ZnCO₃(s) → ZnO(s) + CO₂(g). Already balanced. One compound breaking into two — DECOMPOSITION (thermal, since heat is needed to break down a carbonate).

   Step 3 — (c) H₂(g) + Cl₂(g) → 2HCl(g). Two elements combining into a single product — COMBINATION.

   Step 4 — (d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Magnesium is more reactive than hydrogen, so it displaces hydrogen from the acid — DISPLACEMENT.

   ✦ Answer: (a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — double displacement (b) ZnCO₃(s) → ZnO(s) + CO₂(g) — decomposition (c) H₂(g) + Cl₂(g) → 2HCl(g) — combination (d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — displacement

   Where students slip. Calling (a) just a 'precipitation' reaction without also naming it double displacement — precipitation describes what's observed, but the reaction-type name asked for is double displacement.

9. 92 marksNCERT Cl-10 Science, Exercise Q9

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

   Hint. This question explicitly asks for examples — a definition alone won't earn full marks.

   Step 1 — Define both terms. Exothermic reactions release heat or energy to the surroundings along with the products. Endothermic reactions absorb energy from the surroundings for the reaction to proceed.

   Step 2 — Give one example each, from this chapter. Exothermic: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat (Activity 1.4) — the beaker feels warm because heat is released. Endothermic: heating ferrous sulphate crystals to decompose them, 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) (Activity 1.5) — heat has to be continuously supplied for the reaction to keep going.

   ✦ Answer: Exothermic reactions release heat along with the products, e.g. CaO + H₂O → Ca(OH)₂ + heat. Endothermic reactions absorb energy for the reaction to occur, e.g. heating ferrous sulphate crystals to decompose them.

   Where students slip. Giving only definitions without a worked example each — 'give examples' is graded as a separate requirement here.

10. 102 marksNCERT Cl-10 Science, Exercise Q10

    Why is respiration considered an exothermic reaction? Explain.

    Hint. Look at what the chapter's own equation for respiration shows being produced alongside carbon dioxide and water.

    Step 1 — Write the overall reaction. C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy — glucose combines with oxygen inside body cells.

    Step 2 — Explain why it counts as exothermic. The reaction releases energy as one of its products, shown explicitly in the equation, and this is the energy the body uses to stay alive, move and stay warm. A reaction that gives out energy alongside its products is, by definition, exothermic.

    ✦ Answer: Respiration is exothermic because breaking down glucose with oxygen (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) releases energy rather than absorbing it — this released energy powers the body's activities.

    Where students slip. Confusing respiration with photosynthesis — photosynthesis absorbs energy (endothermic) and runs in the reverse direction; respiration is the exothermic process happening in body cells.

11. 113 marksNCERT Cl-10 Science, Exercise Q11

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

    Hint. The question wants one equation for each type, so the 'opposite' comparison is concrete, not just asserted.

    Step 1 — Compare the two patterns. Combination: two or more reactants → one product (A + B → AB). Decomposition: one reactant → two or more products (AB → A + B) — literally the reverse arrangement of reactants and products.

    Step 2 — One equation each. Combination: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. Decomposition: CaCO₃(s) → CaO(s) + CO₂(g) [on heating]. Notice the calcium oxide made by decomposing limestone is the same substance used as a reactant in the combination example — one process builds a single substance up, the other breaks a single substance down.

    ✦ Answer: Decomposition reactions are the opposite of combination reactions because combination joins two or more substances into one, while decomposition breaks one substance into two or more. Combination: CaO + H₂O → Ca(OH)₂ + heat. Decomposition: CaCO₃ → CaO + CO₂ (on heating).

    Where students slip. Writing only one of the two equations asked for — the question wants an equation for each type to make the comparison concrete.

12. 123 marksNCERT Cl-10 Science, Exercise Q12

    Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

    Hint. All three equations look similar (one reactant, several products) — it's the energy source written above the arrow that tells them apart.

    Step 1 — Heat (thermal decomposition). 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) [heat] (Activity 1.5).

    Step 2 — Light (photochemical decomposition). 2AgCl(s) → 2Ag(s) + Cl₂(g) [sunlight] (Activity 1.8) — the reaction black-and-white photography relies on.

    Step 3 — Electricity (electrolytic decomposition). 2H₂O(l) → 2H₂(g) + O₂(g) [electricity] (Activity 1.7), since it's the electric current passed through the acidified water that splits it, not heat or light.

    ✦ Answer: Heat: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. Light: 2AgCl → 2Ag + Cl₂. Electricity: 2H₂O → 2H₂ + O₂.

    Where students slip. Mixing up which activity used which energy source — the equation pattern looks the same across all three, so the label above the arrow is what actually distinguishes them.

13. 133 marksNCERT Cl-10 Science, Exercise Q13

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

    Hint. Check whether a free element appears anywhere in the reaction — that's what separates the two types.

    Step 1 — State the core difference. In a displacement reaction, one element takes the place of another element within a compound (A + BC → AC + B) — only one element ever moves, and a free element is always involved, since that's what does the displacing. In a double displacement reaction, two compounds swap ions with each other (AB + CD → AD + CB) — no free element is involved at all.

    Step 2 — One equation each. Displacement: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — iron, a free element, displaces copper. Double displacement: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) — both reactants are compounds; their ions simply exchange partners.

    ✦ Answer: Displacement — a single element replaces another element within a compound (Fe + CuSO₄ → FeSO₄ + Cu). Double displacement — two compounds exchange ions with no free element involved (Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl).

    Where students slip. Saying both reaction types 'produce a precipitate' — only some double displacement reactions do; displacement reactions like Fe + CuSO₄ produce no precipitate at all, just a colour change and a metal deposit.

14. 142 marksNCERT Cl-10 Science, Exercise Q14

    In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

    Hint. This is the same idea as Activity 1.9 (iron displacing copper), just with a less reactive metal being displaced this time.

    Step 1 — Identify the pattern. Copper is more reactive than silver, so copper metal displaces silver from silver nitrate solution.

    Step 2 — Write and balance. Skeletal: Cu(s) + AgNO₃(aq) → Cu(NO₃)₂(aq) + Ag(s). Cu(NO₃)₂ needs 2 NO₃⁻, so 2 AgNO₃ are needed, releasing 2 Ag: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s).

    ✦ Answer: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s).

    Where students slip. Writing CuNO₃ instead of Cu(NO₃)₂ — copper here forms its +2 nitrate salt, and missing that subscript-2 bracket throws off the whole balance.

15. 153 marksNCERT Cl-10 Science, Exercise Q15

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

    Hint. Think about which reaction type always makes precipitation possible.

    Step 1 — Define it. A precipitation reaction produces an insoluble solid (a precipitate) directly within a solution, formed when two soluble compounds react and exchange ions.

    Step 2 — Give examples, from this chapter. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) [white precipitate] + 2NaCl(aq) (Activity 1.10). Also Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) [yellow precipitate] + 2KNO₃(aq) (Activity 1.2). Both are double displacement reactions, since it's the ion exchange in each that makes the newly formed compound insoluble.

    ✦ Answer: A precipitation reaction forms an insoluble solid when two solutions react and their ions exchange partners, e.g. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq), where white barium sulphate comes out of solution.

    Where students slip. Describing precipitation as a distinct fifth reaction type alongside the main four — it's a double displacement reaction where the product happens to be insoluble, not a separate category.

16. 164 marksNCERT Cl-10 Science, Exercise Q16

    Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

    Hint. The question wants two separate example reactions per term, not the same reaction analysed twice.

    Step 1 — (a) Oxidation: gain of oxygen. Example 1: 2Mg(s) + O₂(g) → 2MgO(s) — magnesium gains oxygen. Example 2: in CuO + H₂ → Cu + H₂O, hydrogen gains oxygen to become water, so hydrogen is oxidised.

    Step 2 — (b) Reduction: loss of oxygen. Example 1: CuO(s) + H₂(g) → Cu(s) + H₂O(l) — copper oxide loses oxygen to become copper. Example 2: 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) — lead oxide loses oxygen to become lead.

    ✦ Answer: Oxidation = gain of oxygen, e.g. 2Mg + O₂ → 2MgO, and H₂ → H₂O in CuO + H₂ → Cu + H₂O. Reduction = loss of oxygen, e.g. CuO → Cu in that same reaction, and PbO → Pb in 2PbO + C → 2Pb + CO₂.

    Where students slip. Reusing the exact same single reaction for both 'two examples' of one term — the question wants two distinct example reactions per term.

17. 172 marksNCERT Cl-10 Science, Exercise Q17

    A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

    Hint. This exact colour change (shiny brown metal → black surface) was one of the chapter's own activities.

    Step 1 — Match the description to Activity 1.11. The chapter's own Activity 1.11 heats copper powder in air: copper is shiny and brown, and on heating its surface becomes coated with a black substance.

    Step 2 — Name the compound and reaction. The black coating is copper(II) oxide, formed since copper gains oxygen from the air on heating: 2Cu(s) + O₂(g) → 2CuO(s) [heat].

    ✦ Answer: X is copper (Cu). On heating in air it forms black copper(II) oxide, CuO: 2Cu + O₂ → 2CuO.

    Where students slip. Naming iron instead of copper — iron's oxide (rust) is reddish-brown, the opposite colour pattern to the one described (shiny brown metal turning black), which fits copper, not iron.

18. 182 marksNCERT Cl-10 Science, Exercise Q18

    Why do we apply paint on iron articles?

    Hint. Recall exactly which two things need to reach the iron surface for rusting to start.

    Step 1 — Recall what rusting needs. Rusting requires both oxygen and moisture reaching the iron surface (4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O).

    Step 2 — Explain what paint does. A layer of paint forms a physical barrier between the iron surface and the air and moisture around it, so oxygen and water can no longer reach the metal directly.

    ✦ Answer: Paint coats the iron surface and keeps air and moisture from reaching the metal. Since rusting needs both oxygen and water in direct contact with iron, cutting off that contact prevents corrosion.

    Where students slip. Saying paint 'stops iron from reacting with rust' — rust is the product, not a reactant; paint's job is to block iron from oxygen and water, the actual reactants.

19. 192 marksNCERT Cl-10 Science, Exercise Q19

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

    Hint. Nitrogen isn't doing any chemistry here — think about what it's physically pushing out of the packet.

    Step 1 — Recall what causes rancidity. Rancidity is the oxidation of fats and oils by atmospheric oxygen, which changes their smell and taste.

    Step 2 — Explain the role of nitrogen. Nitrogen is unreactive towards the oil and fat in the food. Flushing the packet with nitrogen displaces the oxygen-containing air inside it, so little or no oxygen is left in contact with the food to cause oxidation.

    ✦ Answer: Nitrogen is flushed into the packaging to push out the oxygen that would otherwise oxidise the fats and oils and cause rancidity; nitrogen itself is unreactive, so it doesn't affect the food.

    Where students slip. Saying nitrogen 'absorbs' the oxygen — it doesn't react with or absorb anything; it simply displaces the air, and the oxygen in it, inside the packet.

20. 204 marksNCERT Cl-10 Science, Exercise Q20

    Explain the following terms with one example each. (a) Corrosion (b) Rancidity

    Hint. Corrosion attacks metals; rancidity attacks fats and oils — give each its own kind of example.

    Step 1 — (a) Corrosion. Corrosion is the slow attack of a metal's surface by substances around it, such as moisture, acids or other gases in the air, forming an unwanted compound on the surface. Example: rusting of iron, 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O, forming reddish-brown rust.

    Step 2 — (b) Rancidity. Rancidity is the oxidation of fats and oils in food, changing their smell and taste. Example: an opened packet of chips or fried snacks left out for a long time develops a stale, unpleasant smell, since the oil in it slowly oxidises on contact with air.

    ✦ Answer: (a) Corrosion — slow oxidation/attack of a metal by moisture, acids etc., e.g. rusting of iron. (b) Rancidity — oxidation of fats/oils in food, e.g. stale-smelling oil in old fried snacks.

    Where students slip. Giving corrosion and rancidity the same single example — they act on different kinds of substances (metals vs. fats/oils), so each needs its own distinct example.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc101.pdf) — three in-text question sets (8 questions total) plus one end-of-chapter Exercise (20 questions); the electron-transfer ('OIL RIG') definition of oxidation/reduction and the terms oxidising/reducing agent do not appear anywhere in the current chapter text. Questions are referenced from the NCERT textbook for identification.