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NCERT Class 10 Science Chemical Reactions and Equations: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 10 Science book, chapter “Chemical Reactions and Equations”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   For 2PbO(s) + C(s) → 2Pb(s) + CO₂(g), which statements 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.

   1. In the reaction 2PbO(s) + C(s) → 2Pb(s) + CO₂(g), lead oxide (PbO) loses oxygen to form lead (Pb), so lead oxide is reduced.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   2. Carbon (C) gains oxygen to form carbon dioxide (CO₂), so carbon is oxidised.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   3. Statement (a) says lead is getting reduced, which is incorrect because lead is the product of reduction, not the species being reduced.
   4. Statement (b) says carbon dioxide is getting oxidised, which is incorrect because carbon dioxide is the product of oxidation.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   5. Statement (c) says carbon is getting oxidised, which is correct.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   6. Statement (d) says lead oxide is getting reduced, which is correct.
      why?

      Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

   7. The incorrect statements are (a) and (b).

   Answer: (i): statements (a) and (b) are incorrect

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2. Exercises Q2

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

   1. Identify the reactants as iron(III) oxide (Fe₂O₃) and aluminium (Al).
   2. Observe that aluminium is more reactive than iron in the reactivity series.
   3. Note that aluminium displaces iron from its oxide compound to form aluminium oxide (Al₂O₃).
   4. Recognize that iron is released as a free element in the product side.
   5. Classify this reaction as a displacement reaction because a more reactive element replaces a less reactive one.
      why?

      A more reactive metal displaces a less reactive metal from a solution of its salt (reactivity series); a less reactive metal cannot displace a more reactive one.

   Answer: (d) displacement reaction

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3. Exercises Q3

   What happens when dilute hydrochloric acid is added to iron filings? (a) hydrogen gas and iron chloride form (b) chlorine gas and iron hydroxide form (c) no reaction (d) an iron salt and water form.

   1. Iron is a reactive metal that displaces hydrogen from dilute acids.
      why?

      A more reactive metal displaces a less reactive metal from a solution of its salt (reactivity series); a less reactive metal cannot displace a more reactive one.

   2. When dilute hydrochloric acid (HCl) is added to iron filings (Fe), a single displacement reaction occurs.
   3. The iron reacts with the acid to produce iron(II) chloride and hydrogen gas.
   4. The balanced chemical equation for this reaction is Fe + 2HCl → FeCl₂ + H₂.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   5. Therefore, hydrogen gas and iron chloride are formed.

   Answer: (a) hydrogen gas and iron chloride form

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4. Exercises Q4

   What is a balanced chemical equation, and why should chemical equations be balanced?

   1. A balanced chemical equation is one where the total number of atoms of each element is the same on both the reactant and product sides.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   2. This balance is required to satisfy the Law of Conservation of Mass.
   3. According to this law, mass can neither be created nor destroyed during a chemical reaction.
   4. Therefore, the total mass of the reactants must equal the total mass of the products.
   5. Balancing ensures that the number of atoms of every element is conserved in the reaction.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   Answer: A balanced chemical equation has equal numbers of atoms of each element on both sides; equations are balanced to obey the Law of Conservation of Mass.

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5. Exercises Q5

   Write and balance: (a) hydrogen combines with nitrogen to form ammonia (b) hydrogen sulphide 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 reacts with water to give potassium hydroxide and hydrogen.

   1. (a) Write the unbalanced equation: N2 + H2 → NH3
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   2. (a) Balance nitrogen atoms by placing a coefficient of 2 before NH3: N2 + H2 → 2NH3
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   3. (a) Balance hydrogen atoms by placing a coefficient of 3 before H2: N2 + 3H2 → 2NH3
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   4. (b) Write the unbalanced equation: H2S + O2 → H2O + SO2
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   5. (b) Balance sulfur atoms by placing a coefficient of 2 before H2S and SO2: 2H2S + O2 → H2O + 2SO2
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   6. (b) Balance oxygen atoms by placing a coefficient of 3 before O2: 2H2S + 3O2 → H2O + 2SO2
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   7. (b) Balance hydrogen atoms by placing a coefficient of 2 before H2O: 2H2S + 3O2 → 2H2O + 2SO2
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   8. (c) Write the unbalanced equation: BaCl2 + Al2(SO4)3 → AlCl3 + BaSO4
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   9. (c) Balance aluminum atoms by placing a coefficient of 2 before AlCl3: BaCl2 + Al2(SO4)3 → 2AlCl3 + BaSO4
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   10. (c) Balance sulfate groups by placing a coefficient of 3 before BaSO4: BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   11. (c) Balance barium and chlorine atoms by placing a coefficient of 3 before BaCl2: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   12. (d) Write the unbalanced equation: K + H2O → KOH + H2
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   13. (d) Balance hydrogen atoms by placing a coefficient of 2 before H2O and KOH: K + 2H2O → 2KOH + H2
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   14. (d) Balance potassium atoms by placing a coefficient of 2 before K: 2K + 2H2O → 2KOH + H2
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

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

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6. Exercises Q6

   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.

   1. (a) Count atoms on both sides: H=3, N=1, O=5, Ca=1 on left; Ca=1, N=2, O=7, H=2 on right.
   2. (a) Nitrogen is unbalanced (1 vs 2), so put a coefficient of 2 before HNO3.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   3. (a) Now H is 4 on left and 2 on right, so put a coefficient of 2 before H2O.
   4. (a) Final balanced equation: 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   5. (b) Count atoms: Na=1, O=1, H=1, S=1, O=4 on left; Na=2, S=1, O=4, H=2, O=1 on right.
   6. (b) Sodium is unbalanced (1 vs 2), so put a coefficient of 2 before NaOH.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   7. (b) Hydrogen is now 2 on left and 2 on right, but oxygen in water needs balancing; put 2 before H2O.
   8. (b) Final balanced equation: 2NaOH + H2SO4 → Na2SO4 + 2H2O.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   9. (c) Count atoms: Na=1, Cl=1, Ag=1, N=1, O=3 on both sides.
   10. (c) All atoms are already balanced with coefficient 1 for all compounds.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   11. (c) Final balanced equation: NaCl + AgNO3 → AgCl + NaNO3.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   12. (d) Count atoms: Ba=1, Cl=2, H=2, S=1, O=4 on left; Ba=1, S=1, O=4, H=1, Cl=1 on right.
   13. (d) Chlorine and Hydrogen are unbalanced (2 vs 1), so put a coefficient of 2 before HCl.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   14. (d) Final balanced equation: BaCl2 + H2SO4 → BaSO4 + 2HCl.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

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

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7. Exercises Q7

   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.

   1. (a) Write the unbalanced equation: Ca(OH)2 + CO2 → CaCO3 + H2O.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   2. (a) Count atoms on both sides: Ca=1, O=4, H=2, C=1 on left; Ca=1, O=4, H=2, C=1 on right.
   3. (a) The equation is already balanced.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   4. (b) Write the unbalanced equation: Zn + AgNO3 → Zn(NO3)2 + Ag.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   5. (b) Balance nitrate groups by placing coefficient 2 before AgNO3: Zn + 2AgNO3 → Zn(NO3)2 + Ag.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   6. (b) Balance silver atoms by placing coefficient 2 before Ag: Zn + 2AgNO3 → Zn(NO3)2 + 2Ag.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   7. (c) Write the unbalanced equation: Al + CuCl2 → AlCl3 + Cu.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   8. (c) Balance chlorine atoms by placing coefficient 3 before CuCl2 and 2 before AlCl3: Al + 3CuCl2 → 2AlCl3 + Cu.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   9. (c) Balance aluminium atoms by placing coefficient 2 before Al: 2Al + 3CuCl2 → 2AlCl3 + Cu.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   10. (c) Balance copper atoms by placing coefficient 3 before Cu: 2Al + 3CuCl2 → 2AlCl3 + 3Cu.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   11. (d) Write the unbalanced equation: BaCl2 + K2SO4 → BaSO4 + KCl.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   12. (d) Balance potassium and chlorine atoms by placing coefficient 2 before KCl: BaCl2 + K2SO4 → BaSO4 + 2KCl.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   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

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8. Exercises Q8

   Write balanced equations and name the reaction 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).

   1. (a) Write the unbalanced equation: KBr + BaI2 → KI + BaBr2.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   2. (a) Balance the bromine and iodine atoms by adding coefficients: 2KBr + BaI2 → 2KI + BaBr2.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   3. (a) Identify the reaction type as double displacement because ions exchange between reactants.
   4. (b) Write the unbalanced equation: ZnCO3 → ZnO + CO2.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   5. (b) Check atom counts; zinc, carbon, and oxygen are already balanced, so the equation is ZnCO3 → ZnO + CO2.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   6. (b) Identify the reaction type as decomposition because one reactant breaks into simpler products.
   7. (c) Write the unbalanced equation: H2 + Cl2 → HCl.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   8. (c) Balance the hydrogen and chlorine atoms by placing a coefficient of 2 before HCl: H2 + Cl2 → 2HCl.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   9. (c) Identify the reaction type as combination because two elements join to form a single compound.
   10. (d) Write the unbalanced equation: Mg + HCl → MgCl2 + H2.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   11. (d) Balance the chlorine and hydrogen atoms by placing a coefficient of 2 before HCl: Mg + 2HCl → MgCl2 + H2.
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   12. (d) Identify the reaction type as displacement because magnesium displaces hydrogen from the acid.

   Answer: (a) 2KBr + BaI2 → 2KI + BaBr2, double displacement; (b) ZnCO3 → ZnO + CO2, decomposition; (c) H2 + Cl2 → 2HCl, combination; (d) Mg + 2HCl → MgCl2 + H2, displacement

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9. Exercises Q9

   What are exothermic and endothermic reactions? Give examples.

   1. An exothermic reaction is a chemical change where heat energy is released to the surroundings.
   2. In these reactions, the total energy of the products is lower than the energy of the reactants.
   3. A common example is the burning of methane gas, which produces carbon dioxide, water, and heat.
   4. An endothermic reaction is a chemical change where heat energy is absorbed from the surroundings.
   5. In these reactions, the total energy of the products is higher than the energy of the reactants.
   6. A common example is the thermal decomposition of calcium carbonate, which requires heat to form calcium oxide and carbon dioxide.

   Answer: Exothermic reactions release heat (e.g., combustion of methane); endothermic reactions absorb heat (e.g., decomposition of calcium carbonate).

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10. Exercises Q10

    Why is respiration called an exothermic reaction?

    1. Respiration is the process where glucose reacts with oxygen in our body cells to release energy.
    2. The chemical equation for this process is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy.
    3. During this reaction, chemical energy stored in glucose is converted into heat and usable energy.
    4. Since heat energy is released to the surroundings during the reaction, it is classified as an exothermic reaction.

    Answer: Respiration is called an exothermic reaction because it releases energy in the form of heat.

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11. Exercises Q11

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

    1. A combination reaction is a process where two or more reactants combine to form a single new product.
    2. A decomposition reaction is a process where a single reactant breaks down into two or more simpler products.
    3. Since one reaction joins substances together and the other splits a substance apart, they are considered opposites.
    4. An example of a combination reaction is the formation of calcium oxide from calcium and oxygen: 2Ca + O₂ → 2CaO.
    5. An example of a decomposition reaction is the breakdown of calcium carbonate into calcium oxide and carbon dioxide: CaCO₃ → CaO + CO₂.

    Answer: Decomposition reactions are the opposite of combination reactions because they involve the breakdown of a single compound into simpler substances, whereas combination reactions involve the joining of two or more substances to form a single product. Combination: 2Ca + O₂ → 2CaO; Decomposition: CaCO₃ → CaO + CO₂.

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12. Exercises Q12

    Write one decomposition reaction each where energy is supplied as heat, light and electricity.

    1. Thermal decomposition: Calcium carbonate breaks down into calcium oxide and carbon dioxide when heated.
    2. Light decomposition: Silver chloride decomposes into silver and chlorine gas when exposed to sunlight.
    3. Electrolytic decomposition: Water splits into hydrogen and oxygen gases when electric current is passed through it.

    Answer: (i) CaCO₃(s) → CaO(s) + CO₂(g); (ii) 2AgCl(s) → 2Ag(s) + Cl₂(g); (iii) 2H₂O(l) → 2H₂(g) + O₂(g)

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13. Exercises Q13

    How do displacement and double displacement reactions differ? Write an equation for each.

    1. A displacement reaction is a type of chemical reaction in which a more reactive element displaces a less reactive element from its compound.
       why?

       A more reactive metal displaces a less reactive metal from a solution of its salt (reactivity series); a less reactive metal cannot displace a more reactive one.

    2. In a single displacement reaction, only one element is replaced, such as when iron reacts with copper sulfate to form iron sulfate and copper.
    3. The chemical equation for this single displacement reaction is Fe + CuSO₄ → FeSO₄ + Cu.
    4. A double displacement reaction involves the exchange of ions between two compounds, resulting in the formation of two new compounds.
    5. In a double displacement reaction, both cations and anions swap partners, often leading to the formation of a precipitate.
    6. The chemical equation for a double displacement reaction is AgNO₃ + NaCl → AgCl + NaNO₃.
    7. The key difference is that displacement involves one element replacing another, while double displacement involves the mutual exchange of ions between two reactants.

    Answer: Displacement reactions involve one element displacing another from a compound, while double displacement reactions involve the exchange of ions between two compounds; equations: Fe + CuSO₄ → FeSO₄ + Cu and AgNO₃ + NaCl → AgCl + NaNO₃

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14. Exercises Q14

    Silver is recovered from silver nitrate solution by displacement with copper. Write the reaction.

    1. Copper is more reactive than silver, so it displaces silver from its salt solution.
       why?

       A more reactive metal displaces a less reactive metal from a solution of its salt (reactivity series); a less reactive metal cannot displace a more reactive one.

    2. The reactants are copper metal and silver nitrate solution.
    3. Copper forms a +2 ion, so it combines with two nitrate ions to form copper(II) nitrate.
    4. Silver is displaced as a solid metal.
       why?

       A more reactive metal displaces a less reactive metal from a solution of its salt (reactivity series); a less reactive metal cannot displace a more reactive one.

    5. The balanced chemical equation is Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

    Answer: Cu + 2AgNO3 → Cu(NO3)2 + 2Ag

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15. Exercises Q15

    What is a precipitation reaction? Explain with examples.

    1. A precipitation reaction is a double displacement reaction where two soluble reactants in aqueous solution form an insoluble solid product.
    2. This insoluble solid is called a precipitate and it separates from the solution.
    3. For example, when aqueous solutions of barium chloride and sodium sulfate are mixed, they react to form barium sulfate and sodium chloride.
    4. The balanced chemical equation is BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
       why?

       A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

    5. In this reaction, barium sulfate (BaSO₄) is insoluble in water and appears as a white precipitate.
    6. Another example is the reaction between lead nitrate and potassium iodide, which forms a yellow precipitate of lead iodide.
    7. The equation for this is Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).

    Answer: A precipitation reaction is a double displacement reaction in which an insoluble solid (precipitate) forms from two aqueous reactants, such as BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl.

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16. Exercises Q16

    Explain (a) oxidation and (b) reduction in terms of gain or loss of oxygen, with two examples each.

    1. (a) Oxidation is the chemical process in which a substance gains oxygen.
    2. Example 1: When carbon burns in oxygen, it forms carbon dioxide, so carbon is oxidized.
       why?

       Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

    3. Example 2: When hydrogen reacts with oxygen to form water, hydrogen gains oxygen and is oxidized.
       why?

       Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

    4. (b) Reduction is the chemical process in which a substance loses oxygen.
       why?

       Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

    5. Example 1: When copper oxide is heated with hydrogen, it loses oxygen to form copper, so copper oxide is reduced.
       why?

       Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

    6. Example 2: When lead oxide reacts with carbon, it loses oxygen to form lead, so lead oxide is reduced.
       why?

       Oxidation is gain of oxygen (or loss of hydrogen); reduction is loss of oxygen (or gain of hydrogen). In a redox reaction one substance is oxidised while another is reduced.

    Answer: (a) Oxidation is gain of oxygen; (b) Reduction is loss of oxygen.

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17. Exercises Q17

    A shiny brown element X turns black when heated in air. Name X and the black compound.

    1. The shiny brown element X is copper (Cu).
    2. When copper is heated in air, it reacts with oxygen to form a black compound.
    3. The black compound formed is copper(II) oxide (CuO).
    4. The chemical equation for this reaction is 2Cu + O₂ → 2CuO.

    Answer: X is copper (Cu) and the black compound is copper(II) oxide (CuO).

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18. Exercises Q18

    Why do we paint iron articles?

    1. Iron reacts with oxygen and moisture in the air to form iron oxide, commonly known as rust.
    2. Rust is porous and flaky, which allows more air and moisture to reach the inner iron surface.
    3. This continuous exposure causes the iron to corrode and eventually break down.
    4. Painting creates a protective layer that acts as a barrier.
    5. This barrier prevents oxygen and moisture from coming into direct contact with the iron surface.
    6. By blocking these elements, the chemical reaction that causes rusting is stopped.

    Answer: We paint iron articles to prevent rusting by creating a barrier that stops oxygen and moisture from contacting the iron.

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19. Exercises Q19

    Why are oil and fat containing foods flushed with nitrogen?

    1. Oil and fat containing foods undergo oxidation when exposed to air.
    2. This oxidation process is commonly known as rancidity.
    3. Rancidity causes the food to develop an unpleasant smell and taste.
    4. Nitrogen is an inert gas that does not react with the food.
    5. Flushing with nitrogen displaces the oxygen present in the packaging.
    6. Without oxygen, the oxidation of fats and oils is prevented.

    Answer: Oil and fat containing foods are flushed with nitrogen to prevent rancidity by removing oxygen and stopping oxidation.

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20. Exercises Q20

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

    1. (a) Corrosion is the slow chemical deterioration of metals, especially iron, when they react with moisture and oxygen in the air.
    2. (a) Example: Iron objects like gates or nails turn into reddish-brown rust (hydrated iron oxide) over time when exposed to damp air.
    3. (b) Rancidity is the oxidation of fats and oils present in food, which leads to an unpleasant smell and taste.
    4. (b) Example: When chips or fried snacks are left open in air for a long time, they become stale and develop a bad odor due to the oxidation of their oil content.

    Answer: (a) Corrosion is the slow chemical deterioration of metals, e.g., rusting of iron; (b) Rancidity is the oxidation of fats and oils in food, e.g., stale chips.

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