Unit-1 : Chemical Reactions and Equations(Questions)

Unit-1 : Chemical Reactions and Equations

📘 Learning Points

• Meaning of Chemical Equation

• Balancing Chemical Equations

• Types of Chemical Reactions

• Endothermic & Exothermic Reactions

• Oxidation, Reduction & Redox Reactions

• Effects of Oxidation Reactions in Daily Life

• Key Answers are given below 

I. Multiple-Choice Questions (1 mark each)

1. An example for chemical change among the following is (A)
   A. Dissolving salt in water B. Tearing of paper C. Rusting of iron D. Boiling of water.

2. This is not a property of a chemical reaction among the following (A)
   A. Change in colour B. Release of heat C. Change in shape D. Release of gas.

3. Fe₂O₃ + 2 Al → Al₂O₃ + 2 Fe (Sup 2019)
   The above chemical reaction is an example for (A)
   A. Combination reaction B. Decomposition reaction C. Displacement reaction D. Double-displacement reaction.

4. The group of reactants among the following which react with each other to exchange ions and form a precipitate is (D) (Mar-2023)
   A. BaCl₂ and Na₂SO₄ B. Al₂O₃ and HCl C. NaOH and H₂SO₄ D. Na₂O and CO₂

5. The evolution of oxygen and hydrogen gases in the electrolysis of water is (E)
   A. Double-displacement reaction B. Exothermic reaction C. Displacement reaction D. Decomposition reaction (Model 1 2025)

6. The chemical reaction that takes place when reactants exchange their ions mutually is (E)
   A. Double-displacement reaction B. Dissociation reaction C. Dissociation reaction D. Combination reaction

7. CuO + H₂ → Cu + H₂O In this reaction, the reactants that undergo oxidation and reduction respectively are (D) (Mar 2020 / Apr 2025-1)
   A. CuO + H₂ B. H₂ + CuO C. Cu + H₂O D. H₂O + Cu

8. Silver nitrate turns into grey colour in the presence of sunlight because (D)
   A. Silver chloride decomposes to form silver. B. Silver chloride decomposes to form chlorine. C. Silver chloride undergoes oxidation. D. Silver chloride undergoes reduction.

9. The ratio of hydrogen and oxygen gases released in the electrolysis of water is (D) (Aug 2024)
   A. Hydrogen : Oxygen :: 1 : 2 B. Oxygen : Hydrogen :: 2 : 3 C. Hydrogen : Oxygen :: 2 : 1 D. Oxygen : Hydrogen :: 3 : 1

10. The conversion of vegetable waste into compost is an example for (E)
    A. Reduction reaction B. Exothermic reaction C. Endothermic reaction D. Redox reaction.

11. The gas released at the cathode in the electrolysis of water is (Apr 2022) (A)
    A. Oxygen B. Hydrogen C. Chlorine D. Nitrogen

12. Identify the chemical equation in which the chemical reaction takes place (Jun 2019) (D)
    A. FeSO₄ + Pb → PbSO₄ + Fe B. ZnSO₄ + Fe → FeSO₄ + Zn C. 2 AgNO₃ + Cu → Cu(NO₃)₂ + Ag D. PbCl₂ + Cu → CuCl₂ + Pb

13. When dilute hydrochloric acid is added to iron fillings (E)
    A. Hydrogen gas and iron chloride are produced B. Chlorine gas and ferric hydroxide are produced C. No reaction takes place D. Ferrous salt and water are produced.

14. The metal that displaces copper from copper sulphate (June 2022 / Apr 2022) (D)
    A. Gold B. Silver C. Copper D. Iron

15. Ferrous sulphate crystals lose their green colour when heated because this compound (Apr 2024-1) (D)
    A. Breaks down into simpler compounds B. Loses water molecules C. Releases sulphur dioxide gas D. Produces brown smoke

16. Chip manufacturers flush bags of chips with nitrogen gas because (June 2023) (E)
    A. To prevent corrosion B. To prevent oxidation C. To cause corrosion D. To prevent reduction.

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II. Answer the following (1-mark questions)

17. What is a chemical reaction?

18. What is a chemical equation?

19. Why should a magnesium ribbon be cleaned with sandpaper before burning it in air?

20. Chemical equations are to be balanced — Why?

21. The decomposition of vegetable matter into compost and the respiration process are exothermic reactions. Why?

22. What is precipitation reaction? Give an example.

23. What is corrosion?

24. Manufacturers of chips flush the packets of chips with nitrogen — why?

25. Iron articles are to be painted — why?

26. What is rancidity?

27. Suggest any two methods to prevent rancidity.

28. ZnO + C → Zn + CO — Identify the substances that are oxidised and reduced in the above reaction.

29. When sodium sulphate solution is added to barium chloride solution, a white precipitate of barium sulphate is formed. Which are the ions responsible for this precipitate formation?

30. Name the product formed when calcium oxide reacts with water.

31. The magnesium ribbon burns with a dazzling white flame in air and changes into magnesium oxide. Identify the type of reaction.

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III. Two-mark Questions

32. List out the observations that help us determine that a chemical reaction has taken place.

33. What is a combination reaction? Give an example.

34. What is a decomposition reaction? Give an example.

35. What is a displacement reaction? Give an example.

36. Why does an iron nail change its colour when immersed in copper-sulphate solution? Write the chemical equation for this reaction.

37. FeSO₄ + Cu → CuSO₄ + Fe — Is it possible for the above reaction to take place? Justify your answer.

38. What is a double-displacement reaction? Give an example.

39. Which coloured precipitate is formed when lead nitrate solution is mixed with potassium iodide solution? Name the precipitate and identify the type of chemical reaction.

40. Name the brown fumes liberated when lead nitrate is heated. Write the balanced chemical equation for this reaction.

41. What are endothermic and exothermic reactions? Give an example for each.

42. When calcium carbonate is heated, calcium oxide and carbon dioxide are produced. Write the balanced chemical equation and state the type of reaction.

43. What is a redox reaction? Give an example.

44. The chemical reaction that takes place between sodium-sulphate solution and barium-chloride solution is called double-displacement reaction – why? Write the balanced chemical equation.

45. Which type of reaction gives quicklime from limestone (calcium carbonate)? Write the chemical equation.

46. Mention any two measures to prevent corrosion (rusting of iron).

47. A shiny brown-coloured element X on heating in air becomes a black-coloured compound Y.
     a) Name the element X. b) Name the compound Y.

48. The given equation represents CuSO₄ + X → Cu + Y.
     a) Which element is X among Fe and Ag? Justify. b) Write the molecular formula of Y.

49. Draw a diagram of the apparatus used in electrolysis of water and label a) Graphite rod b) Oxygen.

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IV. Three-mark Questions

50. To a solution of sodium sulphate if the same amount of barium chloride is added:
     i) Name the white precipitate formed. ii) Name the ions responsible. iii) Mention the type of reaction.

51. Strips of zinc, iron, magnesium and copper are taken in test tubes A, B, C and D respectively. Equal amounts of ferrous-sulphate solution are added. In which test tubes does reaction take place? Why? Write the equations.

52. Explain with an example each for the decomposition reactions that take place when energy is supplied in the form of (i) heat (ii) light (iii) electricity.

53. Name the salts used in black-and-white photography. Write the equations for the reactions when they are exposed to sunlight.

54. A solution of substance Z is used for white-washing — i) Name Z ii) Write its formula iii) Write the equation when Z reacts with water.

55. What type of chemical reaction takes place when an iron nail is immersed in copper-sulphate solution? Why? Write the balanced equation.

56. The reaction of barium chloride with aluminium-sulphate solution is an example for which type of reaction? Why? Write the balanced equation.

57. Write balanced chemical equations for the following:
     i) Calcium carbonate → Calcium oxide + Carbon dioxide
     ii) Hydrogen + Chlorine → Hydrogen chloride
     iii) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

58. Balance the following chemical equations (Apr 2025):
     i) H₂ + O₂ → H₂O  ii) Na₂CO₃ + HCl → NaCl + H₂O + CO₂  iii) N₂ + H₂ → NH₃

59. Write balanced equations for the following:
     i) Quicklime reacts with water ii) Zinc flakes added to CuSO₄ solution iii) NaCl solution added to AgNO₃ solution.

60. Translate the following statements into balanced equations:
     i) Hydrogen sulphide burns in air to give water and SO₂.
     ii) BaCl₂ reacts with Al₂(SO₄)₃ to give AlCl₃ and BaSO₄.
     iii) Potassium reacts with water to give KOH and H₂.

61. Write the balanced equation for:
     i) Combustion of natural gas ii) Reaction of potassium with water iii) Reaction of iron with copper sulphate.

62. Explain the following with chemical equations: (i) Oxidation (ii) Reduction.

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V. Four-mark Questions

63. “Lead nitrate is mixed with potassium iodide solution.” Answer the following:
     i) Write the balanced equation. ii) Name the precipitate. iii) Name its colour. iv) State the type of reaction.

64. Write the balanced equations for the following and identify the type of each reaction:
     i) Hydrogen + Oxygen → Water ii) Zinc sulphate + Copper → Copper sulphate + Zinc iii) Zinc carbonate → Zinc oxide + Carbon dioxide iv) Sodium chloride + Silver nitrate → Silver chloride + Sodium nitrate.

65. Give reasons:
     i) Objects made of copper lose lustre when exposed to air.
     ii) An iron nail placed in CuSO₄ solution slowly turns brown.

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Key Answers

📝 I. Multiple Choice Questions (1 mark each)

1. Example of a chemical change — (C) Rusting of iron

2. Not a property of a chemical reaction — (C) Change in shape

3. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe — (C) Displacement reaction

4. Reactants that exchange ions — (A) BaCl₂ and Na₂SO₄

5. Evolution of O₂ and H₂ in electrolysis — (C) Endothermic reaction

6. Reactants exchange ions — (A) Double displacement reaction

7. CuO + H₂ → Cu + H₂O → (B) H₂ + CuO

8. Silver nitrate turns grey because — (A) Silver chloride decomposes to form silver

9. Ratio of H₂ : O₂ in electrolysis — (C) 2 : 1

10. Burning of fuel — (B) Exothermic reaction

11. Gas released at cathode — (B) Hydrogen

12. Displacement equation — (C) 2AgNO₃ + Cu → Cu(NO₃)₂ + Ag

13. Fe + HCl → Hydrogen gas and iron chloride are formed

14. Metal that displaces copper — (D) Iron

15. FeSO₄ loses green colour on heating because — (B) Loses water molecules

16. Chips flushed with nitrogen — (B) To prevent oxidation

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🧭 II. Answer the following – 1 Mark Questions

17. Chemical Reaction – A process involving rearrangement of atoms/molecules to form a new substance.

18. Chemical Equation – Symbolic representation of a chemical reaction.

19. Magnesium ribbon is cleaned to remove the oxide layer.

20. Why balance equations? To justify the Law of Conservation of Mass.

21. Decomposition of vegetable matter and respiration are exothermic because heat is liberated.

22. Precipitation Reaction – Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.

23. Corrosion – Gradual deterioration of metals by air, moisture or acids.

24. Why nitrogen in chips? To prevent rancidity (oxidation of oils).

25. Why paint iron articles? To prevent rusting.

26. Rancidity – Undesirable change in taste/smell of oily foods due to oxidation.

27. Prevent rancidity by – (i) Using antioxidants (ii) Airtight containers.

28. In ZnO + C → Zn + CO → ZnO reduced, C oxidized.

29. Ions forming white precipitate — SO₄²⁻ and Ba²⁺.

30. CaO + H₂O → Ca(OH)₂ – Calcium Hydroxide.

31. Burning of magnesium – Combination reaction / Oxidation reaction.

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🧩 III. Answer the following – 2 Mark Questions

32. Evidences of Chemical Reaction: Change of state, colour, temperature, and gas evolution.

33. Combination Reaction: 2Mg + O₂ → 2MgO.

34. Decomposition Reaction: CaCO₃ → CaO + CO₂.

35. Displacement Reaction: CuSO₄ + Fe → FeSO₄ + Cu.

36. Iron nail in CuSO₄ solution → blue colour fades because iron displaces copper.

37. Reaction FeSO₄ + Cu → CuSO₄ + Fe does not occur as iron is more reactive.

38. Double Displacement Reaction: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.

39. Lead nitrate + KI → Yellow precipitate of PbI₂ (Double Displacement).

40. Lead nitrate heated: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes).

41. Endothermic reaction: CaCO₃ → CaO + CO₂ (absorbs heat).
      Exothermic reaction: CH₄ + 2O₂ → CO₂ + 2H₂O + heat.

42. CaCO₃ → CaO + CO₂ – Thermal decomposition.

43. Redox Reaction: ZnO + C → Zn + CO (C oxidized, ZnO reduced).

44. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl – Double Displacement.

45. CaCO₃ → CaO + CO₂ – Decomposition Reaction.

46. Methods to prevent corrosion: Painting, greasing, oil coating, anodizing, galvanizing (any two).

47. X = Copper, Y = Copper oxide (CuO).

48. Fe reacts with CuSO₄ (because Fe is more reactive than Ag). Y = FeSO₄.

49. Diagram of Electrolysis of Water – Label (a) Graphite rod (b) Oxygen gas.

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⚛️ IV. Answer the following – 3 Mark Questions

50. BaCl₂ + Na₂SO₄ → BaSO₄ (white ppt), ions: Ba²⁺ & SO₄²⁻, type: Double Displacement.

51. Reaction in Zn and Mg test tubes (FeSO₄ solution):
      Zn + FeSO₄ → ZnSO₄ + Fe
      Mg + FeSO₄ → MgSO₄ + Fe

52. Decomposition Examples:
      a) Thermal: CaCO₃ → CaO + CO₂
      b) Photolytic: 2AgCl → 2Ag + Cl₂
      c) Electrolytic: 2H₂O → 2H₂ + O₂

53. Salts in Photography: AgCl, AgBr → (2AgCl → 2Ag + Cl₂) & (2AgBr → 2Ag + Br₂).

54. White washing: Z = CaO, CaO + H₂O → Ca(OH)₂.

55. Fe + CuSO₄ → FeSO₄ + Cu – Displacement Reaction.

56. 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ – Double Displacement.

57. Balanced Equations:
      (i) CaCO₃ → CaO + CO₂
      (ii) H₂ + Cl₂ → 2HCl
      (iii) Mg + 2HCl → MgCl₂ + H₂

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🧮 V. Balance the following Equations

58. 2H₂ + O₂ → 2H₂O
      Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
      N₂ + 3H₂ → 2NH₃

59. CaO + H₂O → Ca(OH)₂
      CuSO₄ + Zn → ZnSO₄ + Cu
      NaCl + AgNO₃ → NaNO₃ + AgCl

60. 2H₂S + 3O₂ → 2H₂O + 2SO₂
      3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
      2K + 2H₂O → 2KOH + H₂

61. CH₄ + 2O₂ → CO₂ + 2H₂O
      2K + 2H₂O → 2KOH + H₂
      CuSO₄ + Fe → FeSO₄ + Cu

62. Oxidation – 2Cu + O₂ → 2CuO  Reduction – 2MgO → 2Mg + O₂

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🧠 V. Four Mark Questions

63. Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
      Precipitate: Lead iodide (PbI₂), Colour: Yellow, Type: Double Displacement.

  (i) 2H₂ + O₂ → 2H₂O
  (ii) ZnSO₄ + Cu → CuSO₄ + Zn
  (iii) ZnCO₃ → ZnO + CO₂
  (iv) NaCl + AgNO₃ → NaNO₃ + AgCl

  (i) Copper objects lose lustre due to formation of CuO/CuCO₃ (corrosion).
  (ii) Fe in CuSO₄ → brown deposit of Cu (displacement reaction).