Class 9th Science Atoms and Molecules Practice Questions

Section A – Multiple Choice Questions (MCQs) [Q.1–20]

1. Who is regarded as the father of modern atomic theory?
   a) Dalton
   b) Thomson
   c) Rutherford
   d) Bohr
2. The chemical symbol of sodium is:
   a) So
   b) Sd
   c) Na
   d) Sa
3. Which of the following represents a molecule of an element?
   a) H₂O
   b) H₂
   c) CO₂
   d) NaCl
4. The smallest particle of an element which takes part in a chemical reaction is called:
   a) Atom
   b) Molecule
   c) Proton
   d) Neutron
5. Atomicity of phosphorus is:
   a) 2
   b) 3
   c) 4
   d) 5
6. Which is a triatomic molecule?
   a) O₂
   b) H₂O
   c) NaCl
   d) N₂
7. Valency of calcium is:
   a) 1
   b) 2
   c) 3
   d) 4
8. The formula of aluminium oxide is:
   a) AlO
   b) AlO₂
   c) Al₂O₃
   d) AlO₃
9. The molecular mass of CO₂ is:
   a) 28
   b) 44
   c) 32
   d) 46
10. Which law states that “In a chemical reaction, mass is neither created nor destroyed”?
    a) Law of constant proportion
    b) Law of conservation of mass
    c) Dalton’s law
    d) Avogadro’s law
11. Which diagram represents a molecule of oxygen?
    (Fig: O = O)
12. Which diagram represents water molecule?
    (Fig: H–O–H)
13. The valency of nitrogen in NH₃ is:
    a) 1
    b) 2
    c) 3
    d) 4
14. The number of atoms in Na₂CO₃ is:
    a) 5
    b) 6
    c) 7
    d) 8
15. Avogadro’s number is:
    a) 6.022 × 10²³
    b) 3.022 × 10²³
    c) 9.022 × 10²³
    d) 1.022 × 10²³
16. The formula of sulphuric acid is:
    a) HSO₃
    b) H₂SO₄
    c) H₂SO₃
    d) HSO₄
17. Which of the following is an empirical formula?
    a) H₂O₂
    b) CH₄
    c) C₆H₁₂O₆
    d) H₂O
18. Molecular formula of glucose is:
    a) C₂H₄O₂
    b) C₆H₁₂O₆
    c) CH₂O
    d) C₆H₆
19. Who proposed the law of constant proportions?
    a) Dalton
    b) Proust
    c) Lavoisier
    d) Avogadro
20. The molar mass of NaOH is:
    a) 23 g/mol
    b) 40 g/mol
    c) 56 g/mol
    d) 44 g/mol

Section B – True/False [Q.21–30]

21. Atoms can be created or destroyed in a chemical reaction.
22. Hydrogen molecule is monoatomic.
23. Formula mass of NaCl is 58.5 g/mol.
24. Law of constant proportion was given by Lavoisier.
25. One mole of water contains 6.022 × 10²³ molecules.
26. The chemical formula of ammonium ion is NH₄⁺.
27. Oxygen gas is diatomic.
28. Mass of 1 mole of carbon atoms is 12 g.
29. The atomicity of ozone is 3.
30. One mole of any gas at STP occupies 22.4 L.

Section C – Fill in the Blanks [Q.31–40]

31. The smallest particle of matter that cannot be divided chemically is called __________.
32. The symbol of iron is __________.
33. The chemical formula of calcium hydroxide is __________.
34. The combining capacity of an atom is called __________.
35. One mole of oxygen molecules contains __________ molecules.
36. Atomic mass of chlorine is __________.
37. A molecule of methane contains __________ atoms.
38. The symbol of potassium is __________.
39. The number of atoms in H₂SO₄ molecule is __________.
40. __________ proposed the law of conservation of mass.

Section D – Assertion & Reason [Q.41–50]

41. Assertion: An atom is indivisible.
    Reason: Atoms cannot be divided by any means.
42. Assertion: H₂O has 2 hydrogen atoms and 1 oxygen atom.
    Reason: The formula represents fixed ratio of elements.
43. Assertion: 1 mole of Na contains 6.022 × 10²³ atoms.
    Reason: Avogadro’s number is 6.022 × 10²³.
44. Assertion: Compounds are formed by physical combination.
    Reason: Compounds have variable composition.
45. Assertion: Valency of carbon is 4.
    Reason: Carbon forms four bonds with other atoms.
46. Assertion: Molecular mass of CO₂ is 44 u.
    Reason: Atomic mass of C = 12 u, O = 16 u.
47. Assertion: Law of constant proportion is applicable to compounds.
    Reason: Compounds always have fixed ratio of elements.
48. Assertion: Hydrogen molecule is diatomic.
    Reason: Hydrogen has valency 1.
49. Assertion: The chemical formula of aluminium chloride is AlCl₃.
    Reason: Valency of Al = 3, Cl = 1.
50. Assertion: 1 mole of water weighs 18 g.
    Reason: Molecular mass of water is 18 u.

Section E – Very Short Answer (1 Mark Each) [Q.51–60]

51. Define atom.
52. Define molecule.
53. Give the chemical formula of calcium carbonate.
54. Name the law given by Lavoisier.
55. Define atomicity.
56. Write the symbol of copper.
57. What is the valency of oxygen?
58. Write the molecular formula of ammonia.
59. Define mole.
60. Write the molecular formula of methane.

Section F – Short Answer (2 Marks Each) [Q.61–70]

61. State Dalton’s atomic theory (any two postulates).
62. Write the differences between atom and molecule.
63. Explain with example: monoatomic and diatomic molecule.
64. Why do atoms combine?
65. Write the formula and molar mass of Na₂O.
66. Give two examples of polyatomic molecules.
67. Write the names of radicals: NO₃⁻, SO₄²⁻.
68. Differentiate between valency and atomic number.
69. Calculate the molecular mass of H₂SO₄.
70. What is the significance of a chemical formula?

Section G – Short Answer (3 Marks Each) [Q.71–80]

71. Explain the law of conservation of mass with an experiment.
72. Calculate the number of molecules in 18 g of water.
73. Write the steps to calculate the formula unit mass of NaCl.
74. Balance the following equation: H₂ + O₂ → H₂O (fig with O=O)
75. Balance the following equation: N₂ + H₂ → NH₃ (fig with N≡N)
76. Write molecular formulas of: aluminium oxide, calcium hydroxide, ammonium nitrate.
77. How is valency used to write chemical formula? Give two examples.
78. Write differences between empirical and molecular formula.
79. Explain the law of constant proportion with an example.
80. Calculate the number of moles in 88 g of CO₂.

Section H – Long Answer (4–5 Marks Each) [Q.81–90]

81. State Dalton’s atomic theory with limitations.
82. Explain laws of chemical combination with examples.
83. Calculate molecular mass of C₆H₁₂O₆ and explain the steps.
84. Derive the chemical formula of Na₂SO₄ using valency.
85. What is Avogadro’s hypothesis? How does it help in understanding molecules?
86. Compare atoms, molecules, and ions with examples.
87. Calculate number of atoms in 4.4 g of CO₂.
88. Balance the following chemical equation step by step:
    Fe + H₂O → Fe₃O₄ + H₂ (include skeletal eqn fig)
89. Explain the difference between compound and mixture with examples.
90. Calculate number of moles and molecules in 36 g of water.

Section I – Case Study Based [Q.91–100]

Case Study 1 (Q.91–95):
In a lab experiment, a student mixes hydrogen and oxygen gases in the ratio 2:1 by volume and ignites them. Water is formed.

91. Write the balanced chemical equation.
92. Which law of chemical combination is illustrated?
93. How many moles of water are formed when 2 moles of hydrogen react with 1 mole of oxygen?
94. Draw the figure of water molecule showing bonding (H–O–H).
95. How many atoms are present in one molecule of water?

Case Study 2 (Q.96–100):
A sample of NaCl weighing 58.5 g is taken. (Na = 23, Cl = 35.5)

96. Calculate the molar mass of NaCl.
97. Find the number of moles present in 58.5 g NaCl.
98. How many molecules are present in 58.5 g NaCl?
99. How many Na⁺ and Cl⁻ ions are present in 1 mole of NaCl?
100. Draw the lattice structure of NaCl (Fig: alternating Na⁺, Cl⁻ ions).