Question Bank — Atomic Foundations of Matter

NCERT Class 9 Science · Chapter 9 · CBSE Board Pattern

Class 9 Science Chapter 9 Question Bank CBSE Board Pattern

Section A — MCQs (10 questions, 1 mark each)

  1. Which law states that mass can neither be created nor destroyed in a chemical reaction?
    (a) Law of Constant Proportions
    (b) Law of Conservation of Mass
    (c) Dalton’s Atomic Theory
    (d) Proust’s Law

  2. According to the Law of Constant Proportions, the mass ratio of hydrogen to oxygen in water is:
    (a) 1:2
    (b) 2:1
    (c) 1:8
    (d) 8:1

  3. Which postulate of Dalton’s Atomic Theory explains why atoms of different elements have different chemical properties?
    (a) Atoms are indivisible
    (b) Atoms of a given element are identical
    (c) Atoms of different elements have different masses and chemical properties
    (d) Atoms combine in simple whole-number ratios

  4. In the formation of a hydrogen molecule, the type of bond formed is:
    (a) Ionic bond
    (b) Covalent bond (single)
    (c) Covalent bond (double)
    (d) Metallic bond

  5. An oxygen molecule (O₂) contains:
    (a) A single covalent bond
    (b) A double covalent bond
    (c) An ionic bond
    (d) No bond

6. Assertion (A): The total mass of reactants equals the total mass of products in a chemical reaction.
Reason (R): Matter can neither be created nor destroyed in a chemical reaction.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

7. Assertion (A): Sodium chloride conducts electricity in aqueous solution but not in solid state.
Reason (R): In solid state, ions are fixed in the crystal lattice and cannot move freely.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

  1. The chemical formula of magnesium hydroxide is written using brackets because:
    (a) It contains two magnesium ions
    (b) It contains more than one polyatomic hydroxide ion
    (c) Magnesium has variable valency
    (d) It is a covalent compound

  2. The formula unit mass of Na₂O (Atomic mass: Na = 23 u, O = 16 u) is:
    (a) 39 u
    (b) 62 u
    (c) 78 u
    (d) 46 u

  3. Which of the following is a covalent compound?
    (a) Sodium chloride
    (b) Copper sulfate
    (c) Camphor
    (d) Magnesium oxide

Section B — Very Short Answer (6 questions, 2 marks each)

  1. State the Law of Conservation of Mass. Give one example from daily life.
  2. What is the difference between O and O₂?
  3. Define a molecule. Give one example of a molecule of an element and one of a compound.
  4. Differentiate between a single covalent bond and a double covalent bond with one example each.
  5. What are polyatomic ions? Give two examples.
  6. Calculate the molecular mass of H₂O (Atomic mass: H = 1 u, O = 16 u).

Section C — Short Answer (5 questions, 3 marks each)

  1. State any three postulates of Dalton’s Atomic Theory. How do these postulates explain the Law of Conservation of Mass?
  2. Explain the formation of a water molecule with the help of electron sharing. Why is it a covalent compound?
  3. Write the steps to write the chemical formula of an ionic compound. Give the formula of calcium chloride using these steps.
  4. Why do ionic compounds conduct electricity in aqueous solution but not in solid state? Give reason.
  5. Distinguish between molecular mass and formula unit mass with suitable examples.

Section D — Long Answer (3 questions, 5 marks each)

  1. (a) Describe the formation of a sodium chloride crystal with a labelled diagram showing the arrangement of Na⁺ and Cl⁻ ions. (3 marks)
    (b) Write the chemical formula of sodium chloride and explain why brackets are not needed. (2 marks)

  2. Verify the Law of Conservation of Mass using the reaction between 4.0 g calcium carbonate and 2.92 g hydrochloric acid that produces 1.76 g carbon dioxide, 0.72 g water and 4.44 g calcium chloride. Show full calculation. (Multi-step numerical)

  3. (a) State the Law of Constant Proportions. Explain with the example of water obtained from different sources. (2 marks)
    (b) If 12 g carbon combines with 32 g oxygen to form 44 g carbon dioxide, calculate the mass of carbon dioxide formed when 2.4 g carbon reacts completely with oxygen. Show working. (3 marks)

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1: Students performed Activity 9.2 using vinegar and baking soda in two experimental set-ups. In set-up 1, baking soda was added directly and mass decreased after reaction. In set-up 2, the balloon was tied to the flask and mass remained constant.

(i) Why did the mass decrease in set-up 1? (1 mark)
(ii) Name the gas evolved and write the word equation. (1 mark)
(iii) Which law is verified in set-up 2? State the law. (1 mark)
(iv) How does fixing the balloon prevent error in measurement? (1 mark)

Case 2: Water collected from a river, borewell and ocean was purified and analysed. It always contained hydrogen and oxygen in the mass ratio 1:8. When 9 g of this water was decomposed, 1 g hydrogen and 8 g oxygen were obtained.

(i) Which law is illustrated here? Who proposed it? (1 mark)
(ii) If 18 g of water is decomposed, what mass of oxygen will be obtained? (1 mark)
(iii) Why is this ratio constant irrespective of the source of water? (1 mark)
(iv) Name the scientist who proposed that atoms combine in simple whole-number ratios. (1 mark)

▶ Answer Key Attempt all questions first,
then tap to reveal

Section A

  1. (b)
  2. (c)
  3. (c)
  4. (b)
  5. (b)
  6. (a) — Both true, R explains A (Law of Conservation of Mass)
  7. (a) — Both true, R explains A (ions fixed in lattice)
  8. (b)
  9. (b) — (23×2) + 16 = 62 u
  10. (c)

Section B

  1. Mass neither created nor destroyed — 1 mark; example (e.g., solution formation) — 1 mark.
  2. O = atom; O₂ = molecule (two atoms) — 2 marks.
  3. Electrically neutral entity of >1 atom, independent existence — 1 mark; examples — 1 mark.
  4. Single: H–H/H–Cl (one shared pair) — 1 mark; Double: O=O (two shared pairs) — 1 mark.
  5. Ions from ≥2 elements — 1 mark; OH⁻, SO₄²⁻, NH₄⁺ (any two) — 1 mark.
  6. (1×2) + 16 = 18 u — 2 marks.

Section C

  1. Any three postulates (atoms indivisible, identical for element, different for elements, etc.) — 2 marks; link to mass conservation — 1 mark.
  2. Electron dot structure of H₂O (O shares 2 electrons with 2 H) — 2 marks; covalent because sharing — 1 mark.
  3. Steps (cation first, charges, criss-cross, simplify) — 2 marks; CaCl₂ correct — 1 mark.
  4. Ions fixed in solid lattice — 2 marks; free to move in solution — 1 mark.
  5. Molecular mass = sum for covalent molecule — 1.5 marks; formula unit mass = sum for ionic simplest ratio — 1.5 marks.

Section D

  1. (a) Labelled diagram of NaCl crystal lattice (Na⁺ surrounded by 6 Cl⁻ and vice versa) — 3 marks; (b) NaCl, no brackets needed as monoatomic ions — 2 marks.
  2. Reactants total = 6.92 g; products total = 6.92 g; equal → law obeyed (full addition shown) — 5 marks.
  3. (a) Law stated + water 1:8 ratio from any source — 2 marks; (b) 2.4 g C gives 8.8 g CO₂ (proportion 44/12 × 2.4) — 3 marks.

Section E

Case 1: (i) CO₂ escapes; (ii) Carbon dioxide, Vinegar + Baking soda → CO₂ + …; (iii) Law of Conservation of Mass; (iv) Prevents loss of traces.
Case 2: (i) Law of Constant Proportions, Proust; (ii) 16 g oxygen; (iii) Fixed ratio by mass; (iv) Dalton.

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All questions are answerable from the NCERT chapter text. Reviewed by GFIS faculty.