🧪 Atoms and Molecules – MCQs for Class 9 (NCERT)

Section 1: Laws of Chemical Combination & Dalton’s Atomic Theory (Q1–Q25)

Q1. Which law states that in a chemical reaction, the mass of reactants equals the mass of products?

A. Law of Constant Proportion
B. Law of Conservation of Mass
C. Dalton’s Atomic Theory
D. Avogadro’s Law

✅ Correct Answer: B. Law of Conservation of Mass

Explanation:

• A. Law of Constant Proportion → Wrong, it states that elements combine in a fixed ratio.
• B. Law of Conservation of Mass → Correct, no mass is lost or gained during a chemical reaction (Lavoisier).
• C. Dalton’s Atomic Theory → Wrong, it explains the nature of atoms, not conservation of mass.
• D. Avogadro’s Law → Wrong, it relates volume and number of molecules at constant T & P.

Q2. The Law of Constant Proportion was given by:

A. Lavoisier
B. Proust
C. Dalton
D. Avogadro

✅ Correct Answer: B. Proust

Explanation:

• A. Lavoisier → Wrong, he gave Law of Conservation of Mass.
• B. Proust → Correct, he stated that elements combine in definite proportion by mass.
• C. Dalton → Wrong, proposed atomic theory.
• D. Avogadro → Wrong, gave Avogadro’s law.

Q3. Which statement is NOT part of Dalton’s atomic theory?

A. Atoms are indivisible.
B. Atoms of a given element are identical.
C. Atoms can be created or destroyed in chemical reactions.
D. Atoms combine in simple whole-number ratios.

✅ Correct Answer: C. Atoms can be created or destroyed in chemical reactions.

Explanation:

• A. Atoms are indivisible → Correct statement of Dalton’s theory.
• B. Atoms of a given element are identical → Correct, as per Dalton.
• C. Atoms can be created or destroyed → Wrong, Dalton said they cannot be created or destroyed.
• D. Atoms combine in whole numbers → Correct, fundamental idea of molecules.

Q4. Which scientist is known as the ‘Father of Modern Chemistry’?

A. Dalton
B. Rutherford
C. Lavoisier
D. Thomson

✅ Correct Answer: C. Lavoisier

Explanation:

• A. Dalton → Wrong, father of atomic theory.
• B. Rutherford → Wrong, discovered nucleus.
• C. Lavoisier → Correct, father of modern chemistry for Law of Conservation of Mass.
• D. Thomson → Wrong, discovered electron.

Q5. In CO₂, the ratio of mass of carbon to oxygen is always 3:8. This proves which law?

A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportions
D. Avogadro’s Law

✅ Correct Answer: B. Law of Constant Proportion

Explanation:

• A. Conservation of Mass → Wrong, deals with total mass, not ratio.
• B. Constant Proportion → Correct, CO₂ always has 3:8 ratio.
• C. Multiple Proportions → Wrong, deals with same elements forming different compounds.
• D. Avogadro’s Law → Wrong, relates volumes and molecules.

Q6. Water from different sources like river, rain, well shows the same composition (H₂O). This proves:

A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportions
D. Dalton’s Atomic Theory

✅ Correct Answer: B. Law of Constant Proportion

Explanation:

• A. Conservation of Mass → Wrong, deals with mass balance.
• B. Constant Proportion → Correct, H and O always combine in 1:8 mass ratio.
• C. Multiple Proportions → Wrong, deals with CO and CO₂ type cases.
• D. Dalton’s Theory → Wrong, general theory, not specific to water.

Q7. Which of the following is true according to Dalton’s atomic theory?

A. Atoms can be subdivided.
B. Atoms of the same element have different masses.
C. Atoms combine in simple ratios to form compounds.
D. Atoms can be created by chemical reactions.

✅ Correct Answer: C. Atoms combine in simple ratios to form compounds.

Explanation:

• A. Atoms subdivided → Wrong, Dalton said indivisible.
• B. Different masses → Wrong, Dalton said same mass.
• C. Simple ratios → Correct, molecules are formed this way.
• D. Created in reactions → Wrong, atoms can’t be created.

Q8. When two elements form two or more compounds, the masses of one element combine with a fixed mass of another in small whole number ratios. This is:

A. Law of Constant Proportion
B. Law of Multiple Proportions
C. Law of Conservation of Mass
D. Dalton’s Atomic Theory

✅ Correct Answer: B. Law of Multiple Proportions

Explanation:

• A. Constant Proportion → Wrong, fixed ratio, not multiple.
• B. Multiple Proportions → Correct, e.g., CO and CO₂.
• C. Conservation of Mass → Wrong, unrelated.
• D. Dalton’s Theory → Wrong, this is a specific law.

Q9. Which of the following is NOT a postulate of Dalton’s atomic theory?

A. Atoms are indivisible.
B. Atoms of different elements are different.
C. Atoms can be destroyed by chemical reaction.
D. Compounds are formed by combination of atoms.

✅ Correct Answer: C. Atoms can be destroyed by chemical reaction.

Explanation:

• A. Indivisible atoms → Dalton’s statement.
• B. Different atoms differ → Dalton’s statement.
• C. Destroyed by chemical reaction → Wrong, Dalton opposed this.
• D. Compounds are formed by atoms → Dalton’s statement.

Q10. Which of the following laws is not explained by Dalton’s atomic theory?

A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportions
D. Law of Gravitation

✅ Correct Answer: D. Law of Gravitation

Explanation:

• A, B, C → All are explained by Dalton’s theory.
• D. Gravitation → Not related to chemistry, hence incorrect.

Q11. The mass of 1 atom of oxygen (atomic mass = 16 u) is:

A. 16 g
B. 16 u
C. 16×1.66×10−24 16 \times 1.66 \times 10^{-24} g
D. 16×6.022×1023 16 \times 6.022 \times 10^{23} g

✅ Correct Answer: C. 16×1.66×10−24 16 \times 1.66 \times 10^{-24} g

Explanation:

• A. 16 g → Wrong, this is 1 mole, not 1 atom.
• B. 16 u → Wrong, u is a relative unit, not actual mass in grams.
• C. Correct, since 1 u = 1.66×10−241.66 \times 10^{-24} g. So, 16×1.66×10−2416 \times 1.66 \times 10^{-24} g.
• D. 16×6.022×102316 \times 6.022 \times 10^{23} g → Wrong, this is absurdly large.

Q12. Which law can be verified by the reaction of hydrogen and oxygen forming water?

A. Law of Multiple Proportions
B. Law of Constant Proportion
C. Dalton’s Atomic Theory
D. Avogadro’s Law

✅ Correct Answer: B. Law of Constant Proportion

Explanation:

• A. Multiple Proportions → Wrong, needs two compounds of same elements.
• B. Constant Proportion → Correct, H₂O always has H:O = 1:8 by mass.
• C. Dalton’s Theory → Wrong, general theory not verification.
• D. Avogadro’s Law → Wrong, deals with gases, not composition.

Q13. Which of the following is a limitation of Dalton’s atomic theory?

A. Atoms are indivisible.
B. Atoms combine in whole number ratios.
C. Atoms of same element have identical mass.
D. It does not explain isotopes.

✅ Correct Answer: D. It does not explain isotopes.

Explanation:

• A. Indivisible → Postulate, not limitation.
• B. Whole number ratios → Postulate, not limitation.
• C. Identical mass → Postulate, but limited.
• D. Correct, Dalton’s theory could not explain isotopes (same element, different mass).

Q14. Which of the following pairs shows law of multiple proportions?

A. CO and CO₂
B. H₂O and H₂S
C. NaCl and KCl
D. HCl and NaCl

✅ Correct Answer: A. CO and CO₂

Explanation:

• A. CO & CO₂ → Correct, same elements (C and O) combine in ratios 12:16 and 12:32.
• B. H₂O & H₂S → Wrong, different elements (O and S).
• C. NaCl & KCl → Wrong, different elements (Na and K).
• D. HCl & NaCl → Wrong, different compounds altogether.

Q15. Which of the following is correct regarding Dalton’s theory?

A. It explains isotopes and isobars.
B. It explains Law of Gravitation.
C. It explains Law of Constant Proportion.
D. It explains Law of Inertia.

✅ Correct Answer: C. It explains Law of Constant Proportion

Explanation:

• A. Isotopes & isobars → Wrong, limitation of Dalton’s theory.
• B. Gravitation → Wrong, unrelated.
• C. Constant Proportion → Correct, atoms combine in fixed ratios.
• D. Inertia → Wrong, related to Newton’s laws.

Q16. Which postulate of Dalton’s theory is false today?

A. Atoms are indivisible.
B. Atoms of an element are identical.
C. Atoms combine in whole numbers.
D. Atoms of different elements are different.

✅ Correct Answer: A. Atoms are indivisible.

Explanation:

• A. Indivisible → Wrong today; atoms divisible into protons, neutrons, electrons.
• B. Identical atoms → Still mostly correct (except isotopes).
• C. Whole numbers → Still correct.
• D. Different atoms differ → Correct.

Q17. If 9 g water is decomposed, it gives 1 g H₂ and 8 g O₂. Which law is verified?

A. Law of Conservation of Mass
B. Law of Constant Proportion
C. Law of Multiple Proportion
D. Avogadro’s Law

✅ Correct Answer: A. Law of Conservation of Mass

Explanation:

• A. Conservation of Mass → Correct, total mass of products = 9 g (same as reactant).
• B. Constant Proportion → Wrong, deals with fixed ratio (H:O = 1:8).
• C. Multiple Proportion → Wrong, not two compounds.
• D. Avogadro’s Law → Wrong.

Q18. Which of the following is NOT explained by Dalton’s theory?

A. Conservation of Mass
B. Constant Proportion
C. Formation of isotopes
D. Multiple Proportion

✅ Correct Answer: C. Formation of isotopes

Explanation:

• A. Conservation of Mass → Explained by indivisible atoms.
• B. Constant Proportion → Explained.
• C. Isotopes → Correct, limitation.
• D. Multiple Proportion → Explained.

Q19. The ratio of mass of nitrogen to oxygen in NO and NO₂ is 14:16 and 14:32. Which law is verified?

A. Conservation of Mass
B. Constant Proportion
C. Multiple Proportion
D. Avogadro’s Law

✅ Correct Answer: C. Multiple Proportion

Explanation:

• A. Conservation → Wrong, mass balance not shown here.
• B. Constant Proportion → Wrong, ratio changes.
• C. Multiple Proportion → Correct, small whole number ratio (1:2).
• D. Avogadro’s Law → Wrong.

Q20. Dalton proposed that atoms of different elements differ in:

A. Size and shape only
B. Mass only
C. Mass and chemical properties
D. Mass and volume

✅ Correct Answer: C. Mass and chemical properties

Explanation:

• A. Size & shape → Wrong, not emphasized by Dalton.
• B. Mass only → Wrong, also chemical properties differ.
• C. Correct, both mass & properties differ.
• D. Mass & volume → Wrong, volume not in Dalton’s theory.

Q21. Which law is supported by chemical analysis of compounds from different sources showing same composition?

A. Conservation of Mass
B. Constant Proportion
C. Multiple Proportion
D. Isotopic Law

✅ Correct Answer: B. Constant Proportion

Explanation:

• A. Conservation → Wrong, deals with mass balance.
• B. Constant Proportion → Correct, same compound = same ratio.
• C. Multiple Proportion → Wrong, needs multiple compounds.
• D. Isotopic Law → Wrong, no such law.

Q22. Which is correct according to Dalton’s theory?

A. Atoms are divisible.
B. Atoms of an element can have different masses.
C. Compounds are formed by atoms combining in fixed ratio.
D. Atoms can be destroyed in chemical reactions.

✅ Correct Answer: C. Compounds are formed by atoms combining in fixed ratio

Explanation:

• A. Divisible → Wrong.
• B. Different masses → Wrong, though isotopes exist.
• C. Correct, fixed ratio = compounds.
• D. Destroyed → Wrong.

Q23. Which law is demonstrated when 100 g CaCO₃ decomposes to give 44 g CO₂ and 56 g CaO?

A. Conservation of Mass
B. Constant Proportion
C. Multiple Proportion
D. Avogadro’s Law

✅ Correct Answer: A. Conservation of Mass

Explanation:

• A. Conservation of Mass → Correct, 100 g reactant = 100 g product.
• B. Constant Proportion → Wrong.
• C. Multiple Proportion → Wrong.
• D. Avogadro’s Law → Wrong.

Q24. Dalton’s theory failed to explain:

A. Atoms of elements are identical
B. Atoms combine in simple ratios
C. The existence of isotopes
D. The conservation of mass

✅ Correct Answer: C. The existence of isotopes

Explanation:

• A. Identical atoms → Dalton’s postulate.
• B. Simple ratios → Postulate.
• C. Correct, isotopes not explained.
• D. Conservation → Explained.

Q25. Which scientist formulated the Law of Multiple Proportions?

A. Dalton
B. Lavoisier
C. Proust
D. Avogadro

✅ Correct Answer: A. Dalton

Explanation:

• A. Dalton → Correct, gave Multiple Proportions.
• B. Lavoisier → Wrong, gave Conservation of Mass.
• C. Proust → Wrong, gave Constant Proportion.
• D. Avogadro → Wrong, gave Avogadro’s Law.

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