Atoms and Molecules — Class 9 MCQs with Answers
Class 9 CBSE Science · Chapter 8
90 practice questions · 30 Easy · 30 Medium · 30 Hard · Updated
Practise the most important Class 9 CBSE Science questions from Chapter 8, "Atoms and Molecules". You get 9 timed quizzes made from 90 NCERT-based MCQs, with answers and explanations. The questions are split into 30 Easy, 30 Medium and 30 Hard. Warm up on the basics, then move on to the exam-level questions that set top scorers in CBSE Board exams and the JEE & NEET foundation years apart.
To score well in "Atoms and Molecules", focus on clear concepts in physics, chemistry and biology basics. Each MCQ here is timed and uses exam-style marking (+4 correct, −1 wrong, 0 skipped). This trains you to stay accurate under time pressure, as real papers need. Every question has a short explanation, so a wrong answer becomes a quick lesson. It is the fastest way to fix gaps before a test.
Use this chapter for focused revision. Start with the Easy set to check your basics on Atoms and Molecules, then move to Medium and Hard to practise applying them. Your accuracy, streaks and XP save automatically. This chapter also adds to your overall Class 9 Science mastery score. 14 sample questions are solved in full below, with the answer and a worked explanation. Sign in free to start practising.
Key concepts: Atoms and Molecules (Class 9 Science)
Matter is made of atoms that combine in fixed whole-number ratios, which is what the laws of chemical combination assert. This chapter covers those laws, atomic and molecular masses, chemical formulae, and the mole as the chemist's counting unit.
- Law of conservation of mass
- In a chemical reaction mass is neither created nor destroyed, so the total mass of products equals that of the reactants.
- Law of constant proportions
- A pure compound always contains the same elements in the same proportion by mass, whatever its source or method of preparation.
- Dalton's atomic theory
- Matter is made of indivisible atoms; atoms of an element are identical, and compounds form when atoms combine in fixed whole-number ratios.
- Atom
- The smallest particle of an element that takes part in a chemical reaction; most atoms cannot exist independently.
Atoms and Molecules — important questions & MCQs with answers (Class 9 Science)
14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.
- Q1Easy
The atomic theory was proposed by:
A.Niels BohrB.RutherfordC.John Dalton✓ CorrectD.ThomsonAnswer: C. John Dalton
Explanation: John Dalton proposed the atomic theory in 1808, describing matter as made of indivisible atoms — this predates Rutherford's nucleus and Bohr's orbits by over a century.
- Q2Easy
The chemical formula of water is:
A.HOB.H₂O✓ CorrectC.H₂O₂D.OHAnswer: B. H₂O
Explanation: Water's formula combines two hydrogen atoms with one oxygen atom: H₂O — the subscript 2 applies only to hydrogen, not to both elements.
- Q3Easy
Avogadro's number is approximately:
A.9.81B.6.022 × 10⁻²³C.3.14 × 10⁸D.6.022 × 10²³✓ CorrectAnswer: D. 6.022 × 10²³
Explanation: Avogadro's number is 6.022 × 10²³ (a positive exponent, since it's a huge count of particles) — not 10⁻²³, which would be a vanishingly tiny number.
- Q4Easy
The law of constant (definite) proportions states that in a chemical compound, the elements are always combined in:
A.Variable proportions by massB.A fixed proportion by mass✓ CorrectC.Equal proportions by volumeD.Proportions depending on temperatureAnswer: B. A fixed proportion by mass
Explanation: Proust's law of definite proportions: every sample of a pure compound has its elements combined in the same fixed mass ratio, regardless of source or method of preparation.
- Q5Easy
One atomic mass unit (1 u) is defined as 1/12th the mass of an atom of:
A.Hydrogen-1B.Oxygen-16C.Carbon-12✓ CorrectD.Nitrogen-14Answer: C. Carbon-12
Explanation: 1 u is defined as exactly 1/12th the mass of one carbon-12 atom — the modern reference standard, replacing older oxygen- and hydrogen-based scales.
- Q6Easy
The mass of one mole of any element is equal to its:
A.Atomic mass in grams (molar mass)✓ CorrectB.Atomic mass in kgC.Atomic number in gramsD.Valency in gramsAnswer: A. Atomic mass in grams (molar mass)
Explanation: The molar mass of an element, in grams, is numerically equal to its atomic mass — e.g. sodium (atomic mass 23 u) has a molar mass of 23 g/mol.
- Q7Medium
The law of conservation of mass states that in a chemical reaction, mass is:
A.CreatedB.Neither created nor destroyed✓ CorrectC.DestroyedD.MultipliedAnswer: B. Neither created nor destroyed
Explanation: Lavoisier's law of conservation of mass: total mass of reactants equals total mass of products, so mass is neither created nor destroyed in a chemical reaction.
- Q8Medium
Molecular mass of CO₂ (C = 12, O = 16) is:
A.28 uB.32 uC.44 u✓ CorrectD.48 uAnswer: C. 44 u
Explanation: Molecular mass of CO₂ = 12 + 2(16) = 12 + 32 = 44 u, adding carbon's mass to TWO oxygen atoms, not one.
- Q9Medium
Number of moles in 36 g of water (M = 18 g/mol) is:
A.2✓ CorrectB.1C.0.5D.4Answer: A. 2
Explanation: Moles = mass ÷ molar mass = 36 g ÷ 18 g/mol = 2 mol — dividing, not multiplying, converts grams to moles.
- Q10Medium
Carbon combines with oxygen to form CO and CO₂. The mass ratio of oxygen in these two compounds (per fixed mass of carbon) is 1:2. This is an example of the law of:
A.Conservation of massB.Definite proportionsC.Reciprocal proportionsD.Multiple proportions✓ CorrectAnswer: D. Multiple proportions
Explanation: Dalton's law of multiple proportions applies when two elements form more than one compound: the masses of one element combining with a fixed mass of the other form a simple whole-number ratio, here 1:2.
- Q11Medium
Molecular mass of H₂SO₄ (H = 1, S = 32, O = 16) is:
A.98 u✓ CorrectB.96 uC.49 uD.100 uAnswer: A. 98 u
Explanation: Molecular mass of H₂SO₄ = 2(1) + 32 + 4(16) = 2 + 32 + 64 = 98 u, counting all four oxygen atoms in the formula, not just one.
- Q12Hard
5.6 g of Fe react with 3.2 g of S to form FeS. The mass of FeS produced (no excess) is:
A.5.6 gB.3.2 gC.11.2 gD.8.8 g✓ CorrectAnswer: D. 8.8 g
Explanation: By conservation of mass, mass of FeS = mass of Fe + mass of S = 5.6 g + 3.2 g = 8.8 g.
- Q13Hard
Molecular mass of NH₃ (N = 14, H = 1) is:
A.15 uB.18 uC.17 u✓ CorrectD.42 uAnswer: C. 17 u
Explanation: Molecular mass of NH₃ = 14 + 3(1) = 17 u, counting all three hydrogens bonded to nitrogen, not just one.
- Q14Hard
Number of atoms in 0.5 mole of carbon is:
A.3.011 × 10²³✓ CorrectB.6.022 × 10²³C.1.2044 × 10²⁴D.6.022 × 10²²Answer: A. 3.011 × 10²³
Explanation: Atoms = moles × Avogadro's number = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ — half a mole gives half of Avogadro's number, not the full 6.022 × 10²³.
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Start this chapter free →Atoms and Molecules — FAQs
What are the key concepts in Class 9 Science Atoms and Molecules?+
Matter is made of atoms that combine in fixed whole-number ratios, which is what the laws of chemical combination assert. This chapter covers those laws, atomic and molecular masses, chemical formulae, and the mole as the chemist's counting unit. Key ideas include Law of conservation of mass, Law of constant proportions, Dalton's atomic theory, Atom.
What does Class 9 Science Chapter 8 (Atoms and Molecules) cover on XamBaaz?+
It has 90 NCERT-based MCQs on "Atoms and Molecules": 30 Easy, 30 Medium and 30 Hard. Together they make 9 timed quizzes, and you never get the same set twice. Every question has an instant explanation. They help you prepare for CBSE Board exams and the JEE & NEET foundation years.
Are these "Atoms and Molecules" questions free to practise?+
Yes. Sign in with Google to practise "Atoms and Molecules" free. Full unlimited access is ₹999/year on a launch offer until 1 December 2026. No chapter is charged separately.
How should I revise "Atoms and Molecules" for the exam?+
Start with the Easy quiz to check your basics, then try Medium and Hard to practise applying them. There are 9 timed quizzes on this chapter, so you can come back for a fresh set instead of one you have seen. Read each explanation, retry the questions you miss, and track your accuracy until it stays high.
Are these "Atoms and Molecules" MCQs available with answers?+
Yes. 14 sample questions are shown here in full, each with the correct option and a step-by-step "Why" explanation. Sign in free with Google to start practising, with instant scoring.
Is there negative marking in the "Atoms and Molecules" quizzes?+
Yes. The timed quizzes use exam-style marking: +4 for a right answer, −1 for a wrong one and 0 for a skip. MHT-CET and CBSE board papers have no negative marking. Our mocks for those are scored their way.
What are the important questions from Atoms and Molecules (Class 9 Science)?+
The questions that matter most test Law of conservation of mass, Law of constant proportions, Dalton's atomic theory, Atom. This page shows 14 solved important MCQs with answers and explanations; all 90 questions on the chapter are available as timed quizzes once you sign in.
Is there an online quiz for Atoms and Molecules?+
Yes — Class 9 Science Atoms and Molecules has timed online quizzes at Easy, Medium and Hard levels, with instant scoring and a worked explanation on every question. The first quiz on the chapter is free.