Chapter 9 is Atomic Foundations of Matter. It covers the laws of chemical combination, Dalton’s atomic theory, ionic and covalent compounds, chemical formulae, and molecular and formula unit masses.
Welcome to Science World By Tushar Sir! Practise these 50 solved Class 9 Science Chapter 9 MCQs, Atomic Foundations of Matter, based on the NCERT Exploration textbook. Each question includes four options, the correct answer, and a simple explanation to help you understand important concepts and prepare for school examinations.
50 MCQs with answers and explanations
Section A: Laws of Chemical Combination
Q1. Which law states that mass can neither be created nor destroyed during a chemical reaction?
A. Law of Constant Proportion
B. Law of Conservation of Mass
C. Law of Multiple Proportions
D. Law of Gravitation
Correct Answer: B. Law of Conservation of Mass
Explanation: In a chemical reaction, the total mass of the reactants equals the total mass of the products.
Q2. According to the law of constant proportion, a pure chemical compound always contains:
A. The same elements in a fixed proportion by mass
B. Different elements in any proportion
C. The same elements in equal masses
D. Only one type of atom
Correct Answer: A. The same elements in a fixed proportion by mass
Explanation: A pure compound has a definite composition by mass, regardless of its source or method of preparation.
Q3. In a chemical reaction, 12 g of carbon reacts completely with 32 g of oxygen. What mass of carbon dioxide is formed?
A. 20 g
B. 32 g
C. 44 g
D. 48 g
Correct Answer: C. 44 g
Explanation: By conservation of mass, mass of carbon dioxide = 12 + 32 = 44 g.
Q4. Water always contains hydrogen and oxygen in which mass ratio?
A. 1:1
B. 1:2
C. 2:1
D. 1:8
Correct Answer: D. 1:8
Explanation: Water, H₂O, contains 2 g of hydrogen for every 16 g of oxygen, giving a mass ratio of 1:8.
Q5. Which scientist proposed the atomic theory to explain the laws of chemical combination?
A. John Dalton
B. Ernest Rutherford
C. Niels Bohr
D. J. J. Thomson
Correct Answer: A. John Dalton
Explanation: Dalton proposed that matter is made of atoms and used this idea to explain chemical combination.
Q6. If 10 g of a substance reacts with 15 g of another substance to form a single product, what is the mass of the product if the reaction is complete and no other products form?
A. 5 g
B. 15 g
C. 25 g
D. 150 g
Correct Answer: C. 25 g
Explanation: Total mass of the reactants is 10 + 15 = 25 g.
Q7. Which observation supports the law of constant proportion?
A. Water from different sources has the same hydrogen-to-oxygen mass ratio
B. Water evaporates when heated
C. Ice floats on water
D. Water can dissolve salt
Correct Answer: A. Water from different sources has the same hydrogen-to-oxygen mass ratio
Explanation: The composition of pure water remains constant regardless of its source.
Q8. If 24 g of magnesium combines completely with 16 g of oxygen, what is the total mass of magnesium oxide formed?
A. 8 g
B. 24 g
C. 32 g
D. 40 g
Correct Answer: D. 40 g
Explanation: Total mass = 24 + 16 = 40 g, assuming complete reaction with no other products.
Q9. Which statement best describes the law of conservation of mass?
A. Atoms disappear during reactions
B. Total mass remains constant in a closed system during a chemical reaction
C. Products always have less mass than reactants
D. Chemical reactions always increase mass
Correct Answer: B. Total mass remains constant in a closed system during a chemical reaction
Explanation: Chemical reactions rearrange atoms; they do not destroy or create ordinary atoms.
Q10. A compound contains 3 g of carbon and 8 g of oxygen. What is the mass ratio of carbon to oxygen?
A. 3:8
B. 8:3
C. 3:11
D. 1:3
Correct Answer: A. 3:8
Explanation: The mass ratio is directly obtained from the given masses: 3:8.
Section B: Dalton’s Atomic Theory
Q11. According to Dalton’s atomic theory, all matter is made up of tiny particles called:
A. Cells
B. Molecules only
C. Atoms
D. Ions only
Correct Answer: C. Atoms
Explanation: Dalton proposed that atoms are the fundamental particles of matter involved in chemical reactions.
Q12. According to Dalton’s original theory, atoms of the same element were considered to have:
A. Different chemical identities
B. The same mass and properties
C. No mass
D. Different numbers of protons
Correct Answer: B. The same mass and properties
Explanation: This was a key postulate of Dalton’s original model. The discovery of isotopes later showed that atoms of the same element can have different masses.
Q13. Which discovery showed that atoms are divisible into smaller subatomic particles?
A. Discovery of the electron
B. Discovery of the water cycle
C. Discovery of photosynthesis
D. Discovery of sound waves
Correct Answer: A. Discovery of the electron
Explanation: The discovery of electrons demonstrated that atoms contain smaller particles.
Q14. In a chemical reaction, atoms are generally:
A. Converted entirely into energy
B. Destroyed
C. Created from nothing
D. Rearranged to form new substances
Correct Answer: D. Rearranged to form new substances
Explanation: Chemical reactions involve the rearrangement of atoms and changes in chemical bonds.
Q15. Which statement is consistent with Dalton’s explanation of compounds?
A. Compounds contain elements in random proportions
B. Compounds contain atoms of different elements combined in simple whole-number ratios
C. Compounds contain only one element
D. Compounds cannot be broken down chemically
Correct Answer: B. Compounds contain atoms of different elements combined in simple whole-number ratios
Explanation: Dalton’s theory explained that atoms combine in simple numerical ratios to form compounds.
Section C: Atoms, Molecules and Chemical Formulae
Q16. What is the chemical formula of water?
A. HO
B. H₂O
C. H₂O₂
D. OH₂₂
Correct Answer: B. H₂O
Explanation: One water molecule contains two hydrogen atoms and one oxygen atom.
Q17. What does the formula CO₂ represent?
A. One carbon atom and one oxygen atom
B. Two carbon atoms and one oxygen atom
C. One carbon atom and two oxygen atoms
D. Two carbon atoms and two oxygen atoms
Correct Answer: C. One carbon atom and two oxygen atoms
Explanation: The subscript 2 applies to oxygen, indicating two oxygen atoms per molecule.
Q18. Which of the following is a diatomic molecule of an element?
A. H₂
B. H₂O
C. CO₂
D. NH₃
Correct Answer: A. H₂
Explanation: A hydrogen molecule consists of two hydrogen atoms bonded together.
Q19. How many atoms are present in one molecule of ammonia, NH₃?
A. 2
B. 3
C. 5
D. 4
Correct Answer: D. 4
Explanation: NH₃ contains one nitrogen atom and three hydrogen atoms, making four atoms in total.
Q20. What is the chemical formula of sodium chloride?
A. Na₂Cl
B. NaCl
C. NaCl₂
D. Na₂Cl₂
Correct Answer: B. NaCl
Explanation: Sodium forms Na⁺ ions and chlorine forms Cl⁻ ions. They combine in a 1:1 ratio to produce an electrically neutral compound.
Q21. What is the formula of magnesium oxide?
A. MgO
B. Mg₂O
C. MgO₂
D. Mg₂O₃
Correct Answer: A. MgO
Explanation: Magnesium forms Mg²⁺ and oxygen forms O²⁻. One of each ion balances the charges.
Q22. Which formula represents calcium chloride?
A. CaCl
B. Ca₂Cl
C. CaCl₂
D. Ca₂Cl₃
Correct Answer: C. CaCl₂
Explanation: Ca²⁺ requires two Cl⁻ ions to balance its charge.
Q23. What is the chemical formula of aluminium oxide?
A. AlO
B. AlO₂
C. Al₂O
D. Al₂O₃
Correct Answer: D. Al₂O₃
Explanation: Two Al³⁺ ions provide a total charge of +6, balanced by three O²⁻ ions with a total charge of −6.
Q24. Which of the following is a polyatomic ion?
A. Na⁺
B. SO₄²⁻
C. Cl⁻
D. Mg²⁺
Correct Answer: B. SO₄²⁻
Explanation: The sulphate ion contains several atoms bonded together and carries an overall charge of −2.
Q25. What is the formula of calcium hydroxide?
A. CaOH
B. Ca₂OH
C. Ca(OH)₂
D. CaOH₂
Correct Answer: C. Ca(OH)₂
Explanation: Calcium has a +2 charge and hydroxide has a −1 charge. Two hydroxide ions are needed for each calcium ion.
Q26. What is the formula of ammonium sulphate?
A. NH₄SO₄
B. (NH₄)SO₄
C. NH₄(SO₄)₂
D. (NH₄)₂SO₄
Correct Answer: D. (NH₄)₂SO₄
Explanation: Two NH₄⁺ ions balance the −2 charge of one SO₄²⁻ ion.
Q27. Which of the following is a covalent compound?
A. Sodium chloride
B. Magnesium oxide
C. Carbon dioxide
D. Calcium chloride
Correct Answer: C. Carbon dioxide
Explanation: Carbon and oxygen atoms share electrons to form covalent bonds in CO₂.
Q28. Which type of bond generally forms through the transfer of electrons from one atom to another?
A. Ionic bond
B. Covalent bond
C. Metallic bond only
D. Hydrogen bond only
Correct Answer: A. Ionic bond
Explanation: Electron transfer forms oppositely charged ions that attract each other electrostatically.
Q29. A covalent bond is formed when atoms:
A. Lose all their protons
B. Share electron pairs
C. Lose their nuclei
D. Destroy their electrons
Correct Answer: B. Share electron pairs
Explanation: Covalent bonding involves sharing one or more pairs of electrons between atoms.
Q30. Which formula represents a molecule containing two nitrogen atoms?
A. NO₂
B. N₂O
C. NH₃
D. N₂
Correct Answer: D. N₂
Explanation: The subscript 2 in N₂ indicates that the molecule contains two nitrogen atoms.
Section D: Valency and Writing Chemical Formulae
Q31. What is the usual valency of oxygen in simple compounds?
A. 1
B. 2
C. 3
D. 4
Correct Answer: B. 2
Explanation: Oxygen usually forms two bonds or has a combining capacity of two in common compounds.
Q32. What is the usual valency of sodium?
A. 1
B. 2
C. 3
D. 4
Correct Answer: A. 1
Explanation: Sodium generally loses one electron to form Na⁺.
Q33. What is the usual valency of aluminium?
A. 1
B. 2
C. 3
D. 4
Correct Answer: C. 3
Explanation: Aluminium commonly forms Al³⁺ by losing three electrons.
Q34. Which formula is obtained by combining Mg²⁺ and Cl⁻ ions?
A. MgCl
B. Mg₂Cl
C. MgCl₃
D. MgCl₂
Correct Answer: D. MgCl₂
Explanation: Two chloride ions are required to balance the +2 charge of one magnesium ion.
Q35. Which is the correct formula of sodium carbonate?
A. NaCO₃
B. Na₂CO₃
C. Na(CO₃)₂
D. Na₃CO₂
Correct Answer: B. Na₂CO₃
Explanation: Two Na⁺ ions balance the −2 charge of one carbonate ion, CO₃²⁻.
Q36. Which is the correct formula of potassium sulphate?
A. KSO₄
B. K₂SO₃
C. K₂SO₄
D. KSO₃
Correct Answer: C. K₂SO₄
Explanation: Two K⁺ ions balance the −2 charge of one sulphate ion.
Q37. What is the formula of aluminium chloride?
A. AlCl₃
B. AlCl
C. Al₂Cl
D. AlCl₂
Correct Answer: A. AlCl₃
Explanation: One Al³⁺ ion combines with three Cl⁻ ions to give a neutral compound.
Q38. Which formula correctly represents magnesium hydroxide?
A. MgOH
B. Mg₂OH
C. MgOH₂
D. Mg(OH)₂
Correct Answer: D. Mg(OH)₂
Explanation: One Mg²⁺ ion requires two OH⁻ ions. Brackets indicate that the hydroxide group occurs twice.
Q39. What is the correct formula of calcium phosphate?
A. CaPO₄
B. Ca₃(PO₄)₂
C. Ca₂PO₄
D. Ca(PO₄)₃
Correct Answer: B. Ca₃(PO₄)₂
Explanation: Three Ca²⁺ ions give +6, while two PO₄³⁻ ions give −6, producing a neutral compound.
Q40. Why are brackets used in the formula Mg(OH)₂?
A. To indicate a mixture
B. To show that magnesium is optional
C. To indicate that the hydroxide group occurs twice
D. To show that oxygen is absent
Correct Answer: C. To indicate that the hydroxide group occurs twice
Explanation: The subscript 2 outside the brackets applies to both oxygen and hydrogen in the OH group.
Section E: Atomic Mass, Molecular Mass and Application Questions
Q41. What is the approximate atomic mass of carbon?
A. 12 u
B. 1 u
C. 14 u
D. 16 u
Correct Answer: A. 12 u
Explanation: The relative atomic mass commonly used for carbon in Class 9 calculations is approximately 12 u.
Q42. What is the molecular mass of water (H₂O)? Use H = 1 u and O = 16 u.
A. 16 u
B. 17 u
C. 20 u
D. 18 u
Correct Answer: D. 18 u
Explanation: Molecular mass = \(2(1) + 16 = 18\text{ u}\).
Q43. Calculate the molecular mass of carbon dioxide (CO₂). Use C = 12 u and O = 16 u.
A. 28 u
B. 44 u
C. 32 u
D. 48 u
Correct Answer: B. 44 u
Explanation: Molecular mass = \(12 + 2(16) = 44\text{ u}\).
Q44. What is the molecular mass of ammonia (NH₃)? Use N = 14 u and H = 1 u.
A. 14 u
B. 16 u
C. 17 u
D. 18 u
Correct Answer: C. 17 u
Explanation: Molecular mass = \(14 + 3(1) = 17\text{ u}\).
Q45. Calculate the formula unit mass of sodium chloride (NaCl). Use Na = 23 u and Cl = 35.5 u.
A. 58.5 u
B. 46 u
C. 35.5 u
D. 81.5 u
Correct Answer: A. 58.5 u
Explanation: Formula unit mass = \(23 + 35.5 = 58.5\text{ u}\).
Q46. What is the molecular mass of methane (CH₄)? Use C = 12 u and H = 1 u.
A. 12 u
B. 14 u
C. 18 u
D. 16 u
Correct Answer: D. 16 u
Explanation: Molecular mass = \(12 + 4(1) = 16\text{ u}\).
Q47. Calculate the formula unit mass of magnesium oxide (MgO). Use Mg = 24 u and O = 16 u.
A. 24 u
B. 40 u
C. 32 u
D. 48 u
Correct Answer: B. 40 u
Explanation: Formula unit mass = \(24 + 16 = 40\text{ u}\).
Q48. How many oxygen atoms are present in three molecules of CO₂?
A. 3
B. 4
C. 6
D. 9
Correct Answer: C. 6
Explanation: Each CO₂ molecule contains two oxygen atoms. Therefore, \(3 \times 2 = 6\) oxygen atoms.
Q49. A sample of a pure compound contains 4 g of hydrogen and 32 g of oxygen. What is the hydrogen-to-oxygen mass ratio?
A. 1:8
B. 1:4
C. 2:1
D. 4:32
Correct Answer: A. 1:8
Explanation: Dividing both masses by 4 gives the simplest ratio, 1:8.
Q50. A compound has a formula X₂Y₃. If the atomic mass of X is 10 u and that of Y is 16 u, what is its formula mass?
A. 48 u
B. 58 u
C. 64 u
D. 68 u
Correct Answer: D. 68 u
Disclaimer: These are independently prepared practice MCQs for educational purposes by Science World By Tushar Sir. They are not official NCERT questions. NCERT and its textbook titles are referenced solely to identify the relevant curriculum.