CBSE Class 9 Science Chapter 9: Atomic Foundations of Matter — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 9: Atomic Foundations of Matter from the NCERT Class 9 Science textbook. Each question has four options - read both the Assertion (A) and Reason (R) carefully before selecting your answer.
How to attempt Assertion & Reason questions:
(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.
(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.
Assertion & Reason Questions
1
Assertion (A): Mass is conserved during a chemical reaction carried out in a closed system.
Reason (R): A chemical reaction rearranges atoms without creating or destroying them.
Reason (R): A chemical reaction rearranges atoms without creating or destroying them.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Because every atom present before the reaction remains present afterward, the total mass remains unchanged.
2
Assertion (A): The law of conservation of mass applies to both physical and chemical changes.
Reason (R): A compound always contains its constituent elements in a fixed ratio by mass.
Reason (R): A compound always contains its constituent elements in a fixed ratio by mass.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. The reason states the law of definite proportions; it does not explain why mass is conserved during changes.
3
Assertion (A): Dalton proposed that atoms of different elements combine in simple whole-number ratios to form compounds.
Reason (R): Dalton proposed that atoms of different elements always have identical masses and properties.
Reason (R): Dalton proposed that atoms of different elements always have identical masses and properties.
Answer: (c)
A is true, but R is false. Dalton distinguished the atoms of different elements by their masses and properties.
4
Assertion (A): An ionic compound such as sodium chloride exists as separate, electrically neutral NaCl molecules in its crystal.
Reason (R): The formula of an ionic compound represents the simplest whole-number ratio of its ions.
Reason (R): The formula of an ionic compound represents the simplest whole-number ratio of its ions.
Answer: (d)
A is false, but R is true. An ionic crystal is a three-dimensional lattice of ions rather than a collection of discrete molecules.
5
Assertion (A): Two non-metal atoms can achieve stable outer shells by forming a covalent bond.
Reason (R): A covalent bond forms when atoms share one or more pairs of electrons.
Reason (R): A covalent bond forms when atoms share one or more pairs of electrons.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Sharing allows the bonded atoms to complete their valence shells without producing oppositely charged ions.
6
Assertion (A): Both an oxygen molecule and a carbon dioxide molecule are electrically neutral entities.
Reason (R): An oxygen molecule contains one kind of atom, whereas carbon dioxide contains atoms of different elements.
Reason (R): An oxygen molecule contains one kind of atom, whereas carbon dioxide contains atoms of different elements.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. The reason distinguishes molecules of elements from molecules of compounds, but it does not explain their electrical neutrality.
7
Assertion (A): The formula CO2 represents one carbon atom combined with two oxygen atoms in each molecule.
Reason (R): The subscript 2 in CO2 applies equally to carbon and oxygen.
Reason (R): The subscript 2 in CO2 applies equally to carbon and oxygen.
Answer: (c)
A is true, but R is false. A missing subscript after C means one carbon atom; the written subscript applies only to O.
8
Assertion (A): Sodium chloride is formed when sodium and chlorine share a pair of electrons equally.
Reason (R): Transfer of an electron from sodium to chlorine produces oppositely charged ions that attract each other.
Reason (R): Transfer of an electron from sodium to chlorine produces oppositely charged ions that attract each other.
Answer: (d)
A is false, but R is true. Sodium chloride is ionic and forms through electron transfer, not equal sharing.
9
Assertion (A): The formula of magnesium chloride is MgCl2.
Reason (R): One Mg2+ ion requires two Cl- ions to make the compound electrically neutral.
Reason (R): One Mg2+ ion requires two Cl- ions to make the compound electrically neutral.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The 2+ and two 1- charges balance, giving the simplest ratio 1:2.
10
Assertion (A): Solid sodium chloride does not conduct electricity, whereas molten sodium chloride does.
Reason (R): Ionic compounds generally have high melting points because of strong inter-ionic attractions.
Reason (R): Ionic compounds generally have high melting points because of strong inter-ionic attractions.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the conductivity difference is caused by ion mobility, not by the high melting point.
11
Assertion (A): An ionic compound can conduct electricity in its molten state.
Reason (R): In the molten state, its ions remain fixed at their lattice positions.
Reason (R): In the molten state, its ions remain fixed at their lattice positions.
Answer: (c)
A is true, but R is false. Melting frees the ions to move and carry electric current.
12
Assertion (A): Covalent compounds generally have higher melting and boiling points than ionic compounds.
Reason (R): The attractions between particles of many covalent compounds are weaker than the inter-ionic forces in ionic solids.
Reason (R): The attractions between particles of many covalent compounds are weaker than the inter-ionic forces in ionic solids.
Answer: (d)
A is false, but R is true. Ionic compounds generally have the higher melting and boiling points because their ions are strongly attracted.
13
Assertion (A): A sugar solution does not conduct electricity even though sugar dissolves in water.
Reason (R): Sugar is a covalent compound and does not produce mobile ions when it dissolves.
Reason (R): Sugar is a covalent compound and does not produce mobile ions when it dissolves.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Dissolving alone is insufficient for conduction; charged particles must be available to move.
14
Assertion (A): Camphor and naphthalene are generally insoluble in water but dissolve in solvents such as kerosene.
Reason (R): Covalent compounds usually have lower melting and boiling points than ionic compounds.
Reason (R): Covalent compounds usually have lower melting and boiling points than ionic compounds.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. The reason gives another common covalent property but does not explain the observed solubility pattern.
15
Assertion (A): The formula unit mass of potassium chloride is 74.5 u when K = 39 u and Cl = 35.5 u.
Reason (R): It is calculated by adding the mass of one potassium atom to twice the mass of one chlorine atom.
Reason (R): It is calculated by adding the mass of one potassium atom to twice the mass of one chlorine atom.
Answer: (c)
A is true, but R is false. KCl has a 1:1 ratio, so 39 + 35.5 = 74.5 u; chlorine is not counted twice.
16
Assertion (A): Molecular mass is the appropriate term for the mass of a sodium chloride molecule.
Reason (R): Ionic compounds form extended crystal lattices and are described using formula units.
Reason (R): Ionic compounds form extended crystal lattices and are described using formula units.
Answer: (d)
A is false, but R is true. For NaCl, formula unit mass is used because discrete NaCl molecules are not present in the ionic crystal.
17
Assertion (A): Atoms in a molecule are held together by chemical bonds.
Reason (R): Bond formation can help atoms reach more stable valence-shell arrangements.
Reason (R): Bond formation can help atoms reach more stable valence-shell arrangements.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The attractive force of a chemical bond maintains the combination associated with the more stable arrangement.
18
Assertion (A): The formula of an ionic compound gives the simplest whole-number ratio of its ions.
Reason (R): The formula of a covalent compound shows the number of atoms of each element in a molecule.
Reason (R): The formula of a covalent compound shows the number of atoms of each element in a molecule.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both formula statements are correct, but the description of a covalent formula does not explain the ionic formula convention.
19
Assertion (A): The molecular mass of water is 18 u when H = 1 u and O = 16 u.
Reason (R): The value is obtained because one water molecule contains one hydrogen atom and one oxygen atom.
Reason (R): The value is obtained because one water molecule contains one hydrogen atom and one oxygen atom.
Answer: (c)
A is true, but R is false. Water is H2O, so its mass is (2 x 1) + 16 = 18 u.
20
Assertion (A): A typical ionic bond between two non-metals is produced by equal sharing of electrons.
Reason (R): Covalent bonds commonly form between non-metal atoms by electron sharing.
Reason (R): Covalent bonds commonly form between non-metal atoms by electron sharing.
Answer: (d)
A is false, but R is true. Equal sharing between non-metals describes covalent bonding, not ionic bonding.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 9 Atomic Foundations of Matter?
This chapter covers all key topics from Atomic Foundations of Matter as per CBSE 2026-27 syllabus.
Is this Assertion & Reason useful for CBSE board exams?
Yes, designed for CBSE Class 9 board exam preparation covering the complete syllabus.
Are these CBSE Class 9 Science Assertion & Reason updated for 2026-27?
Yes, all content at eBookPublisher is updated as per the latest 2026-27 CBSE syllabus.
How many chapters are in CBSE Class 9 Science?
All chapters of CBSE Class 9 Science are covered at eBookPublisher with free Assertion & Reason for each chapter.
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