The following 20 Multiple Choice Questions (MCQ) are based on
Chapter 9: Atomic Foundations of Matter from the NCERT Class 9
Science textbook Exploration. Each question has four options - select the
best answer and click to check.
Instructions: Read each question carefully. Choose the most
appropriate option from (a), (b), (c), or (d). Click
Show Answer to reveal the correct answer with explanation.
Multiple Choice Questions
1The law of conservation of mass states that in a closed system mass:
(a) Is created during reactions
(b) Is destroyed during reactions
(c) Remains constant during physical and chemical changes
(d) Always doubles
(c) is correct
The total mass of reactants equals the total mass of products in a closed system.
2If 12 g carbon reacts completely with 32 g oxygen, the mass of carbon dioxide formed is:
(a) 20 g
(b) 32 g
(c) 44 g
(d) 384 g
(c) is correct
Conservation of mass gives 12 + 32 = 44 g.
3The law of constant proportions states that a pure compound always contains elements in:
(a) Any mass ratio
(b) A fixed mass ratio
(c) Equal numbers of atoms only
(d) A ratio that changes with source
(b) is correct
Composition by mass is characteristic of a given compound.
4Water samples from different sources are chemically identical when pure because they have:
(a) Different hydrogen-oxygen ratios
(b) A fixed composition
(c) No oxygen
(d) The same volume only
(b) is correct
Pure water has the same elements combined in a definite proportion.
5Which statement agrees with Dalton's atomic theory?
(a) Matter is made of atoms
(b) Atoms are created in chemical reactions
(c) Atoms of every element are identical to all other elements
(d) Compounds form in arbitrary ratios
(a) is correct
Dalton proposed that matter consists of atoms that rearrange during chemical reactions.
6According to Dalton, compounds form when atoms combine in:
(a) Simple whole-number ratios
(b) Continuously changing ratios
(c) Massless groups
(d) Only equal ratios
(a) is correct
Simple integral ratios account for definite compound composition.
7A covalent bond forms by:
(a) Sharing electron pairs
(b) Transferring protons
(c) Sharing neutrons
(d) Removing the nucleus
(a) is correct
Atoms share valence electrons to reach stable configurations.
8Which molecule contains a covalent bond?
(a) H2
(b) NaCl
(c) MgO
(d) KBr
(a) is correct
The two hydrogen atoms share an electron pair.
9An ionic bond forms mainly through:
(a) Electron transfer and electrostatic attraction
(b) Neutron exchange
(c) Equal sharing only
(d) Melting two elements
(a) is correct
Electron transfer creates oppositely charged ions that attract.
10When sodium forms Na+, it:
(a) Gains one electron
(b) Loses one electron
(c) Gains one proton
(d) Loses one neutron
(b) is correct
Sodium achieves a stable outer shell by losing its single valence electron.
11When chlorine forms Cl-, it:
(a) Loses one electron
(b) Gains one electron
(c) Loses a proton
(d) Gains a neutron
(b) is correct
Chlorine completes its octet by accepting one electron.
12The correct formula for magnesium chloride is:
(a) MgCl
(b) MgCl2
(c) Mg2Cl
(d) Mg2Cl2
(b) is correct
Mg2+ requires two Cl- ions for electrical neutrality.
13The correct formula for aluminium oxide is:
(a) AlO
(b) Al2O
(c) AlO2
(d) Al2O3
(d) is correct
Two Al3+ ions balance three O2- ions.
14Why is solid sodium chloride electrically non-conducting?
(a) It has no charged particles
(b) Its ions are fixed in a crystal lattice
(c) It contains only molecules
(d) It has no sodium
(b) is correct
The ions cannot move through the solid lattice to carry current.
