CBSE Class 9 Science Chapter 09 Atomic Foundations Of Matter MCQs Set 02

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Chapter-wise Objective Questions: Chapter 09 Atomic Foundations Of Matter

Navigate directly to the 50 objective questions for Chapter 09 Atomic Foundations Of Matter using the digital viewer below. Each practice set includes verified answer keys, allowing students to instantly cross-check their work and identify areas requiring further revision.

Question: In Activity 9.2, when baking soda and vinegar are mixed in an open container without sealing the system, the balance reading decreases over time. Which of the following best explains this observation?
A. The Law of Conservation of Mass is violated during this chemical reaction
B. A gas escapes from the system into the air, removing mass from the weighing platform
C. The baking soda absorbs water from the vinegar, increasing its mass temporarily
D. The chemical reaction creates new matter that is lighter than the original reactants
Show Answer & Explanation

Answer: (B) A gas escapes from the system into the air, removing mass from the weighing platform

Explanation:
Gas (carbon dioxide) produced during the reaction escapes into the air. Since the balance only measures what remains on the platform, the loss of gaseous product causes the final reading to be less than the initial reading. This apparent decrease does not violate mass conservation—the escaped gas still exists; it is simply not included in the measurement.

Question: A compound contains magnesium and oxygen atoms. If magnesium atoms have a valency of 2 and oxygen atoms have a valency of 2, what would be the simplest whole number ratio of atoms in the formula after criss-crossing?
A. MgO with subscripts 1:1
B. Mg₂O₂ simplified to MgO
C. MgO₂ showing one magnesium and two oxygens
D. Mg₂O showing two magnesium and one oxygen
Show Answer & Explanation

Answer: (B) Mg₂O₂ simplified to MgO

Explanation:
When criss-crossing the charges (2+ and 2−), you initially get Mg₂O₂. However, the chapter explicitly states that the chemical formula must represent the simplest ratio, so subscripts must be divided by their common factor. Dividing both 2 and 2 by 2 gives 1:1, making the final formula MgO.

Question: Joseph Proust's law states that elements in a compound always combine in a fixed ratio by mass. If a purified water sample from a river contains hydrogen and oxygen in a 1:8 mass ratio, what would be the expected ratio in water obtained from a different source such as an ocean?
A. The ratio would vary depending on the source location
B. It would remain 1:8 regardless of the water's origin
C. Ocean water would have a ratio closer to 1:16 because of dissolved salts
D. The ratio would be unpredictable without analyzing the specific sample
Show Answer & Explanation

Answer: (B) It would remain 1:8 regardless of the water's origin

Explanation:
The chapter explicitly states that water collected from various sources—rivers, borewells, or the ocean—when purified and analysed, always contains hydrogen and oxygen in the same 1:8 mass ratio. This consistency demonstrates Proust's Law of Constant Proportions.

Question: Which of the following correctly describes what happens at the atomic level when hydrogen gas (H₂) forms from two hydrogen atoms?
A. Each hydrogen atom transfers one electron to the other, forming an ionic bond
B. The two hydrogen atoms share one pair of electrons, forming a single covalent bond
C. The hydrogen atoms combine by exchanging their nuclei with each other
D. One hydrogen atom absorbs the other atom completely to create a larger particle
Show Answer & Explanation

Answer: (B) The two hydrogen atoms share one pair of electrons, forming a single covalent bond

Explanation:
The chapter describes hydrogen molecule formation as a sharing process. Each hydrogen atom has one electron and needs one more to fill its K-shell. They achieve stability by sharing a pair of electrons (one from each atom). This shared pair attracts both nuclei, holding the molecule together through a single covalent bond, depicted as H—H.

Question: In naming the covalent compound formed from one carbon atom and two oxygen atoms, which statement correctly applies the naming rules from the chapter?
A. It is named monocarbon dioxide because each element must have a prefix
B. It is named carbon dioxide because the first element does not use the mono- prefix
C. It is named carbon monoxide to indicate two oxygen atoms
D. It is named dicarbon oxide to show the atomic ratio
Show Answer & Explanation

Answer: (B) It is named carbon dioxide because the first element does not use the mono- prefix

Explanation:
According to the chapter's naming system for covalent compounds, the first element retains its name without a prefix (even if there is only one atom), while the second element gets the appropriate prefix and ends in -ide. Carbon dioxide (CO₂) follows this rule: carbon (no prefix) + di- + oxide.

Question: Sodium atoms have 11 electrons and 11 protons when neutral. After sodium loses one valence electron to become Na⁺, how many protons and electrons does the ion contain?
A. 10 protons and 10 electrons
B. 11 protons and 10 electrons
C. 12 protons and 11 electrons
D. 10 protons and 11 electrons
Show Answer & Explanation

Answer: (B) 11 protons and 10 electrons

Explanation:
The number of protons in an atom never changes—sodium always has 11 protons in its nucleus. However, after losing one electron, the sodium atom now has 10 electrons instead of 11. This imbalance (11 positive protons versus 10 negative electrons) results in a net positive charge of +1, making it a cation represented as Na⁺.

Question: In an ionic compound, if the cation is Al³⁺ and the anion is O²⁻, what is the correct formula after criss-crossing the charges?
A. AlO
B. Al₂O₃
C. Al₃O₂
D. AlO₃
Show Answer & Explanation

Answer: (B) Al₂O₃

Explanation:
• Writing symbols with charges: Al³⁺ and O²⁻
• Criss-crossing numbers (ignoring signs): Al₂O₃
• Checking for common factors: no common divisor for 2 and 3
• Final formula: Al₂O₃

Question: According to Dalton's Atomic Theory, when carbon and oxygen atoms combine to form carbon dioxide, what is the ultimate fate of these atoms?
A. They are destroyed and transformed into completely different elements
B. They remain indivisible and are merely rearranged into a new compound
C. They merge into a single large atom that behaves differently
D. They lose their individual properties and blend into a uniform substance
Show Answer & Explanation

Answer: (B) They remain indivisible and are merely rearranged into a new compound

Explanation:
Dalton's postulate states that atoms are indivisible particles that cannot be created or destroyed in a chemical reaction. When carbon and oxygen combine, the individual atoms retain their identity but enter into a new arrangement, forming the compound carbon dioxide (CO₂).

Question: During Activity 9.1, when salt dissolves in water to form a solution, the total mass of the solution equals the mass of the water plus the mass of the salt. What does this observation indicate about the nature of this process?
A. The process is a chemical reaction where mass is created
B. The process is a physical change where mass is conserved
C. The salt undergoes a chemical transformation that affects its mass
D. The water molecules decompose and release mass into the air
Show Answer & Explanation

Answer: (B) The process is a physical change where mass is conserved

Explanation:
Since the mass remains constant and unchanged, dissolving salt in water is a physical change. The chapter demonstrates that during physical changes, the total mass is conserved—no mass is created, destroyed, or lost during the dissolution process.

Question: In the crystal structure of sodium chloride, each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻). What is the name given to this regular, repeating three-dimensional pattern of ions?
A. Molecular lattice
B. Crystal structure
C. Ionic arrangement
D. Atomic configuration
Show Answer & Explanation

Answer: (B) Crystal structure

Explanation:
The chapter specifically defines this repeating three-dimensional arrangement of ions in ionic compounds as a crystal structure. It can also be represented visually as a crystal lattice, where ions are depicted as points or dots to help visualize their spatial arrangement.

Question: Two solutions are tested for electrical conductivity: one containing dissolved sodium chloride and another containing dissolved sugar. Both solutions allow electricity to conduct equally well. Which explanation best accounts for this observation?
A. Both compounds are ionic and produce conducting ions in solution
B. Sugar dissolves to produce ions while sodium chloride does not
C. Both compounds conduct electricity through the water molecules themselves
D. Neither compound actually conducts electricity; the results must be incorrect
Show Answer & Explanation

Answer: (D) Neither compound actually conducts electricity; the results must be incorrect

Explanation:
According to Activity 9.4, dissolved sodium chloride (an ionic compound) conducts electricity because it provides ions in solution. However, sugar (a covalent compound) dissolves in water but does not produce ions, so its solution does not conduct electricity. Therefore, the observation that both conduct equally is inconsistent with what the chapter describes.

Question: When writing the formula for a compound using the criss-cross method, you obtain Ca₂(CO₃)₂ after crossing the charges on Ca²⁺ and CO₃²⁻. According to the chapter's rules, how should this formula be simplified?
A. Leave it as Ca₂(CO₃)₂ because brackets always remain
B. Simplify it to CaCO₃ by dividing both subscripts by their common factor
C. Change it to Ca(CO₃) by removing the outer subscript
D. Keep Ca₂ but remove the subscript from the carbonate to get Ca₂CO₃
Show Answer & Explanation

Answer: (B) Simplify it to CaCO₃ by dividing both subscripts by their common factor

Explanation:
The chapter states that the chemical formula must give the simplest ratio. Since both Ca and CO₃ have the subscript 2, dividing by 2 yields CaCO₃ as the simplified form. The valencies remain balanced (one Ca²⁺ ion bonds with one CO₃²⁻ ion) even in the simplified formula.

Question: In the formation of a water molecule, one oxygen atom combines with two hydrogen atoms. Why is oxygen able to share its two electrons with two different hydrogen atoms rather than with a single hydrogen atom?
A. Oxygen atoms are naturally generous and prefer to share with multiple atoms
B. Oxygen needs two electrons to complete its octet, while each hydrogen needs only one
C. Water molecules require exactly three atoms by nature's design
D. The oxygen atom repels electrons from a single hydrogen, forcing multiple combinations
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Answer: (B) Oxygen needs two electrons to complete its octet, while each hydrogen needs only one

Explanation:
Oxygen has six valence electrons and needs two more to achieve an octet (eight electrons). Each hydrogen has one electron and needs one more for a stable duplet (two electrons in the K-shell). Therefore, oxygen shares one pair of electrons with each of two hydrogen atoms, allowing all three atoms to reach stability simultaneously.

Question: When an element with atomic number 8 gains two electrons, what charge does the resulting ion carry, and why?
A. It becomes 2+ because it has gained positive particles
B. It becomes 2− because it now has two more electrons than protons
C. It becomes neutral because electrons balance protons equally
D. It becomes 2+ because the nucleus gains extra charge
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Answer: (B) It becomes 2− because it now has two more electrons than protons

Explanation:
An element with atomic number 8 (oxygen) has 8 protons and originally 8 electrons. After gaining 2 electrons, it has 10 electrons but still only 8 protons. The excess of 2 negative electrons over positive protons results in a net charge of 2−, making it the oxide anion (O²⁻).

Question: According to the chapter, hydrogen sulfide (H₂S) is a covalent compound where hydrogen atoms share electrons with a sulfur atom. In the formula H₂S, why are there two hydrogen atoms but only one sulfur atom?
A. Hydrogen atoms are smaller and therefore more atoms are needed
B. Sulfur needs two electrons to complete its valence shell, and each hydrogen provides one
C. The compound becomes unstable with only one hydrogen atom
D. Sulfur atoms repel additional sulfur atoms, preventing S₂ formation
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Answer: (B) Sulfur needs two electrons to complete its valence shell, and each hydrogen provides one

Explanation:
Sulfur has six valence electrons and needs two more to complete its octet. Hydrogen has one electron and needs one more. Therefore, two hydrogen atoms (each contributing one electron) pair with one sulfur atom (contributing two electrons), allowing all atoms to achieve stable electronic configurations.

Question: During Activity 9.2, when the balloon remains attached to the flask and is allowed to inflate during the reaction between vinegar and baking soda, the initial and final mass readings on the balance are equal. What principle of chemistry does this observation directly demonstrate?
A. Matter is created during chemical reactions in gaseous form.
B. The total mass of reactants equals the total mass of all products, including gases that remain in the closed system.
C. Gases produced in chemical reactions always stay within containers and never escape.
D. Covalent bonds are stronger than ionic bonds in chemical reactions.
Show Answer & Explanation

Answer: (B) The total mass of reactants equals the total mass of all products, including gases that remain in the closed system.

Explanation:
When the balloon captures the carbon dioxide gas produced by the reaction, no mass escapes from the system. The final reading matches the initial reading because all products—including the gas trapped in the balloon—are accounted for on the balance, proving that mass is conserved.

Question: In the formation of a chlorine molecule (Cl₂), two chlorine atoms each contribute one electron to form a shared pair. Which representation correctly shows this bonding arrangement and its name?
A. Two lines between Cl atoms, representing a double bond.
B. One line between Cl atoms, representing a single covalent bond.
C. Three lines between Cl atoms, representing a triple bond.
D. No line between atoms, representing an ionic bond.
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Answer: (B) One line between Cl atoms, representing a single covalent bond.

Explanation:
When two chlorine atoms share one pair of electrons (one from each atom), they form a single covalent bond, depicted by a single line as Cl—Cl.

Question: According to the Law of Constant Proportions, if pure water samples are obtained from different sources and analyzed, what would be the expected mass ratio of hydrogen to oxygen in all samples?
A. It varies depending on the water's source and purity level.
B. It remains constant at 1:8, regardless of the water's origin.
C. It changes based on the method used to purify the water.
D. It depends on the temperature at which the water was collected.
Show Answer & Explanation

Answer: (B) It remains constant at 1:8, regardless of the water's origin.

Explanation:
The Law of Constant Proportions (or Definite Proportions) states that elements in a compound always combine in a fixed mass ratio. The chapter explicitly states that purified water from rivers, borewells, or oceans always contains hydrogen and oxygen in a 1:8 mass ratio.

Question: When naming the covalent compound formed from nitrogen (five valence electrons) and hydrogen, the correct name is ammonia (NH₃) rather than nitrogen trihydride. What does this exception demonstrate about compound nomenclature?
A. The prefix system is unreliable and should not be used for any covalent compounds.
B. Some binary compounds are known only by common names that differ from systematic naming rules.
C. Hydrogen atoms always precede nitrogen atoms in molecular formulas and names.
D. The number of atoms in a covalent compound determines whether it uses systematic or common names.
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Answer: (B) Some binary compounds are known only by common names that differ from systematic naming rules.

Explanation:
The chapter explains that while most covalent compounds follow the prefix naming system, a few binary compounds—such as H₂O (water) and NH₃ (ammonia)—are known only by their common names, not their systematic names.

Question: When electrons are criss-crossed between a cation with a +3 charge and an anion with a -2 charge, what subscripts result before simplification?
A. 2 and 3 after criss-crossing, no simplification needed.
B. 3 and 2 after criss-crossing, then simplified to their simplest ratio.
C. 6 and 6 after criss-crossing, simplified to 1 and 1.
D. The charges cannot be criss-crossed with these values.
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Answer: (B) 3 and 2 after criss-crossing, then simplified to their simplest ratio.

Explanation:
When criss-crossing charges, the number becomes the subscript of the opposite ion. A +3 cation gets subscript 2 (from the -2 anion), and a -2 anion gets subscript 3 (from the +3 cation), yielding a formula like X₃Y₂ in simplest form.

Question: In Activity 9.4, samples of sodium chloride, copper sulfate, camphor, and naphthalene are tested for solubility in water, kerosene, and petrol. Which observation best distinguishes ionic compounds from covalent compounds based on solubility?
A. Ionic compounds dissolve in all solvents equally well, while covalent compounds dissolve in none.
B. Ionic compounds typically dissolve in water but not in kerosene or petrol, whereas most covalent compounds show the opposite pattern.
C. Both types dissolve equally in water but differ in their behavior with kerosene.
D. Solubility is unrelated to whether a compound is ionic or covalent.
Show Answer & Explanation

Answer: (B) Ionic compounds typically dissolve in water but not in kerosene or petrol, whereas most covalent compounds show the opposite pattern.

Explanation:
The chapter states that ionic compounds like sodium chloride and copper sulfate are generally soluble in water but insoluble in kerosene and petrol, while covalent compounds such as camphor and naphthalene are insoluble in water but dissolve in kerosene and petrol.

Question: A neutral sodium atom (atomic number 11) loses one electron to form Na⁺. Which of the following correctly describes the resulting ion?
A. It has 11 electrons and is negatively charged because it lost an electron.
B. It has 10 electrons and is positively charged because it now has more protons than electrons.
C. It has 12 electrons and is positively charged due to the loss of a neutron.
D. It has 11 electrons and is neutral because the number of protons remains 11.
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Answer: (B) It has 10 electrons and is positively charged because it now has more protons than electrons.

Explanation:
A neutral sodium atom has 11 protons and 11 electrons. When it loses one electron, the ion still has 11 protons but only 10 electrons, making it positively charged (Na⁺).

Question: According to Dalton's Atomic Theory, atoms of a given element are identical in mass and chemical properties, while atoms of different elements have different masses and properties. How does this postulate explain why carbon dioxide always has the same composition regardless of its source?
A. Different sources of carbon dioxide contain different types of carbon and oxygen atoms.
B. Since carbon atoms are always identical to other carbon atoms, and oxygen atoms are always identical to other oxygen atoms, the ratio and properties remain constant.
C. Carbon dioxide molecules can vary internally but appear the same externally.
D. Dalton's theory does not address the composition of compounds like carbon dioxide.
Show Answer & Explanation

Answer: (B) Since carbon atoms are always identical to other carbon atoms, and oxygen atoms are always identical to other oxygen atoms, the ratio and properties remain constant.

Explanation:
• Carbon atoms are always identical in mass and properties to other carbon atoms
• Oxygen atoms are always identical to other oxygen atoms
• When these identical atoms combine in the same ratio (one carbon to two oxygen), they always form the same compound with consistent composition.

Question: In the formula Mg(OH)₂, parentheses are used around the OH⁻ group with a subscript of 2. Why is this notation necessary rather than writing MgOH₂?
A. Parentheses are used randomly in chemical formulas and serve no particular purpose.
B. Parentheses indicate that there are two separate hydroxide ions (each as OH⁻), not individual O and H atoms.
C. Parentheses show that oxygen and hydrogen are bonded together within the compound.
D. The chapter does not explain the use of parentheses in formulas.
Show Answer & Explanation

Answer: (B) Parentheses indicate that there are two separate hydroxide ions (each as OH⁻), not individual O and H atoms.

Explanation:
Parentheses are used when two or more polyatomic ions of the same type are present in a formula. In Mg(OH)₂, the subscript 2 outside the parentheses indicates two complete hydroxide ions (OH⁻), not two separate oxygen atoms and two separate hydrogen atoms.

Question: A solid ionic compound does not conduct electricity, but when melted (heated until liquid), it suddenly conducts electricity. According to the chapter's explanation of ionic compound structure and properties, why does the molten state conduct electricity while the solid state does not?
A. Melting adds electrons to the ions, enabling them to move and conduct electricity.
B. In the solid state, ions are held in fixed positions by strong forces, preventing movement. In the molten state, ions are free to move and carry electric current.
C. Melting converts ionic compounds into covalent compounds, which always conduct electricity.
D. Ionic compounds never truly conduct electricity; the apparent conductivity in molten state is due to thermal energy, not ion movement.
Show Answer & Explanation

Answer: (B) In the solid state, ions are held in fixed positions by strong forces, preventing movement. In the molten state, ions are free to move and carry electric current.

Explanation:
The chapter explains that ionic compounds do not conduct electricity in the solid state because their ions are held in fixed positions by strong electrostatic forces. When melted, ions become mobile and can freely move, allowing them to conduct electricity. Ions must be able to move freely for electrical conductivity to occur.

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Multiple Choice Questions (MCQs) for Class 9 Science Chapter 09 Atomic Foundations Of Matter

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