Multiple Choice Questions (MCQs) for Class 9 Science: Chapter 09 Atomic Foundations Of Matter
Access targeted multiple-choice questions for Chapter 09 Atomic Foundations Of Matter designed to align with the latest CBSE academic syllabus for Class 9 Science. These objective practice sets help students evaluate their conceptual understanding and improve exam readiness.
Practice Chapter 09 Atomic Foundations Of Matter MCQs for Class 9 Science
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.
A. The electrons are transferred from hydrogen to chlorine, creating a positively charged hydrogen ion and a negatively charged chlorine ion.
B. The electrons are shared between the two atoms in a covalent bond, with the shared pair attracted to both nuclei.
C. The hydrogen atom loses its electron completely to the chlorine atom, forming an ionic compound.
D. The chlorine atom donates electrons to the hydrogen atom to complete hydrogen's first shell.
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Answer: (B) The electrons are shared between the two atoms in a covalent bond, with the shared pair attracted to both nuclei.
Explanation:
In hydrogen chloride formation, both atoms need one electron each to achieve stable configurations. Rather than transferring electrons, they share one electron pair. This shared pair is simultaneously attracted to both nuclei, holding them together. This is the defining characteristic of a covalent bond, as described in section 9.4.1 of the chapter.
A. 2 grams of hydrogen and 16 grams of oxygen
B. 3 grams of hydrogen and 15 grams of oxygen
C. 1 gram of hydrogen and 8 grams of oxygen
D. 4 grams of hydrogen and 14 grams of oxygen
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Answer: (A) 2 grams of hydrogen and 16 grams of oxygen
Explanation:
The Law of Constant Proportions states that elements in a compound always combine in a fixed mass ratio, regardless of the source. Since water always contains hydrogen and oxygen in a 1:8 ratio, doubling the mass of water (from 9 g to 18 g) also doubles the amount of each element. Therefore, 18 g of water contains 2 g hydrogen and 16 g oxygen.
A. One pair of electrons; single bond
B. Two pairs of electrons; double bond
C. Three pairs of electrons; triple bond
D. Two single bonds; covalent bond
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Answer: (B) Two pairs of electrons; double bond
Explanation:
As shown in Fig. 9.8 and explained in section 9.4.1A, oxygen atoms have six valence electrons and each needs two more to complete an octet. Two oxygen atoms sharing two electrons each creates two pairs of shared electrons. This arrangement is called a double bond, depicted as O=O.
A. The amount of reactants used differs between the two setups.
B. In set-up 1, the gas escapes from the system, while in set-up 2, the balloon captures the gas and keeps it in the system.
C. The temperature of the reactants is different in the two setups.
D. The type of scale used to measure mass is different between the setups.
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Answer: (B) In set-up 1, the gas escapes from the system, while in set-up 2, the balloon captures the gas and keeps it in the system.
Explanation:
In set-up 1, carbon dioxide gas is produced and escapes into the air, so it is no longer part of the system being weighed, causing a decrease in the recorded mass. In set-up 2, the balloon remains attached and inflates, keeping all the gas products within the closed system. This demonstrates that the total mass of reactants equals the total mass of products when nothing escapes, directly supporting the Law of Conservation of Mass.
A. The number of protons increases by one; the number of electrons decreases by one.
B. The number of protons remains at 11; the number of electrons decreases to 10.
C. Both the number of protons and electrons decrease by one.
D. The number of protons remains at 11; the number of electrons increases to 12.
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Answer: (B) The number of protons remains at 11; the number of electrons decreases to 10.
Explanation:
The number of protons in an atom never changes because they are in the nucleus. A sodium atom has 11 protons and 11 electrons when neutral. When it loses one valence electron, it still has 11 protons, but now only 10 electrons, giving it a +1 charge and the symbol Na⁺, as explained in section 9.4.2.
A. Crystal lattice
B. Molecular structure
C. Ionic bond network
D. Electron configuration
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Answer: (A) Crystal lattice
Explanation:
The regular, repeating three-dimensional arrangement of ions in an ionic compound is called a crystal lattice. In sodium chloride, ions are arranged in a pattern where each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa, forming a stable three-dimensional structure. This is illustrated and explained in section 9.4.2 and Fig. 9.14c.
A. The prefix 'mono-' is always dropped to simplify pronunciation.
B. The first element in a covalent compound never uses a prefix, while the second element always does.
C. Monocarbon is not a recognized element name in chemistry.
D. The prefix 'mono-' can only be used for the second element in a compound.
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Answer: (B) The first element in a covalent compound never uses a prefix, while the second element always does.
Explanation:
• Covalent compound naming convention states that the first element retains its regular name without a prefix
• The second element ends in -ide and receives a prefix indicating the number of atoms
• CO is named carbon monoxide (not monocarbon monoxide) because 'mono-' is omitted for the first element but applied to the second element as 'monoxide'
A. Ionic compounds dissolve to release free ions that can move and carry electric current, while covalent compounds do not produce ions in solution.
B. Covalent compounds absorb too much energy from the electrical source.
C. Ionic compounds are always liquid at room temperature, while covalent compounds are solids.
D. Water molecules interact with ionic compounds but repel covalent compounds.
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Answer: (A) Ionic compounds dissolve to release free ions that can move and carry electric current, while covalent compounds do not produce ions in solution.
Explanation:
Ionic compounds like sodium chloride dissociate into mobile ions (Na⁺ and Cl⁻) when dissolved in water. These free ions can move through the solution and conduct electricity. Covalent compounds like sugar dissolve in water but do not produce ions, so even though they dissolve, the resulting solution does not conduct electricity. This distinction is explained in section 9.6.
A. Keep the formula as Al₂(CO₃)₃ because it is already in simplest form.
B. Divide both subscripts by 2 to get AlCO₃.
C. Check if the subscripts can be divided by a common factor, and since 2 and 3 share no common factor, keep it as Al₂(CO₃)₃.
D. Remove the parentheses to write it as Al₂CO₃.
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Answer: (C) Check if the subscripts can be divided by a common factor, and since 2 and 3 share no common factor, keep it as Al₂(CO₃)₃.
Explanation:
Section 9.5.2 explains that after criss-crossing charges, subscripts should be divided by a common factor if one exists. In this case, the subscripts are 2 and 3. Since these numbers share no common factor other than 1, the formula Al₂(CO₃)₃ is already in its simplest whole number ratio and requires no further simplification.
A. The atoms are destroyed and new atoms are created.
B. The atoms are indivisible particles that rearrange but are neither created nor destroyed.
C. The atoms combine and lose their individual identities entirely.
D. Some atoms are destroyed while new ones are created to maintain mass balance.
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Answer: (B) The atoms are indivisible particles that rearrange but are neither created nor destroyed.
Explanation:
Dalton's Atomic Theory postulates that atoms are indivisible particles that cannot be created or destroyed in a chemical reaction. When hydrogen and oxygen combine to form water, the individual atoms retain their identity but rearrange into a new combination. This postulate explains why the mass is conserved and why compounds always have the same composition, forming the theoretical foundation discussed in section 9.3.
A. Matter is created during physical changes.
B. Mass is conserved during physical changes, though the state or appearance of matter changes.
C. Physical changes cause the loss of some matter to the surroundings.
D. The mass of water increases during dissolution.
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Answer: (B) Mass is conserved during physical changes, though the state or appearance of matter changes.
Explanation:
Activity 9.1 demonstrates that when salt dissolves in water—a physical change—the total mass before dissolving equals the total mass after dissolving. This shows that mass is conserved even though the salt's visible form changes from separate crystals to a dissolved state. The chapter uses this observation to introduce the broader concept that mass is conserved in both physical and chemical changes.
A. Law of Conservation of Mass
B. Dalton's Atomic Theory
C. Law of Constant Proportions
D. Law of Chemical Equilibrium
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Answer: (C) Law of Constant Proportions
Explanation:
The Law of Constant Proportions (also called Law of Definite Proportions) states that a compound always contains the same elements in a fixed mass ratio, regardless of its source or how it was formed. The observation that purified water from different sources always shows the same 1:8 hydrogen-to-oxygen mass ratio is the classic example used in section 9.2 to illustrate this fundamental law of chemistry.
A. Mg had +1 and Cl had -2
B. Mg had +2 and Cl had -1
C. Mg had -2 and Cl had +1
D. Mg had +3 and Cl had -3
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Answer: (B) Mg had +2 and Cl had -1
Explanation:
• In the criss-cross method, the charges (not valencies) are written under element symbols
• Magnesium, a metal with two valence electrons, forms a +2 cation (Mg²⁺)
• Chlorine, a nonmetal needing one electron, forms a -1 anion (Cl⁻)
• Criss-crossing the charges (2 and 1) gives MgCl₂, where the superscript 1 on chlorine is omitted
A. H:Cl with a single bond
B. H—H with a single covalent bond
C. H=H with a double covalent bond
D. H⁺ and H⁻ forming an ionic bond
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Answer: (B) H—H with a single covalent bond
Explanation:
A hydrogen molecule consists of two hydrogen atoms, each contributing one electron to form a shared pair. This is represented as H—H, where the line denotes a single covalent bond (one pair of shared electrons). As explained in section 9.4.1A and shown in Fig. 9.6, when two atoms share one electron pair, they are said to be bonded by a single covalent bond.
A. 16 electrons with configuration 2, 8, 6
B. 17 electrons with configuration 2, 8, 8
C. 18 electrons with configuration 2, 8, 8
D. 18 electrons with configuration 2, 8, 7
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Answer: (C) 18 electrons with configuration 2, 8, 8
Explanation:
A neutral chlorine atom has 17 electrons (matching its 17 protons). When it gains one electron, it acquires 18 electrons total while still having 17 protons, making it a negatively charged ion (Cl⁻). With 18 electrons distributed across shells following the octet rule, the configuration is 2, 8, 8, giving chlorine a stable electron arrangement as explained in section 9.4.2.
A. Parentheses are required for all polyatomic ions in any compound.
B. Parentheses indicate that there are two separate hydroxide ions bonded to one magnesium ion, distinguishing this notation from Mg(OH)₂ written without parentheses.
C. Parentheses are used only when naming compounds aloud.
D. The chapter requires parentheses around all anions without exception.
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Answer: (B) Parentheses indicate that there are two separate hydroxide ions bonded to one magnesium ion, distinguishing this notation from Mg(OH)₂ written without parentheses.
Explanation:
When a polyatomic ion like OH⁻ appears more than once in a formula, parentheses are used with a subscript to show that the entire group is repeated. Mg(OH)₂ clearly shows that two complete hydroxide ions (each with one oxygen and one hydrogen) are bonded to the magnesium ion. Without parentheses, the formula would be ambiguous or incorrectly suggest the atomic composition, as noted in section 9.5.2.
A. The sealed balloon prevents any gas from escaping the system, keeping all products inside where they can be weighed together.
B. The chemical reaction produces different substances when confined in a balloon.
C. The sealed system creates higher pressure that causes atoms to bond more strongly, preventing mass loss.
D. The balloon absorbs the carbon dioxide gas that would otherwise escape into the air.
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Answer: (A) The sealed balloon prevents any gas from escaping the system, keeping all products inside where they can be weighed together.
Explanation:
When the system is sealed (experimental set-up 2), all products of the reaction—including the carbon dioxide gas—remain contained and are weighed together. In the open setup (experimental set-up 1), the gas escapes into the atmosphere during the reaction, so it is no longer present on the balance when the final reading is taken. This demonstrates that if you can capture all products in a closed system, the total mass is conserved.
A. Sodium ion has 10 electrons and chloride ion has 18 electrons.
B. Sodium ion has 11 electrons and chloride ion has 17 electrons.
C. Sodium ion has 12 electrons and chloride ion has 16 electrons.
D. Sodium ion has 9 electrons and chloride ion has 19 electrons.
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Answer: (A) Sodium ion has 10 electrons and chloride ion has 18 electrons.
Explanation:
Sodium begins with 11 electrons but loses one, leaving 10 electrons in Na⁺. Chlorine starts with 17 electrons but gains the electron lost by sodium, resulting in 18 electrons in Cl⁻. Both ions achieve stable octets through this electron transfer.
A. Oxygen has two unpaired electrons in its valence shell that each can be shared with a different hydrogen atom.
B. Hydrogen atoms repel each other strongly, forcing oxygen to bond with two separate atoms.
C. Water molecules naturally form with this specific 2:1 ratio due to gravitational forces between atoms.
D. Oxygen atoms are larger than hydrogen atoms, allowing space for multiple hydrogen atoms to bond.
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Answer: (A) Oxygen has two unpaired electrons in its valence shell that each can be shared with a different hydrogen atom.
Explanation:
Oxygen has six valence electrons, of which two are unpaired. Each unpaired electron can pair with an electron from a separate hydrogen atom, allowing oxygen to form two single covalent bonds. This electron configuration explains why water is H₂O and not H₄O or HO.
A. Sodium chloride breaks apart into ions when dissolved, while sugar dissolves but does not form ions.
B. Sugar molecules are larger and therefore block the flow of electrical current through water.
C. Sodium chloride absorbs electrical energy from the surrounding water, while sugar does not.
D. The ionic bonds in sodium chloride are stronger than the covalent bonds in sugar.
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Answer: (A) Sodium chloride breaks apart into ions when dissolved, while sugar dissolves but does not form ions.
Explanation:
Ionic compounds like sodium chloride dissociate into charged ions (Na⁺ and Cl⁻) when dissolved in water. These mobile ions carry electrical charge through the solution, enabling conductivity. Sugar, a covalent compound, dissolves in water but remains as neutral molecules without producing ions, so electrical current cannot flow. Conductivity requires the presence of mobile ions.
A. The Law of Conservation of Mass, which states that matter cannot be created or destroyed.
B. The Law of Constant Proportions, which states that elements in a compound always combine in a fixed ratio by mass.
C. Dalton's Atomic Theory, which states that all atoms of the same element are identical.
D. The Law of Definite Properties, which states that compounds retain the properties of their elements.
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Answer: (B) The Law of Constant Proportions, which states that elements in a compound always combine in a fixed ratio by mass.
Explanation:
• Elements in a pure compound always combine in the same fixed ratio by mass
• This ratio is independent of the compound's source or method of preparation
• The consistent 1:8 ratio across river, borewell, and ocean water samples illustrates this fundamental principle proposed by Joseph Proust
A. Some binary covalent compounds are known only by common names that differ from systematic nomenclature.
B. Hydrogen atoms cannot be counted using the prefix system when bonding with non-metal elements.
C. The prefix naming system has an error that applies only to compounds containing nitrogen.
D. Ammonia is an ionic compound, not covalent, so different naming rules apply to it.
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Answer: (A) Some binary covalent compounds are known only by common names that differ from systematic nomenclature.
Explanation:
The chapter explicitly states that a few binary covalent compounds are known only by their common names. H₂O is called water (not hydrogen monoxide) and NH₃ is called ammonia (not nitrogen trihydride). These exceptions reflect common usage in everyday language that persists despite systematic nomenclature rules.
A. Subscripts become 2 and 3; they are used as-is in the final formula Al₂O₃.
B. Subscripts become 3 and 2; they are reversed to give O₃Al₂.
C. Subscripts become 6 and 6; they are both divided by 6 to give AlO.
D. Subscripts become 4 and 3; they are divided by their common factor to simplify.
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Answer: (A) Subscripts become 2 and 3; they are used as-is in the final formula Al₂O₃.
Explanation:
Criss-crossing the charge numbers (not the signs) from Al³⁺ and O²⁻ produces Al₂O₃. Since 2 and 3 share no common factor, no further simplification is needed. The final formula is written directly as Al₂O₃. The chapter notes that subscripts are divided by common factors only when simplification is possible.
A. A molecule, because sodium and chloride ions are held together by bonds.
B. A crystal lattice, a regular, repeating three-dimensional pattern of ions.
C. A covalent network, because the ions form an extended three-dimensional structure.
D. An atomic solid, because individual atoms are arranged in a repeating pattern.
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Answer: (B) A crystal lattice, a regular, repeating three-dimensional pattern of ions.
Explanation:
The chapter defines a crystal lattice as the representation of a crystal structure using points or dots to show the arrangement of ions in a regular, repeating three-dimensional pattern. In ionic compounds like NaCl, ions do not exist as discrete molecules but rather form these extended crystal structures. The lattice helps visualize how oppositely charged ions are organized in space.
A. The ions break apart into individual atoms that are better electrical conductors.
B. The ions remain fixed in the crystal lattice but become positively or negatively charged.
C. The ions are no longer held in fixed positions and can move freely through the liquid, allowing charge to flow.
D. The covalent bonds in the compound break apart, releasing electrons that conduct current.
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Answer: (C) The ions are no longer held in fixed positions and can move freely through the liquid, allowing charge to flow.
Explanation:
In solid ionic compounds, ions are locked in fixed positions by strong electrostatic forces, preventing them from moving and thus preventing electrical conduction. When the compound melts, thermal energy overcomes these forces, allowing ions to move freely throughout the liquid. This mobile ion movement enables the flow of electrical charge. Solid compounds cannot conduct for this reason; molten ones can.
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