CBSE Class 9 Science Chapter 09 Atomic Foundations Of Matter MCQs Set 01

Answer: (B) Carbon dioxide gas escapes into the air, reducing the total mass in the system

Explanation:
In Experimental set-up 1 of Activity 9.2, a brisk effervescence occurs and the final reading does not match the initial reading. The chapter explicitly states that the mass difference occurs because the gas produced by the chemical reaction escapes, resulting in a difference between the initial and final readings.

Answer: (C) Mass remains constant in a chemical reaction when all products remain in the closed system

Explanation:
The chapter notes that in experimental set-up 2, the final reading matches the initial reading. This is because the gas produced is trapped in the balloon, keeping all products within the system on the balance, thus verifying that total mass before and after the reaction remains equal.

Answer: (B) An equal quantity of matter exists both before and after the operation

Explanation:
The chapter states that Lavoisier proposed the Law of Conservation of Mass, asserting that "in every operation an equal quantity of matter exists both before and after the operation." This is the foundational principle of mass conservation in chemical reactions.

Answer: (B) The Law of Constant Proportions

Explanation:
The chapter explains the Law of Constant Proportions (also called the Law of Definite Proportions or Proust's Law) by stating that elements in a compound combine in a fixed ratio by mass, irrespective of the source. The consistent 1:8 hydrogen-to-oxygen ratio in water from different sources exemplifies this law perfectly.

Answer: (B) Atoms are indivisible and merely rearrange without being created or destroyed

Explanation:
One of Dalton's key postulates, as presented in the chapter, is that atoms are indivisible particles that cannot be created or destroyed in a chemical reaction. Instead, they rearrange to form new combinations, which explains both the conservation of mass and the formation of compounds with fixed proportions.

Answer: (B) A covalent bond forms because each atom shares one electron with the other

Explanation:
The chapter explains that when hydrogen and chlorine combine to form hydrogen chloride, both atoms need one electron each. Since both atoms have similar electron requirements, they share one electron pair rather than one transferring electrons to the other, forming a covalent bond represented as H—Cl.

Answer: (B) Two lines representing a double bond

Explanation:
• Each oxygen atom contributes two electrons
• Two pairs of electrons are shared between the atoms
• The chapter illustrates this as O=O with two lines
• This is called a double bond

Answer: (A) Carbon tetrachloride

Explanation:
The chapter explains that when naming covalent compounds, the first element retains its regular name without a prefix (mono- is omitted for the first element), while the second element gets a prefix and ends in -ide. Therefore, CCl₄ is named carbon tetrachloride, not monocarbon tetrachloride.

Answer: (B) 10 electrons, because it lost one electron and now has fewer electrons than protons

Explanation:
When sodium loses its one valence electron, it goes from having 11 electrons (matching its 11 protons) to having 10 electrons. Since it still has 11 protons but only 10 electrons, the positive charges from protons outnumber the negative charges from electrons by one, giving it a 1+ charge.

Answer: (B) A crystal lattice

Explanation:
The chapter explicitly describes the arrangement of ions in sodium chloride as a crystal structure, which is represented as a crystal lattice where ions are depicted as points or dots. This visualization helps show how oppositely charged ions are arranged in a regular, repeating three-dimensional pattern.

Answer: (B) Ions in the solid state are fixed in position, but ions in dissolved solution are free to move

Explanation:
The chapter states that ionic compounds do not conduct electricity in the solid state because their ions are held in fixed positions by strong forces. Electricity requires free-moving ions. When dissolved in water, ions become mobile and can conduct electricity, explaining the difference in conductivity between solid and aqueous states.

Answer: (C) Water (H₂O)

Explanation:
The chapter notes that H₂O, which would ordinarily be named hydrogen monoxide according to the prefix system, is commonly known as water. Similarly, NH₃ is known as ammonia instead of nitrogen trihydride. These are exceptions to the regular naming convention for binary compounds.

Answer: (C) Ca²⁺ and Cl⁻

Explanation:
• Calcium has a 2+ charge (Ca²⁺)
• Chlorine has a 1− charge (Cl−)
• When criss-crossing: the 2 from Ca becomes the subscript for Cl, and the 1 from Cl becomes the subscript for Ca
• This gives CaCl₂ (with the 1 subscript omitted)
• The charges must balance to create a neutral compound

Answer: (B) 18 u

Explanation:
The molecular mass is calculated by multiplying each element's atomic mass by the number of atoms of that element, then summing the results. For H₂O: (1 u × 2) + (16 u × 1) = 2 u + 16 u = 18 u. The chapter demonstrates this exact calculation in Example 9.4.

Answer: (B) An ionic bond

Explanation:
Ionic compounds do not conduct electricity in solid form because ions are fixed in place, but they conduct when dissolved in water because the ions become mobile. Covalent compounds like sugar may dissolve in water but do not conduct electricity because they do not produce ions. The described behavior is characteristic of ionic compounds.

Answer: (B) Physical changes do not alter the total mass of matter involved

Explanation:
The activity demonstrates that dissolving salt is a physical change because no mass is lost or gained—the solution's mass equals water mass plus salt mass. This shows the fundamental principle that physical changes rearrange matter without affecting its total quantity.

Answer: (B) Atoms are indivisible and cannot be created or destroyed in a chemical reaction

Explanation:
The burning of magnesium demonstrates that atoms are not destroyed but merely rearrange when magnesium combines with oxygen. This directly illustrates Dalton's postulate about the indestructible nature of atoms during chemical reactions.

Answer: (C) Chemical bond

Explanation:
The force holding atoms together through any type of bonding—whether sharing or transfer of electrons—is called a chemical bond. In the specific case of shared electrons, this creates a covalent bond, which is a type of chemical bond.

Answer: (B) Law of Constant Proportions

Explanation:
The Law of Constant Proportions (or Law of Definite Proportions) states that elements in a compound always combine in a fixed ratio by mass, regardless of the compound's source. The chapter emphasizes that water always contains hydrogen and oxygen in a 1:8 mass ratio, exemplifying this law.

Answer: (B) The balloon in setup 2 traps the gas produced, preventing it from escaping

Explanation:
The key difference is that setup 2 seals the system by fixing the balloon to the flask, capturing any gas produced. In setup 1, the gas escapes into the air, causing an apparent mass loss. This shows why the balloon must contain the products to verify mass conservation in a closed system.

Answer: (B) 18 electrons; 2, 8, 8

Explanation:
Chlorine has 17 protons and originally 17 electrons (2, 8, 7). When it gains one electron to form Cl⁻, it has 18 electrons, achieving a stable octet with the configuration 2, 8, 8. This extra electron accounts for the negative charge on the anion.

Answer: (C) Nitrogen requires a prefix to indicate two atoms; the name is dinitrogen pentoxide

Explanation:
According to the prefix naming system in the chapter, the first element takes a prefix only if there is more than one atom of it (di- in this case for N₂), while the second element always takes a prefix. Since there are five oxygen atoms, we use pentoxide. The name is dinitrogen pentoxide.

Answer: (A) Parentheses indicate that hydroxide is a polyatomic ion, and the subscript 2 shows two hydroxide ions are bonded to one magnesium ion

Explanation:
Brackets are used in formulas when there are two or more polyatomic ions of the same type. In Mg(OH)₂, the subscript 2 indicates that two separate hydroxide ions (each with its own oxygen and hydrogen) are bonded to the single magnesium ion. Without parentheses, Mg(OH)₂ would be confused with a different structure.

Answer: (B) The ions become free to move throughout the liquid, allowing charge flow

Explanation:
In the solid state, sodium and chloride ions are held in fixed positions within the crystal lattice by strong electrostatic forces, preventing them from moving and thus preventing conductivity. In the molten (liquid) state, ions are freed from their fixed positions and can move freely through the liquid, enabling electrical conductivity. The chapter explains this principle when discussing why ionic compounds conduct electricity only when dissolved or melted.

Answer: (B) Divide both subscripts by their greatest common factor (2) to get the simplest ratio of 1 and 2

Explanation:
The chapter explicitly states that after criss-crossing charges, subscripts should be divided by any common factor to obtain the simplest whole number ratio. If you have subscripts 2 and 4, dividing both by 2 gives 1 and 2, which represents the true proportion of ions in the compound. This ensures the formula reflects the simplest ratio of elements.