Class 9 Science Chapter 1 Assertion and Reason Questions - Matter in Our Surroundings

Q1: Assertion (A): The term vapour is used to represent the gaseous state of a substance which is otherwise liquid at room temperature.

Reason (R): It is proper to regard the gaseous state of ammonia as vapours.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (c) Assertion (A) is true but reason (R) is false.

Explanation: The term vapour specifically refers to the gaseous state of substances that are liquid at room temperature, such as water. However, ammonia is a gas at room temperature and does not qualify as vapour. Therefore, while assertion (A) is accurate, reason (R) is incorrect.

Q2: Assertion (A): Naphthalene does not leave any residue when kept open for some time.

Reason (R): The conversion of a solid directly into gas is called condensation.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (c) Assertion (A) is true but reason (R) is false.

Explanation: Naphthalene sublimates, meaning it transitions from a solid directly into a gas without leaving a residue. The correct term for the conversion of a solid to a gas is sublimation, not condensation, which refers to the gas turning into a liquid.

Q3: Assertion (A): During evaporation of liquids, the temperature remains unaffected.

Reason (R): Kinetic energy of the molecules is inversely proportional to absolute temperature.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (d) Assertion (A) is false but reason (R) is true.

Explanation: During evaporation, the temperature of the liquid actually decreases as higher energy molecules escape into the gas phase. The statement regarding kinetic energy and absolute temperature is true; however, it does not explain the assertion correctly.

Q4: Assertion (A): Ice floats on the surface of water.

Reason (R): The density of both water and ice is the same.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (c) Assertion (A) is true but reason (R) is false.

Explanation: Ice floats on water because it is less dense than liquid water, not because their densities are equal. The open crystalline structure of ice results in lower density compared to water.

Q5: Assertion (A): The particles of a gas are in constant random motion and move in all directions.

Reason (R): Gases have negligible intermolecular forces, allowing particles to move freely.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Explanation: The assertion is true because gas particles are in constant random motion due to their high kinetic energy. The reason is also true, as the weak intermolecular forces in gases allow for this free movement. Thus, the reason accurately explains the assertion.

Q6:Ass ertion (A): The conversion of a gas directly into solid is called condensation.

Reason (R): Naphthalene leaves residue when kept open for some time.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Both assertion (A) and reason (R) are false.

Ans: Option (d) Both assertion (A) and reason (R) are false.

Explanation: The conversion of a gas directly into a solid is called deposition, not condensation. Additionally, naphthalene does not leave any residue; it sublimates, indicating that both statements are incorrect.

Q7: Assertion (A): At normal pressure (1 atm) the boiling point of water is 100ºC or 373.15 K.

Reason (R): As the pressure increases, boiling point of water also increases.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

Explanation: Both the assertion and reason are true; however, the reason does not explain the assertion since the boiling point is a property of water at specific pressure conditions, not a direct consequence of pressure alone.

Q8: Assertion (A): Evaporation is a surface phenomenon that occurs at all temperatures.

Reason (R): During evaporation, molecules at the surface of a liquid gain enough energy to overcome intermolecular forces and enter the gaseous state.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Explanation: The assertion holds because evaporation can occur at any temperature when surface molecules gain sufficient energy to escape into the gas phase. The reason accurately describes the mechanism of evaporation, making it a correct explanation of the assertion.

Q9: Assertion (A): Liquids diffuse easily as compared to gases.

Reason (R): Intermolecular forces are greater in gas.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (d) Assertion (A) is false but reason (R) is true.

Explanation: Liquids diffuse more slowly than gases due to stronger intermolecular forces in liquids. Therefore, the assertion is incorrect while the reason is valid.

Q10: Assertion (A): Diffusion rate of oxygen is smaller than nitrogen.

Reason (R): Molecular size of nitrogen is smaller than oxygen.

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

Ans: Option (a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Explanation: The diffusion rate of a gas is inversely proportional to the square root of its molecular mass. Since nitrogen (N₂) has a lower molecular mass than oxygen (O₂), the diffusion rate of nitrogen is indeed higher, making the assertion true.

The diffusion rate of a gas is governed by Graham's law of diffusion, which states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular mass:
Diffusion rate ∝ 1/√M, where M is the molecular mass.

Molecular mass:

• Oxygen (O₂): 32 g/mol
• Nitrogen (N₂): 28 g/mol

Since oxygen has a higher molecular mass than nitrogen, its diffusion rate is slower than that of nitrogen. Thus, the assertion is true.