AIIMS 2006 Chemistry Colligative Properties Assertion Reason Question

To address the question, we must analyze the assertion and reason provided, and determine their truthfulness and relationships.

Assertion:

"The water pouch of instant cold pack for treating athletic injuries breaks when squeezed and NH₄NO₃ dissolves lowering the temperature."

This statement is true. Instant cold packs contain two compartments: one containing water and another containing a solid such as ammonium nitrate (NH₄NO₃). When the pouch is squeezed, the compartment containing water breaks, allowing the water to mix with the solid NH₄NO₃. The dissolution of NH₄NO₃ in water is an endothermic process, meaning it absorbs heat from the surroundings, which results in a decrease in temperature. This is why such packs are used for treating athletic injuries, as the cold reduces swelling and numbs pain.

Reason:

"Addition of non-volatile solute into solvent results into depression of freezing point of solvent."

This statement is also true. According to colligative properties, the addition of a non-volatile solute to a solvent decreases the freezing point of the solvent. This phenomenon is described by the formula:

$$\Delta T_f = i \cdot K_f \cdot m$$

where:

• $\Delta T_f$ is the depression in freezing point,
• $i$ is the van 't Hoff factor (number of particles the solute dissociates into),
• $K_f$ is the freezing point depression constant of the solvent,
• $m$ is the molality of the solute.

In the case of NH₄NO₃, it dissociates into NH₄⁺ and NO₃⁻ ions, thus $i = 2$. The dissolution process absorbs heat, thereby lowering the temperature, which aligns with the assertion.

Conclusion:

Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Why Other Options Are Incorrect:

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion: This option is incorrect because the reason does indeed explain the mechanism behind the temperature drop due to the dissolution of NH₄NO₃.
• C) Assertion is true but Reason is false: This is incorrect because both statements are true.
• D) Both Assertion and Reason are false: This is incorrect as both statements have been shown to be true.

Key Concepts:

• Colligative Properties: Properties that depend on the number of solute particles in a solution, not their identity.
• Endothermic Reaction: A reaction that absorbs heat, resulting in temperature decrease in the surroundings.

Thus, the examination of both the assertion and reason confirms that the correct answer is indeed A.