Question

Assertion: The water pouch of an instant cold pack for treating athletic injuries breaks when squeezed and $N{H_4}N{O_3}$ dissolves thus lowering the temperature.
Reason: Addition of non-volatile solute into solvent results into depression of freezing point of the solvent
(A) Both Assertion and Reason are correct and Reason is the correct Explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct Explanation for Assertion.
(C) Assertion is Correct but Reason is Incorrect.
(D) Assertion is Incorrect but Reason is correct.

Answer 1

Hint : For this question we will inspect both statements carefully and observe if they are correct. The water pouch of an instant cold pack contains both water and Ammonium Nitrate $N{H_4}N{O_3}$ . Ammonium Nitrate is a non-volatile solute. When we add a non-volatile solute to water or any other solvent it lowers the vapour pressure of that solvent. So, the solution will achieve equilibrium at a lower temperature than temperature of pure solvent

Complete Step By Step Answer:
Now we take a look at the first statement that is Assertion
When an instant cold pack is squeezed the ammonium nitrate it contains breaks down into small particles and dissolves into the water, as a result it increases the density of solution overall lowering the temperature of solution. The addition of Ammonium Nitrate lowers the vapour pressure of solvent as pure solvent has higher vapour pressure.
Let’s Understand the Reasoning behind this
Now from the formula of depression in Freezing point,
$\vartriangle {{\text{T}}_{\text{f}}} = {{\text{T}}_{{\text{f}}\left( {solvent} \right)}} - {{\text{T}}_{{\text{f}}\left( {solution} \right)}}$
where $\vartriangle {{\text{T}}_{\text{f}}}$ is Depression in freezing point, ${{\text{T}}_{{\text{f}}\left( {solvent} \right)}}$ is the freezing point of pure Solvent, and ${{\text{T}}_{{\text{f}}\left( {solution} \right)}}$ is the freezing point of Solution.
We observe lowering the freezing point of solution increases the depression in Freezing point.
The Assertion is right that Ammonium nitrate being a non-volatile solute, when dissolved in the water lowers the temperature of the solution and Reason is also correct as depression in freezing point increases due to the lowering of freezing point of the solution.
So, option (A) is Correct.

Note :
Freezing point of a substance is defined as the temperature at which the substance changes its state from liquid to solid, also the vapour pressure of both the solid state and liquid state must be equal.