Question

Assertion
The freezing point of 0.005 M urea solution is different from that of 0.005 M sodium chloride solution.
Reason
The depression in freezing point is directly proportional to the number of species present in the solution.
A. Both Assertion and Reason are correct but 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. Both Assertion and Reason are incorrect.

Answer 1

The freezing point of a substance is defined as the temperature in which it just changes into a solid-state and both solid and liquid coexist and have the same vapor pressure. The freezing point of a solution is always less than that of its pure solvent which is also known as depression in freezing point.
We know that the vapor pressure of a solution is less than that of a pure solvent at each temperature. It means that the vapor pressure of the solution will become equal to that of pure solid solvent only at a lower temperature so that it may start freezing. This is the reason why the freezing point of a solution is less than its pure solvent.

Complete step by step answer:
Depression in the freezing point belongs to colligative properties. We know that colligative properties are such properties that depend upon the number of solute particles or moles.
If we add urea $\left( {N{H_2}CON{H_2}} \right)$ and sodium chloride $\left( {NaCl} \right)$ separately in water then there will be a difference of dissociation between urea and sodium chloride. Sodium chloride is a strong electrolyte and will dissociate into two ions and gives the value of $i = 2$ which is also the number of species present in the solution. Urea is a non-electrolyte and will not dissociate into ions and gives the value of $i = 1$
We know depression in freezing point is
$\Delta {T_f} = i \times {K_f} \times m$
where i is the number of species, ${K_f}$ is the molal depression constant , m is the molality of the solution.
So if we find the depression in the freezing point for urea and sodium chloride it will give a different value for both urea and sodium chloride as there is a difference in the number of species or i value for both urea and sodium chloride.
If the values of depression in freezing for both urea and sodium chloride are different then, the values of freezing point for urea and sodium chloride will also be different.
After discussing we can conclude that both assertion and reason are correct but the reason is the correct explanation for the assertion.

So, the correct answer is Option A.

Note: The properties of the dilute solution which depend upon the number of particles of the solute in a solution are called colligative properties.
A few important colligative properties are:
Relative Lowering in vapor pressure, Elevation in boiling point, Depression in freezing point and Osmotic pressure.