Question

Equimolar solutions of two non-electrolytes in the same solvent have:
A. Same boiling point and same freezing point
B. Same boiling point but different freezing point
C. different boiling point but same freezing point
D. different boiling point and different freezing point

Answer 1

Hint: Boiling point and freezing point are colligative properties i.e. they do not depend on the type of substance but depend on the number of solute particles in the solution. If the number of solute particles for the given non- electrolytes are the same i.e., they are equimolar. It means they have the same boiling and freezing point.

Complete answer:
First of all we will understand what colligative property is.
Changes in the freezing point and boiling point of a solution depend primarily on the number of solute particles present rather than the kind of particles. Such properties of solutions are called colligative properties (from the Latin colligatus, meaning “bound together” as in a quantity). The vapour pressure and osmotic pressure of solutions are also colligative properties.
Now we see the definition of boiling point.
Normal boiling point of a substance is the temperature at which the vapor pressure equals 1 atm. If a nonvolatile solute lowers the vapor pressure of a solvent, it must also affect the boiling point.
Because the vapour pressure of the solution at a given temperature is less than the vapour pressure of the pure solvent. It depends on the number of solute particles in the solution. Achieving a vapour pressure of 1 atm for the solution requires a higher temperature than the normal boiling point of the solvent. Thus the boiling point of a solution is always greater than that of the pure solvent.

Let us take an example to understand the change in boiling and freezing point
 Both NaCl and ${\text{CaC}}{{\text{l}}_2}$are ionic compounds that dissociate in water to yield solvated ions. As a result, a 0.01 M aqueous solution of NaCl contains 0.01 M Na+ ions and 0.01 M Cl- ions, for a total particle concentration of 0.02 M. Similarly, the solution of ${\text{CaC}}{{\text{l}}_2}$contains 0.01 M Ca2+ ions and 0.02 M Cl- ions, for a total particle concentration of 0.03 M.
Therefore, the boiling point of ${\text{CaC}}{{\text{l}}_2}$solution is more than that of NaCl solution and freezing point of ${\text{CaC}}{{\text{l}}_2}$solution is less than that of NaCl solution.
We can say that since boiling point and freezing point are colligative properties. So if two non-electrolytes on forming solution have equal number of solute particles i.e. the two solutions are equimolar then their boiling point and freezing point will remain same irrespective of different types of non- electrolytes.

So, option A is the correct one.

Note: You should remember that the colligative properties depend on the number of solute particles not on the type of non-electrolyte. So we can say that aqueous solutions have both a lower freezing point and a higher boiling point than pure water. Probably one of the most familiar applications of this phenomenon is the addition of ethylene glycol (“antifreeze”) to the water in an automobile radiator. This solute lowers the freezing point of the water, preventing the engine from cracking in very cold weather from the expansion of pure water on freezing.