Question

If the solution boils at a temperature \[{{\rm{T}}_{\rm{1}}}\] and the solvent at a temperature \[{{\rm{T}}_{\rm{2}}}\], the elevation of the boiling point is:
A. \[{{\rm{T}}_{\rm{1}}}{\rm{ + }}{{\rm{T}}_{\rm{2}}}\]
B. \[{{\rm{T}}_{\rm{1}}}{\rm{ - }}{{\rm{T}}_{\rm{2}}}\]
C. \[{{\rm{T}}_{\rm{2}}}{\rm{ - }}{{\rm{T}}_{\rm{1}}}\]
D. \[{{\rm{T}}_{\rm{1}}}{\rm{ \times }}{{\rm{T}}_{\rm{2}}}\]

Answer 1

Hint: Boiling point of a liquid is defined as the temperature at which the vapour pressure is equivalent to the external pressure. The boiling point of a solution is always greater than the boiling point of the solvent.

Complete Step by Step Solution:
The transformation of the liquid state into a vapour state is vaporisation. It occurs at a specific temperature. At equilibrium, vapours exert pressure over the liquid phase which is known as the vapour pressure.

The boiling of a liquid occurs at a temperature at which its vapour pressure is equal to the atmospheric pressure. This temperature is called boiling point. For example:- the boiling of water occurs at 373K temperature at which the vapour pressure of water is 1 atmosphere.

In a pure solvent, the whole surface is retained by solvent molecules. If a non-volatile solute is put into a solvent, then the surface of the liquid acquires the solute and solvent molecules. The solvent molecules present on the surface get reduced. Thus, the solvent molecules exiting from the surface are also reduced. So, vapour pressure reduces.

The vapour pressure of the solution is always less than the vapour pressure of the solvent.
This is called lowering of vapour pressure. With the reduction in vapour pressure, a surplus amount of temperature is needed to exceed the external pressure for the liquid to start boiling. Hence, the boiling point increases. So, the boiling point of a solution is always greater than the boiling point of the solvent. This is called elevation in boiling point.

If the temperature of the solution is T1 and the solvent is at a temperature of \[{{\rm{T}}_{\rm{2}}}\] , the elevation of the boiling point is \[{{\rm{T}}_{\rm{1}}}{\rm{ \times }}{{\rm{T}}_{\rm{2}}}\].
So, option B is correct.

Note: The lowering of vapour pressure leads to an elevation in boiling point.
We know that when the vapour pressure equals external pressure, the liquid starts boiling.
As the vapour pressure above a solution decreases, more temperature is required by the liquid particles for the phase transition into the vapour state to happen. So, the boiling point increases as a result.