How does the presence of a nonvolatile solute affect the boiling point and melting point of a solution relative to the boiling point and melting point of the pure solvent?
Solution: Here, we are going to explain how the pressure of a nonvolatile solute affect the boiling and melting point of a solution relative to the boiling and melting point of the pure solvent.
Step1:
In a pure liquid the entire surface is occupied by the molecules of the liquid. If a non-volatile solute is added to a solvent to give a solution, the vapour pressure of the solution is solely from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. In the solution, the surface has both solute and solvent molecules; thereby the fraction of the surface covered by the solvent molecules gets reduced. Consequently, the number of solvent molecules escaping from the surface is correspondingly reduced, thus, the vapour pressure is also reduced.
A liquid boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. Now, in order to make the solution boil, its vapour pressure must be increased by raising the temperature above the boiling temperature of the pure solvent. Thus, the boiling point of a solution is always higher than that of the boiling point of the pure solvent in which the solution is prepared .
Step2:
Again, at the melting point of a substance, the solid phase is in dynamic equilibrium with the liquid phase. Thus, the melting point of a substance may be defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase.
When a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at lower temperature. Thus, the melting point of the solvent decreases.
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