Question

The addition of solutes to solvents can lower the freezing point, elevate the boiling point and lower the vapour pressure of the solvents. how is this done?

Answer 1

Boiling point of a substance is the temperature where the vapour pressure of the liquid above the liquid equals the external atmospheric pressure. When a solute is added to a solvent, it interferes with the conversion of solvent molecules into vapour form. Hence more energy is required to attain a vapour pressure of the solvent that equals the atmospheric pressure. Thus presence of a solute raises the boiling point and lowers the vapour pressure of a solvent.
In simple words we can say that the heat provided to convert the liquid into vapour phase is taken up by the solute molecules and hence more heat need to be provided before the solution boils.
In a similar manner, presence of a solute lowers the freezing point of a liquid.