Question

Presence of soluble impurities ______ the boiling points of liquid

Answer 1

Hint:
When a soluble impurity is added to a liquid, the particles of the solute mix with that of the solvent and some move to the surface. This prevents some molecules of the liquid from vaporizing, and hence, reduces the vapour pressure of the mixture. And as we know, boiling point is the temperature at which the partial pressure becomes equal to the atmospheric pressure. So reduction in vapour pressure will have an inverse effect on boiling point.

Complete step by step answer:
Whenever a soluble, non-volatile impurity is added to a liquid, some of the solute particles tend to migrate towards the surface of the liquid, and some will remain in the bulk liquid. Due to this, an obstruction is caused to the volatile nature of the liquid. In other words, it becomes harder for the liquid molecules to vaporize due to the obstruction caused by the molecules of the added impurity.
The result is that the net vapour pressure of the mixture becomes lower than that of the pure liquid, as fewer liquid molecules are able to go into the vapour phase.
We know that the boiling point of a substance is the temperature at which the vapour pressure of the substance equals the external pressure. What actually happens during boiling is that we raise the temperature of the liquid, thereby raising its vapour pressure, until it becomes equal to the atmospheric pressure and starts to boil.
So, when the impurity is added, vapour pressure is reduced, and thus, more heat needs to be supplied to raise it to the external pressure and make the mixture boil.
Hence, presence of soluble impurities increases the boiling point of liquids.

Note: Elevation in boiling point is a colligative property, meaning that it is dependent on the amount of the impurity present and not on the nature of the impurity. Higher the concentration of the impurity, lesser will be the vapour pressure of the mixture, and hence, higher will be the elevation in boiling point.