Question

How is a solution freezing point and boiling point affected by addition of a dissolved solute?

Answer 1

Effect on Boiling point :

As we dissolve solute in a pure liquid The Vapour Pressure of the solution decreases compared to pure liquid So To make the solution boil i.e. to increase the vapour pressure​​​​​​​​ to make equal to atmospheric pressure Temperature Needs to be increased.

So there will be an Elevation In Boiling Point.   Example : If a pure liquid boils at 20°C Now will boil at temperature greater than 20°C (T>20°C)

Effect on Freezing Point :

As we dissolve solute in a liquid, it has opposite effect than the effect on boiling point As we add solute , the solute - solvent interactions prevents the solvent from going into solid phase by decreasing the activity of liquid phase. So the solution will freeze at lower temperature than it's normal freezing point of pure solvent.  So temperature needs to be decreased.

So there will be a Depression in Freezing Point. Example : If a liquid Freezes at 0°C So now will freeze at temperature lower than 0°C ( T< 0°C)