Question

why boiling point increases with pressure ??

Answer 1

I think you are referring to atmospheric pressure. So, basically, the boiling point increases as the atmospheric pressure increases.

The reason is because boiling point is the temperature at which the vapor pressure of a solution is equal to the atmospheric pressure. Vapor pressure is dependent on the kinetic energy (which is proportional to temperature) of the molecules in the solution; therefore, if the temperature of the solution remains same, then as the atmospheric pressure goes up, the boiling point will go up.

Answer 2

Roughly speaking, the average kinetic energy of the vapor molecules will be determined by the temperature (which determines the average kinetic energy overall) and the amount of energy they have to expend to get into the vapor phase by breaking free of the intermolecular forces in the liquid phase. This, in turn, determines the amount of pressure bubbles of the vapor can exert on the surrounding fluid. (The more kinetic energy the gas molecules have, the more pressure they exert on confinding walls.)

In order for a liquid to boil, the bubbles of vapor must be able to exert enough pressure on the walls of the bubble to grow, or at least not collapse. That is, they must exert more than the external pressure on the system. Therefore, the higher the external pressure, the higher you must raise the temperature for the vapor bubbles to have enough pressure to survive.

Answer 3

A definition of boiling point is when the vapor pressure of the liquid equals the ambient atmospheric pressure, thusly if you increase the pressure the vapor pressure must also increase and to do this you have to increase the temperature.