The boiling point is the temperature at which the vapor pressure of the liquid equals the pressure at enviornment of liquid and the liquid changes to vapor.
A liquid in a vacuum has a lower boiling point than when that liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies depending upon the surrounding environmental pressure. For a given pressure, different liquids boil at different temperatures
The heat of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure.
Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules of liquid escape into the surroundings as vapor.
Even water will start boiling below 100 degree C when we reduce the pressure.and by furthur decrease in pressure it will start boiling at room temperature.the liquid uses its internal energy to change its phase.