Boiling in a Vacuum

The boiling point of a liquid is not fixed; it is the temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure.

At sea level, standard atmospheric pressure is 1 atm. When the vacuum pump removes air, the external pressure on the liquid is drastically reduced. With less pressure pushing down on the water, the molecules require less thermal energy to escape into a gaseous state. Once the ambient pressure drops low enough, the water begins to boil vigorously even though it feels cold to the touch.

Why Does Water Get Colder?

During the boiling process, the liquid loses energy. Evaporation and boiling are endothermic processes, meaning they absorb energy.

• The most energetic (hottest) molecules are the ones that turn into gas and escape
• The slower-moving (colder) molecules are left behind
• Because the highest-energy molecules leave the system, the average kinetic energy of the remaining liquid drops, lowering the overall temperature

Under a continuous, strong vacuum, the water can drop its temperature so significantly that it will eventually freeze while still boiling