14.6 Boiling Water at Reduced Pressure

Chemical Concept Demonstrated: Pressure's effect on the boiling point of a liquid


  • A few boiling chips are added to a flask half-filled with water.
  • The flask is stoppered and a vacuum hose is inserted in the stopcock.
  • Turn on the vacuum pump.


    Without the aid of additional heat, the water inside the flask begins to boil.  After a certain time, the boiling may cease.


    The boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid is equal to the external pressure.  When the pressure above a liquid is reduced, the vapor pressure needed to induce boiling is also reduced, and the boiling point of the liquid decreases.  At 250 C, water boils at an external pressure of 24 mmHg.  As the water boils, heat is lost due to the heat of vaporization of water, which is 40.88 kJ/mol.  Eventually, the water may cool to a temperature at which the vapor pressure is less than the external pressure provided by the pump, at which point the water will stop boiling.