14.6 Boiling Water at Reduced Pressure

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

Demonstration:

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.

Observations:

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

Explanation:

    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.