Boiling Cold Water Under Reduced Pressure: Phase Diagram of Water

A vacuum pump is used to reduce the ambient pressure above water under a sealed bell jar. By reducing the pressure, the boiling point is lowered so that the water begins to boil at room temperature.  Optional: a digital thermometer can be used to measure the temperature of the boiling water.

Curriculum Notes 

This demonstration would be appropriate for an introductory chemistry course when states of matter are being discussed. Also, this demonstration could be used to illustrate the effect of reducing the pressure on the boiling point of water when discussing phase diagrams.  For advanced courses this demonstration can be used when discussing thermodynamics of phase changes and phase diagrams.

Lead Time 
One day of lead time is required for this project.

A liquid boils when the vapor pressure of the liquid is equal to the ambient pressure. When the ambient pressure is reduced to below the vapor pressure of the liquid, the liquid boils. Water typically boils at 100°C when the pressure is equal to 1 atm, but when the pressure is reduced using a vacuum pump, the water boils at room temperature.

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 Cwater 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.  


Place a 250 mL beaker containing 150 mL of warm water on a vacuum plate. Cover the beaker on the plate with a bell jar. Connect the vacuum pump to the plate with a vacuum hose. Turn on the vacuum pump to reduce the pressure inside the bell jar. As the pressure is reduced, the water begins to boil.

Please keep this demonstration brief so that the vacuum pump does not become saturated with water.

A video camera may be needed to display the demonstration to students sitting in the back of a large lecture hall.

Safety Precautions 

Use a blast shield between the audience and the vacuum pump and bell jar for protection in case of glass implosion due to vacuum. Break the vacuum seal prior to turning off the vacuum so that vacuum oil does not get sucked up the hose.

The demonstrator should wear a face shield while the bell jar is being placed under reduced pressure.



1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers; Volume 2; Wisconsin; 1985; p. 81-84.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group