Franklin Flask

Water boiling point is lowered at reduced pressure.  The Franklin Flask is a seal vessel with a bowl on the underside into which you put ice to condense out water vapor in the container, lowering the pressure so that the vapor pressure exceeds the internal pressure, even down to 30°C or 40°C.


  • Franklin flask
  • boiling chip
  • ringstand plus iron ring
  • thermometer or temperature probe
  • stopper
  • burner or hotplate
  • thermal gloves or towel.


  • Water
  • crushed ice


  1. Partially fill the flask with water and add a boiling chip.
  2. Heat the water until boiling vigorously.
  3. Withdraw the flask from the heat using thermal gloves or a towel.
  4. When no more steam issues from the flask, insert the stopper with the thermometer or temperature probe into the flask firmly. Test security of stopper by inverting the flask OVER THE SINK. If secure, invert flask on ringstand.
  5. Place crushed ice in the concave bottom. Water will boil until ice is melted. Keep replenishing the ice until the water has fallen to within 15-20°C of room temperature.


The pressure inside the flask equals the vapor pressure of the water at any given temperature. The cold surface condenses some of the water vapor and reduces the pressure below the equilibrium vapor pressure of water. The water boils to reestablish equilibrium. The hottest water molecules enter the vapor phase. This lowers the flask's temperature via evaporative cooling.




Special Safety Notes

  • The flask and the hot water can cause burns
  • Seating the stopper too firmly may cause the neck of the flask to break.


  1. Alyea and Dutton, p. 60(4-4s).
Franklin Flask with the ice loaded in the bowl shaped bottom
Franklin Flask boiling with temperature probe pointed at it