Simultaneous Freezing and Boiling of Water



Under certain conditions of low humidity and low pressure, water can both freeze and boil at room temperature. Evaporation of the water at low humidity withdraws heat by removing the fastest of the molecules of the water causing it to freeze. Boiling occurs when the vapor pressure of the water, a function of temperature, equals the atmospheric pressure.



To set up the demonstration, fill a Petri dish two third full of concentrated Sulphuric Acid and place it on the plate of a standard vacuum pump. Place a clay triangle on top of the Petri dish. Fill a watch glass with tap water and place it on top of the clay triangle. Put the vacuum pump vacuum bell over top and turn the vacuum pump on. The setup is shown in the diagram. When the vacuum is reduced to the vapor pressure of water, the water will start to boil. The Sulphuric Acid acts as a decessant to remove water vapor from the system causing it to loose heat and freeze.

see caption above--"to set"

Note: Most manufacturers recommend changing the vacuum pump oil after several repetitions where concentrated acid is involved as the acid fumes can be dissolved in the oil.