Cincinnati Form Franklin Flask

Demonstrate boiling at a reduced temperature and pressure.

This specially designed 1-liter flask is filled with water.
After the water is brought to a boil the neck is closed off with a rubber stopper fitted with a thermometer. Ice is then placed in the special concave bottom. As you do so, the water begins to boil rapidly again. The thermometer clearly shows that the temperature is falling but the water will boil until it is only 15-20° above room temperature.