Atmospheric Pressure and Volume Differences Between Vapor and Liquid (page 2)
Why It Works
As the steam inside the flask begins to cool, the air pressure inside the flask drops. This is primarily the result of the phase change of the steam from vapor to liquid water, which occupies a much smaller volume. The cooling air inside the flask also contracts, adding to the reduced pressure. Atmospheric pressure pushes down on the liquid in the flask, driving up into the glass tube. The cooler the flask gets, the lower the pressure. This process feeds on itself in an accelerating manner, producing the fountain effect.
Other Things to Try
The mechanism that drives the liquid up into the flask is the basis for what is known as a Torricelli barometer. Air pressure is measured by how high a column of water can be supported by air pressure with a vacuum in the flask. Mercury is used instead of water because standard air pressure can support a mercury column roughly 30 inches high, compared with a much-higher column for water. Because of potential difficulties in working with mercury in academic settings, it is probably best just to read about this one.
This project works because of the volume differences between vapor and liquid, and the force exerted by air pressure.
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