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All About Vapor Pressure
Why does boiling a liquid create vapor that can spin a turbine?
Actually, all liquids have a certain "vapor pressure" at certain
temperatures. For instance, a cup of water at room temperature
has vapor pressure even though you don't see steam coming out of
it. So what causes vapor pressure?
Let's go back to the cup of water example. Look at the surface
of the water. Now imagine zooming in really close. If you could
see the individual molecules, you would notice that they were all
moving around slightly, constantly jostling and shoving each
other but staying in the same overall position. The movement is
caused by the energy associated with the given temperature.
At room temperature, the molecules will all have a certain amount
of energy and will therefore be moving around at a certain speed,
whereas at a colder temperature, the molecules will have less
energy and will not be moving around as much. At absolute 0 (0
Kelvin, or -273 degrees Celsius), the lowest possible
temperature, technically the movement of the molecules would be
at a minimum (although there would still be movement).
Now imagine that one of the molecules of water near the surface
is pushed a little harder from below. The molecule gains just
enough energy to "break free" of the other molecules and is
launched into the air. The free molecule will most likely escape
into the open air if there is no lid on the container; if there
is a lid, however, then the molecule will bounce off of the
different surfaces within the system and eventually return to the
surface of the liquid where it "reattaches" itself to the other
molecules.
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