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I am unable to understand the evolution of vapors at atmospheric pressure i.e. if the vapor pressure is 7 psia how can the vapor evolute from the liquid in the first place?
- Furqan (age 25)
MOL Pakistan Oil & Gas, Pakistan
At 7 psi, that vapor pressure would be less than atmospheric pressure
(about 15 psi). So if the liquid were sealed in a closed container with
no air maintained (say with a piston) at 15 psi, there would be no
vapor, as you say. However, in an open container, where only part of
the gas pressure comes from the vapor and the rest comes from air, it's
easy for vapor to leave the liquid surface. Until the vapor
concentration rises to give a total of 7 psi contribution to the gas
pressure above, more vapor will leave the liquid than will rejoin the
liquid. With even a little ventilation, it's likely that the vapor will
never reach 7 psi partial pressure above the liquid, so evaporation
will continue until the liquid is gone.
(republished on 07/24/06)
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