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Q & A: vapor pressure

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Most recent answer: 10/22/2007
Q:
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
A:
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.

mike w

(published on 10/22/2007)

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