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Vapor pressure and atmospheric pressure.
Kwabena Osei-Bonsu
#1 Posted : Saturday, August 14, 2010 1:44:02 AM
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the book says vapor pressure is independent of atmospheric pressure, if that is the case then why must the vapor pressure of a liquid be equal to the atmopheric pressure for it boils. I pictured atmospheric pressure as a column of air sitting above a liquid and therefore pushing down the gas molecules from the liquid ,thus affecting its vapor pressure
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#2 Posted : Thursday, August 19, 2010 9:23:49 PM
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Hi Kwabena, you are correct in your assertion of atmospheric pressure. Boiling occurs only when the vapor pressure of the solution is equal to the atmospheric pressure, and at sea-level, we call this the normal boiling point. If you were to bring a pot of water to the top of Mount Everest, because the atmospheric pressure is lower, the vapor pressure of the solution would reach this point at a lower temperature. Hence water boils at a lower temperature on a mountain (hence pasta would cook slower on Everest).

Conversely, in a pressure cooker, the boiling point of water is raised past the normal boiling point (100C, 1atm) because the atmospheric pressure of the cooker is so much higher (hence, a greater vapour pressure and in turn, temperature, is needed in order for the v.p. of the solution to equal the atmospheric pressure of the cooker). Indeed, pressure cookers cause water to boil at a higher temperature than a simple pot of water on the stove.

I hope these practical examples help....the next layer of complexity is when you add a nonvolatile solute to your solution (*Raoult's law)- have a read of these important points in EK and feel free to contact me if you have any further questions.... 780-988-1845 / e-mail: MatthewWard_Prep101@hotmail.com...

-Matt

Thanks, Matt
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