Is vapor pressure independent of atmospheric pressure? - This is what my instructor told me, but I don't understand why.

Question

anonymous · Answer

No it is completely dependent on atmospheric pressure. Think about the surface of the liquid as a solid plate. In order for the liquid to boil, or its molecules to turn to gas, they must overcome the force that the atmospheric pressure is putting on this imaginary plate (the surface of the liquid). So if the atmosphere puts more pressure on top of the liquid, it needs more force to break out of its liquid form. That force is vapor pressure.

anonymous · Answer

Thanks for your answer. I had the same logic in my mind, but then my instructor said that they are independent of each other, and I was confused. Thanks again.

anonymous · Answer

Your instructor is generally correct: vapor pressure is a characteristic of a substance, and depends only on the temperature.  Theoretically, there is a very, very slight dependence on pressure which is, roughly speaking, comparable to how the chemical potential of the solid or liquid substance depends on pressure.  Since that dependence is extremely small, as a rule, this dependency is essentially zero.

Think of your practical experience: you know some substances have a high vapor pressure at room temperature --  e.g. alcohol, ether, gasoline -- because they smell powerfully, and other substances have a very low vapor pressure -- e.g. cooking oil -- because you don't smell it at all unless it is heated.  Do you notice the "smellyness" of perfume (alcohol) decreasing on sunny days when atmospheric pressure is high?  Hopefully not!

The logic you had in mind -- of the atmosphere exerting a pressure on the surface of the liquid and preventing it from evaporating -- is mistaken.  The atmosphere isn't a solid substance, it's a gas, and can only exert force when a single gas molecule collides with the liquid surface.   You would be better off thinking of it as a very tattered swiss-cheese kind of surface, wtih many holes, pressing down on the liquid surface.  Imagine turning a glass of water upside down, and trying to stop the water from coming out by pressing a surface containg holes against the water.  Would that stop  the water from coming out?  Would it matter if you pressed harder?  Certainly not.