# Why doesn't atmospheric pressure affect vapor pressure?

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#### mcatkillers

##### New Member
2+ Year Member
Hey guys, so Berkeley Review book says that atmospheric pressure affects boiling point but not vapor pressure.

From what I understand, vapor pressure (using water as the example) is the pressure exerted when the rate of evaporation equals the rate of condensation, and that pressure is created by the water molecules in the air above the liquid.

This must mean that at this equilibrium state, there's a certain amount/concentration of water gas molecules in the air just above the liquid surface. So I have 2 questions:

1) What determines the concentration of the water gas molecules in the air just above the liquid surface?

2) If atmospheric pressure increases, more force is pushing down on the air above the liquid surface - wouldn't this make it more difficult for the liquid molecules to evaporate, decreasing the amount of water gas molecules in that air space?

This is really screwing with my head, I don't even know what question to ask. Pls help

Hey guys, so Berkeley Review book says that atmospheric pressure affects boiling point but not vapor pressure.

From what I understand, vapor pressure (using water as the example) is the pressure exerted when the rate of evaporation equals the rate of condensation, and that pressure is created by the water molecules in the air above the liquid.

This must mean that at this equilibrium state, there's a certain amount/concentration of water gas molecules in the air just above the liquid surface. So I have 2 questions:

1) What determines the concentration of the water gas molecules in the air just above the liquid surface?

2) If atmospheric pressure increases, more force is pushing down on the air above the liquid surface - wouldn't this make it more difficult for the liquid molecules to evaporate, decreasing the amount of water gas molecules in that air space?

This is really screwing with my head, I don't even know what question to ask. Pls help
Basically if atmospheric pressure increases then BP will increase since water boils when atmospheric pressure equals vapor pressure. Atmospheric pressure only affects BP and not vapor pressure.

In addition vapor pressure and bp are inversely related. Lower bp means higher vapor pressure and vice versa.

Does this help at all?

In order for a molecule to escape from a liquid, it must move to the surface and overcome the intermolecular forces holding it in the liquid phase. Neither of those two requirements (moving to the surface or overcoming the force holding it to the other nearby molecules is affects by gas molecules above that system.

It requires heat, to provide the energy needed to overcome the forces, so BP correlates to temperature and the intermolecular forces (T and deltaHvap).

The only correlation between BP and atmospheric pressure is that the highest possible vapor pressure that a liquid can generate is equal to the atmospheric pressure, because once it vaporizes it becomes the atmosphere. Kind of a weird thought to wrap your head around.

Think of atmospheric pressure as the target vapor pressure is trying to reach and that BP is the temperature at which enough vapor escapes to reach that target.