Why does external pressure on a liquid increase vapor pressure

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External pressure on a liquid can paradoxically increase its vapor pressure, despite the common understanding that higher pressure makes it harder for the liquid to vaporize. This occurs because adding hydrostatic pressure raises the Gibbs free energy of the liquid, which increases the tendency for solvent molecules to escape into the vapor phase. The discussion highlights the importance of the Poynting correction and the relationship between pressure and Gibbs free energy, particularly at constant temperature. While higher external pressure typically requires higher temperatures for boiling, it can also lead to an increase in vapor pressure at constant temperature due to changes in molecular activity. Understanding these dynamics is crucial for grasping liquid-gas transitions and the effects of pressure on vaporization.
  • #31
realitysickness said:
Hi DrDu, do you mind responding to my earlier question: "My only question remains then: given the information you just stated, for situations where temperature is not a constant, why are the boiling point temperatures of a liquid higher at higher pressures?
The boiling point is defined as the temperature where the vapour pressure is equal to external pressure. So it will always rise with pressure.
 
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  • #32
OldYat47 said:
Increasing the pressure in a gaseous atmosphere above a liquid increases the amount of energy needed for individual molecules to escape to a vapor state. The first effect is much smaller than the second.
No, it does not. Ideal gases don't know of each other. So when in the gas phase, the energy of the vapour will only depend on temperature, but not on pressure. The energy of the liquid on the other hand is slightly increased due to compression. So the energy a molecule needs to go from the liquid to the gas phase decreases. For real gases there are slight deviations from ideality at high pressures, but these are weaker than the energy change due to pressure of the liquid.
 

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