Why does external pressure on a liquid increase vapor pressure

  • #26
DrDu
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If you increase external pressure on the liquid, wouldn't you be making it more difficult for the liquid to transition into a gas?"
Yes, but this is a kinetic effect, not a thermodynamic one.
 
  • #27
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I didn't say that, it's a quote from the OP. And the statement is correct regardless of why. Still, the key point is that the increase in vapor pressure due to the increase in "atmospheric" pressure is not as great as the increase in energy required for the liquid's molecules to go into vapor state.
 
  • #28
DrDu
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Still, the key point is that the increase in vapor pressure due to the increase in "atmospheric" pressure is not as great as the increase in energy required for the liquid's molecules to go into vapor state.
You are comparing apples and pears here, or increase in pressure to increase in energy.
 
  • #29
You are comparing apples and pears here, or increase in pressure to increase in energy.
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? Shouldn't the favorable increase in entropy be enough to promote liquid evaporating and thus, boiling? Why is additional heat required then?"

Based off your responses to OldYat47, is it that one situation is referring to a thermodynamic phenomenon (@ constant T, increase in external pressure on liquid increases evaporation) and the other is referring to a kinetic phenomenon (when T is not constant, increase in external pressure on liquid requires increase in T for liquid molecules to evaporate)? Some clarification would help. Thank you!
 
  • #30
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I'm not comparing anything. There are two separate things happening. Increasing the pressure in a liquid increases its vapor pressure. 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.
 
  • #31
DrDu
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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.
 
  • #32
DrDu
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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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