To summarize from my textbook:

rate of evaporation:

  • proportional to surface area
  • but essentially independent from pressure (of surrounding gas)

rate of condensation

  • proportional to both surface area
  • and concentration (#molecules/volume) of (appropriate) molecules in the gas

At some concentration the rate of evaporation = rate of condensation. The pressure corresponding to this concentration is called "vapor pressure of the crystal".

(There is some example about iodine in a sealed flask in the same chapter, but there is no explicit statement of assumptions about sealed container or not having another gas, like air "around". Textbook: College Chem by Pauling.)

My question:

For a liquid/crystal in a closed vacuumed container, this seems straightforward, since pressure would be a function of concentration (and temperature), I believe. So there is 1 pressure corresponding to the concentration (at the given temperature). But then why is it not called "vapor concentration"? Which the mentioning of its proportionality, as the only relevant one, would suggest.

The only reason for the name "vapor pressure", I can imagine, is that not only concentration matters.

For example, considering the cases where the sealed container has initially near vacuum or 1atm or 2atm of air besides the liquid/crystal. Then the pressure of the gas at equilibrium in those cases will surely be different, even if the concentration of the evaporated substance is the same.

The book does not mention "partial pressure" or "pressure contribution".

My alternative thinking is that "vapor pressure" does not describe the equilibrium state of arbitrary mixed gas of arbitrary container and initial pressure, but confusingly enough, just the property of liquid/crystal which only depends on chem composition and temperature. Which I would call "in-vacuum evaporation rate". And the reason for the naming is the special experimental scenario used to measure/define this property. Neither the total pressure of the possibly mixed surrounding gas, nor the partial pressure of any component is equivalent to it, just merely equal in certain scenarios.

Is this correct?

I am confused. Isn't the book a bit incomplete with the explanation?

  • 2
    $\begingroup$ Welcome to Chemistry SE! Thank you for a well-formed question! You might want to register at the site, so as not to lose your points. $\endgroup$ May 7 '16 at 18:21
  • $\begingroup$ >But then why is it not called "vapor concentration"? || both vapor concentration and vapor pressure are defined and bound by ideal gas law.. But pressures are, your know, easier to measure, then concentrations. $\endgroup$
    – permeakra
    May 7 '16 at 20:11
  • 1
    $\begingroup$ Both processes do NOT depend on the absolute pressure in the gas phase, but the partial pressure of the substance in the gas phase. The "vapour pressure" is not a variable but the property of the substance at given temperature, i.e. its partial pressure above the condensed substance in equilibrium. $\endgroup$
    – Karl
    May 7 '16 at 23:05
  • $\begingroup$ @Karl - i disagree because of latent heat.... e.g. condensation will be faster at higher pressure if surrounding gas pressure is higher for more collisions with surface to help remove heat from surface - this is like blowing on surface to help cool it $\endgroup$
    – tom
    May 7 '16 at 23:24
  • 1
    $\begingroup$ @tom The heat conductvity of gas does only strongly depend on pressure at very low pressures ("Dewar"). Also the heat conductivity of the condensate is magnitudes higher than that of the gas phase. Plus if we are not talking "isothermic" you get convection etc. and it becomes a LOT more complicated. $\endgroup$
    – Karl
    May 8 '16 at 0:33

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