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Vapour pressure?

  1. Feb 5, 2012 #1
    Hi all,

    I have a question on vapor pressure. As far as I understood, a liquid will flashes into gas when the absolute pressure it is experiencing (in a pipe / in the atmosphere) is lower than the liquid's vapor pressure (assuming temperature is constant).
    Everyone knows that gasoline forms a vapor cloud when it is released into the atmosphere. But a lots of sources I came across on the internet states that the vapor pressure of gasoline ranges between 20kPa - 80 kPa at room temperature on average, so if that is true, how does gasoline form a vapor in the atmosphere?

    Thank you~
  2. jcsd
  3. Feb 5, 2012 #2
    The way I have understood it, is that temperature is just an average. Some of the molecules will have enough energy by chance to escape as vapor at any temperature. The numbers you state are average, and if the average is high enough for vaporization, you get a large scale effect, ie boiling.
  4. Feb 5, 2012 #3


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    Gold Member

    Well then, how does water from a vapour in air at room temperature?
    The vapor pressure of water at 20 deg C or room temperature is 17.5 mm Hg, yet liquid water still evaporates at atmospheric pressure.

    But boiling water at 100 deg C has a vapour vapor pressure is 760.0 mm Hg - ie atmospheric pressure.

    The vapour pressure is a function of temperature. If you put a substance in a closed container, an equilibrium will be reached after some time where the pressure of the gas of the substance in the container reaches a value depending upon the temperature. That is the vapour pressure at that temperature. Since gasoline is not a pure substance its vapour pressure varies depending upon its volatile constituents.

    Here is an excerpt fom Wiki regarding dalton's Law
    It takes some time to get your head around all of this vapour pressure stuff I know.
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