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Question regarding temperature

  1. Mar 29, 2008 #1
    I have a question.

    Is the gas state of element X having a higher kinetic energy than the liquid sate of element X? This is what my high school texts told me.

    But kinetic theory says that "The average molecular kinetic energy is proportional to the absolute temperature"

    If yes, then does that implies that gas is always having a higher temperature than liquid?
    but then how can water vapour be cooler than boiling water?
    Last edited: Mar 29, 2008
  2. jcsd
  3. Mar 29, 2008 #2


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    the kinetic energy of each atom of element X will be greater in a gas than a liquid, but of course the liquid is far more dense and so per volume there is more thermal energy.

    Your statement: "The average molecular kinetic energy is proportional to the absolute temperature" is correct.

    Why do you think that water vapour is cooler than boiling water?

    Water vapour can attain much higher temperatures than liquid water; at the point of evapouration it has the thermal energy of the boiling water plus the energy required for vapourisation.

    Perhaps from personal experience steam may appear cooler than boiling water (although often not) but this is because the water molecules are very diffuse in air.

    There's a whole host of additional caveats like pressure, humidity impureities etc that affect things, but generaly the higher T the higher KE.
  4. Mar 29, 2008 #3
    yes, you're right.

    My question was arisen when I was reading about air conditioning system.

    When the high pressure and temperature liquid refrigerant passes through a thing called "expansion valve", it expands and become gas and hence becoming cold. The I relate it with the kinetic theory, I see contradiction... how can gas be cooler than liquid? Maybe it has something to do with pressure...
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