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Phase Change: What is actually going on?

  1. Mar 4, 2006 #1
    When water changes phase from liquid to gas, what is actually happening at a molecular level?

    I have heard the general explanation that enough kinetic energy is imparted to the surface water molecules to allow them to "breaK" free from the dipole forces that hold them in the liquid. Sort of like they reach an escape velocity, maybe?

    What I don't understand, however, is why phase changes occur at a constant temperature. If I add energy to the water, and some of its surface molecules start to break free and form gas, I would think that those molecules would have more kinetic energy (temperature) than they did before (thats what allowed them to break free, right?).

    Any help is greatly appreciated.

  2. jcsd
  3. Mar 4, 2006 #2
    Heat is basically asynchonous vibration of molecules. If the magnitude of vibration is sufficient to break the weak bonding between molecules in the liquid state, the "VanDeWaals" bonding, then the material will assume the gaseous state.
    At a critical temperature, which is nonuniform throughout the liquid, certain groups of molecules will have sufficient energy to break free of these weak bonding forces and become gaseous. These molecules have just as much kinetic energy as before, but nothing to bind them to adjoining molecules, as their high relative velocity does not allow sufficient time for new VanDeWaals bonds to form. Eventually, the temperature across the entire macro volume evens out, and all molecules assume the gaseous state.
    Think of temperature as more of a local phenomena than a bulk measure.
    R. Indech
  4. Mar 4, 2006 #3
    Thanks for the reply, Rindech.

    Prior to boiling, the surface water molecules do not have enough kinetic energy to overcome the vanderwaals forces in the liquid (as well as the external pressure on the liquid). Boiling starts to occur when the surface water molecules gain enough kinetic energy to overcome such forces. For this reason, I can't help but think that the kinetic energy, and thus the temperature, of the surface water molecules would have to be greater during boiling than immediately prior to.

    Maybe my understanding of temperature during a phase change is incorrect. I add energy to a liquid at constant pressure. Its temperature increases. Eventually, it reaches the boiling temperature for that pressure and begins to change phase. My understanding is that during the actual phase change, despite the addition of energy, the temperature of the fluid does not increase as it did prior to boiling. Rather, the temperature stays constant and the energy addition manifests itself in the form of expansion of the fluid, which does work on the outside. Is this correct?

    I might be able to understand how the average kinetic energy of the whole fluid stays constant, but wouldn't the kinetic energy of the specific water molecules that boil off have to increase during the phase change? Otherwise, what allows them to boil off when they weren't boiling off earlier?

    Thanks again for your help.
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