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Hi there! So i have a q from the realms of thermodynamics , should be

  1. Jan 31, 2012 #1
    Hi there! So i have a q from the realms of thermodynamics , should be simple for you guys because I'm' in high school.
    So ,i stumbled over a problem(NOT HW POST) and the text was rather interesting it said that in a vessel from which the air is quickly evacuated contains water at 0 Celsius,now the interesting part , the water gets an intensive vaporization which causes the left water to gradually freeze, anyway the problem asked what fraction of water(from the initial quantity) can be turned to ice knowing just λ for vaporization and λ for melting , and that 0C, no masses no nothing!
    However , my question is how can the left water freeze because of an vaporization.
    Just to be clear what they want to say is that , the water is quickly heated causing vaporization,right? And if they HEAT the water how can a fraction of it freeze?

    It seems odd to me.


    TY
     
  2. jcsd
  3. Jan 31, 2012 #2

    Philip Wood

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    Re: Thermodynamics

    Water will evaporate slowly at 0°C. Indeed even ice evaporates. The trick is to make the evaporation quick. The usual way to do this is to induce boiling (evaporation from inside the liquid as well as from the surface) by reducing the pressure on the surface. I've succeeded at about 30° using an ordinary rotary vacuum pump; whether it could be done in practice at 0°C I'm not sure...
     
  4. Jan 31, 2012 #3

    Ken G

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    Re: Thermodynamics

    No, the key to the problem is that you don't need to heat the water to get it to evaporate. Even at 0 C, water has some kinetic energy in its molecules, and a small number of those molecules will, just by chance, be moving much faster than the rest. Those are the ones that will evaporate, taking with them more than their fair share of kinetic energy (heat). This is why we sweat to cool us down-- the evaporation selects the particles with the lion's share of the kinetic energy, since they are the ones that have the energy to evaporate. Then, the ones left behind will have less kinetic energy on average, so the temperature drops further. In this case, the T can't drop because it's already at the freezing point, so instead some freezing occurs. The key issue for finding the final proportion of evaporated vs. frozen is simply the ratio of the energy taken away by the evaporation and the energy released by freezing, because the total energy stays the same. That's why you don't want to heat this at all-- you want to keep the same energy in there.
     
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