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Evaporation through vacuum

  1. Mar 21, 2014 #1
    Is it possible to make a liquid evaporate solely with vacuum?

    If you were to reduce the pressure in a container of liquid to 0, while also insulating it from any outside heat, would it remain a liquid or turn into a gas? If it turns into a gas via lack of pressure alone, does the auto-refrigeration effect still occur?

  2. jcsd
  3. Mar 22, 2014 #2


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    Staff: Mentor

    Yes. It's easy to boil water at room temperature in a vacuum chamber, and even if you cool the chamber enough to freeze liquid water, the ice will cheerfully sublimate away in a vacuum.
  4. Mar 22, 2014 #3


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    Staff: Mentor

    Absolutely. If you expose water to a vacuum it will start to boil until the temperature of the liquid water falls below freezing. (At which point it will sublimate as Nugatory said)
  5. Mar 22, 2014 #4
    thank you for the explanations.

    so, am i right in saying that heat is not necessary for a phase change? for instance, if a substance was in a 0 pressure container and also at absolute zero, it would exist as a gas?

    im asking because ive been operating under the assumption that a phase change would require at least some amount of particle KE to break any attraction that caused the substance to exist in a more solid state in the first place. for example, water molecules as a liquid would require some movement proportional to the amount of pressure in order to turn into a gas. but the way it sounds, water molecules will use whatever movement they have to evaporate, and beyond that if the pressure is sufficiently low, any remaining non-gas substance will be pulled apart by the vacuum solely (which would not result in a temperature decrease).

    again, thanks for helping me understand this!
  6. Mar 22, 2014 #5


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    Correct ... pressure is just as important as temperature in a phase diagram.
  7. Mar 22, 2014 #6
    No, you are not so right.
    The evaporating molecules take away thermal energy form the environment, including the rest of the liquid, which cools down.
    The fact that you don't have to provide heat does not mean that thermal energy is not involved in phase transitions.

    And in your closed box you will have a vacuum only in the first instant. The liquid evaporates until the pressure of the vapors reaches equilibrium with the rest of the liquid.
    The vacuum (or lower pressure) has an effect on the speed of the evaporation. It does not change the vapor pressure or latent heat of the material.
  8. Mar 22, 2014 #7
    No, that's not correct. The temperature will in fact decrease when a liquid evaporates in a vacuum due to the latent heat of evaporation. Latent heat is like taxes - there is no way around it.
  9. Mar 22, 2014 #8
    so that means if something were at absolute 0 and in a vacuum, it couldnt change phases??
  10. Mar 22, 2014 #9


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    Staff: Mentor

    Neither absolute zero nor a perfect vacuum are achievable, so this question isn't well-formed. However, i can try answering a similar question that might be what you really mean...

    If you expose a substance to a very low temperature and pressure, it will go through whatever phase changes are needed to reach equilibrium with the near-perfect vacuum and low temperature that you've surrounded it with. Because absolute zero is not achievable there will always be some residual thermal energy available to drive theh phase transition to equilibrium.
  11. Mar 22, 2014 #10
    i guess what im getting at is whether or not heat is necessary for a phase change. and it sounds like youre saying yes, heat is necessary for phase change. lack of pressure alone cannot drive a substance to change phase.
  12. Mar 22, 2014 #11
    He was talking about liquids in general, not water specifically. His question is actually pretty interesting. I'm not sure if all liquids would evaporate in a vacuum. If the temperature is close enough to 0K maybe some of them wouldn't, but I'm not quite sure

  13. Mar 22, 2014 #12


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    Heat may be needed. But that does not place a lower bound on the temperature. For any given temperature, there is a quantity of material that will have enough heat to evaporate something. With a hard enough vacuum, the rate at which that happens will be greater than the rate at which condensation occurs.
    Last edited: Mar 22, 2014
  14. Mar 22, 2014 #13
    The vacuum is just contributing to confusion, it's a side track.
    The vacuum is not necessary and is not a "replacement" for thermal energy.
    If it evaporates in vacuum it evaporates in atmosphere. Just that it take a much longer time.
  15. Mar 22, 2014 #14
    seems like the consensus is that heat is the key component of a phase change. a lack of pressure only hastens that process, but cannot cause it on its own. but because we cant make anything absolute zero, there no way to really test. i just wanted to understand what components contributed to evaporation, and it seems that heat is necessary and lack of pressure is only a component that accelerates it.
  16. Mar 23, 2014 #15
    How is the zero K necessary? To test what?
    Are you confusing heat with temperature, by any chance?
  17. Mar 23, 2014 #16
    if the temperature is absolute zero, then that means if a liquid changes into a gas then it could do so without any heat. but it seems as though that is not possible and there always needs to be some heat in order for a phase change to occur.
  18. Mar 23, 2014 #17
    The role done by pressure is really related to the partial vapor pressure of the liquid that is evaporating. The pressure of other gases that might be present in the atmosphere is not relevant
  19. Mar 23, 2014 #18
    You don't need zero K to test it. The heat absorbed during evaporation can be measured.
  20. Mar 23, 2014 #19
    i think there might be a misunderstanding. my question is regarding if it is possible to change a liquid into a gas even if there were 0 heat in the substance trying to be evaporated. it seems like it is not though, based on the responses in this thread.
  21. Mar 23, 2014 #20
    Yes, it may be. The part in bold does not make much sense.
    The heat cannot be in a substance. By the definition of heat.
    The substance may have thermal energy. And this thermal energy may be changed by transferring heat to or from the substance. Even at zero K the thermal energy does not have to be zero.

    But is this relevant to your question about evaporation and vacuum?
    The heat involved in evaporation can be measured at any temperature.
    At very low temperature most (or almost all) of the substances are solid anyway. So your problem regarding evaporation of liquid is not in this range of temperatures.
    Evaporation is a well known and studied phenomenon. What is the problem, after all?
  22. Mar 23, 2014 #21
    ah, thanks for the explanation. i had been using heat and thermal energy interchangeably. i also didnt realize something could have any thermal energy at absolute zero.

    usually evaporation is taught as having a pressure and temperature component. that is why im asking if something can evaporate *solely* based on the pressure component. so yes, most substances might be a solid at absolute zero, but that is not what im asking about. im asking if evaporation could be induced by only pressure, in the complete absence of thermal energy.

    im not trying to challenge what we know about evaporation but trying to understand its limits better.
  23. Mar 23, 2014 #22
    This (bold) is a vague and imprecise statement which is encountered often in business and politics statements. I don't think it helps understanding.
    Evaporation is a phenomenon. It does not have temperature or pressure as "components".
    We may say that both temperature and pressure (of the atmosphere above liquid) determine the rate of evaporation, for example. Or that vapor pressure depends on temperature.

    Evaporation is not "induced" by parameters (temperature, pressure) but by thermal motion of the molecules in the liquid. By a phenomenon, not by a parameter.
    Then it become obvious that without thermal motion there will be no evaporation. All molecules will be frozen, at rest. So no evaporation. Vacuum or not.

    But this is a hypothetical situation.

    Of course, the molecules may have some motion which is not the thermal motion but it induced by external fields. This may induce (or facilitate) the molecules breaking free from the liquid.
    Last edited: Mar 23, 2014
  24. Mar 23, 2014 #23
    that is exactly what i needed to know. thanks!!!
  25. Feb 23, 2017 #24
    Old thred but anyway:
    A comet may illustrate this.
    It obviously contains water for a long time, but some evaporate when hit by some sunschine.
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