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Homework Help: Sucking water from vacuum!

  1. Dec 17, 2007 #1
    [SOLVED] Sucking water from vacuum!

    Suppose we have a rigid box half filled of water and the rest is vacuum; admit that all the water is in liquid phase. The box walls are strong enough and don't collapse due to the atmospheric pressure; through the walls of the box there is a robust sering with which we want to suck water from inside the box. Is it possible to suck water to the inside of the sering?
  2. jcsd
  3. Dec 17, 2007 #2


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    If it were a true vacuum, then the water would boil and then freeze. So, no you could not suck it out with a pump.
  4. Dec 17, 2007 #3
    Ok, but suppose that water don't change state.
  5. Dec 17, 2007 #4


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    Yes it would be possible provided you had a more perfect vacuum on the other side. If the pressure on both sides is equal, then no. In your hypothetical world, the laws would be crazy.
  6. Dec 17, 2007 #5


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    I don't know what a 'robust sering' is but I would say it is very possible to remove water from a half filled box that contains no atmosphere. You must realize that liquid water must be at greater than 0C and that water at that temperature has a finite vapor pressure. Thus, you set up a problem that is not possible (a half filled box of liquid water, the rest is vacuum) without active pumping from the vapor side. At 0C, liquid water has a vapor pressure of roughly 0.6 kPa which isn't quite a vacuum. Pumping at this temperature would require a special pump (certainly not one that 'sucks') to avoid cavitation, but it should be possible.
  7. Dec 17, 2007 #6


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    Well if you are asking a hypothetical question that doesn't obey the laws of physics as we know them, then you can just pick any answer you like.

    Like I said before, if you place water in a vacuum, it will cause it to boil, and it will eventually freeze. So you can't suck the water out.

    Here is a link that shows how the boiling point of water varies as the pressure is reduced. Notice how the boiling point becomes lower as the pressure is decreased.

  8. Dec 18, 2007 #7

    If you check the URL carefully it doesn't make much sense.
  9. Dec 18, 2007 #8

    D H

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    Sure it does. The site displays a table with heading "Boiling Temperature of Water in a Vacuum" and two columns labeled "Temperature oF" and "Vacuum inHg". The first column is obviously the boiling temperature of water. The second column, maybe not so obviously, is the quality of the vacuum relative to one atmosphere, or 29.925 inches of mercury. The pressure decreases as the vacuum quality increases.

    Going back to the original post: You posed a nonsensical problem in the sense that it breaks the laws of physics. In a real box, water will evaporate until either the relative humidity in the vapor part of the box reaches 100% or all of the water has evaporated.
  10. Dec 18, 2007 #9
    Ok, thanks for your collaboration; I knew was a non real problem. I now understand why is never possible to find liquid water in the moon, for example :)
  11. Dec 18, 2007 #10


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    Doesn't make much sense to who?

    I think DH explained it very clearly. If you still don't understand let us know and we'll try again.
    Last edited: Dec 19, 2007
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