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Boiling water without heat

  1. Feb 15, 2016 #1
    Hey everyone,

    I found this video on Youtube of this guy boiling water by reducing the pressure:

    I'm not sure how the gauge he's looking at works but I think this is the vacuum instrument he's using: https://us.vwr.com/store/catalog/product.jsp?product_id=4789427

    ^It says that it provides vacuum to 723.9mm Hg.

    I looked at a water phase diagram and noticed that you could only have 1kPa (7.5mm Hg) of pressure at 25C to make water evaporate.

    In conclusion, I don't really understand how his water is managing to boil and was wondering if any of you knew why.
     
  2. jcsd
  3. Feb 15, 2016 #2

    Borek

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    Vacuum produced with water aspirator is limited by the vapor pressure of water, and can go much lower. Must be a mistake in the description.
     
  4. Feb 15, 2016 #3
    Thanks! I was also wondering if it would be possible to have another tube inserted into the jar that led to another jar where the vapor could be collected? (should have put it in the post)
     
  5. Feb 15, 2016 #4

    rbelli1

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    Yes. That is essentially how heat pipes work. Also bubble lights and those novelty gizmos where when you hold it it boils continuously.

    You are not really "boiling without heat". When you lower the pressure then the boiling point drops accordingly. The boiling proceeds exactly the same way as if you put a flame underneath. Heat enters the vessel where the liquid is in contact and causes boiling. If you cool the top sufficiently you will condense the contents on the top and the boiling will continue as long as heat flows through the system.

    BoB
     
  6. Feb 15, 2016 #5
  7. Feb 15, 2016 #6

    russ_watters

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    If you are suggesting the pressure is nowhere near low enough, I suspect you are reading the gauge upside-down. :wink:

    Vacuums are often measured from zero gauge pressure (the word "vacuum" is equivalent to a negative symbol).

    [edit]
    Also, I looked up the saturation pressure of water at 25C and what you have in there isn't correct either.
     
  8. Feb 16, 2016 #7
    if you put a thermometer in what would it say?
     
  9. Feb 16, 2016 #8

    Borek

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    Please elaborate, the question is pretty ambiguous.
     
  10. Feb 16, 2016 #9
    it will show that the water is boiling at a lower temperature than 100C, which is the temperature water boils at for normal atmospheric pressure.
     
  11. Feb 16, 2016 #10
    i though it may be less than 100c - but how much less?
     
  12. Feb 16, 2016 #11

    Borek

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    If you stick the thermometer in water it will show water temperature regardless of whether it boils or not. I am still not sure what your question is.
     
  13. Feb 16, 2016 #12
    The lower the pressure, the lower temperature the water will boil at.
    At sufficiently low pressure, (and assuming pure water), solid ice will evaporate and there is no intermediate liquid state of the water.
     
  14. Feb 16, 2016 #13

    russ_watters

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    Heat flows from warm areas to cold areas. Which way is the heat flowing here (from what to what?)? Answer that and you'll be able to guess pretty close to what the temperature of the water is.
     
  15. Feb 16, 2016 #14
    Boiling is a phase change, not temperture dependent. The water temperature does not need to change for the water to boil. We live above 9,000 feet and water boils at about 190 degrees F making it difficult to cook vegetables since the water will not increase beyond the boiling point. But if we use a pressure cooker we can heat the water inside above the 212 F temperature and cook quickly due to the pressure increase. This is the reverse of boiling water by lowering the pressure.
     
  16. Feb 17, 2016 #15

    Mark Harder

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    [QUOTE="
    I looked at a water phase diagram and noticed that you could only have 1kPa (7.5mm Hg) of pressure at 25C to make water evaporate.
    In conclusion, I don't really understand how his water is managing to boil and was wondering if any of you knew why.[/QUOTE]

    You and other folks here cite the vapor pressures at a given temperature, 298K. The guy places his hand on the jar to show it isn't hot, but was it any cooler than ambient? If he holds the jar in his hand, does the rate of evaporation increase? The temperature in this case will decrease as the vacuum is pulled and the water evaporates. In order to maintain a constant temperature in such a case, don't you need to add heat? If you don't, then if you want to keep the cooling water boiling, don't you need to keep decreasing the pressure as the water temperature drops? If so, then performing the experiment in a temperature bath at 298K and performing it under adiabatic or semi-adiabatic conditions (i.e. when heat in the apparatus is removed faster than it gains from the air around it) will show different vapor pressures.

    To take a similar example, I have one of those "Pocket Rocket" camp stoves that sit on a liquid butane canister. When the stove valve is opened, butane evaporates, the butane gas burns and the canister cools down. At ambient temperatures below 40 F or so, the stove stops working and the canister becomes coated in ice. I assume this is the case because the ambient air can't supply enough heat to keep the butane in the canister evaporating. Cooking in the cold requires that the canister be warm when I turn the stove on, so I sleep with the canister the night before. Then I have to bootstrap the stove by heating water in the lid of my camp pot and standing the stove in the hot water. It's not a very good heat bath, but I can at least prepare a proper pot of tea for breakfast. I suppose I could be less lazy and pack in a stove that burns gasoline under pressure.
     
  17. Feb 17, 2016 #16
    i was wondering what-or if-there is a relation between presure and boiling i.e.@x temp.= y pressure?
     
  18. Feb 17, 2016 #17

    rbelli1

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  19. Feb 17, 2016 #18
    thank you bob
     
  20. Feb 17, 2016 #19

    Mark Harder

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    No. Absolutely not. The lhs of your equation is in units of temperature, Kelvins is the standard. The rhs is in units of pressure. I think Pascals are the standard units of pressure, so is kg/m2. You can't equate apples and oranges. Pressure is not the same thing as temperature. You can't use them interchangeably. The same holds for addition. You can't add apples and oranges, or pressure and temperature for that matter. Dimensional analysis is a very handy way to check calculations and derived equations; like has to equal like. Multiplication is another matter.
     
  21. Feb 17, 2016 #20

    rbelli1

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    For a particular material the temperature/pressure curve holds. In a general sense no you can't make that statement. We are talking about water here so the specific works.

    BoB
     
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