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Boiling Water ?

  1. Nov 10, 2007 #1
    If you suspend an open-topped container of water in a pot of boiling water, water in the inner container will reach 100 degrees C but will not boil. Why is this?
     
  2. jcsd
  3. Nov 10, 2007 #2

    turbo

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    You can bring the open-topped container to equilibrium (212deg F) with the water in the larger pot, but since the water in the larger pot never gets hotter than 212 deg, the water in the suspended pot cannot boil. What's happening is that you are adding heat energy to the bigger pot, and that energy is not increasing the temperature of the water in that pot past the boiling point - the heat is being used to satisfy the latent heat of vaporization to make saturated steam at 212 deg which bubbles up (boiling).
     
  4. Nov 10, 2007 #3
    this is kind of on the other side of this question
    I was doing this in my 8th grade science class room
    but we did not get a chance to finish it.
    we put a cup of water into an airtight container
    and slowly started to suck out all of the air and after like 5 mins the water started to look like it was boiling

    with out the heat
    now i can under stand the air being pulled out of the water but i am wondering after some time what would happen to the water
     
  5. Nov 10, 2007 #4

    turbo

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    You were not pulling air out of the water. You were reducing the pressure over the water so that it would go to its vapor phase (boil) at less than boiling temperature. Boiling temperature is a function of both temperature and pressure. Large boilers are often highly pressurized so that the water will store LOTS of heat energy and not boil until the water is very hot. If you boil water on top of a mountain, it will boil at a lower temperature than 212 deg F and the offset is proportional to the ambient pressure.
     
  6. Nov 11, 2007 #5
    that is pretty hard to understand but ok
     
  7. Nov 11, 2007 #6

    turbo

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    Try this.

    http://en.wikipedia.org/wiki/Boiling_point

    You can also run a Google search on "boiling", "pressure", "temperature", etc. I'm sure that there are simple tutorials on the web that illustrate this.
     
  8. Nov 11, 2007 #7

    DaveC426913

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    Remember what boiling is: when the molecules of the water have enough energy to escape the loose bonds of other water molecules to fly off. That can be achieved by either
    a] giving the molecules more enrrgy
    b] making it easier for the molecules to escape (by lowering the air pressure above the water), or
    c] both.
     
  9. Nov 11, 2007 #8
    My Mom used the "pot floating in a pot" method to melt chocolate. One physical requirement possibly overlooked here is that the conduction of escaping steam's heat from the outer pot onto the inner pot must be negligible, which the experiment usually obeys.
     
  10. Nov 11, 2007 #9
    thanks now i see where this is going
     
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