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Sudden boiling of coffee in cup after heat source removed

  1. Jun 20, 2004 #1

    Jenab

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    I just now heated a cup of coffee in a microwave oven for 2 minutes. When I opened the oven door to get the cup, the surface of the coffee was placid. But the moment I picked up the cup and moved it, there was a brief moment of boiling lasting maybe one second. What might have caused the coffee to boil after the heating energy had been turned off?

    Jerry Abbott
     
  2. jcsd
  3. Jun 20, 2004 #2

    Alkatran

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    Maybe the heat was still heading towards the outside, maybe the kinetic energy you gave it caused friction, and just a bit more heat (to boil it), maybe the change of atmosphere, maybe the last few rays were heading out of the microwave, maybe there were air bubbles, maybe ...
     
  4. Jun 20, 2004 #3

    chroot

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    The water was superheated -- i.e. heated beyond its boiling point without actually boiling. The reason? Boiling requires nucleation sites -- little irregularities, bumps, or other debris in the water to serve as places where bubbles of steam can form. Without any nucleation sites, water can be heated significantly above its boiling point without actually boiling. When you remove it from the microwave and slosh it a bit, the cavitation can prompt the onset of boiling. So can the addition of a spoon, or some coffee grinds.

    Always be extremely careful when removing hot water from a microwave, especially when using a smooth glass or ceramic container.

    - Warren
     
  5. Jun 21, 2004 #4
    Safety note

    Actually, to be on the safe side, never heat water in a microwave without making sure there is something in it that allows nucleation, such as a wooden stick or other non-smooth object.
     
  6. Jun 21, 2004 #5
    Part of the problem not mentioned by CHROOT is that water boils from the center out in a microwave. So, a single bubble can form in the center of the cup, below the surface of the water and burst on its own or when the cup is moved. This can occur after only a few seconds in a microwave.

    I put a cup of coffee in the microwave to be heated, turned around for about 15 seconds, turned back and half of the coffee was sitting in the glass dish on the bottom of the microwave oven. I thought the cup had broken, but it hadn't. I think a huge bubble had burst low in the cup.
     
  7. Jun 21, 2004 #6

    chroot

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    Artman,

    I have never seen evidence of the "huge bubble at the bottom" mechanism you described. In fact, I'd bet money it doesn't happen.

    - Warren
     
  8. Jun 22, 2004 #7
    ooosh chroot your the dude
    will a piece of chicken cause a nucleation site?
     
  9. Jun 22, 2004 #8

    chroot

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    FUNKER,

    Definitely. :smile:

    - Warren
     
  10. Jun 22, 2004 #9
    This article describes the occurance. Flash steam in the center of the cup provides the force to propel the water upwards. The mechanism is as you described, superheating the water. My point is that it can do far more than just boil, but can literaly explode from the cup. It is not a slow bubble formation, but the creation of flash steam expanding in the cup.

    Exploding cup of water
     
  11. Jun 22, 2004 #10
    Heh this remind me of a story my physics teacher in high school told me.

    He was heading to school one winter morning, and he had left 2 bottles of water in his truck over night. One was frozen, and the other one wasn't. I guess they were different brands(assuming). Anyways, one was purified. Must have been distilled.
     
  12. Jun 22, 2004 #11

    chroot

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    It's also possible one had a smaller air bubble inside it, or the bottles were of different compositions and elasticities, and thus the pressure was different in one bottle than in the other.

    - Warren
     
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