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Strange water boiling phenomenon

  1. Jul 26, 2009 #1
    Since I'm around that old question about an observed phenomenon came to my mind:

    - a cup is filled with water and heated up using a microwave oven. When I see it starting to boil, the oven is stopped abruptly. And while there is no more energy injected into the water, it boils even more for a second when the oven is stopped. So the boiling peak is (just) after the oven is stopped. Looks strange to me.

    - one can remark a similar phenomenon using gas.


    Something (probably) similar:

    - a saucepan is filled with water and heated up using a gas cooker. Then the water boils linearly, and some steam escapes from the top of the saucepan. When the gas is stopped, much more steam comes out of the saucepan for 1-2 seconds. Again, it looks like there is suddenly more heat going to the water, while, on the opposite, the gas was cut and no more (or much less) heat goes to the water.

    Does somebody know the reason? Thanks.
     
  2. jcsd
  3. Jul 26, 2009 #2

    mathman

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    Gold Member

    I wont comment on the microwave.

    However for the stovetop I think it is somewhat of an illusion. When the heat is on the water turns to vapor and stays that way because of the heat. When the heat stops the vapor above the saucepan begins to condense and is visible as tiny droplets.
     
  4. Jul 26, 2009 #3

    negitron

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    Microwave ovens can superheat small regions of liquid, which will continue to dump energy into the rest of the water after the microwaves are cut off.
     
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