Microwaving Water

Discussion in 'Off-Topic' started by mozikluv, May 11, 2007.

  1. mozikluv

    Thread Starter AAC Fanatic!

    Jan 22, 2004
    1,437
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    here's something i would to share:)

    moz

    Microwaving Water!


    (I did not know this, did you?) I microwave water for coffee all the time. Now ,I don't think I will be doing it this way anymore .


    A 26-year old man decided to have a cup of coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for, but he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup, he noted that the! water was not boiling, but suddenly the water in the cup "blew up" into his face. The cup remained intact until he threw it out of his hand, but all the water had flown out into his face due to the build up of energy. His whole face is blistered and he has 1st and 2nd degree burns to his face which may leave scarring.

    He also may have lost partial sight in his left eye. While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc, (nothing metal).

    It is however a much safer choice to boil the water in a tea kettle.


    General Electric's Response:

    Thanks for contacting us, I will be happy to assist you The e-mail that you received is correct. Microwaved water and other liquids do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it.

    To prevent this from happening and causing injury, do not heat any liquid for more than two minutes per cup. After heating, let the cup stand in t he microwave for thirty seconds! before moving it or adding anything into it.

    Here is what our local science teacher had to say on the matter: "Thanks for the microwave warning. I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new, or when heating a small amount of water (less than half a cup).

    What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point.

    What then usually happens is that the liquid is bumped or jarred, which is just enough of a shock to cause the bubbles to rapidly for m and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken."
     
  2. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    seems like a case of DNB (departure from nucleate boiling).
    when heat flux increases the water starts to form a film instead of bubble because rate of heating is high.this film does the work of heat transfer.
    i will have to study a little abt what actually happens in case of microwave heating and how it actually heats (i think it is different than heating in presence of flame)
    cant say much abt superheating since as far as i know (not much btw)
    it always occurs after steam formation.
    a 'liquid' in superheated state is somewhat new for me, even in a nuclear power plant steam formation rate is quite high.
    a similar but oppsosite in nature phenomenon occurs if steam is expanded
    at the cost of enthalpy(adiabatic process) at a very high speed/rate,
    its enthalpy goes beyond what is req for steam at that pressure(latent +sensible heat) but it does not condense to water instead remains a steam
    (somewhat like cold steam iwud like to call it) its like having a steam at 90
    degree celcius at standard atm pressure.

    edit: i wud like to share a funny event (though not that much informative)
    in winter which lasts 2 months in our country, i wud boil water for having a bath (its much more economical this way)
    i spilled abt 4 litres of hot boiling water (sensible + some dryness fraction*latent) on my legs and in pain i threw myself about two meters in air. it was a funny incident now that i recall it ,though scars remain it was really something to remember and learn from.

    btw :speaking of winter england comes to our country for cricket when winter is at its peak in our country its quite funny to see them sunbathing with goggles on next to an indian covered from head to toe in sweaters scarf mufflers and gloves rubbing his hands :)
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Thanks for the alert moz. I will be sure to exercise added caution in the future when microwaving water.

    hgmjr
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Microwaves now have a bump spot on the turntable to try and prevent that. Just stick a long spoon in there and stir. I've only seen it with reasonably clean water.
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    The water, in the right conditions, can be heated to 110ºC without boiling. This is known in science and it is called a metastable state (that happens if there us no agitation or thermal convection in the system while heating). But pure compounds are more prone of reaching such state.
    This also happens when overcooling water. I saw an experiment with a recipient of glass containing overcooled water. The water was liquid but froze instantly when the recipient was hit (the recipient even exploded).
     
  6. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    That happened to me once....... luckily I wasn't looking down into the cup at the time, but it still burnt my hands. Gave me quite a fright more than anything else.... as it exploded up and out of the cup as I was moving the cup over to the bench just before I was about to add soup.
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    145
    The concept of superheating and microwave ovens has been around for many years, and has been a subject for debate in the RF and microwave community because it is one of those cases where some disgression must be applied as opposed to something that can be elimintaed by design.

    Reference http://www.phys.unsw.edu.au/~jw/superheating.html for a general FAQ on superheating in microwave ovens. There are also hundreds of papers on this subject.

    Dave
     
  8. gbm46

    Active Member

    May 6, 2007
    46
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    i remember my thermodynamics lecturer saying something about the record for superheating was something over 200 C, in extremely controlled conditions of course. or something. maybe.
     
  9. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    in power plants (pulverised fuel fired)
    we heat steam to tempratures of abt 570 degree celcius at pressure of
    140 and 40 bars where the boiling temp are abt 336 and 250 degree celcius
    resp. tempratures n a laboratory basis can still go higher if a good grade mateial
    is used as container. (do u know at abt 374 degree celcius and 221 bar water flashes into steam without any addition of latent heat.which means at that point the density of water and the enthalpy equals steam's )
     
  10. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    That is not impossible if you consider the effects of pressure on the heating process.

    Dave
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    When I hear superheating, I think of "dry" steam being heated for more energy content. The last generation of Navy steam plants put out steam at 1200 psi & 900 deg F. Still big numbers after conversion to MKS.
     
  12. el_khem

    New Member

    Dec 14, 2007
    2
    0
    In addition to the potential hazzard of superheated water from the microwave oven, microwaved water may be unfit for long term human consumption - it seems to be no good for plants!

    Have a look at this site http://www.execonn.com/sf/

    There can be many criticisms leveled at the project, however it just may be the tip of the proverbial iceberg.

    Feel free to replicate the experiment with protocols at your convenience.
     
  13. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    Maybe I'm a cynic, but I'm not buying the health-related issues with microwave heating. In fact the burning of food products through conventional heating (something which isn't possible by microwave heating alone) has more evidence of health issues than any related microwaving mechanism. Although I am not disputing the results obtained for that experiment, I wouldn't take a 6th grade experiment to mean anything as evidence of any long term health benefits of the techniques.

    Dave
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Looks like an urban myth. No scientific proof on that. Besides, looks like that a undocumented scissor went somewhere in the process.
     
  15. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    Lol! That did cross my mind. Whilst reading it I did think, at what point did the plant watered with microwave water get pruned! The text offered no answers :D

    Dave
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
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    I'd be a bit suspicious about the quality of the experiment. Like, did the kid let the microwaved water cool completely before pouring on the plant? And it does look like some trimming took place.

    For another experiment, go to You Tube and search on spiders. Watch the spiders on drugs video. (Ah, the joys of finally having much higher bandwidth).

    I've been nuking cold coffee for many years and am not aware of any withering. Rather the opposite.
     
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