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I Boiling point of water heated via microwave

  1. Nov 10, 2015 #1
    I took about 100 ml of distilled water, put a small, clean rock in it (to prevent "bumping" or explosion) and heated it repeatedly in a microwave oven on high for 45 seconds.

    After each heating I checked the temperature of the water with a digital temperature probe. I did this 7 or 8 times. I saw bubbles coming off of the rock and the sides of the glass.

    The highest reading I ever got was 93.5 degrees C. I gave up when the quantity of water become noticeably reduced (50%).

    So, I thought, the thermometer must be off. So I boiled tap water in a large pot to roiling and checked the temperature with the probe. 99.5 degrees C was the highest value I could read. That's about right for my altitude.

    My question is why was the boiling point in the microwave lower? Shouldn't it have actually been higher due to superheating? Does this have anything to do with hydrogen bonding (or the breaking of hydrogen bonds)?
     
  2. jcsd
  3. Nov 10, 2015 #2

    DrClaude

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    Microwave ovens are notorious for producing uneven heating. You may be observing local hot spots where you have boiling water, while the average temperature of the water is lower.
     
  4. Nov 10, 2015 #3
    Possible, but I stirred vigorously with the probe as I took the readings. I did the same when I checked the stove heated water (a much larger volume).
     
  5. Nov 10, 2015 #4
    I see now several problems with my experiment.

    1. I should have used distilled water in both tests.

    2. I should have used the little rock in both tests.

    3. I should have used the same container in both tests (difficult under the circumstances).

    4. I should have removed the stove heated water from the heat source before taking the reading exactly as I did in microwave case.

    5. I should have brought the stove heated water to a boil repeatedly (same number of times, but I doubt I can use the same time of heating exposure).

    6. I should have recorded each individual temperature measurement.

    Not too great a design I guess.
     
  6. Nov 10, 2015 #5

    russ_watters

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    But you didn't stir it while it was being heated! You stirred it after removing it from the microwave, resulting in evening-out the temperature to something just below boiling.

    When on a stove, you heat water from the bottom and sides, which is nearly perfect for uniform temperatures because as the steam bubbles rise, they will condense back into water if the water above is too cool, heating it up and resulting in very uniform temperatures.
     
  7. Nov 10, 2015 #6
    The rock......its the variable in your experiment that probably caused the anomalies in your experiment. The microwave produces heat via electromagnetic field oscillation of water molecules, the stove top creates heat through conduction. The rock in your experiment was added to prevent the distilled water from exploding when super heated but it also was acting like a (heat sink) absorbing heat from the water. Since the microwave is going to heat the rock slower if at all the rock is always going to be cooler than the surrounding water. So you should preheat your rock and repeat the experiment. That is if your goal is to get a higher temperature from the microwaved distilled water.
     
  8. Nov 10, 2015 #7
    Ha! Very nice indeed. Thanks.

    Totally irrelevant, but it was 50 years ago that my mother had me collect quarter-size gravel rocks to put in her tea kettle so it wouldn't "bump", as she called it, as the water began to boil. That was way before microwave ovens were in common use.
     
  9. Nov 10, 2015 #8
    You still may not measure anything higher than 100c to observe a "super heating" effect, do to heat transfer. Either from the glass container or the heat probe.(Thermodynamic equilibrium)
     
  10. Nov 10, 2015 #9
    I certainly hope someone smarter than me has performed this little experiment (and the right way). I'm having difficulty locating research papers, however.

    I did read one post on another forum where someone claimed to have reached 105° C via the microwave.
     
  11. Nov 10, 2015 #10

    sophiecentaur

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    Perhaps what is needed if a method of having the water stirred whilst it's actually in the microwave and also a method of measuring the temperature. If the oven has a turntable, perhaps a stirring paddle could be dipped in the water. The paddle could be on a rod, stuck to the roof, just off centre of the turntable axis. (That's something I would have expected in domestic microwaves, for stirring sauces and soups etc but never seen one so far) An alcohol in glass thermometer could be read through the door and should not affect the performance.
     
  12. Nov 10, 2015 #11

    russ_watters

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    I'm not sure I follow. This seems like a very basic experiment: why would there be research papers about it?
     
  13. Nov 10, 2015 #12
    I think the best way to preform this particular experiment would be to remove the rock and just heat the distilled water in a glass container and use an infrared thermometer to take your readings. By removing the rock and not disturbing the water with a heat probe there is less heat transfer. You could get a super heating result you could measure. But this method could be dangerous if you touch the glass container...if you take it out of the microwave and accidentally dropped anything in it.
     
  14. Nov 11, 2015 #13

    sophiecentaur

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    Yes, agreed. Great care needs to be taken where superheated water is involved. Putting coffee granules in pre-heated milk can make a real mess and scald you in a volcano of white froth.
     
  15. Nov 11, 2015 #14
    I really wasn't trying to get higher than
    You could study water your entire life and never even scratch the surface. Start with its isotope effects and that will get you through the first 50 years. Why investigate the diameter of the earth? Everybody knows it's flat.
     
  16. Nov 11, 2015 #15

    russ_watters

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    That's a really odd response that doesn't answer my question at all. Sure, you could study water your entire life and never scratch the surface - but this experiment and the issue it investigates are junior-high basic. "Isotope effects" would be a decade beyond it. So it still begs the question as to why you think there would be professional research papers about it and why you are investigating it. Your flat-earth comment is bizarre and wrong, but it suggests you think there is something wrong about our current understanding of how water works. If there is another question here that you are not asking, just ask it.
     
  17. Nov 11, 2015 #16
    Your flat-earth comment is bizarre and wrong


    I think sceptic was just being facetious russ_watters. Personally I could care less if it for a highschool science fair project or if it's serious work in fluid dynamics. I'm a novice lending my knowledge. Curiosity is the root of discovery. And if sceptic learns something from this discussion then it was worth the time to talk to him. Wouldn't you agree?

    Reference https://www.physicsforums.com/threa...ter-heated-via-microwave.842368/#post-5285611
     
  18. Nov 11, 2015 #17

    russ_watters

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    I'm not sure he was, but yes I certainly agree. And if the thread had ended after your post #8, that would have been great. But he asked for more and all I did was ask for a clarification of what he wanted.
     
  19. Nov 12, 2015 #18
    Okay I have a confession to make. Two confessions actually. The reason I did this was to check the calibration of my recently acquired digital thermometer probe, not to advance the state of our knowledge in fluid mechanics.

    I thought a simple way would be to boil water in the microwave and check that it read somewhere very close to 100 degrees.

    The first time I put the bottle in with 100 milliliters of water and the water exploded ( but not the bottle ) after about a minute and 30 seconds. That clearly didn't help.

    So then I read that I had to add the rock or something to make it so that it wouldn't superheat and explode. So I added the rock.

    Then, I heated again. I didn't want to put too much time on the microwave because I didn't want it to explode again. So I chose to do it repeatedly for 45 seconds on high.

    But as often as I tried. I could not get the temperature above 93.5. And I thought maybe somebody on this forum would know why. Actually it should have taken one post to answer the question.

    My question for those who object to the very asking of a question, or take offense, would be why? If you know the answer give it. If you don't, silence would be in order. Emotion and insult have nothing to do with it.
     
  20. Nov 12, 2015 #19
    Another idea for one of my "high school" level science experiments - superheat the water to explosion in the microwave then measure the temperature of the water left in the container and the temperature of the water splattered in various places and ways.

    I'm going to make russ_watters really mad at me for being ignorant. But I don't think any temperature you'll measure in this situation will be above the boiling point. I don't think water anywhere in the container reached the boiling point.
     
  21. Nov 12, 2015 #20

    russ_watters

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    And it did: you were provided the correct answer in post #2 (and additional details in several other posts, particularly my answer to your follow-up question in the next post).
    Nobody has objected to asking the question or taken offense to the question itself. Any hint of offense would be the result confusion/frustration over your seeming unwillingness to accept the answer. We can only help people learn if they accept the answer they are given!
    Agreed. So please do not bring them into this discussion.

    By the way, I've done a similar experiment because of the need for accurate temperature measurement for beer brewing. I choose to use water boiling on a stove because I knew it would provide more even heating than water boiled in a microwave and continuous application of heating while taking the reading. If you google "how to calibrate a thermometer boiling water" you will see that the technique requires you to insert the thermometer and take the reading while the water is at a rolling boil, which can only mean it is on the stove.
     
    Last edited: Nov 12, 2015
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