1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I What happened to the boiling water

Tags:
  1. Jun 28, 2017 #1
    Hello!

    This question is not about a particular homework exercise, or any exercise, but rather an astonishing real life fact, which I just experienced, and, as I am still on my way to becoming educated in science, I would like to ask for your help on how to explain this phenomena.

    I had a rather warm pure water, and I wanted to warm it up, almost to a boiling point. I put a small uncovered flat round pan (radius about 10, or less, cm; and the hight of edges is about 4 - 5 cm), filled with water of about 3 cm in hight, on the electric stove and turned on the heat. As the water was already warm, it should have come to a boiling point within 2 - 3 minutes, given the way my stove works. But in 5 minutes I didn't see any boiling, nor even any air tiny bubble, which usually form before the water reaches the boiling point; the water was flat and tranquil, with a tiny steam coming out of it. I decided to use it as it is. But! When I lifted the pan, and thus stirred the water a bit by starting to move the pan away from the stove, the water suddenly bursted out, started bubbling heavily, really heavily, spilling all over. It looked as if there was something on top of it that prevented it from showing the physical boiling while it was quietly sitting on the hot stove, even though it had reached the boiling temperature.

    What could it be? What type of physical / chemical phenomena is it?

    Thank you very much!
     
  2. jcsd
  3. Jun 28, 2017 #2

    DrClaude

    User Avatar

    Staff: Mentor

    The phenomenon is called superheating. The temperature of the water went over the boiling point without starting to boil, such that the water was in a metastable state. As you moved the pan, the perturbation was enough to allow the first steam bubbles to form, and all the extra heat that water had accumulated suddenly went into vaporization, hence the violent boiling. You are lucky to 1) have observed that phenomenon, 2) haven't burn yourself! The phenomenon is quite rare, and usually more easily observable in water heated up in a microwave oven than on a stove top.
     
  4. Jun 28, 2017 #3
    Thank you very much for this explanation! This was really interesting, astonishing, staggering. And, yes, great that I didn't burn myself - that thought arrived only upon the first lasting astonishment. :-) I will study this phenomena in more details, as now I know what it is. Thank you for the link.
     
  5. Jun 28, 2017 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It's a potential hazard in many situations. Heating milk in a microwave oven and then sprinkling coffee granules in can cover you and the floor with scalding froth. It happens too quickly for any change in boiling point due to the mixing.
     
  6. Jun 28, 2017 #5

    anorlunda

    Staff: Mentor

    Do steam tables provide numbers for superheated/supercooled water regions? I wonder if ASME has even measured those properties.

    It is rare. When I make models of boilers and reactors, I have never made provisions for superheating or supercooling.

    This thread made me curious. What are the temperature limits for supercooling and superheating? I found nothing about superheating, but for supercooling I found this.

    I can't resist a supercooling anecdote. At college, we used to leave Coke bottles out on the window sill to cool. One night when it was -40 out, my roommate brought in a bottle and opened it as I watched. When he popped the cap, a geyser of liquid shot out and froze mid air leaving an icicle (Cokesicle? :sorry:) in the shape of a perfect parabola.
     
  7. Jun 28, 2017 #6
    I chanced upon "The Myth of the Boiling Point" a while back which provides a historical insight to the behavior of water subject to temperature variation.
    http://www.sites.hps.cam.ac.uk/boiling/
     
  8. Jun 28, 2017 #7

    russ_watters

    User Avatar

    Staff: Mentor

    Since it requires a) cleanliness and b) stillness, you aren't going to see it in a boiler.
     
  9. Jun 28, 2017 #8

    anorlunda

    Staff: Mentor

    That article you linked says 109 °C for the onset of boiling. Does anyone have a higher number? I'm still curious about how far superheating can be pushed.
     
  10. Jun 28, 2017 #9

    hilbert2

    User Avatar
    Science Advisor
    Gold Member

    This kind of superheating is much more likely to happen if you use a Bunsen burner to heat a glass test tube that contains liquid, probably because there are less perturbations in a small volume of liquid and because the glass surface in a tube is quite smooth compared to the interior surface of a kettle.
     
  11. Jun 28, 2017 #10
    The highest temperatures I've seen mentioned are from 240°C to 280°C.

    From http://www1.lsbu.ac.uk/water/phase_anomalies.html
     
  12. Jun 29, 2017 #11

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    Can also be caused by an oily film either on the pan surfaces or floating on the water surface .

    Can actually be oil - like residual cooking oil - or can be a film of the treatment chemicals commonly used in tap water .
     
  13. Jun 29, 2017 #12
    Both phenomena are nonequilibrium ones, so difficult to reproduce and even observe. When 2 phases coexists they can be in equilibrium (in this case the dynamic one) or one of them disappears. To be in the true equilibrium, they must exchange a substance under experiment with the same velocity in both directions. This is possible only through perfect fitting of the substance molecules on the border between phases. This is highly improbable as phases differ in symmetry at molecular level and in symmetry of molecular interactions. In the microscopic scale both phases consist of the clusters of a substance. Any cluster should be in aqulibrium, i.e. inside of it gradients of temperature, pressure and composition should not exist. In contact with cluster in another phase or even composition, both clusters exchange substance in different rates. So one of them should disappear. In a dynamic equilibria another cluster should appear. These processes are relatively fast, driven by a local gradients of temperature, pressure and composition. The transport of a mass requires also inequilibria in any cluster actually changing mass.
    Appearance of a new phase goes through nucleation, i.e. formation of a microscopic clusten of this new phase in a bulk volume of preexisting one. The nucleus should grow or disappear, mainly as a result of preexisting inequilibria. Nucleation in crystallization may be initiated electrostatically (scratching the glass vial, etc.) or through insertion of tiny amount of new phase (seeding). In boiling usually we have the bubbles of an ambient gas in our liquid so they can import molecules or clusters from liquid and grow. Air bubbles appear from "boiling stones", etc. When our liquid is degassed, filtered from tiny solid impurities, etc., the appearance of a new phase depends on accident, and probability of such one should be equal in any volume unit of homogenous liquid. So do not expect large overheating inside industrial boiler. In small or even microscale samples overheating and overcooling frequently are in the scale of tens Celsius degrees.
    Best regards,
    zbikraw
     
  14. Jun 30, 2017 #13

    anorlunda

    Staff: Mentor

    Thank you @zbikraw for a detailed and very clear explanation.

    I love PF. I learn something new every day. In this case, the word "aqulibrium." It sounds like a sensible word and you even defined us for it. But it is the first time in my experience that neither Google nor Merriam Webster's Dictionary could find a link to the word.
     
  15. Jun 30, 2017 #14

    Nugatory

    User Avatar

    Staff: Mentor

    I would guess that "aqulibrium" is a typo for "equilibrium"... but it SHOULD be a word, which makes it a sniglet. :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted