# Boiling water conundrum

I found this website and was prompted to post this question after seeing John Rennie’s elegant answer to a “boiling water” problem from some time ago.

I recall my physics teacher (from over 50 years ago) explaining the phenomenon whereby if you boiled a jug of water and turned off the power, allowing the boiling to settle, it would sometimes happen that after some water was then poured out of the jug, the remaining water would suddenly re-boil! I think the explanation was related to latent heat, but....I have forgotten the details, which I was recently trying to discuss with my daughter, now a physics student.

I would be most grateful if someone could explain the details of this phenomenon to me (again).

Regards, Peter

• Thank you very much for your detailed responses. The second answer seems closer to the scenario as I remembered it being described. However, rather than the phenomenon being related to exposure of the element when the jug is decanted, I had it in my mind that something about the energy residual in the remaining water was somehow responsible for a momentary surge causing the “re-boiling” behaviour. – Peter C Jul 13 '19 at 21:40

There is a real effect along these lines, which I will recount below, but it sounds different from the one you cited. Here goes:

If you have a container with extremely smooth sides (free of pits or scratches) and you fill it with water that is pure and free of any sort of suspended solids, and you add heat steadily to it without any stirring or shaking action at all, it is possible to raise its temperature above the boiling point without having it start boiling. This metastable state is called superheat, and if the superheated water is then gently perturbed by, for example, picking up or otherwise moving the container, the entire volume of water will burst into violent boiling in what is called a superheat vapor explosion.

How is this possible? If the container has very smooth sides, there will be no tiny amounts of air trapped in surface pores to act as nucleation seed sites for the phase change to get started at. Ditto for water containing no particles of dust or whatnot. And if you do not shake the hot water, you will not form any transient local low-pressure spots in the water which could similarly furnish an excuse for the superheated water to blow up into steam.

Then if you strike the side of the container with a spoon- or even just pick it up and move it- you'll likely form a transient nucleation site within the bulk of the water and then ka-boom! The whole thing blows up and you get a geyser of hot water and steam shooting out the top of the container.

The higher you raise the water temperature, the smaller becomes the seed site required to trigger boiling and the less time you have to wait for the system to experience a random fluctuation on its own that sets off the boiling. So as superheat goes up, the initiation lag time goes down and approaches zero. When it is zero, the attendant temperature is called the thermodynamic limit of superheat which for water is about 340C.

Can you do this at home? Yes, with a little luck, if you use a freshly-emptied Snapple bottle and put an inch or so of filtered water in it, and set it in a microwave oven which does not have a turntable, and set it on high.

• An additional example: a charged atomic particle travelling through super-heated liquid hydrogen. You've just invented the bubble chamber... – DJohnM Jul 13 '19 at 4:19
• Or you test-ride a moped in the middle of a below-freezing oregonian winter night and encounter a fog bank, arriving at your destination with your beard completely clogged up with ice. – niels nielsen Jul 13 '19 at 4:50
• Or you perform environmental freeze testing on a batch of inkjet printheads and find they will not freeze until you accidentally drop a tray full of them on the concrete floor of the test lab- at which time they all freeze instantly. – niels nielsen Jul 13 '19 at 4:52
• by the way, superheat vapor explosions are what drive droplets of ink out the nozzles of a thermal inkjet printhead! I worked in that field for 28 years. – niels nielsen Jul 13 '19 at 4:53
• Maybe the latent heat the OP is talking about, is the latent heat in the heating element? I can imagine that for a given electric burner of 50 years ago , that this could happen ( we have different burners now, I tried the experiment but it failed :)) – anna v Jul 13 '19 at 5:52

When I do this experiment, or one that sounds like it, what is happening is this:

• The part of the heating element that gets hot is electrically insulated from the water and so to some extent thermally insulated as well. So the core of it is well above boiling point.

• When you turn the kettle off, the element stops heating itself and starts to cool. Heat flows from the hot core to the water.

• Water is a good coolant, so very quickly the temperature at the surface of the element drops below boiling point and boiling stops. But the core is still quite hot.

• As you pour the water out, the element is exposed to the air.

• Air is a very bad coolant, so, the heat flow from the core continuing, the surface of the element is no longer cooled and it becomes hotter, eventually becoming hotter than boiling point because there is nothing to cool it.

• When you put the kettle down the water covers the element again and boils briefly until it has cooled the element back down to below boiling point.

For all this to work best, you need an element whose core gets hot enough, whose core’s thermal capacity is large enough, and which does not conduct heat to the surface too fast.

Analogous experiment: Fill your sink with water. Take a heavy cast iron or cast aluminium frying pan and fry yourself some bacon. Take the frying pan and plunge it into the water until the water in the sink stops boiling. Then immediately take it out and hold it in the air. Any drops of water will dry immediately, and if you have the right combination of heavy pan, well heated (if it isn’t non-stick and you were planning to throw it away anyway you could get it nearly red hot), and a brave assistant, you might even find that new water dripped onto the frying pan will actually boil.

• You missed making the bacon sarnie... making me feel hungry... plus 1 though... – user207455 Jul 13 '19 at 7:11