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I have just boiled a half dozen eggs and wanted to ask about a phenomenon I have witnessed for years but have bottled up inside for so long. Why, when the water is near boiling and nice and hot, does it suddenly get more violent when I rotate the pot? I just did it now when the water was very calm. Rotated it 180 degrees and it went into full jet engine mode. Why does it do this?

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3 Answers 3

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When you rotate the pot, you are jiggling the pot and stirring up the boundary layer between the pot surface and the bulk water. Shaking & stirring the water increases the chances of temporarily forming a population of seed nuclei which are voids between adjacent liquid water molecules, at which the phase change liquid -> vapor can occur. Those voids then grow explosively, turning into a cloud of big vapor bubbles.

It is possible to heat the water so carefully that there are no phase change nuclei present and the bulk water temperature then can rise to exceed the boiling point- an unstable situation which can lead to a dangerous superheat vapor explosion if the superheated liquid is given even a tiny perturbation. Then the entire volume of superheated water blows up into vapor all at the same time, forming a geyser of boiling-hot water and vapor that shoots upwards out of the container and burns your face!

Note also the the time constant for conduction of heat from an electric element through the wall of the pot is of order ~a few seconds. If the boiling is triggered faster than that then the mechanism is not connected to touching hot spots on the burner coils.

In any case, note also that the time constant for conduction of heat laterally from one point on the hot surface to another nearby is also of order ~a few seconds which means that significant hot spots on the bottom of the pan get averaged out fairly promptly. Pot designers know this.

The only exception to this hot spot issue is if the pot is made solely from stainless steel, instead of aluminum. Stainless conducts heat more poorly than aluminum and because it is made thinner than an aluminum pot, a hot spot in the bottom of a stainless pot bulges outward as it expands and lifts the rest of the pot out of contact with the electric coils by a fraction of a millimeter, worsening the hot spot development.

To prevent this, stainless pots are typically clad underneath with a layer of copper which conducts heat much better than stainless, and quickly averages out the hot spot(s). Cheaply clad stainless pots do not have enough copper thickness to be useful and they develop hot spots readily.

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  • $\begingroup$ It's not the entire volume of water blowing up into vapour. It's generally only heated to slightly above boiling point, which means it only has enough energy to convert a small part of the water to vapour. But maybe that makes it even more dangerous, flying boiling hot water can perhaps pose a danger at a longer range than steam. $\endgroup$
    – bdsl
    Commented May 25 at 11:04
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    $\begingroup$ @bdsl, look up "bump boiling" for a description of the hazards of superheat enthalpy. -NN $\endgroup$ Commented May 25 at 16:40
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Bubbles generally need a focus point to determine where they form to nucleate the formation of the bubble. If you happen to have a pot that has an imperfection such as a scratch on the bottom, you might notice that bubbles form at the scratch long before they start forming elsewhere. It's a bit like the way that raindrops need to nucleate before they form rather than a whole layer of water vapour becoming a sheet of water. The raindrops tend to form about tiny dust particles in the air.

When you rotate the pot, it is not so much the rotation, as the disturbance causing eddy currents and localised areas of low and high pressure. The bubbles tend to form where the local pressure is lowest. Once a few bubbles have formed, their motion through the water causes further disturbance and breaking up of thermal layers, leading to the rapid increase in the number of bubbles formed.

As Niels mentioned in his answer, it is possible in very calm conditions to form superheated layers where the bubbles fail to form at the normal boiling temperature and the slightest disturbance will cause a violent formation of bubbles every where all at once. The same can happen for water vapour failing to cause raindrops at the normal pressure and temperature, until the local humidity layer builds up to such an extent, that all the water more or less gets released at the same time and forms a cloudburst.

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What is the type of stove you have? Gas flame? Electric burner? Other?

Your description fits excellently with an exposed electric burner stove. The pan does not sit perfectly on the burner. Some bits are in contact with the pan. Those bits are able to do heat transfer through conduction and will have a lower temperature than the portions of the burner that aren't touching the pan. On my old stove, you could easily see that any point in direct contact would stay a gray color, while points on the burner not touching would glow red.

When you rotate or otherwise shift the pan, this changes which bits touch and which don't. You'll now have some portions of the burner that were much hotter now directly in contact with the pan. This will (temporarily) drive more energy into the pan, increasing the rate of boiling.

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  • $\begingroup$ My stovetop is electric. Your explanation makes a lot of sense. When I rotate the pot, it feels like I'm giving the boiling/heating process a boost, like I'm indeed driving more energy into it. $\endgroup$ Commented May 25 at 14:32
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    $\begingroup$ I don't really think that this effect is something that would immediately lead to visibly more violent boiling. It should take a few seconds for the additional heat to be convected through the bottom of the pot into the water in enough quantity to make such a change. $\endgroup$
    – Arthur
    Commented May 27 at 9:50
  • $\begingroup$ @Arthur, the time it takes should be similar for heat up or heat down. Removing the pan from the burner results in boiling stopping almost immediately. So the time for a change in temperature on the outside to affect the boiling rate is very short. $\endgroup$
    – BowlOfRed
    Commented May 27 at 20:52

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