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Yesterday, I put a pot of water on the stove and set it to boil to make some pasta. I have done this many times but this time the boiling was qualitatively different than normal. Why would it be different from normal? I'll describe it below.

  • First, the pot is a small steel saucepan about 2/3rds full of water with a metal lid.
  • The handle on the top of the lid was scalding hot to the touch. It's usually only warm.
  • Usually I observe two states of boiling, the "loud-but-little happening" start where nucleating bubbles first start but don't rise to the surface and you can see into the water clearly. Then the "rolling boil" where the surface is covered in bubbles.
  • Instead, the water surface was still enough to see through but shimmered everywhere. Individual bubble columns formed but wouldn't last too long, with only one two columns at a time. Isolated bubbles formed everywhere but were small enough to mostly not break the surface of the water.
  • Letting the water sit on high heat did not kick it into a rolling boil.
  • Adding linguine to the water caused slightly explosive bubble column formation around the noodles, at least at first. The boiling remained unusual through the duration of cooking the linguine.

I struggled to identify what exactly was different from normal, mostly it was just an impression that this was not usual.

On a related note, I ended up watching a series of lectures on boiling online to try to answer this question myself and I would recommend them to anyone interested. However, my observations don't match the descriptions and recordings provided in those lectures, leaving my question unanswered.

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  • $\begingroup$ I actually have a guess for a solution to this puzzle myself, but I'm interested to see what others think first. I'll post my potential solution if this isn't resolved. $\endgroup$ – user32157 Apr 7 '20 at 15:55
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It looks like this time round your saucepan and/or water was unusually clean.

Bubbles nucleate on even the smallest specks of dust, scale or other flaws. (In passing, the early "loud" ones are often air coming out of solution). If either the pan or the water has dust around, you will get many bubbles nucleating everywhere. As the main body of water reaches boiling point, these bubbles increase in size and create the familiar vigorous boiling.

Only a few columns suggests that this time round there were only a few nucleation sites present, i.e. unusually little dust. In this situation the few columns of bubbles can carry only a small amount of heat away, so the water "superheats" above boiling point. This exaggerates the shimmering little convection movements over the rest of the surface.

When you plunged the linguine in, they were covered in nucleation sites. Bubbles immediately formed on them and, because the water was superheated, grew almost explosively. This is actually a dangerous experiment to try and replicate, because the water can genuinely erupt and scald you badly. You seem to have been lucky.

I am a bit surprised that the boiling remained abnormal, I can only suggest that as the linguine softened, its hard edges rounded off and the nucleation sites disappeared. Or, maybe the bubbles' clinging around the linguine was visibly abnormal.

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  • $\begingroup$ I should clarify that I struggled to find the right word when I said "explosively" and I think that overstates the situation - there was a healthy amount of bubbles forming around them but it didn't disrupt the surface of the water any more than the already existing bubble columns. And I assume that it would be impossible for superheating to occur while still having widespread bubble formation without going into a rolling boil (even after adding pasta). As such, I'm skeptical that it was significantly superheated. Am I mistaken? $\endgroup$ – user32157 Apr 7 '20 at 17:52
  • $\begingroup$ It depends what you mean by "significantly" superheated. It was just enough to explain what you saw and raise the "explosive" meme in your mind, thankfully not enough to explode scalding water all over you. Is that significant? $\endgroup$ – Guy Inchbald Apr 7 '20 at 18:28
  • $\begingroup$ I guess what I mean is that I doubt that there could have been a lack of nucleation sites if there was, in fact, wide-spread nucleation throughout the pot and adding new nucleation sites (pasta) didn't trigger a pot-wide rolling boiling. For example, why would bubble columns sometimes disappear? If nucleation sites were in short-supply, I'd expect them to rapidly grow once they managed to form. $\endgroup$ – user32157 Apr 7 '20 at 18:40
  • $\begingroup$ Nucleation sites in general do not "manage to form". They are either there or they are not. The fact that you observed only a few columns of bubbles is pretty much by definition a solid demonstration that the sites were only present at the bottom of the columns - and nowhere else. The phenomenon is most familiar to chemists, who tend to keep their vessels and water scrupulously clean. $\endgroup$ – Guy Inchbald Apr 7 '20 at 18:44
  • $\begingroup$ In "I'd expect them to rapidly grow..." I meant for "them" to refer to the bubble columns, not the nucleation sites per se. Does adding a new object (with abundant nucleation sites) to superheated water cause the entire liquid to start boiling? I did not observe that, but I did observe widespread nucleation, just that most bubbles were not forming columns (another reason to think the water wasn't superheated). $\endgroup$ – user32157 Apr 7 '20 at 18:54

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