Again - the issue is all about nucleation sites. These are “points where phase transition is favored.” Nucleation sites are necessary for the heated water to change from liquid to gas.
As you heat water, it will phase shift from liquid to gas beneath the surface at these nucleation sites. That’s where the bubbles and the rolling boil comes from in boiling water. So long as there are sufficient nucleation sites within the water holding on to tiny bits of air - whether those are tiny variations in the surface of the container, or particulate matter - there is no danger. If the water in the container is bubbling or rolling, there is no danger of “superheating.”
The danger comes when all the little air bubbles held by those nucleation sites have already been freed and left the liquid. The water is extremely hot, but it is unable to phase shift beneath its surface. Now there is danger, because there aren’t any nucleation sites left. Introducing new nucleation sites (making contact with previously uncontacted upper parts of the container, adding something like instant coffee, or pouring into another container) causes the phase shift from liquid to gas to happen again, and if the added number of nucleation sites is high enough, the whole container will try to phase shift at the same time. Because the water is in a container, with an open top, the only place for it all to go is out that top, explosively, like a bullet exiting a gun.
None of the previous paragraph will happen with potable water in a household container microwaved for three minutes.
Myth # 3 - Exploding Water
The Myth - If a glass of water is microwaved, removed, and an additive placed in it, it can explode due to superheating.
Verdict - True
Notes - If the water had no impurities in it at the time of superheating (for instance, distilled water), then any sort of additive placed within will make the water flash to steam and violently spray.
Again - the issue is all about nucleation sites. These are “points where phase transition is favored.” Nucleation sites are necessary for the heated water to change from liquid to gas.
As you heat water, it will phase shift from liquid to gas beneath the surface at these nucleation sites. That’s where the bubbles and the rolling boil comes from in boiling water. So long as there are sufficient nucleation sites within the water holding on to tiny bits of air - whether those are tiny variations in the surface of the container, or particulate matter - there is no danger. If the water in the container is bubbling or rolling, there is no danger of “superheating.”
The danger comes when all the little air bubbles held by those nucleation sites have already been freed and left the liquid. The water is extremely hot, but it is unable to phase shift beneath its surface. Now there is danger, because there aren’t any nucleation sites left. Introducing new nucleation sites (making contact with previously uncontacted upper parts of the container, adding something like instant coffee, or pouring into another container) causes the phase shift from liquid to gas to happen again, and if the added number of nucleation sites is high enough, the whole container will try to phase shift at the same time. Because the water is in a container, with an open top, the only place for it all to go is out that top, explosively, like a bullet exiting a gun.
None of the previous paragraph will happen with potable water in a household container microwaved for three minutes.
https://mythbusters.fandom.com/wiki/Microwave_Madness