I was told that dd-water will superheat when heated up in a microwave because of a lack of ions.

1) Doesn't water auto-ionize? I asked this; the answer was it auto-ionizes "too slowly" in dd-water to prevent superheating.

2) This begs the question: what are the kinetics of the auto-ionization of water? What's the rate?

3) This also begs the question - what is superheating? What does ions have anything to do with superheating?


This has a lot to do with nucleation. Just like crystallization and initial rain drop formation, for boiling to occur an initial site within the solution for a bubble of vapor to form on is required. So if you have very clean water that has been deaerated in a highly polished vessel there is an increased chance for superheating to occur due to the reduction in potential nucleation sites. Tap water can superheat too, it is just less likely to do so because it typically contains more impurities that can serve as nucleation sites. This why boiling chips are commonly used in lab distillations. They provide a site for vapor nucleation and usually prevent superheating of the material to be distilled. Introducing other sites for nucleation, such as dissolved bubbles, solid contaminants, rough spots on the wall of the container all serve to lessen the probability of superheating. I'm not sure what ions would have to do with the superheating process other than they are often representative of microscopic solid impurities that can provide that surface for the initial nucleation event.


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