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Name: Greg
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
I recently got an email warning of the dangers of heating water in the microwave. THe warning suggested that upon removal from the microwave, water can "explode" and cause serious burns. One person descibed this:

"It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new. What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat that has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point. What then usually happens is that the liquid is bumped or jarred, which is just enough of a shock to cause the bubbles to rapidly form and expel the hot liquid."

Is there any merit to this warning or this explanation? Seems a little hokey to me.


Replies:
As water is heated up and its temperature approaches boiling point (at the prevailing pressure), bubbles are formed to transfer heat from water by evaporation. For bubbles to form, there has to be sites for them to nucleate (initiate). An ideal surface having ideally smooth and degasses surfaces and an ideal fluid (e.g., devoid of any impurity and gas), the instability you described can be created. There would be none or few bubbles and the temperature can exceed boiling point - with no boiling. You can get violent burst of bubbles if you suddenly drop an ordinary object into this container because bubbles will suddenly form on the object and move to the surface in a sudden burst.

These conditions can arise regardless of the source of heating. A smooth glass container on the stove or inside a microwave oven can do, except that on the stove the heat is imparted to water through the container wall which is the hottest part of the system and is likely to have more nucleation sites that the liquid. In the microwave oven, it is water that is likely to be the hottest part of the system, and with fewer and smaller nucleation sites it will have to get hotter before it starts generating bubbles.

If I remember correctly, the smaller the nucleation sites the more energy it takes to create a bubble. This explain why water in a microwave oven will get hotter than on the stove before it starts boiling.

If you want to read more about this topic, you may want to refer to an introductory text book on "boiling heat transfer."

AK

--
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory



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