Microwave and Heating Water
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.
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
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."
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
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Update: June 2012