Re: Hot water in microwave

[William Beaty <billb@ESKIMO.COM>]

On Sat, 5 Feb 2000, Hugh Haskell wrote:

> I have this effect happen to me almost every time I heat water for
> tea in the microwave oven. It isn't rare and it isn't hard to do (and
> it was recently discussed in some depth on Physhare). What I have
> found is that you have to get the water boiling vigorously before you
> shut off the oven and remove the water. If you get it just boiling or
> just before it boils, you don't get the effect, or if you get it it
> isn't very dramatic. The fact that it must be boiling vigorously in
> order to see the effect (at least in my experience) makes it hard for
> me to believe the superheating hypothesis, although until I thought
> about this fact, I was a proponent of that idea. Maybe someone can
> explain to me how vigorously boiling water can be hotter than 100° C,

Maybe all the apparent boiling is on the surface?  When I boil water in a
glass mug, I see bubbles appear and expand all within about 1cm of the
top.  Perhaps this doesn't provide enough additional surface area to
significantly cool the water, yet it might stir the contents and allow the
entire volume to be exposed to the microwave "hot zone."  If bubbles were
triggered at the bottom, and rose all the way to the surface (the way they
do on a stove), then I'd expect the water not to superheat.

Separate question: I wonder if the microwave radiation significantly heats
the steam?  Once a steam-bubble has appeared near the top of the water
surface, does it expand because of evaporation of the liquid, or does the
absorbtion of microwave energy play any significant role?


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L