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Re: superheated water



Two comments here: (1) top/down heating (2) boiling-chips / gas-liquid
interfaces.

(1) Some private e-mails have questioned my earlier comment that water heats
from top down in a microwave oven. I say, do the experiment. I had done it
many years ago, and I repeated it last night.

I used a tempered glass and put about 5 inches (13 cm) water in it. I
placed it in the center of my Panasonic 700-watt microwave oven with
turntable. I also put a few bits of Kleenex in the water so I could look
for convection currents. I have a digital thermometer with a long
stainless-steel probe that is sensitive at the tip.

After one minute in the oven, the bottom of the water was 40 C and the top
of the water was 75 C. The tissue was immobile on the bottom.

I restarted the microwave. At some point the tissues floated to the top
because they picked up air that was being driven out of solution. Then the
top cm of water began to boil. I stopped the oven and measured temperatures
again. I measured about 60 C at the bottom and 95 C at the top.

I let this water cool so I could have water without dissolved air. Once the
water cooled I pushed the tissues back down to the bottom and put the water
back into the oven. After about 1.5 minutes the top cm of water began to
boil. The tissues never even budged from the bottom. There was clearly no
convection going on. The turntable was operational. Upon removal, water at
the top was 93 C and water at the bottom was 67 C.

Clearly some microwave energy makes it to the bottom... probably mostly from
the sides. But my main point remains... water being heated in a microwave
does not set up convection currents because it is heating from the top down.
This is what I see in three different microwave ovens I have tried.
Microwaves do not penetrate as far into water and into food as many people
think they do. Of course this makes sense... as the waves are absorbed we
expect an exponential decay in intensity something like Beer's Law.

(2) The fact that an existing gas/liquid interface helps initiate boiling is
well known. This is how a boiling chip works. It is also how cracks and
fissures work. It is also well known that once these channels, cracks,
fissures are full of liquid, the gas/liquid interface is gone and can no
longer help initiate boiling. That indeed is the reason that boiling chips
are typically thrown away after one use. They could be dried and used
again, but if you use them in a different experiment you risk cross
contamination of your chemicals. I would throw them away.

It is a common physical chemistry experiment to measure the vapor pressure
of a liquid at various temperatures by flame-sealing a short glass capillary
tube at one end, then invert the tube in the liquid with the closed end up
so that air is trapped inside. Heat the liquid and tube. Before the
boiling point is reached, expanded air begins to bubble from the tube. When
the boiling point is reached, air keeps coming out, but boiling also occurs
at that interface. At this point the primary trick is to keep the liquid
hot and boiling for long enough to replace all the air in the tube with
vapor. This might take two or three minutes of boiling. Then slowly lower
the temperature and watch the tube. When the boiling temperature has been
reached (going from above boiling down to the boiling temperature) the vapor
in the tube begins to condense and draws liquid into the tube. The
temperature at which that first happens is the boiling point at the pressure
of the system. If you have precise control you might be able to hold the
temperature steady so the tube remains full of vapor without bubbling or
filling. If you can do this, you are holding right at the boiling point.
If you go just a tenth degree above boiling it starts to bubble again; just
a tenth degree below boiling and the tube will fill with liquid. If all air
had been excluded, the tube will fill completely. Once filled completely it
no longer "works" as a boiling initiator. If there is still a bit of air
inside, you need to heat it back up and repeat the experiment to get a
better boiling point.

This procedure is written in a fair number of both physical chemistry lab
manuals as well as organic chemistry lab manuals. I have a lab write up
describing in detail how to use this technique with a vacuum pump to take
data for the Clausius-Clapeyron equation for finding the heat of
vaporization for liquids.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817