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Re: superheated water



Another point and question: Is the specific heat of water above 100 C greater than
one? If so, that would make it especially dangerous!

bc

At first I thought that the heat of condensation would be added -- but it's subtracted
first.




Michael Edmiston wrote:

I've often seen water superheat in a microwave. I think it is common in a
microwave (compared to stove top) for two reasons.

(1) A coffee mug, especially new, is pretty smooth on the inside. John
Denker says cups get scratched up. I suppose they do, but ceramic and glass
mugs don't get scratched up very much.

(2) Most microwave ovens bring the waves in from the top. Therefore the
water heats first at the top; just the opposite of water on a stove. Stove
heated water sets up convection currents. Microwave heated water can remain
quite still during heating.

Still liquid in smooth containers would be the prescription for
superheating. Upon removal, sudden movement could initiate the boil, or
addition of a teabag, instant coffee, hot chocolate mix, etc. could initiate
the sudden boiling.

I have had the water boil over onto my hand as I am removing it from the
oven. After that happened a couple times I started flicking the side with
my finger before picking it up. I think that has saved my hand from a burn
perhaps two or three times. But nowadays I rarely heat cups of water in the
microwave.

I saw this same question on another list. A person on that list suggested
adding the teabag (etc.) at the beginning rather than after the water is
hot. That way the tea can serve as a boiling initiator. I say "Yuck!" to
that. I'm enough of a tea connoisseur that I would never do it again after
having tried it once... the tea tastes terrible. It even tastes bad if you
put it in the cup after the water is hot, and that action initiates boiling.
Tea should not be boiled. Therefore, for taste reasons, I always heat my
water in a teakettle on the stove. After boiling is reached, I remove the
kettle from the stove, wait 5-10 seconds, then pour it into the cup with
teabag. The water needs to be just below boiling temperature. I think
every tea vendor will tell you to make tea that way... and there's good
reason.

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