H2O Microwave Criticality

[Tad Blanchard <Tad.M.Blanchard.1@gsfc.nasa.gov>]

I came across the following and had to post it here for everyone to enjoy:


"Subject: Microwaving Water

I received the following from my cousin who works at the CDC in Atlanta.
It's worth sharing. I feel that the following is information that any one
who uses a microwave oven to heat water should be made aware of.  About
five days ago my 26-year old son decided to have a cup of instant coffee.
He took a cup
of water and put it in the microwave to heat it up (something that he
had done numerous times before).  I am not sure how long he set the timer
for but he told me he wanted to bring the water to a boil.  When the timer
shut the oven off, he removed the cup from the oven.  As he looked into the
cup he noted that the water was not boiling but instantly the water in the
cup "blew up" into his face.  The cup remained intact until he threw it out
of his hand but all the water had flew out into his face due to the buildup
of energy.  His whole face is blistered and he has 1st and 2nd degree burns
to his face which may leave scarring.  He also may have lost partial sight
in his left eye. While at the hospital, the doctor who was attending to him
stated that this a fairly common occurrence and water (alone) should never
be heated in a microwave oven.  If water is heated in this manner, something
should be placed in the cup to diffuse the energy such as a wooden stir
stick, tea bag, etc. It is however a much safer choice to boil the water in a
tea kettle.  Please pass this information on to friends and family.

Here's exactly what happened: When water boils, bubbles form.  We all
know that.  It takes energy to form a bubble and they usually form on rough
corners because that takes less energy.  When I took organic chemistry,
we'd put "boiling stones" (small, rough piece of calcium carbonate) in
the bottoms of beakers.  This gave "nucleation points" for bubbles to form
and kept the liquid from overheating (heating a few degrees above the
boiling point) and "bumping." Because the cup was apparently smooth, there
were no nucleation points and the water overheated. Shaking the water allowed
the bubbles to form and it "bumped" with serious results.

Rule:  When boiling water in a microwave, use a clear container so you
can see the bubbles. (I usually use a Pyrex measuring cup.) And if the
container has scratches all the better.  A non-metal object with sharp
corners in the bottom of the cup would also be helpful."

(HOW MUCH OF THIS IS TRUE??)



Tad Blanchard                   NASA-Goddard Space Flight Center
Sr Health Physics Tech           Code 205.9, Greenbelt, MD 20771
Parallax, Inc                                Phone: 301-286-9157
A Member of the OHI Team                     Fax:   301-286-1618
            mailto:tmblanch@pop200.gsfc.nasa.gov
http://gsfc-aphrodite.gsfc.nasa.gov/205/205-2/Health/RADPROT.HTM

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