Phil, from snopes, true, but rare. I put in for
a search string, heating water in a
microwave. After clicking on a link, I snipped this:
Origins: Exploding water? In a nutshell,
Too hot to handle
yes, water can "explode" in the fashion
described above. However, it takes near
perfect conditions to bring this about, thus
"exploding water" is not something the
average hot beverage drinker who would otherwise
now be eyeing his microwave with
trepidation need fear. Odds are, you'll go
through life without ever viewing this
phenomenon first-hand, and if you're one of the
rare few who does get to see it,
you will likely not be harmed by the experience
(that would take your standing right
over the cup at the instant it happened, and the
liquid's bolting up and hitting
your skin).
The Food and Drug Administration (FDA) has advised consumers:
This type of phenomenon occurs if water is
heated in a clean cup. If foreign materials
such as instant coffee or sugar are added before
heating, the risk is greatly reduced.
If superheating has occurred, a slight
disturbance or movement such as picking up
the cup, or pouring in a spoon full of instant
coffee, may result in a violent eruption
with the boiling water exploding out of the cup.
What Can Consumers Do to Avoid Super-Heated Water?
Follow the precautions and recommendations
found in the microwave oven instruction
manuals, specifically the heating time.
Do not use excessive amounts of time when
heating water or liquids in the microwave
oven.
Determine the best time setting to heat the
water just to the desired temperature
and use that time setting regularly.
The e-mailed warning quoted above a model of
non-specificity (no names, date, or
locale; only the detail that the "victim" was 26
years old, presumably to enforce
the idea that this accident could not be blamed
on a child's typical lack of caution),
leaving us able to address only its theoretical
aspects. Nearly every science writer
who has tackled the topic notes that the "severe
superheating" phenomenon is real
but likely not nearly as common as the message cited above might make it sound.
In an article for New Scientist, Richard Barton wrote:
A portion of the water in the cup is becoming
superheated — the liquid temperature
is actually slightly above the boiling point,
where it would normally form a gas.
In this case, the boiling is hindered by a lack
of nucleation sites needed to form
the bubbles
. . .
I imagine that by keeping the cup still and
microwaving for a long time, one could
blow the entire contents of the cup into the
interior of the microwave as soon as
you introduced any nucleation sites. It is this
sometimes explosive rate of steam
production that means you should take great care when using a microwave oven.
From the
How Things Work
web site:
Glass containers are clearly the most likely to
superheat water because their surfaces
are essentially perfect. Glasses have the
characteristics of frozen liquids and a
glass surface is as smooth as... well, glass.
When you overheat water in a clean
glass measuring cup, your chances of superheating
it at least mildly are surprisingly
high. The spontaneous bubbling that occurs when
you add sugar, coffee powder, or
a tea bag to microwave-heated water is the result
of such mild superheating. Fortunately,
severe superheating is much less common because
defects, dirt, or other impurities
usually help the water boil before it becomes
truly dangerous. That's why most of
us avoid serious injuries.
From the
Unwise Microwave Oven Experiments
web site:
Things are different in a microwave oven. The
water gets hot but the container usually
does not. There are no "boiling-bubbles"
triggered by a hot metal pot. Without those
bubbles to cool it, the temperature of the water
rises far higher than 100C°. We
call this "superheated water." Superheated water
is just waiting for some sort of
trigger which will let bubbles form and allow
boiling to commence. If the water becomes
hot enough, a few bubbles will appear, but these
quickly rise and burst, and the
water isn't cooled much at all. In the microwave
oven, even if your mug of water
is bubbling slightly, don't trust it, since it's
temperature has risen so high above
100C° that bubbles are appearing spontaneously.
If some unwitting victim should pour
a soluble powder into the superheated water, this
will carry thousands of tiny air
bubbles into the water. Each of these micro
bubbles expands into a 1cm steam bubble,
and the result is a huge "explosion" of hot
froth. It's just like dumping ice cream
into root beer, but the froth can be so violent
that the hot water sprays into the
air.
If you're worried about explosions over
superheated liquid, you can virtually eliminate
the possibility of its happening to you by simply
leaving some non-metallic object
(such as a wooden spoon or stir stick) in the cup
or bowl when you boil liquids in
your microwave. It may not be necessary, but it
won't hurt anything either. Peace
of mind rarely comes so cheaply.
earlier, Phil Scovell, wrote:
>This is one to look up on snopes I guess but I have heard of this before and
>things falling apart after being super heatened in the microwave.
>
>Phil.
John
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