On Fri, 4 Feb 2005 20:27:19 +0900 "kuri" > wrote:
>
> How does water "explose" ? A microwave is basically a water heating device.
> If it is not fit to safely heat water in a pyrex glass, you are entitled to
> sue the maker and that of the pyrex.

I agree that one should be careful.

Here's what I (a layman) think about it:

"Boiling" is when the liquid (e.g., water) is heated enough
that parts of it start changing phase from liquid (less
energetic molecularly) to gaseous (more energetic).

In a kettle, heat is applied by LOCAL contact with the liquid.
The heated parts of the liquid rise, creating convection.
Eventually, at local "hot spots", parts of the liquid receive
enough energy to start becoming gaseous (e.g., steam). The
gas bubbles are not "trapped" by the "energy state" of the
remaining liquid -- the bubbles escape to the outside air.

In a microwave, heat is applied THROUGHOUT the mass of the
liquid. "Hot spots" have much less of a chance to develop.


In a microwave, the liquid *might* sit "still" enough that
when parts of it have received enough energy to start
becoming gaseous, all neighboring parts are in the same
energy state. The "vapor pressure" of the gas bubble to be
formed from one (still liquid) part is TRAPPED by an equal
"vapor pressure" existing in the (still liquid) neighboring
parts. In such a state, the liquid is called "superheated"
-- it has received enough energy for parts of it to become
converted to the gaseous phase, but because of the "stillness"
of the liquid, these phase changes have not yet occurred.

Normally there will be enough "stirring" within the liquid
in the microwave for the "superheated" condition not to
develop - gas will start escaping rather than being trapped.
Also, even if the liquid becomes "superheated", further
energy input would usually cause a random part of the liquid
to start "bubbling" (though perhaps if a large amount of gas
is suddenly released it might be called "exploding").

But if the liquid is still "superheated" when taken from the
microwave, that jostling ought to be enough to cause the
phase changes to take place all at once. So much gas might
get formed that the remaining liquid is "exploded" out of
its container.


It is not a question of sueing the maker of the microwave or of
the pyrex. It is the __nature__ of microwave energy to penetrate
water, thereby causing it to be heated THROUGHOUT its mass.


mikus