On Fri, 4 Feb 2005 09:08:56 -0600, Derek > wrote:

>On Fri, 4 Feb 2005 20:27:19 +0900, kuri wrote:
>
>> "Bluesea" > wrote in message
>>
>>>> >WHOA! Stop right there 'cause you're scaring me. Are you aware that a
>>>> >microwave oven can super-heat water beyond the 212F boiling point w/o
>>>> >it ever creating a shimmer much less a bubble?
>>
>> No, it can't. Use your thermometer to check (I did). Microvawe is not magic.
>> The bubbling occurs at the same temp. whatever you use to heat your water,
>> the exception is when you do it in a pressure cooker.
>
>Yes, it actually can. Yes, it has been documented.
>
>A clean, smooth container in the microwave inhibits nucleation, the
>transition from liquid to gas. Temperatures as high as 241 degrees
>Fahrenheit have been achieved using a microwave without ever having a
>bubble form. The jostling of the water when removing typically prompts
>the sudden formation of bubbles, as evidenced in the boil over.

OK, I believe that this is "possible". It's just very unlikely -- at
least in my situation for several reasons. I am boiling a lot of water
(3-4 cups). It takes about 7 minutes for my microwave to bring 35
ounces of cold water to a boil. I set the timer for 10 minutes. It
never goes on and on and so is unlikely to get superheated. For some
reason, I have never experienced the lack of bubbles. Always, there
are a few bubbles well before it starts a real boil. I have one of
those turntables so maybe the turning causes a little jiggling which
kicks off the nucleation.

>The same effect can actually be accomplished with a kettle. But most
>kettles have seams in the metal, or patterns embossed in the bottom.
>These are enough to prompt nucleation, so at 212/100 degrees, water
>boils at sea level in a kettle.

Maybe my pyrex is not really clean so there are little particles on
the sides that kick off nucleation.

Remembering way back to high school chemistry, I am guessing that
keeping the container still is more important that keeping the edges
smooth. The chemistry teacher did a little experiment with
supersaturation one day. He took a beaker of water and heated it up to
80C while dissolving sugar in the water. He told us that hot water can
dissolve more suger/gram of water than colder water. When he had
dissolved as much sugar as he could, he put a pencil over the beaker
with a string with a little weight dangling down into the water. He
then turned off the bunson burner and went on with his lecture. He
told us that we had to be careful not to bump the counter where the
beaker was sitting.

After 30 minutes or so, he came back to the beaker which was now much
cooler. He took a knife or something and tapped the side of the beaker
one time. In a flash, the string was covered with sugar crystals --
like the rock candy you sometimes see in novelty stores. He explained
that when the water cooled, it became super saturated. There was more
sugar in the water than the water could handle, but the crystalization
couldn't get started. The tap got it started.

The string was to give it a place with some rough edges to get started
on. Without the string, he said, it mght not form crystals or it wold
form on any little rough spot or impurity on the edges of the beaker.

I don't know if this relevant, but something similar might be
happening with the water and the formation of bubbles.

>I have "tested" several different microwaves over the years, and on
>several instances, I produced a cup of water that was over 212/100
>when measured with a thermometer but never produced a bubble.
>
>However, to accomplish the "boilover" in a microwave typically
>requires overcooking the water by a couple of times beyond what was
>necessary to achieve a boiling temperature.

That makes sense. I never let it go much more than a minute or so past
the boiling point.

--
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(11/09/04)