On Fri, 04 Feb 2005 08:49:38 -0800, Top Spin wrote:

> 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 point is that it is "possible." But, like you say, it's very
unlikely.

The turntable likely "helps," as does the natural convection of the
heating water. It's one of those things - if all of the elements are
right, it can happen. But the improbability factor of all of the
elements be right is so great that it'd probably take you to Alpha
Centauri and back.

(Kudos to anyone who gets the reference.)


>>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.

Or not completely smooth. Or any number of other issues - like the
fact that your microwave has a turntable.

> 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.

That's always a fun experiment. I can't wait to do it with my kid in a
couple of years. You know, opening young minds to science and all
that.

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

Same idea, I would suspect.

>>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.

See? So you're safe.

--
Derek

When birds fly in the right formation, they need only exert half the
effort. Even in nature, teamwork results in collective laziness.