The reason is because you need nucleation sites for boiling to start. The
teabag adds them.
----- Original Message -----
From: "Stephen A. Lawrence" <[email protected]>
To: <[email protected]>
Sent: Friday, July 15, 2011 5:55 PM
Subject: Re: [Vo]:They say liquid water can't be hotter than boiling...
>
----- Original Message -----
From: Stephen A. Lawrence<[email protected]>
To: [email protected]
Cc:
Sent: Friday, July 15, 2011 5:03:11 PM
Subject: [Vo]:They say liquid water can't be hotter than boiling...
So here's a cute experiment, done by accident while on vacation.
Take a smooth china mug, and fill it with water.
Stir the water, so it's swirling nicely (if you don't do this only the
surface will get hot and the experiment probably won't work).
Put it in a microwave on high power for a minute or two. I used 2
minutes, but
the microwave in question was probably not very high power.
Take it out, stir it *again* so it's swirling nicely, and pop it back
into
the microwave for another minute or two.
Take it out. There may be a few bubbles, but on a good morning, it will
*not*
be boiling, not what most of us mean by boiling, anyway.
Drop a teabag into the cup of water which isn't boiling.
Whoa, nelly -- bubbles galore! Now it's boiling!
Gosh, what was in the cup before I put the teabag in?
I've heard you can use a microwave oven to superheat distilled water in
a smooth china mug.
This news to me that you can superheat ordinary water as long as the
water is swirling in the mug.
Harry
It surprised me, too, that's for sure. (All I was trying to do was make a
cup of tea...)
The reason for swirling it was just that a lot of microwave ovens seem to
heat from the top, and if you don't get it swirling, you end up with a cup
of tepid water with a layer of boiling water a fraction of an inch thick
floating on the top. I'd actually have guessed that water in motion was
*less* likely to superheat than still water.
It got me thinking about drops of water entrained in a stream of water
vapor, and made me wonder just how hot the steam must be before you can
*assume* that there is no liquid water hitching a ride in it. Somewhere I
recall reading that, at 1 atm, 110C is hot enough that you can safely
assume dryness with no further tests. 101C, OTOH, probably isn't.
