Re: superheated water

[William Beaty <billb@ESKIMO.COM>]

On Thu, 3 Feb 2000, John S. Denker wrote:

> The idea of superheated water in a coffee mug seems pretty far-fetched to
> me, because every coffee mug I've ever seen has more than enough scratches
> in the surface to provide plenty of nucleation sites.

One possibility:  if it's the liquid/gas interfaces in the scratches which
causes the nucleation, then danger might increase if the water was boiled
TWICE, and allowed to cool between boilings.  On boiling the first time,
air within the nucleation sites would be diluted away by steam.  Upon
cooling, the trapped steam bubbles would collapse to nothing.  When the
temperature was again raised, there might be no nuleation sites remaining.

Somebody mentioned that boiling can be triggered in a beaker by scratching
the surface.  If bubbles cause nucleation and sharp edges do not, then
scratches might still cause nucleation.  When the glass fractures during
the scratching process, the cracks would momentarily contain vacuum, but
then would immediately fill with steam.

Suggested experiment: superheat water in a clean beaker, scratch the
surface to provide nucleation sites, let it boil for awhile, then cool the
water to collapse all the steam bubbles (maybe wait a few minutes so the
water can ooze into all the remaining nano-sized crevices.)  When the
water is again heated, does the same scratch still act as a trigger?
Perhaps a "wet scratch" cannot trigger boiling, even if the scratch
contains many sharp edges.


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