Re: mechanism of de-superheating water

Lofty Goat wrote:
On Mon, 05 Jan 2009 13:27:25 -0600, Matt wrote:

When water is superheated, its temperature is above the boiling point,
yet it is not boiling. Boiling may be set off by starting
nucleation---by adding boiling chips, for instance.

Please consider whether superheating can be caused by hydrostatic
pressure in the body of water. In the depths of the water, the
hydrostatic pressure is greater, and a higher vapor pressure is required
for boiling. So the temperature can be greater in the depths than it is
at the surface.

When the water is stirred, the hotter water from the depths is brought
near the surface and boils as the pressure is reduced.

Please criticize.

When water is "superheated" it isn't being prevented from boiling by pressure. If that were so then any water could be said to be superheated because if its pressure were reduced by enough then it would begin to boil. In a vacuum water at room temperature will boil, near its surface, and at zero-G will boil throughout its volume.


Okay, thanks. Apparently I was misusing 'superheating'. You should be able to tell from my OP that I understand what you have written above.

But what about my alternate or supplementary explanation of the phenomenon that people see when heating distilled water in a microwave oven or on a stovetop, namely sudden, violent, and unexpected boiling when the water is disturbed e.g. by stirring or by a concussion?

My explanation is in effect a claim that the phenomenon might not be due to superheating.


Water at a great depth might not be superheated, as its boiling point will have increased with pressure, but it may still be "supercritical". At a pressure of a few hundred atm. and a temp of a few hundred degrees (I forget the exact numbers) it will behave like steam, but will be incompressible and still the density of liquid water.

Both are quite interesting, and a Web search on either term will turn up plenty of material comprehensible even to a layman. (E.g. myself.)

-- RLW
.

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