Do you recall our exchange of ideas about ice melting under pressure? The
discussion was triggered by C. Swartz who wrote about the subject in his
new book. I finally had a day cold enough to perform the promissed
experiment.
A copper wire (diameter 1 mm), loaded with 4.5 kg masses on each side, was
placed over a cylindrical block of ice. First I did this indoor (+27 C)
then outdoor (+7 C). The difference was dramatic.
1. Indoor data. After about 20 minutes a groove below the wire was already
as deep as its diameter. An hour later the top of the wire was already
about 4 mm below ice surface and a layer of ice was formed above it
(regelation). The water was dripping. You probably saw this demonstration
or read about it.
2. Outdoor data. After one hour the wire was still sitting on top of ice
and no groove was formed. The same status at midnight, two hours later,
when the temperature was still +7C. The groove, if any, was too shalow
to be noticed. I left the wire hanging and went to sleep. The morning
temperature (at 7 a.m.) was still the same and I expected the wire to
be on top of ice, as I left it. But that is not what happened. The wire
was deep inside ice (penetration and regelation of 80 mm). The entire
surface was wet but the diameter of my ice cylinder did not change by
more than 2 mm. The wire would probably go through ice but the weights
reached the ground before this happened. It is unlikely that the outdoor
temerature went up for a short time during the night.
Conclusion? Swartz was right; lowering of the melting point of water by
pressure does exist but an enormous pressure is needed to observe A wire
penetration into ice is due to the conduction of heat along it, from the
warmer section below to the colder section above. The driving difference
of temperature was 27 C indoor and only 7 C outdoor.
During the first two hours the surface temperature of my block was still
below the melting point (ice, taken from the freezer at t= -8 C, was dry).
But during the night the ice temperature did come to 0 C and the wire
started to penetrate into ice (thermal contact improves considerably after
the surface becomes wet.) Progressive penetration was not observed but I
believe that its rate of was much slower than indoor. I predict that the
loaded wire will not penetrate ice when the outdoor temerature is less
than zero degrees C.
Here is a typical quotation from a textbook (Physics by A. Beiser, 3rd
edition, page 334). Ice "is unusual in that its melting point decreases
with increased pressure. Hence, it is possible to melt ice by applying
pressure to it as well as by heating it. An ice skater makes use of this
fact in an interresting way. ..." You know the story.... I used to say,
incorrectly, that "crystals are crushed to the liquid state by pressure".
My blocks of ice were made in common plastic soda bottles (2 liters) filled
with water and placed into a freezer for two days. I cut away plastic and
had nice cyliners of ice. Two tables was used to support the structure indoor;
outdoor it was supported by a simple wooden stand. A
IT IS FUN TO PLAY A SCIENTIST. A student project can easily be formulated
on the basis of a common textbook description of melting under pressure.
Ludwik Kowalski kowalskiL@alpha.montclair.edu
P.S. A person spending too much time on a computer is a CLICK POTATO.
HAPPY NEW YEAR, DEAR CLICKING FRIENDS.