Re: Solid State Physics Q about ice melting point
- From: "Igor Khavkine" <igor.kh@xxxxxxxxx>
- Date: Thu, 23 Feb 2006 22:36:26 +0000 (UTC)
Norm Dresner wrote:
In today's New York Times Science Times section -- which you can also read
on-line at
http://www.nytimes.com/2006/02/21/science/21ice.html?_r=1&oref=slogin
in an article about the unknown reasons for the slipperiness of ice occurs
this statement:
"The lower density of ice [relative to liquid water] also means that the
melting temperature of ice can be lowered below the usual 32 degrees by
squeezing on it."
Since Solid State Physics was the subject I did worst in during my
undergraduate days over 4 decades ago, I can't even begin to understand the
causal relationship between the quantities in this statement. I'm hoping
that someone can illuminate the underlying physics for me.
Actually, this situation can be explained by simple thermodynamic
(macroscopic arguments). We already know that, say close to water's
tripple point, the density of the liquid phase is higher than of the
crystal phase. Common sense tells us that applying pressure will tend
to increase the density of a substance. So if we take ice that is close
to the melting point and apply pressure to it, then it will have no
choice but increase in density. But at a sufficiently higher density,
the only stable phase will be the liquid one. Now, keeping this
pressure constant, we can recover ice by lowering the temperature. This
is equivalent to saying that at higher pressure we've lowered the
melting point (which can also be called the freezing point).
At first glance, it might seem that for high enough pressures we can
get the melting point as low as we want. But that idea runs into
trouble there are crystalline phases of water that have higher density
than the liquid phase and are stable at high pressures. Solid state
physics (or rather condensed matter physics, as it is fashionable to
call it nowadays) comes in only one tries to reproduce these properties
of water from molecular models. I don't know much about how that's
done, but there seems to be some information at the following site,
including a nice P-T phase diagram for water:
http://www.lsbu.ac.uk/water/phase.html
Hope this helps.
Igor
.
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