Re: Sublimation

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From: Marc Afifi (marc_afifi@yahoo.com)
Date: Wed Dec 18 2002 - 09:07:00 PST 

Message-ID: <20021218170700.40551.qmail@web40609.mail.yahoo.com>
Date: Wed, 18 Dec 2002 09:07:00 -0800 (PST)
From: Marc Afifi <marc_afifi@yahoo.com>
Subject: Re: Sublimation

Paul D calls the gas to solid transition sublimation.
This is how scientists refer to the transition but it
is a bit confusing to students who like to have
different terms for different phase transitions. I use
the term deposition to refer to the change from gas to
solid but I advise the students that the term
sublimation really refers to both.

Also, water sublimates here on Earth, too. If you
leave a tray of ice cubes in the freezer for a long
time you'll notice that the cubes get smaller and
smaller as time passes. This is because ice has an
appreciable vapor pressure. In fact, under the right
conditions, many, many substances will sublimate. I
hesitate to say all because in chemistry there is
usually an exception to every "rule." You can find
more information about specific substances by looking
at their phase diagrams.

Melting only occurs within a specific range of
temperature-pressure combinations for any particular
substance. The reason ice will not melt on mars is
that the atmospheric pressure is too low to support
the formation of a liquid. The reason that ice will
not melt in your freezer is that the temperature is
too low. Ice can be made to melt at below zero
temperatures if the pressure is high enough.

One of my favorite demonstrations is the formation of
liquid carbon dioxide. This is done by taking crushed
dry ice and putting it into a clear, strong, pressure
release protected, sealable plastic tube. As the dry
ice sublimates the pressure in the tube eventually
reaches a high enough value that the dry ice melts.
Opening the tube releases the pressure and the dry ice
reappears as the fastest molecules escape leaving the
slowest ones behind. It's a very cool demo...pardon
the pun.

I hope this helps and doesn't confuse, but as Paul D
always says...it's more complicated than that.

Happy holidays! You've earned this vacation.

-M

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