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Melting Dry Ice
Name: Michael
Status: student
Grade: 4-5
Location: MD
Country: N/A
Date: May 2006
Question:
Why doesn't dry ice melt like regular ice? I know
dry ice goes from a solid to a gas, but why does it not melt like regular ice?
Replies:
Dry ice (solid carbon dioxide) DOES melt like regular ice. The
difference is that the pressure of the vapor of dry ice at its
melting point is about 5 atmospheres. This occurs at a temperature
of -57 C. Now when you have a lump of dry ice in a container of
some sort, the pressure applied by the atmosphere on the dry ice is
only 1 atmosphere, rather than 5 atmospheres. The temperature at
which the vapor of dry ice is 1 atmosphere is -78 C, which is lower
than its melting point. So the dry ice passes directly from solid
to vapor because you don't have the needed 5 atmospheres for it to
form a "normal" melting liquid.
This is not all that mysterious. It happens with water all the
time in the winter where often we can observe frost disappearing
from a window or driveway in the morning without first melting. The
process is a little different, but the concept is the same. In the
case of frost, the wind blows water vapor away at temperatures less
than 0 C., and eventually all the frost (ice) disappears without
ever turning into a liquid.
Vince Calder
Michael,
I did a web-search for the "phase diagram of carbon dioxide" and
this site was the first one to come up:
http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html
You may have to scroll down a bit to get to the phase diagram of
carbon dioxide. I will refer to this diagram.
As you can see from the diagram, at any temperature below -56.4
deg-C (if you draw a line straight up from the x-axis where it says
"-56.4") you will find that at no atmospheric pressure is the carbon
dioxide ever a liquid (the liquid state is in blue). This means that
if the block of dry ice is at or below -56.4 deg-C, then the
substance will either be a solid or a gas depending on the
atmospheric pressure. Why carbon dioxide behaves like this (whereas
most of the substance we encounter do not) is due to many
interacting reasons - all of which just happen to be at the right
levels to give us this property of carbon dioxide.
Greg (Roberto Gregorius)
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Update: February 2012
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