Melting Dry Ice

name         Michael
status       student
grade        4-5
location     MD

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?
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   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
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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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