Normally, a solid will melt into a liquid, and
then the liquid will evaporate into a gas. Dry
ice is special because it skips the liquid phase:
it is a solid that sublimates into a gas.
Why does dry ice sublimate instead of
melting? It's because at room temperature and
normal pressure (atmospheric pressure), carbon
dioxide is usually a gas. So when you take dry
ice (solid carbon dioxide) and expose it to this
temperature and pressure, it will try to return to
the gas phase.
If you specify a temperature and a pressure, we
can tell what phase of a substance is stable there
(solid, liquid, or gas) using a phase diagram,
this one for carbon dioxide . From the diagram
you can see that at room temperature (T = 273 Kelvin)
and atmospheric pressure (P = 1 bar), carbon
dioxide is a gas. But if we went to a much higher
pressure, say P = 100 bar, carbon dioxide would be
a liquid. At that pressure, dry ice would melt!
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