Dry Ice

Dry ice is the common name for solid carbon dioxide.

One of the things that many people know about dry ice is that it sublimes, that is it goes directly from the solid phase to the gas phase.

We can understand this if we look at the structure of CO₂.

The Lewis Dot Structure looks like this:

The bonds in this molecule are polar, since oxygen is more electronegative than carbon, but the molecule is non-polar since both ends of the molecule are somewhat negative.

However, because CO₂ is linear, the positive carbon is exposed. So, if the CO₂ molecules are lined up properly, there will be the equivalent of dipole-dipole attractions between the molecules.

As the temperature rises, the molecules begin to move faster. For most solids, this means that the attractions become less effective. However, for dry ice, if one of the molecules moves at all, the the dipole-dipole attraction becomes what could be called dipole-dipole repulsion.

As a result that molecule will leave and become a gas.