[P-man]

We had a big discussion today about this in science class. So is it physically possible to go from a gas to a solid without going through the liquid phase?

[Yggdrasil]

Yes.  At temperatures and pressures below the triple point of your substance, a liquid cannot exist.  For more information see:

http://en.wikipedia.org/wiki/Phase_diagram

[enahs]

See also Dry Ice.

[biospy]

The gas-to-solid transition is called deposition, sublimation is the other way...just so you are clear on that..

[jennielynn_1980]

That is how they freeze dry food (like astronaut food and instant coffee)

[P-man]

I don't really understand. How can the molecules suddenly jump to solid from gas?

[Yggdrasil]

It's the same process as molecules jumping from liquid to gas.  The molecules break the intermolecular bonds which join them together to enter the gas phase.  The only difference is that there isn't enough pressure to hold together weakly interacting molecules which would at higher pressures form a liquid.  Therefore, when the intermolecular bonds are broken in the solid they go directly to the gas phase because the intermolecular forces aren't strong enough to hold together a molecules in a fluid state.

[P-man]

Not even for a split second?

[Yggdrasil]

There may be some weak interactions between gas molecules during sublimation which approximate the interactions found in a liquid, but a liquid is a macroscopic state, not a microscopic one.

[P-man]

So there actually may be an un-recordable amount of time where the molecules are a gas.