Why do sublime solids sublimate in the first place? Is it that their melting point and their liquid's boiling point are the same so that they manage to magically skip the liquid phase? (if so, why does it have to be so?)And is the reverse possible?
Solids may sublimate because they are below the critical pressure, at which the solid has to liquefy upon heating. The boiling point changes depending on pressure. If the solid is at a pressure below which it liquefies, it can go directly to gas. And yes, the reverse is possible. Deposition is the name for this process. Dry ice is deposited carbon gas, which solidifies because the pressure is not high enough for carbon to liquefy at low or ordinary temperatures.
$CO_2$ sublimates at atmospheric pressure, lets have a look at the phase diagram.
Now if you notice the triple point exists above atmospheric pressure. Below this the system can only transform directly from the solid to the gas phase and vice versa by changing the temperature. At the triple point the substance can coexist in all three phases at one distinct temperature and pressure. Beyond the triple point the three phases are distinct with respect to temperature, so via heating or cooling you can witness all three different states.
The opposite of sublimation exists and is called deposition, an example of which would be frost.
A link to a deeper explanation: http://www.av8n.com/physics/melt-sublimate.htm
$\begingroup$ While helpful, the phase diagram isn't so much an explanation for phase changes as a concise collection of phase change data. I don't mean to put you down, I'm just hoping someone touches on the thermodynamic explanation based on free energy. $\endgroup$ Apr 21, 2013 at 19:03
$\begingroup$ yeah, It would be better if yo hit upon the thermodynamics directly $\endgroup$– neweraApr 22, 2013 at 0:20
$\begingroup$ The answers given above are not the explanations, but more the statements of this same fact in more accurate terms. The explanation should appeal to a theory of the phenomenon. I, however, have never seen such a theory before. I may guess that this is due to the fact that the crystallization transition is very poorly understood. As much as I know, only a so-called, weak crystallization theory is developed (see e.g. Kats et al, Physics Reports, 228, pp. 1–91 (1993)). The weak crystallization, however, does not describe such cases as the CO2 mentioned above. $\endgroup$ Apr 24, 2013 at 15:23
$\begingroup$ Without such a theory, one cannot explain the triple point gas-liquid-solid. This was, at least, a status about 10 years ago, when I stopped to follow these works. I would be delighted, if something have changed since that time. $\endgroup$ Apr 24, 2013 at 15:25