Understanding Sublimation: Principle and Process Explained

[zd1899]

On what principle does a substance sublime?
I mean , does it work towards thermal equillibrium and hence the gain in heat raises the Kinetic energy to hence change the state to vapour?
But then sublime substances still sublime when equillibrium has been attained.

 

[Mapes]

All substances sublime to some degree because some of the atoms have enough energy to leave the solid. This is a spontaneous process because the entropy increase is large as atoms leave to form a gas. As the gas pressure builds, though, atoms from the gas will occasionally reattach to the solid. When this process reaches equilibrium, as many atoms are reattaching as detaching; this equilibrium gas pressure is called the vapor pressure. Does this answer your question?

 

[Renge Ishyo]

The most important issue concerning sublimation is the external pressure on the system. This can be seen on a phase diagram most clearly such as the following for water:

http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg

Notice that on the atmospheric pressure (1 atm) line as you move from left to right ice turns to water and then to steam as we know. But notice that if you lowered the external pressure of the system so that the pressure line appeared lower on the diagram (at pressures below 0.006 atm) you can have solid ice directly sublimate into water vapor...skipping the liquid phase entirely.

This is what happens with Carbon dioxide:

http://www.teamonslaught.fsnet.co.uk/co2 phase diagram.GIF

If you look at the phase diagram for carbon dioxide in the region of patmospheric you will see that the transition goes from solid carbon dioxide to vapor with no liquid phase. To get a liquid carbon dioxide phase you have to increase the pressure on the system above atmospheric pressure (from 1 atmosphere to at least above 5.11 atmospheres of pressure as shown on the diagram) so that the "line" is raised up on the phase diagram and the liquid phase appears upon a temperature increase.

 

[zd1899]

Thankyou! This helped clearing my doubt. Its not right to say so , but my teacher at school said , Sublimation is phenomena observed until thermal equillibrium is attained ONLY and does not depend on other factors.