Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Sublimation and Lantent Heat 
Name: pat j bushong
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
It is like this : Sublimation happens all the time (eg: snow, and frost) so what is the latent heat of sublimation? I know from the books that this is supposed to be a triple point deal, but if that e were so then snow and frost would be extremely special! it takes 80 c/g to turn ice into water and 540 c/g to turn water into steam, how many calories per gram does it take to turn water vapor into ice???? (as in frost and snow) this is driving us all mad here in the earth science/physics department - all from a students question!!! please HELP!!! thanks...


Replies:
You release energy in going from vapor to solid, right? So somewhere around 600 calories per gram is released, and it should be a very favorable reaction (especially at low temperatures of course!) The energy released is presumably quickly taken away by the air or whatever else is keeping the forming frost at that low temperature. I assume the actually energy difference depends strongly on temperature (the energy of the vapor is certainly more temperature dependent than the energy of the solid or liquid)...

Arthur Smith

Yours is really a chemistry question, but I will take a stab at it. There is something called Hess's law that says that a net enthalpy change does not depend upon the steps between the initial and final states. Enthalpy is a "state variable". The heat of sublimation is essentially the enthalpy change; it measures the amount of energy required to break up the ionic bonding in an ice crystal, and free up the molecules to make a gas. This energy should therefore be the same as the net energy required to go from solid -> liquid -> gas; the sum of the heats of fusion and vaporization. Sublimation occurs because you are below the triple point; the relevant pressure is the partial pressure of the H2O in the atmosphere. Thanks for the question. Let me know if I got the answer right.

Jack L. Uretsky


Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory