Dihydrogen Sulfide Melting

Name: Sharon
Status: educator
Grade: 9-12
Location: CO

Question: I am tutoring a young lady in Science, and in her textbook there is a 
table that gives the melting and boiling points of various substances. For the gas 
dihydrogen sulfide, it gives a melting point of -85 degrees/C. She asked how can a 
gas "melt", and what state is it in after it melts? I have no idea. Can you help?
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You are caught up in a matter of semantics. A gas doesn't "melt". Rather a solid 
or liquid turns from a solid or liquid phase into a gaseous phase. Often there is 
an intermediate solid to liquid to vapor phase --  but that is not always the case. 
On a cold, dry, day water (in the form of ice crystals) sublimes directly from 
the solid phase to the vapor phase. "Dry ice" sublimes from the solid into the 
vapor without entering the intermediate liquid phase. Usually, it is the 
evaporation of a liquid phase at a pressure of 1 atm is stated as "the boiling 
point"; however, a substance can evaporate from either the liquid or solid phase 
at some other applied pressure less than 1 atmosphere.

Vince Calder
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