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Name: Davis
Status: student
Grade: 12+
Location: Outside U.S.
Country: Taiwan
Date: January 2009

I searched "caffeine" and found that it has both sublimation point and melting point. Why is there a melting point after the sublimation point? How is it determined?

Hi Davis,

Look up a phase diagram of any substance. Here is the first non-specific (not of any particular compound) I found:

Notice the line in between solid and gas? This line indicates the combination of pressure and temperature where the solid is in equilibrium with its own gas - the system is at its sublimation point.

Now, notice the line in between the solid and liquid? This line indicates the combination of pressure and temperature where the solid is in equilibrium with its own liquid - the system is at its melting point.

The solid-gas line is always at low pressures and low temperatures, and, in most cases, with the notable exception of water, the solid-liquid line is always at high pressures and temperatures that are higher than the solid-gas line. This makes sense because we expect there to be gases at high temperatures and low pressures, and we expect there to be solids at low temperatures. We also expect that if we apply high pressures to gases, that it will condense and be liquid. Thus, the placement of the phases make sense, and the placement of the lines make sense.

Hopefully, this indicates why melting points are almost always at higher temperatures and pressures.

Greg (Roberto Gregorius)

Caffeine sublimes nicely under vacuum. It will not so well at atmospheric pressure because air molecules basically push the evaporating caffeine molecules back onto the condensed phase. At atmospheric pressure, caffeine melts just like an ordinary molecular solid.

Richard Barrans Jr., Ph.D., M.Ed

There are two different phase transitions occurring. Solid -----> vapor (sublimation) and Solid -----> liquid (melting). There is even a third transition Liquid -----> vapor (evaporation/boiling). You have to be careful to keep in mind that transitions from condensed phases to vapor are usually tabulated at a vapor pressure of 1 atmosphere, but that is not necessary. Water, for example, has a vapor pressure of about 25 mm of Hg at about 25 C. Water will evaporate against that applied pressure, even though water's "boiling point" is 100 C.

Similarly, the sublimation temperature of CO2 (solid to vapor) is about -78 C. It does not have a "boiling point" in the usual sense of the usage that the pressure of the vapor is 1 atmosphere. The distinction that has to be understood is that "boiling points" are tabulated as the temperature at which the vapor pressure is 1 atmosphere (760mm of Hg). But this specification is one of convenience, not of any particular property of the substance.

Vince Calder

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