The melting point (m.p.) of rhombic sulfur is 385.8 K while the m.p. for monoclinic sulfur is 392 K. Also, the transition temperature of rhombic to monoclinic sulfur is 369 K.

Now, consider heating the rhombic form starting at room temperature. As the temperature reaches 369 K, it gets converted into monoclinic form with m.p. 392 K. Then what does the fact "the m.p. of rhombic sulfur is 385.8 K" signify?

Is the direct conversion of rhombic to monoclinic sulfur by heating not possible?


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Maurice has a plausible explanation: solid-solid phase transformations are generally sluggish or require a strong nonequilibrium condition to force them because atoms and molecules in a solid are not very mobile. In the case of the sulfur transformation, the activation energy required is relatively low because only a rearrangement of relatively weak intermolecular forces is needed. But the temperature range is also low, so the solid-solid transformation may still not be realized fast enough to match up with typical heating procedures.

But there is a twist. When the rhombic sulfur starts to melt, the liquid has much more mobile molecules, so they can easily rearrange to the more stable monoclinic structure faster than a direct solid-solid transformation would have occurred. In effect, we have a recrystallization process in which the sulfur generates its own solvent. This mechanism is thermodynamically capable as long as rhombic sulfur remains and the temperature is in the proper range between 385 and 392 kelvins.

Some years ago one of the steel galvanizing lines where I work had a problem with its amorphous silica furnace rolls: the silica was crystallizing and this was causing the rolls to spall off material. It turned out to be the very process described above, not directly with molten silica (the furnace is not that hot) but with a molten sodium silicate that was forming when caustic soda from a cleaning section was getting into the furnace. They solved the problem by fixing the rinsing part of the cleaner to keep the caustic soda out of the furnace.


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