Sulfur exhibits a very complicated phase behavior that has puzzled chemists for over a century; what you see here is the greatly simplified phase map shown in most textbooks. The difficulty arises from the tendency of S8 molecules to break up into chains (especially in the liquid above 159°C) or to rearrange into rings of various sizes (S6 to S20). Even the vapor can contain a mixture of species S2 through S10.
sulfur phase diagram
The phase diagram of sulfur contains a new feature: there are two solid phases, rhombic and monoclinic. The names refer to the crystal structures in which the S8 molecules arrange themselves. This gives rise to three triple points, indicated by the numbers on the diagram. [Question: which three phases can never coexist?]
When rhombic sulfur (the stable low-temperature phase) is heated slowly, it changes to the monoclinic form at 114°C, which then melts at 119°. But if the monoclinic form is heated rapidly the molecules do not have time to rearrange themselves, so the rhombic arrangement persists as a metastable phase until it melts at 119-120°.
Formation of more than one solid phase is not uncommon — in fact, if one explores into the very high pressures (see below), it seems to be the rule.
Sulfur exhibits a very complicated phase behavior that has puzzled chemists for over a century; what you see here is the greatly simplified phase map shown in most textbooks. The difficulty arises from the tendency of S8 molecules to break up into chains (especially in the liquid above 159°C) or to rearrange into rings of various sizes (S6 to S20). Even the vapor can contain a mixture of species S2 through S10.sulfur phase diagramThe phase diagram of sulfur contains a new feature: there are two solid phases, rhombic and monoclinic. The names refer to the crystal structures in which the S8 molecules arrange themselves. This gives rise to three triple points, indicated by the numbers on the diagram. [Question: which three phases can never coexist?]When rhombic sulfur (the stable low-temperature phase) is heated slowly, it changes to the monoclinic form at 114°C, which then melts at 119°. But if the monoclinic form is heated rapidly the molecules do not have time to rearrange themselves, so the rhombic arrangement persists as a metastable phase until it melts at 119-120°.Formation of more than one solid phase is not uncommon — in fact, if one explores into the very high pressures (see below), it seems to be the rule.
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