Sulfur (atomic number 16) naturally forms molecules composed of eight atoms in a puckered ring structure, denoted as S\(_8\). When liquid sulfur cools, its atoms can arrange themselves into different crystalline and non-crystalline solids, known as allotropes. This variety of physical forms makes the solidification point variable. The exact temperature at which the liquid turns to solid depends on which of these multiple solid structures is forming.
The Standard Freezing Temperature
Under standard thermodynamic conditions, when a stable liquid is cooled slowly enough to maintain equilibrium, it solidifies by first forming the allotrope that is stable at the highest temperature. For sulfur, the liquid state initially transitions into the monoclinic form, often referred to as \(\beta\)-sulfur, at approximately \(119^\circ\text{C}\) (\(246^\circ\text{F}\)). This temperature marks the initial freezing point where the liquid S\(_8\) molecules begin to arrange into the needle-like crystalline structure. The monoclinic form is the one that forms first from the liquid upon cooling, although it is not the most stable solid state at room temperature.
This monoclinic solid then undergoes a second, lower temperature transformation to reach the most stable form of solid sulfur. Below \(95.5^\circ\text{C}\), the stable form is the rhombic form, or \(\alpha\)-sulfur, which is a pale-yellow, octahedral crystal. If the rhombic solid is heated, it melts at about \(112.8^\circ\text{C}\) (\(235^\circ\text{F}\)), representing the melting point of the most stable structure. The solidification process thus involves the liquid freezing at \(119^\circ\text{C}\) to form an intermediate solid, which then structurally changes into the stable rhombic solid below \(95.5^\circ\text{C}\).
Why Sulfur Has Multiple Solid Forms
The existence of multiple solid forms, known as allotropes, arises from the way sulfur atoms can pack together. Rhombic sulfur, the most stable form below \(95.5^\circ\text{C}\), is characterized by its atoms arranging into a specific octahedral crystal lattice. This arrangement is the lowest energy configuration, giving it the highest density and stability.
When heated above \(95.5^\circ\text{C}\), the rhombic structure becomes unstable and converts into monoclinic sulfur. Although the S\(_8\) ring molecule remains the same, the molecules pack differently, resulting in a prismatic crystal structure. This change requires little energy, allowing the two forms to transition back and forth at the \(95.5^\circ\text{C}\) transition temperature. The differing melting points (\(112.8^\circ\text{C}\) and \(119^\circ\text{C}\)) reflect the slight differences in energy required to break their distinct atomic packing arrangements.
How Cooling Rate Influences Solidification
The rate at which liquid sulfur is cooled significantly impacts which solid form is produced, introducing a factor known as kinetics. When liquid sulfur is cooled very slowly, the atoms have sufficient time to align themselves into the ordered, crystalline monoclinic and then rhombic structures. This slow process allows the system to follow the thermodynamic path toward the most stable solid states.
A rapid cooling process, however, prevents the sulfur atoms from settling into an ordered crystal lattice. This creates a metastable solid known as amorphous or “plastic sulfur.” This plastic form is rubber-like and results from the liquid S\(_8\) rings breaking apart and linking into long, tangled polymer chains, which are then frozen in place by the rapid cooling.
Because this amorphous form is frozen before it can crystallize, it solidifies at a different, less defined temperature and lacks the sharp melting point of the crystalline allotropes. The liquid is cooled below its standard freezing point without solidifying into the most stable phase. Over time, plastic sulfur will slowly revert to the most stable rhombic form, demonstrating its temporary, non-equilibrium nature.