Sulfur
Formula: S₈ — Cyclooctasulfur, brimstone
Molar mass: 32.07 (S); 256.5 (S₈) g/mol
Appearance: Bright yellow crystalline solid or powder; odorless in pure form
Hazard: Flammable · Burns produce toxic SO₂ · Keep away from oxidizers
Summary
Elemental sulfur is a yellow nonmetal that exists at room temperature as the rhombic allotrope — crown-shaped S₈ rings packed into an orthorhombic crystal lattice. Above 96°C a second solid form, monoclinic sulfur, becomes stable; it grows as pale yellow needles and reverts slowly to rhombic on cooling. Above 119°C sulfur melts to a yellow mobile liquid; above ~160°C the rings break and chain up into long polymers, producing a viscous dark-red mass. Quenching this hot viscous liquid in cold water traps the polymer chains as plastic sulfur — a rubbery, stretchy, amber solid.
Density 2.07 g/cm³ (rhombic). Practically insoluble in water; dissolves well in carbon disulfide, benzene, and toluene. Burns with a pale blue flame producing sulfur dioxide (SO₂):
\[\ce{S8 + 8O2 -> 8SO2}\]
History
Sulfur has been known since antiquity. The Bible’s “brimstone” is elemental sulfur, and the ancient Romans burned it as a fumigant. Gunpowder — invented in China around the 9th century — required sulfur as fuel (charcoal + potassium nitrate + sulfur). For centuries, Sicily held a near-monopoly on natural sulfur deposits formed by volcanic activity.
The modern chemical industry was built on sulfur. The lead chamber process (1746) and later the contact process (1831) converted sulfur to sulfuric acid, which became so central to industrial chemistry that production of sulfuric acid was used as an index of a nation’s industrial strength.
In 1839 Charles Goodyear accidentally discovered vulcanization: heating natural rubber with sulfur created cross-links between polymer chains, producing a durable, elastic material. This discovery enabled tires, hoses, boots, and most other rubber goods.
Sulfur is also biochemically essential. The amino acids cysteine and methionine contain sulfur; disulfide bridges (-S-S-) between cysteine residues give proteins their three-dimensional shape.
How to get it
Sold as flowers of sulfur (fine powder) or roll sulfur at agricultural suppliers, garden centres, and online chemical suppliers. It is used as a fungicide and soil acidifier, so it is widely available at low cost. Purity is typically 99%+ for agricultural grade, sufficient for all experiments described here.
Experiments
Crystal Polymorphism: Melt sulfur slowly in a test tube, then pour it onto paper and let it solidify at room temperature — monoclinic needles form. Compare their shape to rhombic sulfur powder viewed through a magnifier. Then quench a portion in cold water to make plastic sulfur.
Combustion and SO₂: Burn a small pile of sulfur powder in a heatproof dish outdoors. The pale blue flame and pungent SO₂ smell are unmistakable. Test the gas by holding moist blue litmus paper above the flame — it turns red as SO₂ dissolves to form sulfurous acid.
Iron + Sulfur → Iron Sulfide: Mix iron filings with sulfur powder (7:4 mass ratio), place in an evaporating dish, and heat. The mixture glows red and reacts to form black iron(II) sulfide, demonstrating that the product has completely different properties from the two starting elements: it is non-magnetic and not attracted to a magnet.
Experiments using this chemical:
- Sulfur Crystal Polymorphism - Rhombic, monoclinic, and plastic sulfur from one element
Safety
Flammable — keep away from open flames during storage and weighing. Sulfur ignites easily and the resulting SO₂ is a respiratory irritant (EU WEL: 0.5 ppm TWA). Always work outdoors or with excellent ventilation when burning sulfur.
Do not mix sulfur with oxidizing agents (chlorates, nitrates, peroxides) — mixtures can ignite or explode from friction.
Do not heat above ~160°C in a closed vessel — pressure builds and the viscous polymer is difficult to clean up.
Fine sulfur dust is mildly irritating to eyes and skin; wear safety glasses.
First aid: rinse dust from skin and eyes with water. Move to fresh air if SO₂ from burning sulfur is inhaled; seek medical care if breathing is affected.
Disposal: unreacted sulfur is harmless — bin the solid. Do not wash quantities into drains.