Sulfur Crystal Polymorphism
Summary
One element — sulfur — can take three different solid forms: blocky rhombic crystals, needle-like monoclinic crystals, and rubbery plastic sulfur, an example of polymorphism1. You’ll be able to explain why the same S₈ molecule packs differently, why molten sulfur gets more viscous as it heats, and what “metastable” means.
History
In 1821 Eilhard Mitscherlich coined the term polymorphism to describe how a single chemical compound can form distinct crystal structures depending on conditions. Sulfur became one of the clearest illustrations: at room temperature it crystallizes in the rhombic system (tabular, blocky crystals); above 96°C the monoclinic form is stable (long needles). Cool the needle crystals slowly and they revert to rhombic within hours as individual S₈ rings rearrange. Quench the hot liquid in cold water and you freeze the disordered polymer chains mid-rearrangement, producing rubbery plastic sulfur that snaps back toward rhombic over hours to days.
This experiment requires no special reagents beyond sulfur powder — one of humanity’s most familiar chemicals — and shows in minutes the same phenomenon that takes geologists millennia to observe in minerals.
Hazards & preparation
PPE: safety glasses; use tongs for the hot tube.
- Burning or overheated sulfur gives off choking sulfur dioxide (SO₂). Work outdoors or under an extraction fan, and heat gently — never let the sulfur catch fire (it burns with a blue flame).
- Molten sulfur is hot (>119 °C) and sticky — handle the tube with tongs and pour carefully.
Disposal: cooled sulfur solids are inert — bin them. Don’t wash molten sulfur down a drain (it solidifies and clogs). See the Safety page.
Materials
| What | How much |
|---|---|
| Sulfur powder (flowers of sulfur) | 5–10 g |
| Pyrex or borosilicate test tube | 1 |
| Bunsen2 burner or candle flame | |
| Sheet of white paper or aluminium foil | 1 |
| Glass of cold water | 1 |
| Forceps / wooden tongs | |
| Safety glasses |
Work outdoors or directly under an extraction fan — small amounts of SO₂ are produced if sulfur scorches.
Procedure
Part 1 — Monoclinic Needles
- Add 5 g sulfur powder to the test tube — the stable rhombic form. Note its pale-yellow, opaque, powdery look.
- Hold the tube with forceps and heat gently in the flame. Sulfur melts at 119°C to a clear yellow mobile liquid.
- Keep heating carefully, swirling, until the liquid turns slightly amber (the onset of polymer formation). Do not overheat: for needles, stay in the clear-to-amber stage.
- Remove from the flame. Pour the clear/amber liquid slowly onto white paper or foil in a thin stream. As it cools, pale yellow monoclinic needles form within seconds to minutes.
- Compare the elongated, prismatic needles to the powder you started with.
Part 2 — Plastic Sulfur
- Return the remaining liquid to the flame and heat until dark reddish-brown and distinctly viscous (~160–200°C). The S₈ rings have opened into long tangled chains.
- Pour the viscous liquid in a thin stream directly into the glass of cold water.
- Retrieve the amber solid — soft, rubbery, and stretchable: plastic sulfur, a partly amorphous polymer.
- Stretch and bend it. Over the next hour it stiffens and yellows as the chains relax back into S₈ rings and convert toward rhombic.
Part 3 — Comparison
| Form | Colour | Structure | Stability |
|---|---|---|---|
| Rhombic | Pale yellow | Ordered S₈ rings, orthorhombic lattice | Stable below 96°C |
| Monoclinic | Yellow, translucent | S₈ rings, different packing | Stable 96–119°C; reverts slowly |
| Plastic | Amber/brown, rubbery | Disordered polymeric chains | Metastable; reverts in hours–days |
What you should see
The pale powder melts to a runny yellow liquid, then thickens and darkens to a syrupy red-brown as you keep heating — so stiff it barely pours. Poured amber onto foil it sets into needle crystals; poured red-brown into water it freezes into a rubbery amber blob you can stretch, which slowly turns pale and brittle over the next hour.
| Symptom | Likely cause | Fix |
|---|---|---|
| Sulfur catches fire (blue flame, SO₂) | Overheated | Remove from heat, cover to smother; heat more gently next time |
| No needles, just powder | Cooled from too-hot (polymerized) liquid | Reheat to a clear/amber melt, not red-brown, for needles |
| Plastic sulfur crumbles immediately | Not heated hot enough to polymerize | Heat to the viscous red-brown stage before quenching |
The reactions
The fundamental unit in all three forms is the same molecule, but arranged differently:
Rhombic → Monoclinic (reversible, Tₜ = 96°C):
\[\ce{S8(rhombic) <=>[\Delta][cool] S8(monoclinic)}\]
Monoclinic → liquid → polymer (on continued heating):
\[\ce{S8(l) ->[>160°C] (-S-)_n (polymer)}\]
Polymer → rhombic (slow, at room temperature):
\[\ce{(-S-)_n(amorphous) ->[slow] S8(rhombic)}\]
the same S₈ molecule re-packs into different crystals with temperature, and above ~160 °C the rings open into long chains
The Science
Allotropy vs. polymorphism: All three forms are allotropes of sulfur (different structural forms of the same element). Rhombic and monoclinic are additionally polymorphs of each other — different crystal packings of the same S₈ ring molecule — whereas plastic sulfur differs more deeply, built from open-chain polymer strands rather than closed S₈ rings.
Why does sulfur change viscosity on heating? Most liquids become less viscous as they get hotter. Liquid sulfur does the reverse between 160°C and 200°C because the S₈ rings open and polymerize into chains thousands of units long — the entanglement dramatically increases viscosity. Above ~200°C, thermal energy breaks the chains faster than they form, and viscosity falls again.
Metastability: Plastic sulfur and monoclinic crystals both represent metastable states at room temperature — they exist only because the conversion to the stable rhombic form is slow, not because it does not happen. Given time, all three forms at room temperature end up as rhombic sulfur — a direct demonstration of kinetic vs. thermodynamic control of crystal structure.
Questions to Explore
Why does sulfur exist in multiple solid forms at all? What does it mean for crystal structures to have different energies, and what determines which form is stable?
Hint / answer
Different packings of the S₈ rings have slightly different energies, and the most stable one changes with temperature — rhombic below 96 °C, monoclinic above. Sulfur is unusual in that these energies are close enough that both are accessible, and the S₈ ring can also break into chains.
Why does liquid sulfur get more viscous as it heats? What happens to the S₈ rings, and why does that thicken the liquid?
Hint / answer
Between 160 and 200 °C the eight-membered rings snap open and link into enormously long chains. Those chains tangle around each other and resist flow, so the liquid thickens — the opposite of normal liquids, whose molecules just move faster when heated.
What is the difference between metastability and instability? Plastic sulfur reverts over hours yet exists. What barrier holds it there?
Hint / answer
Metastable means it’s not the lowest-energy form, but a kinetic barrier (rearranging tangled chains back into ordered rings takes time) keeps it stuck there temporarily. An unstable form would have no such barrier and convert at once.
Why do monoclinic crystals revert to rhombic at room temperature? What’s the driving force, and why isn’t it instant?
Hint / answer
Below 96 °C rhombic is the lower-energy, stable packing, so monoclinic slowly rearranges toward it. It isn’t instant because the S₈ rings must physically shuffle into new positions in the solid, which is sluggish at room temperature.
What other elements show allotropy? Carbon (diamond, graphite, fullerenes), tin (white, grey)… what do they share, and is there a periodic pattern?
Hint / answer
Allotropy shows up in elements that can bond to themselves in more than one way — often the nonmetals and metalloids (carbon, sulfur, phosphorus) and some borderline metals (tin). Flexible bonding options let the same atoms build different stable structures.
Explore Further
Reversion timer: Weigh a strip of plastic sulfur immediately after quenching, then check every 30 minutes. As it crystallizes it becomes opaque and brittle. Time how long full conversion takes — it varies with temperature and thickness.
Monoclinic needles from solution: Dissolve a little sulfur in warm toluene or acetone in a fume hood. Let it evaporate slowly in a covered dish for larger, more regular monoclinic needles than from the melt.
Connect to crystal growing: For the seed-crystal method of growing one large, well-formed crystal, see Growing a Large Single Crystal. Sulfur is one of the few accessible examples of orthorhombic symmetry, generating both orthorhombic (rhombic) and monoclinic forms side by side.
Chemicals used in this experiment:
- Sulfur — all three physical forms
Going further
- Grow from solution. Try the slow toluene/acetone evaporation (fume hood) for larger, more perfect monoclinic needles.
- Next in the Crystals & Solutions track: grow mineral “plants” in the Chemical Garden.
Footnotes
Polymorphism — The ability of a solid to exist in more than one crystal structure.↩︎
Robert Bunsen — German chemist (1811–1899) who, with Kirchhoff, developed flame spectroscopy and the Bunsen burner.↩︎