How Scientists Found Two Liquid States in Sulfur

This guide explains the classic experiment that revealed sulfur can exist in two distinct liquid forms. You’ll learn the exact heating and cooling steps, what to observe at each temperature, and how to interpret the dramatic viscosity changes. No special equipment beyond standard lab glassware and a heat source is required.

What You’ll Need

• Pure sulfur powder or roll sulfur (flowers of sulfur works but may contain impurities)
• A heat-resistant test tube or small beaker (borosilicate glass)
• A Bunsen burner or hot plate capable of reaching 250°C (482°F)
• A thermometer that reads up to 300°C (572°F)
• Heat-resistant tongs or a test tube holder
• Safety goggles and heat-resistant gloves
• A heat-proof surface or sand bath
• Optional: a glass stirring rod

Step-by-Step

1. Set up your workspace. Place the heat source on a stable, heat-proof surface. Ensure good ventilation—sulfur dioxide gas can form if the sulfur burns, so work in a fume hood or near an open window.
2. Add sulfur to the test tube. Fill the test tube about one-third full with sulfur powder. Do not overfill—molten sulfur expands and can bubble over.
3. Heat slowly. Using a low flame or low heat setting, warm the test tube gently. Tilt it slightly and rotate it to heat evenly. Sulfur melts at about 115°C (239°F). You’ll see a thin, amber-colored liquid form—this is the first liquid state, often called λ-sulfur (lambda-sulfur).
4. Observe the first liquid state. Between 115°C and about 159°C (318°F), the liquid remains free-flowing and water-like. This is a molecular liquid composed of S8 rings. Record the appearance and viscosity.
5. Continue heating past 159°C. As the temperature rises above 159°C, the color darkens from yellow to orange and then to deep red-brown. The liquid becomes increasingly viscous. This marks the transition to the second liquid state, often called μ-sulfur (mu-sulfur).
6. Watch for the viscosity peak. Around 187°C (369°F), the sulfur becomes so thick that it may be difficult to pour. If you tilt the test tube, the liquid barely moves. This is the maximum viscosity point. The S8 rings have broken open and polymerized into long chains of sulfur atoms.
7. Heat beyond the peak (optional). If you continue heating to about 250°C (482°F), the viscosity decreases again as the polymer chains begin to break down into shorter fragments. The liquid becomes more fluid but remains dark.
8. Cool slowly and observe reversibility. Remove the heat and let the sulfur cool. As it cools, the viscosity increases again, then drops as it passes back through the transition range. At around 159°C, the liquid returns to the thin, yellow state. If you cool quickly (quench), the sulfur may become a rubbery, amorphous solid—a glassy form that slowly reverts to crystalline sulfur over time.
9. Record your observations. Note the temperatures at which you see color changes, viscosity changes, and any solidification behavior. Compare your observations to the known transition points.

Common Mistakes

• Heating too fast. Rapid heating can cause the sulfur to boil or ignite, producing toxic sulfur dioxide. Always heat gradually.
• Using a closed container. Sulfur expands when it melts and polymerizes. If the container is sealed, pressure can build and cause an explosion.
• Overfilling the test tube. Molten sulfur can foam and overflow, creating a mess and a fire hazard.
• Ignoring the temperature range. The two liquid states are only clearly distinguishable between 115°C and about 250°C. Heating beyond that may cause decomposition or boiling.
• Not stirring. Without gentle stirring, the temperature may be uneven, and you might miss the viscosity peak.

Tips

• Use a sand bath or an oil bath for more even heating and better temperature control than a direct flame.
• If you don’t have a thermometer, you can estimate the transition by observing the color change and the way the liquid flows, but a thermometer is essential for precise work.
• To demonstrate the two states clearly, pour a small amount of the thin liquid at 130°C into cold water—it will form a solid that is still yellow and brittle. Then pour some of the thick liquid at 190°C into cold water—it will form a dark, rubbery mass. This contrast makes the difference obvious.
• Clean the test tube while the sulfur is still warm—cooled sulfur is hard and difficult to remove. If it solidifies, you can melt it again or dissolve it in carbon disulfide (use extreme caution—carbon disulfide is highly flammable and toxic).

Troubleshooting

• No visible viscosity change. If the sulfur remains thin even above 159°C, your thermometer may be inaccurate, or you may not be heating evenly. Stir gently and check the thermometer calibration.
• Sulfur turns black or ignites. This indicates overheating or burning. Turn off the heat immediately, move the test tube away from the flame, and allow it to cool in a safe area. Do not inhale any fumes.
• The test tube cracks. Thermal shock from rapid heating or cooling can break glass. Use borosilicate glass and heat gradually. If it cracks, let it cool completely and carefully dispose of the glass and sulfur.
• The sulfur sticks to the glass. This is normal, especially for the polymerized form. Heat the tube gently to remelt the sulfur and pour it out, or use a solvent like carbon disulfide (with proper safety precautions).
• You can’t reach 187°C. If your heat source isn’t powerful enough, use a hotter flame or a higher setting on the hot plate. Ensure the test tube is not too large—a smaller volume heats faster.

Frequently Asked Questions

Why does sulfur have two liquid states?

The first liquid state consists of intact S8 ring molecules. When heated above 159°C, these rings break open and link into long polymer chains. This polymer melt is the second liquid state. The two states have different molecular structures and physical properties.

At what exact temperature does the transition occur?

The transition is gradual, but the onset of polymerization is around 159°C (318°F). The viscosity peaks at about 187°C (369°F). These values can vary slightly with heating rate and impurities.

Is the second liquid state a different chemical substance?

No, both states are elemental sulfur (S8). The difference is physical—the arrangement and bonding of the atoms. The polymerized form is a different allotrope, but it’s still sulfur.

Can I do this experiment at home?

With proper safety equipment and adult supervision, it can be done, but it involves high temperatures and produces toxic fumes if overheated. It’s safer in a school lab or fume hood. Always wear goggles and gloves.

What happens if I cool the sulfur very quickly?

Rapid cooling (quenching) traps the polymer chains in a disordered state, forming a rubbery, amorphous solid. This is not stable—over time (hours to days) it slowly reverts to the crystalline yellow sulfur.

Why does the color change from yellow to red-brown?

The color change is due to the formation of longer chains and the absorption of light at different wavelengths. The polymer chains absorb more visible light, making the liquid appear darker. This is a visual indicator of the transition.

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