Sulfur was used by the Ancient Indians in medicines. In Sanskrit, it is known as sulveri.
Sulfur occurs in nature, both in the free state and in the combined state. In the free state it is found on the surface of earth in volcanic regions of Italy and Japan. There are huge deposits of elemental sulfur burried underground in certain parts of the world, especially Poland, Mexico, and the USA (Texas and Louisiana). In the combined state, it is the constituent of many important metallic minerals, such as copper pyrites (CuFeS2 ), zinc blende (ZnS), galena (PbS), cinnabar (HgS), etc. Crude oil and natural gas also contain a significant amount of sulfur in the form of hydrogen sulfide.
1. Extraction of Sulfur
Sulfur is extracted directly out of the ground by using a process known as Frasch Process. This process is based on the fact that sulfur has relatively low melting point. In this process, a hole is drilled down to the sulfur beds. A set of three concentric pipes is then placed in the hole down to the underground sulfur deposits. Superheated water, at about 443 K, is forced down through the outer pipe into the sulfur beds. Due to the relatively low melting point of sulfur, it gets melted. Through the innermost pipe, compressed air is forced down. The compressed air forces the mixture of molten sulfur and water up to the surface through the middle pipe. Sulfur being insoluble in water, is easily separated.
2. Properties of Sulfur
Sulfur is a non-metallic, brittle, yellow solid. It is insoluble in water, but dissolves in organic solvents such as carbon disulfide, methylbenzene, etc. It does not conduct electricity in solid, molten or dissolved state. It has a low melting point (388 K).
Action of Heat on Sulfur
Take some powdered sulfur in a test tube and heat it gently. Observe carefully the changes through which sulfur undergoes. Do you observe any unusual changes?
A molecule of sulfur consists of 8 atoms of sulfur. These atoms are arranged in a ring alternating up and down. The various rings are stacked over each other. On heating, it melts at 388 K to a yellow liquid. The atoms are still in S8 rings, but the rings can slide over each other. As it is heated further, the S8 rings break to form long chains which tangle up to form a thick dark, viscous liquid. On further heating, the long chains break into small fragments and the mobility of the liquid increases. It starts boiling at 717 K.
Sulfur exists in two crystalline solid forms – rhombic sulfur and monoclinic sulfur. These different forms are known as allotropes and the property is known as allotropy. Rhombic sulfur is stable below 369 K, whereas monoclinic sulfur is stable above 369 K. The two forms have the same chemical properties, but different physical properties. The allotropes have different physical properties because they have different crystal shapes. Crystals of rhombic sulfur have octahedral shape, whereas monoclinic sulfur forms fine needle shaped crystals.
The difference in the shapes of the two forms is due to the difference in the packing of molecules. Even though both the allotropes of sulfur contain S8 rings, the molecules are stacked in different ways. In rhombic sulfur, the S8 rings fit snugly into each other, whereas in monoclinic sulfur, the S8 rings are stacked on top of each other.
We can now define allotropy as follows. The existence of an element in two or more different forms (allotropes) in the same physical state by having different arrangement of atoms or molecules is known as allotropy.