Monoclinic sulfur is a crystalline allotrope of sulfur obtained when rhombic sulfur is heated to 94.5°C. This form is stable only above 96°C. When left at room temperature it reverts back to rhombic form. It has S8 ring molecules in crystalline structure.
Preparation:
sulfur is heated slowly in an evaporating (porcelain) dish till it melts. The molten sulfur is then allowed to cool slowly.
During the cooling process, a solid crust will be formed over the surface. As this crust is being formed, two holes are made in
the crust. The molten sulfur is then poured out of these holes. The crust is then removed. On the lower side of the crust, long
needle–shaped crystals of monoclinic sulfur are formed.
Monoclinic sulfur
Properties:
- Monoclinic sulfur exists as long needle –like prisms, hence it is also called prismatic sulfur.
- Its density is 1.98 g/mL.
- It melts at 119°C.
- It is stable between 96 and 119°C. Below 96°C it slowly changes into rhombic sulfur.
Sa
Sb - It is insoluble in water but readily dissolves in carbon disulphide.