forms of sulphur
Sulphur exists in
crystalline as well as amorphous allotrophic forms. The crystalline form
includes rhombic sulphur (α sulphur) and monoclinic sulphur (β sulphur).
Amorphous allotropic form includes plastic sulphur (γ sulphur), milk of sulphur
and colloidal sulphur.
Rhombic sulphur also known as α sulphur,
is the only thermodynamically stable allotropic form at ordinary
temperature and pressure. The crystals have a characteristic yellow colour and
composed of S8 molecules. When heated slowly above 96 ⁰C, it converts into
monoclinic sulphur. Upon cooling below 96 ⁰C
the β form converts back to α form. Monoclinic sulphur also contains S8
molecules in addition to small amount of S6 molecules. It exists as
a long needle like prism and is also called as prismatic sulphur. It is stable
between 96 ⁰ - 119 ⁰C and slowly changes
into rhombic sulphur.
When molten sulphur is
poured into cold water a yellow rubbery ribbon of plastic sulphur is produced.
They are very soft and can be stretched easily. On standing (cooling slowly) it
slowly becomes hard and changes to stable rhombic sulphur.
Sulphur also exists in
liquid and gaseous states. At around 140 ⁰C
the monoclinic sulphur melts to form mobile pale yellow liquid called λ
sulphur. The vapour over the liquid sulphur consists of 90 % of S8,
S7 & S6 and small amount of mixture of S2,
S3, S4, S5 molecules.