Question

Which is the most stable allotrope of sulphur?
A. Octahedral sulphur
B. Monoclinic sulphur
C. Plastic sulphur
D. Colloidal sulphur.

Answer 1

Hint: The two most commonly allotropes of sulphur are:-
- Rhombic sulphur:- it is also known as octahedral sulphur. It is yellow in colour. It is a stable form of sulphur at room temperature.
-Monoclinic sulphur:- it is prepared by melting rhombic sulphur in a dish and cooling , till the crust is formed. Two holes are made in the crust and remaining liquid is poured out . On removing crust, colourless needle shape crystals of monoclinic sulphur are formed.

Complete step by step answer:
- Octahedral sulphur:- this is also known as rhombic sulphur and is the most stable form as it is insoluble in water and is prepared by evaporating the solution of roll sulphur in carbon disulphide. Hence this option is correct.
- Monoclinic sulphur:- this allotrope is only stable at temperature above 369K . So this option is not correct.
- Plastic sulphur:- it is a type of amorphous sulphur and is not stable at all temperatures. Hence this option is also incorrect.
- Colloidal sulphur:- it is prepared by mixing the solution of alcohol and sulphur in water. It is not stable at every temp or room temp. So the option goes wrong.

Our correct option is A that is, octahedral sulphur.

Note:
Allotropes are different as the one or more different physical forms of an element.
There are two types of sulphur namely
- Crystalline:- it comprises two allotropes monoclinic and rhombic
- Amorphous:- it comprises three allotropes namely plastic sulphur, colloidal sulphur and milk of sulphur.
- Milk of sulphur:- it is prepared by the action of weak hydrochloric acid on ammonium sulphide. This compound is non crystalline and white in colour.