Question

Hardening of plaster of Paris is because of……….
A) the formation of $CaC{O_3}$
B) the formation of $CaSi{O_3}$
C) the transition of orthorhombic gypsum to monoclinic gypsum
D) dehydration

Answer 1

Plaster of Paris, quick setting gypsum plaster consisting of a fine white powder (Calcium sulphate hemihydrate), which hardens when moistened and allowed to dry. Plaster of Paris is prepared by heating calcium sulphate dihydrate or gypsum to $120^oC - 180^oC$.

Complete step by step answer:
Plaster of Paris formula is $CaS{O_4}.\dfrac{1}{2}{H_2}O$. It is white powder. It has a very remarkable property of setting into a hard mass on wetting with the water. So, when water is added to plaster of Paris, it sets into a hard mass in about half an hour. The setting of plaster of Paris is due to its hydration to form crystals of gypsum which set to form a hard solid mass.
$CaS{O_4}.\dfrac{1}{2}{H_2}O + 1\dfrac{1}{2}{H_2}O \to CaS{O_4}.2{H_2}O$
The plaster of Paris absorbs water to form orthorhombic calcium sulphate dihydrate which sets to form a hard mass containing monoclinic calcium sulphate dihydrate.
$CaS{O_4}.\dfrac{1}{2}{H_2}O\xrightarrow[{water}]{{setting}}CaS{O_4}.2{H_2}O\xrightarrow{{hardening}}CaS{O_4}.2{H_2}O$
Hence, option C is correct.

Additional information: The setting of plaster of Paris is accompanied by a slight expansion in volume due to which it is used in making castes of statues, toys etc.
It finds extensive use in surgical bandages in casting and moulding. It is also employed in dentistry, in ornamental work and for taking castes of statues and busts.
It exists in two crystalline forms-
1. Calcite
2. Aragonne
It is insoluble in water but dissolves in the presence of $C{O_2}$ (carbon dioxide) due to the formation of calcium bicarbonate.

Note:
Plaster of Paris is also used in preparation of radiotherapy when fabricating individualized immobilization shells for patients of gypsum plasters, leavened with polystyrene beads as well as chemical expansion agents to decrease the density of the finished product.