Question

Plaster of paris hardens by:
A. Giving $C{O_2}$
B. Changing into carbonates
C. Uniting with water
D. Giving out water

Answer 1

The Plaster of Paris is a quick setting gypsum plaster with a wide range of application in production of bandages. The Plaster of Paris is prepared by heating gypsum at 423 k.

Complete step by step answer:
Plaster of Paris is used for making sculpture and has many applications in everyday lives. Plaster of Paris is a white chemical compound with powder like texture which is a hydrated calcium sulphate which is obtained by calcining gypsum.
The chemical formula of gypsum is $CaS{O_4}.2{H_2}O$. Plaster of paris is also known as Gypsum plaster.
The plaster of paris is formed by heating gypsum at 423 k.
The reaction for the formation of Plaster of Paris is shown below.
$CaS{O_4}.2{H_2}O + Heat \to CaS{O_4}.0.5{H_2}O + 1.5{H_2}O$
On heating gypsum the water molecule is removed and forms a product calcium sulfate hemihydrate. This is known as Plaster of Paris.
The Plaster of Paris on wetting settles into a hard mass. The setting of Plaster of Paris is because of hydration which takes half an hour to get hard.
The reaction is shown below.
$CaS{O_4}.\dfrac{1}{2}{H_2}O + 1.\dfrac{1}{2}{H_2}O \to CaS{O_4}.2{H_2}O$
The Plaster of Paris on uniting with water again forms a crystal of gypsum from where it is formed and hardens into solid mass.
Thus, the Plaster of Paris hardens by uniting with water.

Therefore, the correct option is C.

Note: The Plaster of Paris exists in two crystalline form Calcite and Aragonite. They are insoluble in water but dissolves in the presence of carbon dioxide as they form sodium bicarbonate.