Question

What is plaster of paris? How is it prepared? Give it any two important uses.

Answer 1

Hint: Plaster of paris is a rapid-setting gypsum plaster that consists of a fine white powder and it has a tendency to get hard when it is moistened and allowed to dry. It is named as plaster of paris because it is composed from a huge amount of gypsum that is found near Paris.

Complete answer:
The chemical name of plaster of paris is calcium sulphate hemihydrate and its molecular formula is $CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O$. Its strength is not as strong as other solid compounds and it generally requires an external support if used in a large amount. It is always stored in dry containers or moist proof containers because in the presence of moisture, it gets hydrated and can lead to slow setting of its content. Therefore, it will make it useless after sometime.
Preparation:
When gypsum is heated at ${{120}^{o}}\text{ C}$ in rotary kilns, it undergoes a dehydration reaction and then plaster of paris is formed. The following reaction takes place during the process:
$CaS{{O}_{4}}\cdot 2{{H}_{2}}O\to CaS{{O}_{4}}\cdot \dfrac{1}{2}{{H}_{2}}O+\dfrac{3}{2}{{H}_{2}}O$
Important uses of plaster of paris:
Used in medical treatments for supporting bone fractures in its right position.
It is used in the field of dentistry for making moulds for teeth that are to be replaced.
It is used as metal casting and sculptures which are used as decorative items in building.

Note:
Gypsum is a water-soluble compound which is also known as calcium sulphate dihydrate. Its chemical formula is $CaS{{O}_{4}}\cdot 2{{H}_{2}}O$. The natural form of the gypsum plaster is the bassanite mineral.