Question

Write any two uses of Plaster of Paris.

Answer 1

Hint: Plaster of Paris is made from gypsum. The use of the Plaster of Paris is based upon its properties. When the plaster of Paris is mixed with water, an exothermic reaction occurs. Heat evolves and Plaster of Paris settles down. The volume increases, so it gains the shape of the container and becomes a hard and porous mass.

Complete answer:
When calcium sulphate dihydrate is heated at $\text{120}{}^{\text{o}}\text{C}$ , some amount of water escapes from the calcium sulphate. Calcium sulphate dihydrate is known as gypsum.
The reaction is as follows:
$$\text{CaS}{\text{O}}_{\text{4}}\text{.2}{\text{H}}_{\text{2}}\text{O}\stackrel{\text{120}{}^{\text{o}}\text{C}}{\to }\underset{}{\text{CaS}{\text{O}}_{\text{4}}.\frac{1}{2}{\text{H}}_{\text{2}}\text{O}}+\frac{3}{2}{\text{H}}_{\text{2}}\text{O}$$

Uses of Plaster of Paris:
- Plaster of Paris is used in filling cracks in walls and gaps.
- It is stable at high temperatures, so it is used as fireproofing material.
- Plaster of Paris is used in preparing mould, statues and artificial decorative items.
- Plaster of Paris is used to make the cast that is used in case of bone fracturing.
- It is used in dentistry to fill the gaps in teeth.
- It is used to make blackboard chalks.

Note: When gypsum is heated at low temperature, some amount of water escapes from the calcium sulphate and Plaster of Paris forms. On increasing temperature, complete water escapes from gypsum and dead burnt plaster forms. When anhydrous calcium sulphate (dead burnt plaster) is strongly heated it converts into an oxide of calcium that is known as lime. Both dead burnt plaster and Plaster of Paris vary only in water content but they have many different properties.