Question

Plaster of paris is prepared from:
A) Quick lime
B) Slaked lime
C)Gypsum
D) Limestone

Answer 1

To solve this we must know the formula of plaster of paris. The calcium sulphate with half a molecule of water per molecule of salt is known as plaster of paris. The chemical formula for plaster of paris is ${\text{CaS}}{{\text{O}}_{\text{4}}} \cdot {\text{1/2 }}{{\text{H}}_{\text{2}}}{\text{O}}$.

Complete step by step answer:
We know that the calcium sulphate with half a molecule of water per molecule of salt is known as plaster of paris. The chemical formula for plaster of paris is ${\text{CaS}}{{\text{O}}_{\text{4}}} \cdot {\text{1/2 }}{{\text{H}}_{\text{2}}}{\text{O}}$.
Plaster of paris is a white powder. When plaster of paris is mixed with water it sets into hard mass. This is because the crystals of gypsum are produced and heat is evolved.
Gypsum is also known as calcium sulphate dihydrate. The chemical formula for gypsum is ${\text{CaS}}{{\text{O}}_{\text{4}}} \cdot {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$. Gypsum is a water soluble compound. The solubility of gypsum decreases as the temperature increases. When we heat gypsum, gypsum loses water.
Plaster of paris is prepared by heating gypsum at ${120^ \circ }{\text{C}}$ in rotary kilns. During heating, gypsum gets partially dehydrated.
The reaction of gypsum when gypsum is heated is as follows:
$2\left( {{\text{CaS}}{{\text{O}}_{\text{4}}} \cdot 2{{\text{H}}_{\text{2}}}{\text{O}}} \right)\xrightarrow{{{{120}^ \circ }{\text{C}}}}{\text{2}}\left( {{\text{CaS}}{{\text{O}}_{\text{4}}}} \right) \cdot {{\text{H}}_{\text{2}}}{\text{O}} + 2{{\text{H}}_{\text{2}}}{\text{O}}$
Thus, plaster of paris is prepared from gypsum.
Thus, the correct option is (C) gypsum.

Note: Plaster of paris is a white powder. When plaster of paris is mixed with water it sets into hard mass. This is because the crystals of gypsum are produced and heat is evolved. Plaster of paris is used in the setting of plaster for fixing the broken bones. Plaster of paris is also used in making many decorative items, toys and chemical plugging.