Question

Setting of plaster of paris is:
A ) Oxidation with atmospheric oxygen.
B ) Combustion with atmospheric carbon dioxide.
C ) Dehydration.
D ) Hydration to yield another hydrate.

Answer 1

When plaster of paris is mixed with water and left for a few minutes, a hard porous mass is obtained. The mass of water mixed is about one third of the mass of plaster of paris.

Complete answer:
Plaster of paris is used in everyday life for decoration of domestic items. Plaster of paris is commonly referred to as POP. Plaster of paris is calcium sulphate hemihydrate. One molecule of calcium sulphate and half molecule of water are present in one molecule of plaster of paris. The chemical formula of plaster of paris is ${\text{CaS}}{{\text{O}}_4}.\dfrac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}}$.
Plaster of paris (calcium sulphate hemihydrate) absorbs some water to form calcium sulphate dihydrate. The chemical formula of calcium sulphate dihydrate is ${\text{CaS}}{{\text{O}}_4}.2{{\text{H}}_{\text{2}}}{\text{O}}$ . It has orthorhombic structure.
When calcium sulphate dihydrate is allowed to stand for some time (around 5-15 minutes), it becomes harder. This process is called setting. Hardened calcium sulphate dihydrate is called gypsum. It has monoclinic structure.
Setting plaster of paris is an exothermic process. Heat energy is given out during the process. During setting, there is some (around 1%) increase in the volume.

Hence, the correct option is D ) Hydration to yield another hydrate.

Note: To prepare plaster of paris, gypsum (calcium sulfate dihydrate) is heated at ${\text{393 K}}$ in rotary kilns. Partial dehydration results in conversion of dihydrate to hemihydrate.