Question

Plaster of Paris was first found in Montmartre in Paris in the form of white rocks. Chemically, it can be obtained by heating gypsum carefully to about 373K in a kiln. The temperature must be carefully controlled because at higher temperature, it gets completely dehydrated to form anhydrous calcium sulphate. It is very commonly used in surgical bandages for setting fractured bones. Why should Plaster of Paris be stored in air-tight moisture-proof bags?

Answer 1

Plaster of Paris:

1. It is a white powdery substance, which is obtained by heating gypsum at a higher temperature.
2. Only at high temperatures, it get completely dehydrated to form anhydrous calcium sulphate.

Conditions for the storage of Plaster of Paris:

To keep it dehydrated, it needs to be stored in air-tight moisture bags as it may get hydrated to form gypsum in the presence of moisture.

Reaction involves:

$\underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\underset{}{\overset{}{\leftrightharpoons }}}\underset{\mathrm{Plaster}\mathrm{of}\mathrm{Paris}}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}}$

Thus, Plaster of Paris needs to be stored in air-tight moisture-proof bags.