Question

Setting of Plaster of Paris involves:
A. Oxidisation with atmospheric oxygen.
B. Combination with atmospheric $\text{C}{{\text{O}}_{\text{2}}}$.
C. Dehydration.
D. Hydration to yield another hydrate.

Answer 1

Hint: Plaster of Paris can be represented as $\text{2CaS}{{\text{O}}_{\text{4}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O}$. Setting means adding hydrogen and the process of setting plaster of Paris is exothermic.

Complete step by step answer:
- The setting of plaster of Paris takes place by hydration due to the formation of a solid crystalline hydrate.
- Hardening of plaster of Paris is a hydration reaction, which is the reverse of the dehydration of gypsum.
- Plaster of Paris quickly sets to a hard mass when made into a thin paste with water.
- A slight expansion takes place in the process and heat is evolved.
$\text{2CaS}{{\text{O}}_{\text{4}}}\text{.}{{\text{H}}_{\text{2}}}\text{O}\underset{{{\text{H}}_{\text{2}}}\text{O}}{\overset{\text{setting}}{\mathop{\to }}}\,\text{CaS}{{\text{O}}_{\text{4}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O}$.
Therefore option(D). Hydration to yield another hydrate is the correct answer.

Additional Information:
- Plaster is the common name for calcium sulphate hemihydrate which is made by heating the mineral gypsum (the common name for sulphate of lime). During the 18th century, Paris became the centre of plaster production and hence the name came as plaster of Paris.
- Plaster of Paris is a white powdery slightly hydrated calcium sulphate.
The formula of plaster of Paris is $\text{2CaS}{{\text{O}}_{\text{4}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O}$
- Plaster of Paris is made by heating gypsum and used mainly for casts and molds in the form of quick-setting paste with water.
- The hard plaster produced when this powder is mixed with water which is a fully hydrated form of calcium sulphate.

Note: Whenever we want to dissolve plaster of Paris, dissolve it with the help of potassium cyanide or sodium carbonate. But always remember that plaster of Paris sets in a few minutes, but it takes hours before it is ready to be removed from the mold.