Question

Setting of Plaster of Paris involves:
(A) Oxidation with atmospheric oxygen
(B) Combination with atmospheric $C{{O}_{2}}$
(C) Dehydration
(D) Hydration to yield another hydrate

Answer 1

Hint: The formula to represent plaster of paris is \[CaS{{O}_{4}}.\dfrac{1}{2}{{H}_{2}}O\]. Setting plaster of paris means adding substance to make it hard and the process of setting plaster of paris is exothermic.

Complete step by step solution:
The setting of plaster of paris is due to the formation of calcium hydrate. Hydration of the plaster of paris is the hydration reaction which is the reverse of the reaction of dehydration of gypsum. Plaster of paris when made a thin paste with water it quickly sets to a hard mass.
A slight expansion takes place during this process because this process is exothermic which means that heat is released.
The reaction involves is:
\[2CaS{{O}_{4}}.{{H}_{2}}O\xrightarrow[{{H}_{2}}O]{setting}CaS{{O}_{4}}.2{{H}_{2}}O\]

Therefore the correct answer is option (D) i.e. Setting of plaster of paris involves the hydration to yield another hydrate.

Additional information:
Plaster is the common name for calcium sulphate hemihydrate which is made by heating gypsum which is also the common name of sulphate of lime. Plaster of paris is a white powdery slightly hydrated salt of calcium sulphate.
Plaster of paris is used mainly for casts and moulds in the form of a quick-settling paste with water.
The hard plaster produced after the mixing of the powder is the fully hydrated form of calcium sulphate.

Note: Whenever we want to dissolve plaster of paris it would be easy to dissolve it if we use potassium cyanide or sodium carbonate. It is important to remember that the plaster of paris sets in a few minutes but it generally takes hours before it can be removed from the mould.