Question

Plaster of Paris hardens by:
(A) Giving $C{{O}_{2}}$
(B) Changing into carbonate
(C) Uniting with water
(D) Giving out water

Answer 1

Hint: Plaster of Paris is a compound of calcium element which belongs to group 2 of the s-block. The molecule of Plaster of Paris has half a molecule of water and the hardened form of Plaster of Paris has 2 molecules of water.

Complete step by step solution:
Plaster of Paris is the hemihydrate form of calcium sulfate and it has a formula $CaS{{O}_{4}}.\dfrac12{{H}_{2}}O$.
The preparation of Plaster of Paris takes place from Gypsum which is heated at 373 K. the reaction is:
$\underset{Gypsum}{\mathop CaS{{O}_{4}}.2{{H}_{2}}O}\,\to \underset{Plaster\text{ }of\text{ }Paris}{\mathop CaS{{O}_{4}}.\dfrac12{{H}_{2}}O}\,+3{{H}_{2}}O$
There are some necessary conditions in the formation of Plaster of Paris:
The temperature should not be above 373 K, because above this temperature the whole water of crystallization is lost. The resulting Calcium sulfate is dead burnt plaster because of the loss of properties of setting with water.
The Gypsum should not be allowed to come in contact with carbon-containing fuel because it will reduce the calcium sulfate to calcium sulfite.
Properties of Plaster of Paris:
It has the remarkable property of setting with water. When we mix one-third its weight of water, then it starts to form a plastic mass that sets into a hard mass of interlocking crystals of gypsum within 5 to 15 minutes. That’s why it is called Plaster. The addition of common salt accelerates the rate of setting, while little borax or alum reduces it. The hardening of the Plaster of Paris is believed due to rehydration and its reconversion into gypsum.
$\underset{Plaster\text{ }of\text{ }Paris}{\mathop CaS{{O}_{4}}.\dfrac12{{H}_{2}}O}\,+3{{H}_{2}}O\to \underset{Gypsum}{\mathop CaS{{O}_{4}}.2{{H}_{2}}O}\,$

So, the correct answer is an option (C)- Uniting with water.

Note: Plaster of Paris is used for the production of pottery and ceramics. It is used for making statues, models, and other decorative items. During the fracture of bones, it is used as a surgical bandage. It is also used in dentistry.