Question

What is the chemical formula for Plaster of Paris? How is it prepared? State the common and chemical names of the compounds formed when Plaster of Paris is mixed with water.

Answer 1

Hint: To answer this question, you should recall the concept of preparation of plaster of Paris. The name of this compound i.e. plaster of Paris came from the fact that it was first of all made by heating gypsum which was mainly found in Paris.

Complete step by step answer:
Plaster of Paris is white, powdery in texture and chemically hydrated calcium sulfate that is usually obtained from calcining gypsum. The compound plaster of Paris is obtained when an ore of calcium sulphate i.e. gypsum is heated at \[{120^o}{\text{C}}\]
The reaction can be represented as:
\[{\text{CaS}}{{\text{O}}_{\text{4}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{CaS}}{{\text{O}}_{\text{4}}}{\text{.}}\dfrac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}} + \dfrac{3}{2}{{\text{H}}_{\text{2}}}{\text{O}}\]
The chemical formula for the plaster of Paris is \[{\text{(CaS}}{{\text{O}}_{\text{4}}}{\text{)}}{\text{.}}{{\text{H}}_{\text{2}}}{\text{O}}\] and is better known as calcium sulfate hemihydrate. Plaster of Paris reacts with water liberating heat through crystallization. Gypsum is produced by heating plaster of Paris to about \[{150^o}{\text{C}}\]. When the dry plaster of Paris powder is mixed with water it reforms into gypsum. Depending upon the temperature gypsum converts to hemihydrate or anhydrous form.

Additional information:
 Another type of setting agent used in constructions is cement. We know that components of cement include a chemical combination of calcium, silicon, aluminum, iron and other ingredients. When water is mixed with cement to form a paste, the reaction starts.
In its pure form, the finely ground cement is extremely sensitive to water. It is due to hydration of the molecules of the constituents: dicalcium silicate, tricalcium silicate, tricalcium aluminate. Also, we know those hydration reactions are exothermic, thus, hydration of the above components and their rearrangements generates heat. This change from a fluid state to a solid-state is called setting. The reactions that cause setting and hardening are collectively described as exothermic hydration reactions.
Hence, heat is liberated as the water reacts with the cement during the setting so it’s an exothermic process.

Note:
You should know about the uses of plaster of Paris: Make fine artwork for decoration and beautification of monuments and buildings, numerous morticians and executives of funeral houses to remake the damaged tissue, mould and casts in medical fields, fireproofing products and fire protection systems make use of plaster of Paris.