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Question

(a) What is plaster of Paris? Write the chemical formula of plaster of Paris.
(b) How is plaster of Paris prepared? Write chemical equation of the reaction involved.
(c) Explain why plaster of Paris should be stored in a moisture-proof container.
(d) State two important uses of plaster of Paris.


Solution

(a) Plaster of Paris is calcium sulphate hemihydrate. Its chemical formula is: $$CaSO_4. \dfrac12 H_2O$$.

(b) It is prepared by heating gypsum to a temperature of $$100^0C$$ in a kiln. It loses 3/4th of its water of crystallisation and forms plaster of Paris.

   $$\underset{Gypsum}{CaSO_4.2H_2O} \xrightarrow[373 k]{Heat \ at \ 100^0C} \underset{Plaster \ of \ paris}{CaSO_4.\dfrac12 H_2O} + \underset{water}{1 \dfrac{1}{2} H_2O}$$

(c) Plaster of Paris should be stored in a moisture-proof container because the presence of moisture can cause the slow setting of the plaster of Paris by bringing about its hydration.

(d) Uses of plaster of Paris: 
      (i) It is used as a fire proofing material. 
      (ii) it is used in hospitals for setting fractured bones in the right position to ensure correct healing.

Chemistry

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