Question

(a) What is plaster of Paris? Write the chemical formula of plaster of Paris.
(b) How is plaster of Parish 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.

Answer 1

(a) Plaster of Paris is calcium sulphate hemi-hydrate (calcium sulphate half hydrate). The chemical formula of plaster of Paris is CaSO₄.1/2H₂O.

(b) When gypsum (CaSO₄.2H₂O) is heated to a temperature of 100^⁰C (373K), it loses three-fourths of its water of crystallisation and forms plaster of Paris. ​

CaSO₄.2H₂O $\stackrel{∆}{\to }$ CaSO₄.1/2H₂O + 3/2 H₂O
Gypsum Plaster of Paris

(c) Plaster of Paris should be stored in a moisture-proof container because the presence of moisture can cause its slow setting by the addition of water of hydration, and it would be useless after some time.

(d) Uses:
(i) Plaster of Paris is used in making decorative materials, toys, cosmetics, cheap ornaments, black-board chalks and casts for fractured bone and statues.
(ii) It is used in making surfaces such as walls of houses smooth before they are painted and for creating ornamental designs on the ceilings of houses and other buildings.