Question

(a) How are gypsum and Plaster of Paris different chemically ?
(b) Why should POP be stored in airtight containers?
(c) State two uses of POP.

Answer 1

Hint: POP or Plaster of Paris is a white powder which is mostly used for beautification of ceilings of houses and buildings and for making statues, toys, blackboard chalk whereas gypsum is used for the manufacture of cement, wallboard, plaster of Paris, soil conditioning, a hardening retarder in portland cement etc.

Complete Solution :
(a) The chemical name of Plaster of Paris or POP is Calcium sulfate hemihydrate and the chemical formula can be written as$CaS{{O}_{4}}.{1}/{2}\;{{H}_{2}}O$.
 - The chemical formula of gypsum is $CaS{{O}_{4}}.2{{H}_{2}}O$. The POP is formed by heating gypsum (Calcium sulfate dihydrate) at a temperature around 373 kelvin.
(b) The Plaster of Paris has a property of setting into hard mass when it’s treated with water and this setting is due to the hydration to form crystals of gypsum. This setting of Plaster of Paris into can be represented as follows
\[\underset{Plaster\text{ }of\text{ }Paris}{\mathop{CaS{{O}_{4}}.{1}/{2}\;{{H}_{2}}O}}\,+\underset{Water}{\mathop{{3}/{2}\;{{H}_{2}}O}}\,\to \underset{Gypsum}{\mathop{CaS{{O}_{4}}.2{{H}_{2}}O}}\,\]

Therefore in order to avoid the formation of gypsum in contact with water in air, it should be stored in air tight containers.
  (c) The uses of Plaster of Paris can be given as follows
(i) Used as building materials and as protective coating on walls and ceilings
(ii) Used as casting and moulding agent.

Note: It should be noted that the POP is one of three types of plaster and the other two types of plasters are lime plaster, which is made from calcium hydroxide and sand, and cement plaster, which is the combination of sand, plaster, Portland cement and water.