Chemistry

24. Plaster of Paris (CaSO₄. 1/2 H₂O):
Chemically, Plaster of Paris is calcium sulphate hemihydrate. It is called Plaster of Paris because it is obtained from gypsum which is mainly found in Paris.
Preparation : Plaster of Paris is obtained by heating gypsum (CaSO₄.2H₂O) at 373 K in a kiln. Heating should be done carefully.at 373 K in a kiln. Heating should be done carefully.

25. Properties of Plaster of Paris.
(i) It is white powder.
(ii) When it is mixed with water, crystals of gypsum are produced and set into hard mass.

The setting process is exothermic, i.e., heat is evolved. The setting process may be catalysed by sodium chloride while it can be retarded by borax or alum.
(iii) When Plaster of Pairs is heated at 473 K, if forms anhydrous calcium sulphate which is known as dead burnt plaster of paris. It has no setting property as it takes up water very slowly.

26. Uses of Plaster of Paris.
(i) It is used for plastering fractured bones and dislocated bones so as to set them in proper place.
(ii) It is used in making toys, decorative materials.
(iii) It is used in making casts for statues, toys, surgical instruments, etc.
(iv) It is used in making blackboard chalks.
(v) It is used in dentistry.
(vi) It is used for making smooth surface and ornate designs on walls and ceilings.
(vii) It is used in laboratories for sealing air gaps in apparatus so as to make it airtight.