Question

Why is Plaster of Paris so called ?

Answer 1

Calcium sulphate hemi hydrated[CaSO4.1/2H2O] IS Commonly known as Plaster of Paris.
The name plaster of Paris came from the fact that it was first of all made by heating gypsum which was mainly found in Paris. 
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In 1666, a fire accident occurred in LONDON, destroying many parts of it.
seeing that KING OF FRANCE ordered that all walls made of wood in Paris must be immediately covered with plaster, as a fire protection.
This resulted in large scale mining of Gypsum which was available in Paris in huge quantities,
 From then the Paris became the center of Plaster production
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Hence the name Plaster of Paris.

Answer 2

The name plaster of paris (POP) is derived as the calcium sulphate hemi hydrates are found in large amount deposited in the Montmartre hill in Paris
Plaster of Paris is obtained by heating gypsum or calcium sulphate dihydrate to about 140-180 degree Celsius. When heated to such a temperature, gypsum forms Plaster of Paris. The name is derived from the large deposits of gypsum in the Montmartre hill in Paris.