Question

Why is calcium sulphate hemihydrate called plaster of paris??

Answer 1

Because, Plaster of Paris (POP) is obtained by heating calcium sulphate hemi hydrate, also referred as gypsum for about 140-180 degree Celsius. 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.

Answer 2

Plaster of Paris is a compound prepared by heating Gypsum(CaSO₄.2H₂O) at 373 K. so, it forms CaSO₄. 1/2H₂O.

                           373K  

CaSO₄.2H₂O ------------------>    CaSO₄. 1/2H₂O + 1 1/2H₂O

Since there is 1/2 water molecule so it is called Hemihydrate.

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