Question

why calcium sulphate hemihydrate called as plaster of paris?

Answer 1

It is the common name for sulphate hemihydrate like how we call sodium bicarbonate as baking soda.

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Answer 2

Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris. We know that plaster of Paris, which is chemically calcium sulphate hemihydrate is obtained by heating gypsum. So the origin of the name 'Plaster of Paris' is the fact that the source material gypsum is found in large deposits at Montmartre in Paris.