why is calcium sulphate hemihydrate called plaster of Paris?

Calcium sulfate hemihydrate is commonly known as plaster of Paris due to its historical association with the large deposits of gypsum found near Paris, France. The name "plaster of Paris" has been in use for centuries and is rooted in the historical use of this material in the field of art and construction.

The process of making plaster of Paris involves heating gypsum (calcium sulfate dihydrate) to a certain temperature. When gypsum is heated, it undergoes a chemical reaction in which it loses water molecules and forms calcium sulfate hemihydrate. This partially dehydrated form of calcium sulfate has the ability to rehydrate and harden when mixed with water again, making it useful for casting and molding.

The term "plaster of Paris" likely originated because, in the 17th century, large deposits of gypsum were discovered near Paris. The gypsum from this region became widely known for its quality and suitability for various applications, including sculpture and construction. As a result, the name "plaster of Paris" became synonymous with calcium sulfate hemihydrate, regardless of its source.

Plaster of Paris has been used for casting and molding in art and construction for centuries, and the name has persisted as a common term for this material in many parts of the world.