Why is Calcium Sulphate Hemihydrate called Plaster of Paris?

Calcium Sulphate Hemihydrate is called Plaster of Paris because it is derived by heating gypsum (calcium sulfate dihydrate) to around 150°C, which removes water and forms the hemihydrate. This process was historically centred in Paris, where large gypsum deposits were found and widely used for making quick-setting materials. The name reflects its origin and primary usage in construction and sculpting.

The chemical formula for Plaster of Paris is CaSO4·½H2O.

Complete Answer:

Calcium Sulphate Hemihydrate is commonly known as Plaster of Paris (POP) because it was historically produced in large quantities in Paris, particularly from the abundant gypsum deposits in the Montmartre area.

Here’s the process and reasoning behind the name:

1. Gypsum Source in Paris: Gypsum, a natural mineral made of calcium sulfate dihydrate (CaSO₄·2H₂O), was widely available near Paris.
2. Heating Gypsum: When gypsum is heated to about 150°C (300°F), it loses water and forms calcium sulfate hemihydrate (CaSO₄·½H₂O). This process is called calcination:
   
3. Paris’ Historical Contribution: During the 18th century, this calcined gypsum was extensively manufactured in Paris and used in construction, sculpting, and medical applications. Its association with the city gave it the name “Plaster of Paris.”
4. Rehydration Property: When mixed with water, Plaster of Paris rehydrates into gypsum and hardens quickly, making it useful for moulds, casts, and decorative work.

Application of Plaster of Paris

Plaster of Paris (POP) is widely used across various fields due to its quick-setting property, smooth finish, and versatility. Here are some of its common uses:

• Used to create lightweight, elegant false ceilings and decorative mouldings.
• Provides a smooth finish to walls and ceilings before painting.
• Used for creating intricate designs, statues, and art pieces.
• Used for immobilising broken bones and fractures as it hardens quickly and moulds easily.
• Used to create impressions of teeth for dental procedures.
• Ideal for creating detailed sculptures and models due to its malleability when wet.
• Used in schools and workshops for making craft items like masks and decorative moulds.
• Used for creating replicas and intricate designs in moulds.
• Used to make moulds for ceramics, jewellery, and metal casting.
• Applied as a protective layer to walls and structures to enhance fire resistance.
• Used in building construction for thermal and acoustic insulation.
• Used for repairing small cracks and holes in walls and ceilings.
• Used to restore broken artefacts, sculptures, and historical structures.
• Used in making models for educational purposes in biology and medical studies.
• Used in labs for creating prototypes and experimental moulds.

Common Chemistry Doubts:

What Do You Mean by a Precipitation Reaction?	What is the Atomic Mass in Hydrogen?
What is the Formula Mass of the Iodine Molecule?	What Do You Mean by Polar Protic and Polar Aprotic Solvents?