What is used in the preparation of calcium sulphate hemihydrate? 

The correct answer is Gypsum.

Key Points

• Gypsum (CaSO₄·2H₂O) is used in the preparation of calcium sulphate hemihydrate (CaSO₄·0.5H₂O).
• Calcium sulphate hemihydrate is commonly known as Plaster of Paris.
• The process involves heating gypsum to around 150°C to 180°C, which partially dehydrates it.
• Plaster of Paris is widely used in construction, sculpture, and medical applications for making casts and molds.

Additional Information

• Gypsum:
  • It is a soft sulfate mineral composed of calcium sulfate dihydrate.
  • Gypsum is naturally occurring and is mined from sedimentary deposits.
  • It is used in various industries, including agriculture as a soil conditioner and in the production of cement.
• Plaster of Paris:
  • It is a quick-setting gypsum plaster consisting of a fine white powder (calcium sulfate hemihydrate).
  • When mixed with water, it rehydrates and sets quickly, making it useful for casting molds and medical bandages.
• Dehydration Process:
  • The heating of gypsum to produce Plaster of Paris is a controlled process to ensure partial dehydration.
  • Overheating can lead to complete dehydration, forming anhydrite (CaSO₄), which has different properties.
• Uses of Plaster of Paris:
  • In construction, it is used for making decorative elements and false ceilings.
  • In orthopedics, it is used for making casts for broken limbs.
  • In art, it is used for making sculptures and other decorative items.