POP Chemical Formula — Plaster of Paris Formula and Properties

POP = Plaster of Paris Chemical name: Calcium sulphate hemihydrate Chemical formula: CaSO₄·½H₂O

Alternate ways to write the formula: • (CaSO₄)₂·H₂O (written for 2 formula units) • 2CaSO₄·H₂O

IUPAC name: Calcium sulphate hemihydrate Molar mass: approximately 145.15 g/mol Appearance: White powder

Plaster of Paris is prepared by heating gypsum (calcium sulphate dihydrate) at 120–130°C:

Gypsum → POP + Water CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O

Or written as: 2CaSO₄·2H₂O → 2CaSO₄·H₂O + 3H₂O

Note: If gypsum is heated above 200°C, it forms dead burnt plaster (anhydrous CaSO₄), which cannot set.

When POP is mixed with water, it reacts and sets (hardens) into gypsum:

POP + Water → Gypsum CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

Or: 2CaSO₄·H₂O + 3H₂O → 2CaSO₄·2H₂O (Gypsum)

Key points about setting: • Setting reaction is exothermic (releases heat) • POP expands slightly on setting — useful for making moulds • Sets in about 5–15 minutes depending on water ratio

1. Medical uses: Making casts for fractured bones (orthopaedic plasters)
2. Construction: Plastering walls and ceilings for smooth finish
3. Dentistry: Making dental moulds
4. Art and sculpture: Making statues, decorative items, figurines
5. Fire-proofing: Fireproofing material in buildings
6. Making chalk: Used to make blackboard chalk
7. Ceramics: Making ceramic moulds for pottery