What Is the Chemical Formula of Plaster of Paris?

Plaster of Paris:

• Chemical formula: CaSO4·½H2O
• IUPAC name: Calcium sulphate hemihydrate
• Other name: Hemihydrate of calcium sulphate

The formula CaSO4·½H2O means there is half a molecule of water for every formula unit of CaSO4. Equivalently, it can be written as (CaSO4)2·H2O — one molecule of water for every two formula units of calcium sulphate.

The name 'Plaster of Paris' comes from the large deposits of gypsum found in Montmartre, Paris, which were historically used to make this plaster.

Plaster of Paris is made by carefully controlled heating of gypsum:

Gypsum formula: CaSO4·2H2O (calcium sulphate dihydrate)

Reaction (heating at ~120°C): 2CaSO4·2H2O → 2CaSO4·H2O + 2H2O Or equivalently: CaSO4·2H2O → CaSO4·½H2O + 1½H2O

Conditions: Heating gypsum at 120–130°C drives off 1.5 molecules of water, leaving the hemihydrate.

Important: If heated above 200°C, gypsum loses all its water to form dead burnt plaster (anhydrous CaSO4, also called anhydrite), which no longer sets with water.

Heating above 400°C gives calcium oxide (CaO) and SO3.

When Plaster of Paris is mixed with water, it undergoes a setting reaction:

CaSO4·½H2O + 1½H2O → CaSO4·2H2O (gypsum)

In words: Plaster of Paris + Water → Gypsum

This is the reverse of its preparation. The product is gypsum (calcium sulphate dihydrate), which forms an interlocking crystalline network that gives the set plaster its hardness and rigidity.

Properties of the setting process:

• Setting time: Approximately 5–15 minutes (can be controlled by additives)
• Heat released: Slight exothermic reaction during setting
• Slight expansion: The plaster expands slightly on setting (~1%), which helps it fill moulds precisely
• Final product: Hard, white, smooth solid (gypsum)

Properties of Plaster of Paris (CaSO4·½H2O):

• Appearance: White or off-white fine powder
• Odour: Odourless
• Solubility: Slightly soluble in water
• Molecular mass: CaSO4·½H2O = 40 + 32 + 64 + ½×18 = 145 g/mol

Key characteristics:

• Sets hard within minutes of mixing with water
• Expands slightly on setting (fills moulds accurately)
• Resistant to fire once set
• Smooth surface finish after setting
• Can be ground, carved, and painted when dry

Plaster of Paris has widespread applications:

1. Medical casts: Used to make plaster casts for immobilising broken bones during healing.
2. Dentistry: Used to make dental impressions and models.
3. Sculpting and art: Used by artists and sculptors for making moulds, casts, and statues.
4. Construction: Used for plastering walls and ceilings (though being replaced by dry wall in modern construction).
5. Chalk and blackboard sticks: Chalk used on blackboards is largely calcium sulphate.
6. Fire protection: Applied as a fire-resistant coating on structural elements.
7. Toy making and decorative items: Used in making show-pieces and decorative plasterwork.
8. Laboratory: Used to seal gaps in apparatus.