Give the chemistry of plaster of pairs. [5]

CaSO₄·2H₂O+heat2(CaSO₄·½H₂O)+1½H₂O

Chemistry of Plaster of Paris

1. Chemical Composition and Formula Plaster of Paris (POP) is chemically known as calcium sulfate hemihydrate, with the formula CaSO₄·½H₂O. It is obtained by heating gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) at 100–150°C. The process removes 1.5 moles of water per mole of gypsum, converting it into POP.

2CaSO₄·½H₂O+3H₂OReversible reaction2CaSO₄·2H₂O

gypsumgypsum

2. Preparation When gypsum is heated, it loses water molecules and forms a powdery substance called Plaster of Paris: $$2\mathrm{CaSO}{}_4\cdot 2\mathrm{H}{}_2\mathrm{O}\stackrel{100-150°\mathrm{C}}{\to }2\mathrm{CaSO}{}_4\cdot ½\mathrm{H}{}_2\mathrm{O}+3\mathrm{H}{}_2\mathrm{O}$$ This reaction is endothermic and reversible. If heated beyond 180°C, POP loses all water and forms anhydrous calcium sulfate (CaSO₄), which cannot rehydrate to form POP.

3. Setting Reaction When POP is mixed with water, it rehydrates to form a hard, solid mass of gypsum again: $$2\mathrm{CaSO}{}_4\cdot ½\mathrm{H}{}_2\mathrm{O}+3\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{CaSO}{}_4\cdot 2\mathrm{H}{}_2\mathrm{O}$$ This exothermic reaction is called the setting reaction, which occurs within 5–15 minutes of mixing with water. The process involves:

• Dissolution: POP dissolves in water to form a supersaturated solution.
• Crystallization: Gypsum crystals nucleate and grow, forming a rigid structure.

4. Properties

• White powder with a fine, smooth texture.
• Non-toxic, non-flammable, and odorless.
• Thermally stable up to 180°C (beyond which it becomes anhydrous).
• Hardens rapidly when mixed with water, making it useful for casting and molding.

5. Uses

• Medical: Used in orthopedics for setting broken bones (e.g., plaster casts).
• Construction: For making decorative items, molds, and fire-resistant structures.
• Art and Sculpture: Ideal for creating detailed casts and statues.
• Dental: Used in dentistry for making molds and impressions.

6. Advantages

• Quick setting time (faster than cement).
• Easy to shape before hardening.
• Non-corrosive and durable.

7. Limitations

• Low tensile strength compared to cement.
• Not waterproof (absorbs moisture over time).
• Weakens if exposed to prolonged moisture or high temperatures.

The chemistry of POP relies on its reversible hydration-dehydration process, making it a versatile material in various applications.