Plaster of Paris

📚 Key Concepts

🔹 Real-Life Example

When you break a bone, doctors use Plaster of Paris to make a cast. It starts as a powder, but when mixed with water, it becomes a paste that hardens into a strong solid. This same material is used for making decorative items, sculptures, and even blackboard chalk.

Plaster of Paris (CaSO₄·½H₂O): A white powder obtained by heating gypsum, which hardens when mixed with water. Named after Paris, where large deposits of gypsum were found.

🧪 Preparation and Properties

🔸 Preparation from Gypsum

CaSO₄·2H₂O + Heat (373K) → CaSO₄·½H₂O + 1½H₂O

🔸 Setting of Plaster of Paris

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O + Heat

🔸 Properties

• White powder
• Sets quickly when mixed with water (10-15 minutes)
• Expands slightly on setting
• Good moldability before setting

🔸 Uses

• Medical casts for fractured bones
• Making toys, decorative materials
• False ceiling and wall decoration
• Dental impressions
• Sculpture and art work

🔍 Advanced: Storage

Plaster of Paris must be stored in moisture-proof containers because it absorbs atmospheric moisture and becomes hard, making it unusable