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Activity 2.10 — Plaster of Paris
NCERT Activity 2.10: Heat gypsum (CaSO₄·2H₂O) at 373 K to make Plaster of Paris (CaSO₄·½H₂O). Then add water to PoP — it sets back to gypsum, forming a hard solid. This reversible reaction is the basis of orthopaedic casts and sculpting.
How To Perform
1
Add Gypsum
CaSO₄·2H₂O (white powder) in a crucible.
2
Heat at 373 K
Carefully controlled — too hot gives 'dead burnt plaster'.
3
Get Plaster of Paris
CaSO₄·½H₂O — hemihydrate, can set with water.
4
Add Water to PoP
Paste forms and begins to harden.
5
Observe Setting
PoP hardens into solid gypsum — exothermic.
6
Conclude
Gypsum ⇌ PoP + 1½ H₂O — reversible decomposition.
⚠️ Safety Notes
- •Heat at 373 K (100°C) — NOT higher. Too hot ruins the product.
- •The setting reaction is exothermic — don't touch the setting PoP with bare hands.
- •Avoid inhaling PoP dust — it can irritate the lungs.