Question

Why is it necessary to keep the plaster of Paris away from moisture? Explain.

Hint:

POP is always stored in airtight containers to prevent contact with moisture.

Answer 1

Step 1: Definition of Plaster of Paris (POP)
Plaster of Paris is chemically $CaSO_{4} \cdot \tfrac{1}{2} H_{2}O$. It is obtained by heating gypsum ($CaSO_{4} \cdot 2H_{2}O$).
Step 2: Reaction with Moisture
POP easily absorbs moisture from air. In presence of water, it gets converted back into gypsum: \[ CaSO_{4} \cdot \tfrac{1}{2} H_{2}O + \tfrac{3}{2} H_{2}O \longrightarrow CaSO_{4} \cdot 2H_{2}O \]
Step 3: Reason for Storage Precaution
If POP absorbs moisture, it loses its setting property and hardens prematurely, making it useless for practical applications.
Step 4: Applications of POP
Used in surgical bandages (fractures).
Used for making decorative materials and false ceilings.
Used for smooth finishing of walls.
Final Answer:
POP must be kept away from moisture because it reacts with water and becomes gypsum, losing its usability.