Which of the following compounds is obtained when gypsum is heated at $373 ext{ K}$?

Question

Which of the following compounds is obtained when gypsum is heated at $373	ext{ K}$?

Examoogle · Accepted Answer

The correct answer is Option B: $	ext{CaSO}_4 \cdot \frac{1}{2}	ext{H}_2	ext{O}$. When gypsum is heated at $373	ext{ K}$ ($100^{\circ}	ext{C}$), it loses three-fourths of its water of crystallization to form plaster of Paris, which is chemically calcium sulfate hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$).

Step-by-Step Solution & Concepts:
When gypsum is heated at $373	ext{ K}$ ($100^{\circ}	ext{C}$), it loses three-fourths of its water of crystallization to form plaster of Paris, which is chemically calcium sulfate hemihydrate ($CaSO_4 \cdot \frac{1}{2}H_2O$).

Working Principle: Gypsum ($CaSO_4 \cdot 2H_2O$) contains two molecules of water of crystallization per formula unit. Upon controlled heating at $373	ext{ K}$, a dehydration reaction occurs, releasing $1.5$ molecules of water vapor and leaving behind calcium sulfate hemihydrate, commonly known as plaster of Paris. If heated above $373	ext{ K}$, it loses all its water of crystallization to form anhydrous calcium sulfate ($CaSO_4$), known as 'dead burnt plaster'.

Key Formulas: $CaSO_4 \cdot 2H_2O$ — Chemical formula of Gypsum; $CaSO_4 \cdot \frac{1}{2}H_2O$ — Chemical formula of Plaster of Paris

Key Points: Gypsum is heated at $373	ext{ K}$ ($100^{\circ}	ext{C}$) in manufacturing Plaster of Paris.; It loses $1.5$ molecules of water during this heating process.; Plaster of Paris has a remarkable property of setting into a hard mass when mixed with water.; Heating above $373	ext{ K}$ destroys its setting property completely due to formation of anhydrous $CaSO_4$.

Advantages: Sets rapidly upon mixing with water; Expands slightly on setting, filling minute cavities perfectly

Disadvantages: Overheating beyond $373	ext{ K}$ forms dead-burnt plaster which does not set with water

Applications: Making casts for fractured bones in medical clinics; Making decorative statues; toys

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