Which substance is formed when gypsum is heated to 373 K, resu... | General Science MCQ | Examoogle

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General ScienceChemistry: Acids, Bases and Salts

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Which substance is formed when gypsum is heated to 373 K, resulting in the loss of water molecules?

Calcium carbonate

Calcium sulphate hemihydrate

Calcium chloride

Calcium oxide

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral ScienceChemistry: Acids, Bases and Salts

Option B

Calcium sulphate hemihydrate

Quick Summary:

When gypsum ( $CaSO_{4} \cdot 2H_{2}O$ ) is heated at 373 K, it loses three-fourths of its water of crystallization to form calcium sulphate hemihydrate ( $CaSO_{4} \cdot \frac{1}{2}H_{2}O$ ). This product is widely known as Plaster of Paris (POP).

🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of it like baking a cake where you carefully remove just enough moisture to set the structure without drying it out completely, turning it into a powder that can later be 're-hydrated' to set hard again.

🧠 Memory Mnemonic / Shortcut Aid

G-H-P: Gypsum (2 waters) becomes Hemihydrate (half water) which is POP.

🔢 Key Formulas

$CaSO_{4} \cdot 2H_{2}O \xrightarrow{373 K} CaSO_{4} \cdot \frac{1}{2}H_{2}O + \frac{3}{2}H_{2}O$ — Dehydration reaction of gypsum

$CaSO_{4} \cdot \frac{1}{2}H_{2}O + \frac{3}{2}H_{2}O \rightarrow CaSO_{4} \cdot 2H_{2}O$ — Setting of Plaster of Paris

⚙️ Working Principle

The dehydration process occurs due to the thermal instability of the two water molecules associated with the calcium sulphate lattice. Upon heating to 373 K, the crystal structure sheds $1.5$ moles of water, leaving behind a hemihydrate form which is thermodynamically stable at that specific temperature. If heated above this temperature, all water is lost, resulting in 'dead burnt plaster' (anhydrous calcium sulphate).

📌 Key Points

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Gypsum is chemically known as calcium sulphate dihydrate.
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Plaster of Paris (POP) is chemically known as calcium sulphate hemihydrate.
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The reaction is temperature-sensitive; exceeding 373 K leads to the formation of anhydrous calcium sulphate.
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POP sets into a hard mass upon mixing with water due to the re-formation of gypsum crystals.

✅ Advantages

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Quick setting time when mixed with water
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Excellent for casting molds and surgical casts

❌ Disadvantages / Limitations

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If stored improperly, it absorbs moisture from the air, losing its setting properties
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Must be stored in moisture-proof containers

🛠️ Applications / Uses

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Used by doctors for supporting fractured bones in the right position
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Used for making toys, statues, and decorative materials

📄 Additional Information

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373 K is equivalent to 100 °C.
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Option A (Calcium carbonate) is limestone ( $CaCO_{3}$ ), which is unrelated to the hydration process of gypsum.
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Option C (Calcium chloride) is a salt used as a desiccant, not a product of gypsum heating.
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Option D (Calcium oxide) is quicklime ( $CaO$ ), formed at very high temperatures (above 1000 K).

📊 Diagram / Illustration

✅

B is correct — Heating gypsum to 373 K produces calcium sulphate hemihydrate, commonly called Plaster of Paris.

Core Concepts Used

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Water of crystallization Thermal dehydration Hydrates

💡 EXAM TIP

Remember that gypsum has 2 water molecules and POP has $\frac{1}{2}$ water molecule; the 'hemi' in hemihydrate specifically stands for half.

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