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Engineering Exams RRB-JE CMA (Chemical and Metallurgical Assistant) Technical Abilities 2025 22.04.25 Paper-2 Shift-1 Medium +1 -0.33
On heating gypsum at 373 K, it loses water molecules and becomes _________, called Plaster of Paris.
Correct Answer: A. calcium sulphate hemihydrate
Explanation: When gypsum is heated to 373 K (100°C), it undergoes a chemical reaction where it loses three-quarters of its water of crystallization. Specifically, the dihydrate form of calcium sulphate (CaSO4·2H2O) is converted into calcium sulphate hemihydrate (CaSO4·1/2H2O). This resulting substance is commonly known as Plaster of Paris, which is widely used in construction and medical applications because it hardens into a solid mass when mixed with water.
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