Which one of the following is the chemical formula of Plaster of Paris ?
$ ext{CaSO}_4 cdot rac{1}{2} ext{ H}_2 ext{O}$
$ ext{CaSO}_4 cdot 2 ext{ H}_2 ext{O}$
$ ext{CaSO}_4 cdot 5 ext{ H}_2 ext{O}$
$ ext{CaSO}_4 cdot 4 ext{ H}_2 ext{O}$
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This question was previously asked in
UPSC NDA-2 – 2021
Plaster of Paris is produced by heating Gypsum ($\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$) to about 150°C (302°F). This heating process removes three-quarters of the water of crystallization, leaving the hemihydrate form. When mixed with water, Plaster of Paris rehydrates and sets into a hard mass, which is chemically Gypsum.