Which one of the following statements is correct?
- AThe presence of NaCl increases the rate of setting of Plaster of Paris
- BGypsum is added to the cement to slow down its rate of settingCorrect
- CAll alkaline earth metals form hydrated salts
- DBarium and Strontium are found free in nature
Explanation
Gypsum (calcium sulphate dihydrate) is added to cement specifically to slow down the rate of setting, allowing workers enough time for mixing and application. Without gypsum, cement would set too rapidly to be workable.
Option D is incorrect because Barium and Strontium are highly reactive alkaline earth metals and are never found free in nature; they always occur as compounds.
Option A is actually a correct statement as NaCl does accelerate the setting of Plaster of Paris, but B is the designated answer. Option C is not universally true as not all alkaline earth metals readily form hydrated salts.
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