Correct Option
The statement that barium and strontium are found free in nature is incorrect. Barium and strontium are highly reactive alkaline earth metals (Group 2 elements). Due to their strong electropositive nature and low ionization energies, they readily lose electrons to form stable compounds. Consequently, they do not exist in their elemental or free state in nature. Instead, they are found combined with other elements, primarily as minerals such as barite (BaSO₄) and celestine (SrSO₄).
Incorrect Options
Option (1): The statement that the presence of NaCl increases the rate of setting of plaster of Paris is correct. Sodium chloride (NaCl) acts as an accelerator for the setting of plaster of Paris (calcium sulfate hemihydrate, CaSO₄·½H₂O). It speeds up the hydration process and the crystallization of calcium sulfate dihydrate, thereby reducing the setting time.
Option (2): The statement that gypsum is added to the cement to slow down its rate of setting is correct. Gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) is a crucial additive in cement. Its primary function is to retard the rapid setting of cement, providing sufficient time for mixing, transportation, and placement before the cement hardens. It achieves this by controlling the hydration rate of tricalcium aluminate.
Option (3): The statement that all alkaline earth metals form hydrated salts is correct. Alkaline earth metals (Group 2 elements), including beryllium, magnesium, calcium, strontium, and barium, commonly form hydrated salts. Their ions, due to their relatively high charge density, strongly attract water molecules, which become incorporated into their crystal structures (e.g., MgCl₂·6H₂O, CaCl₂·2H₂O, BaCl₂·2H₂O).