Bone ash contains

$A$ hydrated colorless solid $(A)$ is water soluble and finds use in medicine as a purgative. When a solution of $(A)$ is treated with ammonium phosphate,a white precipitate is formed. $(A)$ gives a pink mass in the cobalt nitrate cavity test. What is $(A)$?

From which of the following reactions can anhydrous $MgCl_2$ be prepared?

The alkaline earth metal that imparts apple green colour to the Bunsen flame when introduced in it in the form of its chloride is

The thermal stability of $BaCO_3$,$CaCO_3$,$SrCO_3$,and $MgCO_3$ decreases in the order:

Describe the importance of the following:
$(i)$ limestone
$(ii)$ cement
$(iii)$ plaster of Paris.