How does Plaster of Paris become hard?

The correct sequence of thermal stability of the following carbonates is:

Which of the following salts is not soluble in water?

The halides of alkaline earth metals form hydrates such as $MgCl_2 \cdot 6H_2O$,$CaCl_2 \cdot 6H_2O$,$BaCl_2 \cdot 2H_2O$,and $SrCl_2 \cdot 2H_2O$. What does this indicate about the halides of group $2$ elements?

Slaked lime is used in the manufacture of

Bleaching powder is obtained by passing chlorine on