What is the difference in the number of water molecules between gypsum and plaster of Paris?

Chemical $A$ is used for water softening to remove temporary hardness. $A$ reacts with $Na_2CO_3$ to generate caustic soda. When $CO_2$ is bubbled through $A$,it turns cloudy. What is the chemical formula of $A$?

$MgCl_2 \cdot 6H_2O$ when heated gives:

Which are the important ingredients present in Portland cement?

Give two uses of beryllium and magnesium.

The correct order of density of $Be, Mg, Ca, Sr$ is