What is the chemical formula of limestone?

Magnesium does not decompose which of the following?

Heating a solid compound $X$ produces $CO_2$ gas and a residue. Adding water to this residue produces compound $Y$. Passing excess $CO_2$ gas through a solution of $Y$ produces a clear solution $Z$. Boiling $Z$ regenerates compound $X$. What is compound $X$?

Compounds of alkaline earth metals are less soluble in water than the corresponding alkali metal compounds due to

$CO_2$ is passed through lime water. Initially,the solution turns milky and then becomes clear upon continued bubbling of $CO_2$. The clear solution is due to the formation of:

Which of the following sulfate salts has the highest solubility in water?