The ionic carbide is

Equimolar solutions of the following were prepared in water separately. Which one of the solutions will record the highest $pH$?

Which one is known as barytes?

Which of the following components of cement is present in the highest amount?

$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms compound $Y$. On passing excess $CO_2$ gas through $Y$,a clear solution $Z$ is formed. On heating $Z$,compound $X$ is recovered. What is compound $X$?
$\mathop X\limits_{solid}$ $\xrightarrow{\text{heating}} CO_2 + \text{residue}$ $\xrightarrow{H_2O} Y$ $\xrightarrow{\text{excess } CO_2} \mathop Z\limits_{\text{solution}}$ $\xrightarrow{\text{heating}} X$

Discuss the trend of the following:
$(i)$ Thermal stability of carbonates of Group $-2$ elements.
$(ii)$ The solubility and the nature of oxides of Group $-2$ elements.