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

$A$ 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 $X$?
$\mathop X\limits_{solid}$ $\xrightarrow{\text{heating}} CO_2 + \text{residue}$ $\xrightarrow{H_2O} Y$ $\xrightarrow{\text{excess } CO_2} \mathop Z\limits_{\text{solution}}$ $\xrightarrow{\text{heating}} X$

Magnesium does not decompose which of the following?

Which of the following statements is incorrect?

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

The element playing a significant role in neuromuscular function and interneuronal transmission is: