The pair of elements that form both oxides and nitrides when burnt in air are

How would you explain the following observations?
$(i)$ $BeO$ is almost insoluble but $BeSO_4$ is soluble in water,
$(ii)$ $BaO$ is soluble but $BaSO_4$ is insoluble in water,
$(iii)$ $LiI$ is more soluble than $KI$ in ethanol.

Which of the following substances is insoluble in water?

Which of the following ions has a higher hydration enthalpy than the $Mg^{2+}$ ion?

Which of the following metals belong to the same group in the periodic table?

$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms $Y$. On passing an excess of $CO_2$ through $Y$ in water,a clear solution $Z$ is obtained. On boiling $Z$,compound $X$ is reformed. The compound $X$ is