The use of Plaster of Paris is ..........

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.

Match the following alkaline earth metals with their respective densities $(g/cm^3)$:

$A$. $Be$	$I$. $1.74$
$B$. $Mg$	$II$. $1.84$
$C$. $Ca$	$III$. $2.63$
$D$. $Sr$	$IV$. $1.55$

The solubility of sulfates in water decreases down the group for $Be$ to $Ba$. The correct order is $BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$. The reason for this is:

In which of the following exothermic reactions,the heat liberated per mole is the highest?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Barium carbonate is insoluble in water and is highly stable.
Reason $R$: The thermal stability of the carbonates increases with increasing cationic size.
Choose the most appropriate answer from the options given below for $A$ and $R$.