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

Explanation

(i) Because of smaller size , higher ionization enthalpy and higher electronegativity , BeO is essentially covalent and hence is insoluble in water . In contrast , `BeSO_(4)` is ionic . Further because of small size of `Be^(2+)` ion , the hydration energy of `BeSO_(4)` is much higher than its lattice energy and hence `BeSO_(4)` is highly soluble in water .
(ii) Both `BaO` and `BaSO_(4)` are ionic compounds . however , the size of `O^(2-)` ion is much smaller than that of the `SO_(4)^(2-)` ion . Since a bigger anion stabilizes a bigger cation more than a smaller anion stabililzes a bigger cation , therefore , the lattice energy of BaO is much smaller than that of `BaSO_(4)` and hence BaO is soluble while `BaSO_(4)` is insoluble in water .
(iii) `Li^(+)` is much smaller than `K^(+)` ion . Therefore , according to Fajan rule , `Li^(+)` ion can polarize bigger `I^(-)` ion to a greater extent than `K^(+)` ion . As a result , LiI is more covalent than KI and hence is more soluble in organic solvents like ethanol.