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

a. Because of smaller size, higher ionisation enthalpy and higher electronegativity, `BeO` is essentially covalent and hence is insoluble in water. In constrast,`BaSO_(4)` is ionic. Further because of small size of `Be^(2+)` ion, the hydration enthalpy of `BaSO_(4)` is much higher than is lattice enthalpy and hence `BaSO_(4)` is highly soluble in water.
b. 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 stabilises a bigger cation more than a smaller anion stabilises a bigger cation, therefore, the lattice enthalpy of `BaO` is much smaller than that of `BaSO_(4)` and hence `BaO` is soluble while `BaSO_(4)` is insoluble in water.
c. `Li^(o+)` is much smaller than `K^(o+)` ion. Therefore, according to Fajan's rule, `Li^(o+)` ion can polarise bigger `I^(ө)` ion to a greater extent than `K^(o+)` ion. As a result, `Lil` is more covalent that `Kl` and hence is more soluble in organic solvents like ethanol.