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) BeO is insoluble in water because of its high lattice enthalpy. It is also covalent in nature and therefore , insoluble in water . On the other hand , `BeSO_4` is ionic .
Its hydration enthalpy is much more than its lattice enthalpy and hence it is 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 as bigger cation more than a smaller cation stabilizes a bigger anion , therefore , the lattice enthalpy of BaO is smaller than `BaSO_4`. Therefore , BaO is soluble while `BaSO_4` is insoluble in water .
(iii) `Li^(+)` is smaller in size than `K^(+)` ion . Therefore , according to Fazan rules , `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.