(i) BeO is almost insoluble but BeSO₄ is soluble in water due to the following reasons:

⇒ Because of the smaller size , higher ionization enthalpy and higher electronegativity, BeO is essentially covalent and hence is insoluble in water.

⇒ On the other hand, BeSO₄ is ionic.

⇒ As we know that, the solubility of a ion in water depends on the balance between lattice energy and hydration energy. The hydration energy of BeSO₄ is much higher than its lattice energy and BeSO₄ is soluble in water.

(ii) BaO is soluble but BaSO₄ is insoluble in water because of the following reasons:

⇒ Both BaO and BaSO₄ are ionic compounds. However, the size of O^2- ion is much smaller than that of SO₄^2- ion.

⇒ Since a bigger anion stabilizes a bigger cation more than a smaller anion stabilizes a bigger cation, therefore, the lattice energy of BaO is much smaller than that of BaSO₄.

⇒ As we know that smaller the lattice energy, greater will be the solubility. Hence, BaO is soluble while BaSO₄ is insoluble in water.

(iii) LiI is more soluble than KI in ethanol because of the following reasons:

⇒ Li⁺ is much smaller than K⁺ ion.

⇒ Therefore, according to the Fajan’s rule, Li⁺ ion can polarise bigger I^- ion to a greater extent than K⁺ ion.

⇒ As a result, LiI is more covalent than KI and hence more soluble in organic solvents like ethanol.