Question

How would you explain the following observations?
(i) $BeO$ is almost insoluble but $BeS{O}_4$ is soluble in water.
(ii) $BaO$ is soluble but $BaS{O}_4$ is insoluble in water.
(iii) $LiI$ is more soluble than $KI$ in ethanol.

Answer 1

(i) Due to high lattice enthalpy and covalent nature, $BeO$ is insoluble in water. In case of ionic $BeS{O}_4$, the hydration enthalpy is much more than its lattice enthalpy. Hence, it is water soluble.
(ii) Oxide ion has smaller size than sulphate ion. $BaO$ has smaller lattice energy than $BaS{O}_4$ as bigger cation stablizes bigger anion to greater extent than smaller cation stabilizes bigger anion. Hence, $BaO$ is water soluble and $BaS{O}_4$ is water insoluble.
(iii) As per Fajan rules, smaller $L{i}^{+}$ ion polarizes bigger $I^{-}$ to a greater extent than $K^{+}$ ion. Hence, $LiI$ is more covalent than KI and hence, more soluble in organic solvents such as ethanol.