How would you expalain the following observation?
(ii)BaO is soluble but `BaSO_(4)` is insoluble in water

Explanation

when small anion `(O^(-2))` combines with large cation `(Ba^(2))` lattice energy is less.But at the same time the hydration energy exceeds the lattice energy .So `BaO` is soluble in water.
In the case of `BaSO_(4)` both cation and anion are large. So the lattice energy is less. But due to the bigger size of `Ba^(2+)` and `SO_(4)^(2-)` their hydration energies are also less. The sum of hydration energy of `Ba^(2+)` and `SO_(4)^(2-)` is less then the lattice energy of `BaSO_(4)` .So `BaSO_(4)` is insoluble in water.