My teacher taught us that higher the ionic character in a compound, higher is its solubility in polar solvents like water. To illustrate this statement, he also gave us the following example-
LiOH< NaOH< KOH< RbOH< CsOH (solublity in water)
According to this example, if the anion is same in all compounds then ionic character increases as we go down the group and thus the compounds become more soluble.
But then our teacher also gave us an exception. He said that if we another example with alkaline earth metals, then ideally according to Fajan's rule BaSO4 should be more soluble than MgSO4. But this is not the case because as we go down the group the decrease in hydration energy is greater than the decrease in lattice energy and thus solublity decreases.
My question is that shouldn"t this explanation also apply for the alkali metals? Shouldn't the decrease in hydration energy be greater than the decrease in lattice energy here as well?