Lithium salts are often much less soluble in water than the corresponding salts of other alkali metals. For example, at 18°C, the concentration of a saturated LiF solution is $\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{2}}$M, whereas that of a saturated KF solution is 1.6 M. How would you explain this behaviour?

it is defined as the ability of an atom to get hydrated in water. As the atomic size increase, the hydration energy increase. So, caesium has high hydration energy, lithium has low hydration energy, and lattice energy is the inverse of hydration energy.

As lithium is less soluble among the alkali metals due to its small size so, in the medium fluoride,both lithium and fluorideions are small in size due to which they have high lattice energy, and the hydration gets decreases,whereas in potassium fluoride; potassium has a large size than lithium and fluoride and ever potassium has high hydration energy than lithium because hydration energy increases down the group so that potassium fluoride is more soluble than lithium fluoride.