EXPLAIN LOW SOLUBILITY OF LiF and CsI IN WATER

The solubility of a salt in water depends upon two factors 

(a) Lattice enthalpy 

(b) Hydration enthalpy

If the hydration enthalpy is greater than lattice enthalpy (hydration enthalpy > lattice enthalpy), then the salt is soluble in water. However if hydration enthalpy < lattice enthalpy, the salt is insoluble in water. In case of CsI, both the ions are large in size. As  a result, both the ions in CsI are less hydrated and have smaller hydration enthalpy. So it is less soluble in water. 

On the other hand, LiF is almost insoluble in water due to its high lattice enthalpy. Both the ions in LiF are small and heavily hydrated. However, because of high charge density on both the ions, the lattice enthalpy is also very high. So in case of LiF, the lattice enthalpy exceeds the hydration enthalpy, because of which it is almost insoluble in water.