The solubility of ionic solids in water differs to a great extent.

Some of the ionic solids like $\mathrm{CaF}_{2}$ are so soluble that they are hygroscopic in nature and absorb even water from the atmosphere on the other extreme there are substances which have so little solubility, like $LiF$ (lithium fluoride), that these are regarded of insoluble. There are some ionic solids which have solubility in between these extreme cases.

The solubility depends on a number of feeling

$(i)$ Lattice enthalpy of the salt.

$(ii)$Solvation enthalpy of ions in the solution.

$(i)$ Lattice enthalpy : Lattice enthalpy is the amount of energy required to break one no. of solid salt into its ions. Greater the lattice enthalpy, more will be the energy required to break the lattice of the salt.

$(ii)$ Solvation enthalpy : Solvation enthalpy is the amount of energy released when one mole of a solid salt gets dissolved in a solvent the for a salt to dissolve in a solvent, the strong forces of attraction between its ions (i.e. lattice enthalpy) must be overcome by the ion – solvent interaction.

Therefore a salt will be soluble in a solvent only when solvation enthalpy is more than lattice enthalpy.

It solvation enthalpy is less than lattice enthalpy, the salt will be insoluble.