We have studied in sec.28.2 that some ionic solids are highly soluble in water whereas others are almost insoluble. The solubility of an ionic solid in water depends mainly on the lattice enthalpy of the salt and the hydration enthalpy of the ions in solution.
The energy required to overcome the attractive forces between ions is referred to as lattice enthalpy whereas the energy released during interaction between ions and the solvent molecules is referred to as solvation enthalpy or hydration energy. Thus, for dissolution process, the lattice enthalpy is always positive while solvation enthalpy is always negative. For a salt to dissolve, its solvation enthalpy must be greater than its lattice enthalpy.
Ionic solids, in general, can be broadly classified into three categories on the basis of their solubility in water,
(i) The solids having solubility greater than 0.1 Mare classified as soluble solids.
(ii) The solids having solubility between 0.01 M and 0.1 M are classified as slightly soluble.
(iii) The ionic solids having solubility less than 0.01 M are termed as sparingly soluble solids. The salts such as BaSO4, AgCI, AgCrO4 etc., belong to this category of salts.