What are ion pairs? What effect does ion-pair formation have on the colligative properties of a solution? How does the ease of ion-pair formation depend on (a) charges on the ions, (b) size of the ions, (c) nature of the solvent (polar versus nonpolar), (d) concentration?

Ion pairs are the pair of ions that are associated together because of the strong electrostatic attraction between them. This mostly occurs in solutions of ionic compounds, where ion-pair forms due to mutual interaction of positive and negative ions. Each ion in an ion pair is surrounded closely by ions of opposite charge, which is the basic foundation of ionic bonding.

Ion-pair formation can severely affect the colligative properties of a solution. The explanation is: Colligative properties including boiling point elevation, freezing point depression, and osmotic pressure are determined by the number of solute particles in the solution rather than their nature. Thus, when ion pairs form, they act as one single entity rather than separate ions, thereby reducing the number of total particles in solution, which in turn decreases the colligative effect.

(a) Charges on the ions: The stronger the charge on the ions, the greater is the electrostatic attraction between them, thereby enhancing the formation of ion-pairs. \n(b) Size of the ions: Smaller ions have high charge density, leading to stronger electrostatic interaction and hence more formation of ion pairs. \n(c) Nature of the solvent (polar vs nonpolar): In polar solvents, the ions get solvated to a greater extent, and the extent of ion-pair formation decreases. Conversely, in nonpolar solvents ion-pairs are more likely to form since there is less solvation of ions. \n(d) Concentration: As concentration increases, the number of potential ion partners increases, hence ion-pair formation becomes more probable.