Ionic salts are used for melting ice on roads for two reasons: they are effective at lowering the freezing point when they dissolve in the water and they sometimes release heat when they dissolve in the water.

the heat of solution depends on the energy involved in breaking apart the salt and the energy involved in dissolving the ions in water (hydration). the enthalpy of hydration for each ion depends on the strength of the intermolecular force between the ion and water, which is electrostatic and follows coulomb's law.
rank the following in order of their predicted ability to melt ice due to their heat of solution:
1 = best at melting ice; 3 = worst at melting ice
a. nacl
b. kcl
c. cacl2