An alternative representation to 0x45's rendition of Coulomb's law accounts for the polarizability of the environment:

                q1 q2
          f = ----------
              4 pi E r2

Where E is actually epsilon. E/E0 is known as the dielectric constant, or relative permittivity. E0 is the permittivity of free space (of a vacuum).

The dielectric constant, subsumed in the constant k in the previous writeup, accounts for screening of electrostatic interactions between charges by the environment. An environment that is highly polarizable will orient around a given charge, thereby effectively neutralizing the effects of electric field at long distances. Water has a high dielectric constant - around 80. As a result, ions in water have a much weaker attraction or repulsion than they do in the gas phase.