IONS IN AQUEOUS SOLUTION

Ionic models:

Ions are charged chemical substance. Positively charged ions are called cations as they travel to the cathode of an electrolytic cell. Negatively charged (-) ions are termed anions as they travel to the anode. A salt dissolves to produce these cations and anions. When a salt (solute) is dispersed in water (solvent) (see Topic D1) to form an aqueous solution of ions, also termed electrolyte solution or an ionic, there is an energetically favorable ion-dipole interaction (Fig. 1).

Fig. 1. The interaction between water

and a cation and water and an anion

in solution.

Due to the different affinities of oxygen and hydrogen for electrons (-), the oxygen has a net fractional negative charge (δ-) and the protons have a net fractional positive charge (δ+), producing a dipole in the water molecule. The charge on the cation produces a electric field (positive gradient of potential with respect to distance) in solution that aligns the water dipoles locally with oxygen closest to the ion. The anion produces a negative electric field that orients the water dipole with the protons closest to the ion.