Question

How do ionization and dissociation differ?

Answer 1

Ionization is a chemical reaction in which some species are transformed in ions. In dissociation ions are getting separate, but they exist from beginning.

As an example of ionization you can think to gaseous hydrogen chloride, HCl(g). It is formed by molecules. These react in water forming ions (`HCl(g) + H_2O(l) -> H_3O^+(aq) + Cl^-` `(aq)`) and dissolving in it to give a solution known as hydrochloric acid.

As an example of dissociation we can consider the process of dissolution in water or melting of a ionic compound. In such a ionic compound as sodium chloride, ions are originally existing in the solid crystal; meting or dissolving the compound in water these ions are separated, becoming free of moving, whereas they are constrained to remain narrowly confined in the crystal. The following aren't considered chemical reactions, although the involved ionic species undergo structural changes due to clustering and forming of dipole-ion bonds with water molecules in the first case.
`Na^+Cl^-` `(s) + H_2O(l) -> Na^+` `(aq) + Cl^-` `(aq)`
`Na^+Cl^-` `(s) -> Na^+Cl^-` `(l)`