When an ionic solid dissolves in water, the polar water molecules surround the crystal. These water molecules physically pull the rigid crystal lattice apart into separate hydrated ions. Strong electrolytes like soluble salts and strong acids dissociate completely in aqueous liquid solutions.
Weak electrolytes only dissociate partially and maintain a dynamic chemical equilibrium in the solution. In a weak electrolyte solution, the forward dissociation reaction balances the reverse reassociation reaction. This delicate balance determines the exact concentration of free ions available in the mixture.
This partial dissociation is mathematically described by a specific value called a dissociation constant. Because dissociation is a reversible process, the separated ions can easily reassociate together again. The overall extent of dissociation heavily depends on the concentration, temperature, and specific solvent.
Chemical dissociation can also occur entirely in the gas phase when molecules are heated. Intense heat energy can literally split a larger gas molecule into smaller chemical fragments. Heating dinitrogen tetroxide gas causes it to thermally dissociate into nitrogen dioxide gas molecules.
Students often confuse dissociation with ionization, but they describe slightly different chemical interaction processes. Dissociation separates already existing ions, while ionization creates entirely new ions from neutral molecules.