For the dissociation of a strong acid, only one arrow is required because the reaction goes to completion. This means that essentially all of the acid molecules dissociate into their respective ions in water. The reaction is essentially irreversible, and the equilibrium lies far to the right.

For example, when hydrochloric acid (HCl) is dissolved in water, it completely dissociates into hydrogen ions (H+) and chloride ions (Cl-). The reaction can be represented as follows:

HCl(aq) → H+(aq) + Cl-(aq)

In this case, only one arrow is needed to show the direction of the reaction because it proceeds essentially to completion.