What type of acid is represented by a double arrow in a chemical equation?

In chemical equations, the use of a double arrow indicates that the reaction is reversible and does not go to completion. This is a characteristic of weak acids, which partially dissociate in solution. In contrast, strong acids completely ionize in water, leading to a one-way reaction indicated by a single arrow.

For example, when a weak acid such as acetic acid (CH₃COOH) is dissolved in water, it establishes an equilibrium with its ions (CH₃COO⁻ and H⁺), represented by a double arrow indicating that both the forward and reverse reactions occur. This is distinct from strong acids, like hydrochloric acid (HCl), which fully dissociate and do not exhibit reversible behavior.

Understanding the behavior of weak acids in equilibrium helps explain why they are represented with a double arrow, highlighting their partial ionization and the fact that they can re-form the original acid from its ions in solution.