Strong acids and weak acids are defined by their ability to ionize. Strong acids usually ionize 100 percent in a solution, while a weak acid does not. For example, a strong acid placed in water immediately donates a hydrogen ion to the water to form a hydroxonium ion.
Hydrogen chloride is a strong acid because it rapidly forms hydrochloric acid when added to water. Since the reaction is less likely to reform the reactants, the reaction is marked with an arrow pointing to the products. Sulfuric acid and nitric acid are strong acids as well. Strong acids have lower pH values due to the high concentration of hydrogen ions in the solution.
On the other hand, a weak acid does not ionize 100 percent when dissolved in water. For example, ethanoic acid is a weak acid that produces hydroxonium and ethanoate ions when placed in water. The acid does not fully dissolve, and the reaction continuously moves forwards and backwards to achieve equilibrium. Thus, the new ions can easily rearrange to form the reactants ethanoic acid and water. Commonly, organic acids are considered weak acids. A weak acid reaction is marked with two arrows pointing in opposite directions. Transition metal cations, hydrofluoric acid and acetic acid are weak acids.