Which statement best describes weak acids?

• A. Acids that ionize partially in water
• B. Acids that ionize completely in water to form H3O+
• C. Acids that do not ionize
• D. Acids that donate protons

Answer: (A)

Weak acids are defined by ionizing only partially in water, forming a balance between the undissociated acid and its ions: HA + H2O ⇌ H3O+ + A−. Because only a small fraction of molecules dissociate, most of the acid remains in the molecular form, and the concentration of hydronium ions is relatively low. This partial ionization is described by a small acid dissociation constant (Ka), which places the equilibrium to the left. In contrast, strong acids dissociate essentially completely, producing a large amount of H3O+ and leaving little undissociated HA. The idea that acids ionize only partially in water captures the distinguishing behavior of weak acids. While acids do donate protons, the key difference here is the extent of ionization, not merely the ability to donate. The other statements describe conditions that don’t match the typical behavior of weak acids in water.