In aqueous solution, which statement best describes a weak acid?

• A. It has a high pH because it produces $O^2-$ ions in water.
• B. It completely ionizes in water, producing only ions and no undissociated acid molecules.
• C. It cannot donate a proton under any conditions.
• D. It partially ionizes in water, establishing an equilibrium between the acid and its conjugate base.

Answer: (D)

Verified step by step guidance

1. Understand the definition of a weak acid: A weak acid is one that does not fully dissociate in aqueous solution, meaning it only partially ionizes.
2. Recognize that partial ionization means an equilibrium is established between the undissociated acid (HA) and its ions (H\_3O\^+ and A\^-), represented by the equation: \(\mathrm{HA + H_2O \rightleftharpoons H_3O^+ + A^-}\).
3. Contrast this with a strong acid, which completely ionizes in water, producing only ions and no undissociated molecules.
4. Note that a weak acid does not produce a high pH by generating oxide ions (O\_2\^-); instead, it produces hydronium ions (H\_3O\^+) but in limited amounts due to partial ionization.
5. Conclude that the best description of a weak acid is that it partially ionizes in water, establishing an equilibrium between the acid and its conjugate base.