Which of the following statements correctly characterizes a strong acid versus a weak acid with examples?

• A. A strong acid completely dissociates in water (e.g., HCl); a weak acid only partially dissociates (e.g., acetic acid).
• B. A strong acid partially dissociates; a weak acid completely dissociates.
• C. Strong acids include acetic acid; weak acids include HCl.
• D. Both dissociate completely in water; there is no difference.

Answer: (A)

In water, acids either ionize completely or only partially. A strong acid donates protons so readily that it dissociates essentially completely into hydronium ions and its conjugate base. A weak acid, by contrast, only partially ionizes, so some of the molecules remain undissociated. This behavior is reflected in the acid dissociation constant: strong acids have very large effective dissociation, while weak acids have much smaller Ka values.

The statement that best captures this is that a strong acid completely dissociates in water (for example, HCl), whereas a weak acid only partially dissociates (for example, acetic acid). This aligns with how these acids behave in solution and explains why HCl solutions are typically far more conductive and have lower pH than solutions of acetic acid at the same concentration.

The other options misstate this behavior: partial dissociation for a strong acid, or complete dissociation for a weak acid, or placing acetic acid in the strong category and HCl in the weak category, or claiming both dissociate completely with no difference.