What is the main difference between a strong acid and a weak acid?

The main difference between a strong acid and a weak acid lies in their ionization in aqueous solutions. A strong acid completely ionizes in water, meaning that when dissolved, it dissociates entirely into its constituent ions. For example, hydrochloric acid (HCl) will break down completely into H⁺ and Cl⁻ ions when mixed with water. This complete ionization results in a higher concentration of hydrogen ions in the solution, leading to a lower pH and a more potent acidic environment.

In contrast, a weak acid only partially ionizes in solution. This means that only a fraction of the acid molecules dissociate into ions, leaving a significant amount of the undissociated acid present in the solution. An example of a weak acid is acetic acid (CH₃COOH), which does not completely dissociate, resulting in both dissociated ions (CH₃COO⁻ and H⁺) and undissociated molecules being present in equilibrium.

Understanding this distinction is crucial for predicting the behavior of acids in chemical reactions and their impact on pH. Therefore, option C accurately captures this fundamental difference, confirming that a strong acid completely ionizes while a weak acid only does so partially.