Strong acids fully ionize in water, whereas weak acids only partially ionize. This fundamental difference significantly impacts their reactivity and pH levels.

Strong acids are defined as substances that completely ionize in aqueous solutions. When a strong acid is dissolved in water, all of its molecules dissociate, releasing hydrogen ions ($\text{H}^+$) into the solution. Examples of strong acids include hydrochloric acid ($\text{HCl}$), nitric acid ($\text{HNO}_3$), and sulfuric acid ($\text{H}_2\text{SO}_4$). Due to their complete ionization, strong acids typically have a low pH value (below 7) and exhibit high reactivity, making them valuable in numerous chemical reactions.

Conversely, weak acids only partially ionize in water. When a weak acid is introduced into an aqueous environment, only a small proportion of its molecules dissociate to release hydrogen ions, while the majority remain intact. Common examples of weak acids include acetic acid (the main component of vinegar) and citric acid (found in citrus fruits). Because weak acids only partially ionize, they generally have a pH closer to 7 and display lower reactivity compared to strong acids.

The difference in ionization also affects the conductivity of the resulting solutions. Ionization generates ions, which are charged particles; therefore, a solution of a strong acid will conduct electricity more effectively than a solution of a weak acid. This is attributable to the larger number of ions present in the strong acid solution, facilitating the movement of electric charge.

In summary, the key distinction between strong and weak acids is their degree of ionization in water. This difference influences their reactivity, pH levels, and conductivity in solution.