A strong acid is defined by its chemical ability to completely dissociate, or break apart, into its component ions when dissolved in water. In an aqueous solution, acid molecules release hydrogen ions (H<sup>+</sup>), which immediately attach to water molecules to form hydronium ions (H3O<sup>+</sup>).
The defining characteristic of a strong acid is that essentially 100 percent of its molecules undergo this separation. If you dissolve one mole of a strong acid in water, you will generate exactly one mole of hydrogen ions, leaving virtually no intact, undissociated acid molecules behind in the beaker.
Because they dissociate so thoroughly, strong acids produce incredibly high concentrations of hydrogen ions, which directly translates to very low pH values. They are also considered strong electrolytes because the high number of free-floating ions makes their solutions excellent conductors of electricity. However, it is a common misconception that "strong" means the same thing as "dangerous" or "corrosive." While highly concentrated strong acids are indeed dangerous, the term "strong" strictly refers to the chemical property of complete dissociation.
A heavily diluted strong acid can easily have a higher, safer pH than a highly concentrated weak acid.
There are surprisingly few common strong acids, and chemistry students are generally encouraged to memorize the list. The most universally recognized strong acids are hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), hydrobromic acid (HBr), hydroiodic acid (HI), and perchloric acid (HClO4).
Any acid that does not appear on this very short list is generally assumed to be a weak acid, meaning it only partially dissociates in water and establishes a dynamic chemical equilibrium.