What determines a strong acid as compared to a weak acid?

A strong acid is defined primarily by its ability to ionize completely in solution, meaning that it dissociates to produce a high concentration of hydrogen ions (H⁺) when dissolved in water. This degree of ionization directly affects the acid's strength; a strong acid is one that, when mixed with water, releases almost all of its H⁺ ions, resulting in a solution with a low pH.

Weak acids, in contrast, only partially ionize in solution, meaning that only a small fraction of their molecules dissociate to release H⁺ ions. This leads to a much higher pH compared to a strong acid at the same concentration.

The acidity constant (Ka) does provide quantitative insight into acid strength, as a larger Ka indicates a stronger acid due to higher dissociation; however, it is the degree of ionization that is the most direct and conceptual understanding of what makes an acid "strong" or "weak" in practical situations. Physical state and the color of the acid in solution are not relevant factors in determining acid strength.