The strength of an acid is determined by the concentration of hydrogen ions (H+) it releases when dissolved in water. The more hydrogen ions an acid releases, the stronger it is. Acids that completely dissociate in water, releasing all of their hydrogen ions, are called strong acids. Acids that only partially dissociate in water, releasing only a small fraction of their hydrogen ions, are called weak acids.

For example, hydrochloric acid (HCl) is a strong acid because it completely dissociates in water, releasing all of its hydrogen ions. Acetic acid (CH₃COOH) is a weak acid because it only partially dissociates in water, releasing only a small fraction of its hydrogen ions.

The strength of an acid can be measured using a pH scale. The pH scale measures the concentration of hydrogen ions in a solution. A pH value of 0 indicates that the solution has a very high concentration of hydrogen ions and is therefore very acidic. A pH value of 14 indicates that the solution has a very low concentration of hydrogen ions and is therefore very basic. A pH value of 7 indicates that the solution has a neutral pH and contains equal concentrations of hydrogen ions and hydroxide ions (OH-).

Acids have a pH value below 7, while bases have a pH value above 7. The higher the concentration of hydrogen ions in a solution, the lower the pH value.