Dissociation of acids

Let’s look at the dissociation of a common acid – hydrochloric acid or HCl. When HCl molecules dissolve they dissociate into H+ ions and Cl- ions.

HCl is a strong acid because it dissociates almost completely.

By contrast, a weak acid like acetic acid (CH3COOH) does not dissociate well in water – many H+ ions remain bound-up within the molecule.

In summary: the stronger the acid the more free H+ ions are released into solution. The greater the number of free H+, the lower the pH value for that acid.

Acids are sometimes referred to as H+ ion donors because they release H+ ions in solution for other molecules to bind to.