Acids increase the number of hydrogen ions, which are symbolized as H+ in chemical formulas, in a solution. Stronger acids more readily donate their hydrogen ions, creating an unequal balance of ions. One example is hydrochloric acid, written as HCl, which completely dissociates in solution into hydrogen and chloride ions.
In pure water, the ionization of a small percentage of water molecules spontaneously creates hydrogen ions when the molecules dissociate into an equal number of hydrogen and hydroxyl, or OH-, ions.
High concentrations of hydrogen ions produce a lower pH number. Acid solutions range in pH from 0 to a little less than 7.