Topic 8 Acids and bases - 8.3 The pH scale

Nature of science: Occam's razor-the pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number. (2.7)


Essential idea: The pH scale is an artificial scale used to distinguish between acid, neutral and basic/alkaline solutions.

pH = - log[H+(aq)] and [H+] = 10-pH.

A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+].

pH values distinguish between acidic, neutral and alkaline solutions.

The ionic product constant, Kw = [H+][OH-] = 10-14 at 298 K.

Applications and skills

Solving problems involving pH, [H+] and [OH-].

Students should be familiar with the use of a pH meter and universal indicator.


Students will not be assessed on pOH values.

Students should be concerned only with strong acids and bases in this subtopic.

Knowing the temperature dependence of Kw is not required.

Equations involving H3O+ instead of H+ may be applied.