The pH scale usually ranges from 0 to 14. Solutions with a pH lower than 7 are acidic. A pH of 7 is neutral and values higher than 7 indicate a basic solution.

The pH value depends on the concentration of hydronium ions which are written as H3O⁺. These ions form directly in water.

Water molecules have the formula H2O. In liquid water the molecules constantly interact with each other. Occasionally one water molecule transfers a proton to another. When this happens a hydronium ion and a hydroxide ion form.

This reaction can be written as
2 H2O ⇌ H3O⁺ + OH⁻

This means that even pure water always contains a small number of ions. These ions form and recombine continuously.

If a solution contains many hydronium ions it is acidic. If the number of hydronium ions is low the solution becomes basic.

In neutral water the concentrations of hydronium ions and hydroxide ions are equal. Under normal conditions this corresponds to a pH value of 7.

The pH value therefore describes how many hydronium ions are present in a solution.