Pure water has the same concentration of hydrogen and hydroxide ions; this is 7 (neutral) on a pH scale from 0 to 14.

If hydrogen ion concentrations increase, the pH is lower than 7 and is an acid.

If hydrogen ion concentrations decrease, the pH is greater than 7; it is a base.

An acidic solution has a high concentration of hydrogen ions with a low pH.

A basic (alkaline) solution has a low concentration of hydrogen ions with a high pH.