It turns red in an acid solution and blue in a basic solution. A solution that gives litmus paper a color midway between red and blue is called a neutral solution. This is a solution that contains hydrogen ions and hydroxide ions in equal amounts. Thus, a solution with an excess of hydrogen ions is an acid solution. A solution with an excess of hydroxide ions is a basic solution.
Pure water is a neutral solution.
It ionizes slightly and releases an equal number of hydrogen and hydroxide ions. The pH is the logarithm of the reciprocal of the hydrogen ion concentration. The Learning Company, Inc.