Acids and bases can cause many organic substances to change color. For example, if lemon juice is added to tea, the tea becomes lighter in color. This occurs because the acid in the lemon juice changes the color of a substance in the tea from dark brown to light brown. This addition restores the original color. A substance that changes color when an acid or base is added to it is called an indicator.
Litmus paper is a common indicator.
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.