Hide a few drops of a concentrated acid in the bottom of a third glass. But knowing how to do the trick doesn't make it any less mysterious. Just why the color change occurs when the pH changes is the subject of this article. Since color change often accompanies chemical change , you might suspect that a chemical reaction is responsible for indicator action. This indeed the case.
Indicators are weak acids or bases with differently colored acid and base forms.
The two forms of the indicator molecule have noticeably different colors. For example, bromocresol green has a yellow HIn form and a blue In form. In, the solution looks bright green. Adding a drop of base converts HIn to In, and the solution becomes more blue. To understand the answer, we'll have to know a little about how color and molecular structure are related.