Substances like phenolphthalein or thymol blue change color depending on what pH they are mixed with. They arepH indicators. This allows them to be used to broadly identify the pH of a substance - but how does it work?

Firstly,the pH can will change the shape of the indicator molecule.Acids and basesare solutions with lots of free H⁺or OH^-ions. When mixed with an indicator, these reactive ions will add themselves onto or strip the molecule. With new atoms, charges, and bonds, the molecule will form a new shape.

Secondly,分子的形状决定了它的颜色. White light is made up of all the different frequencies and colors of light. When passing through a sample, the light is constantly reflected, absorbed, and re-emitted. The amount that any frequency of light is absorbed depends on the shape of the molecule and the bonds it forms.

So the pH changes the shape then the shape changes the color.