17.5 The Rainbow Connection Demonstration

Chemical Concept Demonstrated: Acid-base indicators


A colorless solution is poured into six glasses.  The liquid in the glasses is then poured back into the pitcher.


    Each glass produces a differently colored liquid.  When these liquids are poured back into the pitcher, the result is a colorless solution.


    Before the experiment took place, each glass was pretreated with six different mixtures of indicator, as shown below:

    Red: 1.5 g phenolphthalein and 3.0 g m-nitrophenol

    Orange: .45 g phenolphthalein and 6.0 g m-nitrophenol

    Yellow: 6.0 g m-nitrophenol

    Green: .6 g thymolphthalein and 6.0 g m-nitrophenol

    Blue: 1.5 g thymolphthalein

    Violet: .9 g phenolphthalein and .4 g thymolphthalein

    These indicator solutions were all then allowed to evaporate into a thin film at the bottom of each glass, rendering them invisible.

    The solutions change color because a basic solution is added to each glass.  All of the indicator mixtures are colorless under acidic conditions and change color under basic conditions.

    At the end of the demonstration, the solutions are poured together into a second pitcher that contains a powerful acidic solution.  Because the indicators are colorless under acidic conditions, the contents of all of the glasses mix with the acidic solution to form a single, colorless solution.