The famous blue bottle experiment a visually dramatic way to teach reduction-oxidation (redox) chemistry. Students from grade school to grad school find this reaction memorable and it is considered a classic staple in chemical demonstration shows. A half-full bottle of colorless liquid turns blue when shaken, and when the bottle is allowed to sit still, the color fades. Shaking the bottle again causes the color to reappear like magic! What’s going on?
On the molecular level, the blue bottle experiment is a complex system composed of ethanol, the simple sugar glucose, the dye methylene blue, the hydroxide ion, and oxygen from the atmosphere. The color change occurs do to a pair of competing reduction-oxidation reactions. Hence, the blue bottle experiment is a wonderful tool for introducing the key concepts of reduction and oxidation.
All redox reactions involve electrons being transferred from one compound, the reducing agent, to another compound, the oxidizing agent. The term “reduction” means “gain of electrons”. This seems like an odd choice of terminology since “gain” and “reduce” are usually considered antonyms. However, because the electron has an electrical charge of negative one, gaining electrons will reduce the charge of a species. The term “oxidation” means “loss of electrons” and often, but not always, involves reaction with oxygen. A common mnemonic is the phrase OIL RIG, which stands for “Oxidation Is Loss, Reduction Is Gain”.
In first stage of the blue bottle experiment, the methylene blue dye acts an oxidizing agent and the glucose acts as a reducing agent. The methylene blue oxidizes the glucose to gluconic acid and the glucose reduces the methylene blue to its colorless form. The result is a bottle of colorless solution.
When the bottle is shaken, the surface are of the liquid temporarily increases, causing more oxygen to dissolve in the ethanol. The additional oxygen acts as an oxidizing agent and changes methylene blue to its blue, oxidized form. The result is a dramatic color change from colorless to blue.
When the shaking is stopped, the oxygen levels in solution begin to drop. With less oxygen present, the methylene blue once again is reduced to its colorless form by the glucose, and observers will see the color fade and disappear. The color change can be repeated many times simply by shaking the bottle to induce the blue color and then allowing it to sit still in order to make it disappear.