The Blue Bottle Chemistry Experiment (J.Chem.Ed., V75 N12, p.1580, December 1998) is one of my favorite chemistry or forensics class demonstrations. It is most effective if I have the flask sitting at the teacher demonstration table as the students enter the classroom. I start the class in the usual way with announcements, handing out graded papers, collecting homework, asking if anyone has questions. When the blue color has faded, I shake the clear solution until the blue color returns and resume routine class activities. (http://www.youtube.com/watch?v=wVuSz06qdoo)
It is amusing to watch the rate at which different classes catch on that there is a demonstration going on. It can take up to twenty minutes for all twenty-five to thirty students to start paying attention to the flask that goes from blue to clear to blue again when shaken.
I lead the class through a discussion of the use of indicators. I ask what colors we’ve observed if a solution is acidic or basic. I ask what color indicated the presence of starch with iodine. Then I talk about oxidation and reduction reactions. I finally tell the class the indicator in use is methylene blue.
In this reaction glucose in a basic solution is oxidized by oxygen dissolved in the solution to form gluconic acid. The gluconic acid reacts in the sodium hydroxide to form sodium gluconate. Methylene blue is reduced and goes colorless. Methylene blue is re-oxidized with shaking and goes blue again.
Here is how I prepare the demonstration flask. I fill a 500 ml or one liter Erlenmeyer flask half full of water. I dissolve 2.5 grams of glucose in the water. I dissolve 2.5 grams of sodium hydroxide in the flask; this makes the glucose solution basic or alkaline. I add about 1 milliliter of 0.1% methylene blue indicator to the flask. I add a stopper to the flask and shake to dissolve the dye. The solution turns blue. I let the flask stand for awhile and it gradually turns colorless. Shaking the flask turns the solution blue again. When the reaction stops, add more glucose and sodium hydroxide and shake. The blue color will return.
Then I can talk about other redox reactions (oxidation reduction) that might show a color change such as the formation of rust on an iron nail or iron staircase or the tarnishing of silver tea sets or table ware. Rust is iron oxide formed by the reaction of iron with water and air. The black substance on silver jewelry or silver tea sets or table ware is formed from the reaction of silver with sulfur to form silver sulfide.
