Science Concept: The color of a solution changes as a result of changing levels of oxygen in the solution.
Materials & Pre-Demo Preparations:
3 g dextrose
5 g sodium hydroxide pellets
5-10 ml indigo carmine indicator solution, (1% solution - 1 g per 100 ml water)
250 ml distilled water
500 ml round-bottom flask
Before the demonstration, dissolve 3 g of dextrose and 5 g of sodium hydroxide in 250 ml distilled water. After adding the indigo carmine indicator, the solution needs to sit for at least 10 minutes before you can begin the demonstration.
Directions: In a beaker containing 250 ml of distilled water, add 3 g of dextrose and stir until it dissolves. Next, add 5 g of sodium hydroxide pellets into the same beaker, and use a stirring rod to break apart the pellets so that they will dissolve quickly. Once this is done, pour the contents of the beaker into the glass flask, and add between 5-10 ml of the indigo carmine indicator. Enough should be added to the solution so that it turns a yellow color. Put the stopper in the flask, and let it sit for ten minutes. After ten minutes, the solution should be ready for the demonstration. Shake the flask once or twice, and it should turn a red color. Shake once or twice again, and it should turn green. Then, if it sits, it should turn back to the yellow color.
Introduction & Commentary: Who knows how traffic lights work? What are the three colors that you see on a traffic light? Well, I'm going to make a traffic light on my own in this bottle, and I'm not even going to add any new chemicals like everyone else has been doing.
Explanation: The indigo carmine indicator changes color as a result of the changing levels of oxygen in the solution. The solution is originally yellow in color, but when the flask is shaken, oxygen dissolves into the solution, therefore oxidizing the indicator, and changing the color to red. When the flask is shaken again, the levels of oxygen increase more, and will oxidize the indicator further, causing it to turn green. When the solution is left alone, however, the oxygen concentration drops due to a reaction with the dextrose, so the solution will turn back to its original yellow color.
Safety Precautions: Since sodium hydroxide is a corrosive chemical, use caution when shaking the flask. Make sure the stopper is on tight, and put your hand on top of it when shaking.
Waste Disposal: Dispose of the contents of the flask in the sink.
Reference: Flinn Scientific, Inc., CHEMFAX, Pub. No. 402.00