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Feeling Blue

Students may have witnessed the effects of a chemical reaction when chemicals are mixed: heat is released or absorbed, bubbles form or fizzing occurs, a noise produced, or the colour changes.

During this demo, however, no new chemicals are added to the flask. All you do is shake the liquid in the flask and the colour changes!

Although it doesn't look like you have added any chemicals to cause the reaction, shaking up the bottle does mix the liquid with another substance: Oxygen (air).

The mysterious liquid in the flask contains a chemical — a dye called methylene blue. The methylene blue is reduced (loses oxygen) by the potassium hydroxide and glucose solution and as a result loses its colour.

When you shake the bottle, the oxygen in the air mixes with the solution and re-oxidizes the methylene blue, turning the liquid blue. When the shaking stops and the liquid stands still, the blue colour disappears again as the potassium hydroxide and glucose solution continues to reduce the dye.

Fun Fact

Methylene blue was identified by Paul Ehrlich in 1891 as a successful treatment for malaria. It was replaced by other drugs, because patients disliked its two side effects: turning the urine green, and the sclera (the whites of the eyes) blue. Methylene blue is still used to treat cyanide poisoning and fungal infections in fish.

Objectives

  • Name colour change as one of the possible outcomes of a chemical reaction.

Materials

  • Per Demo:
    a graduated cylinder
    a balance
    250 mL water
    6.5 g potassium hydroxide (KOH)
    5 g dextrose (sugar)
    5 drops methylene blue
    500 mL flask with a rubber stopper (or any bottle/container with a tight-fitting lid)

Key Questions

  • What are some of the ways we can tell that a reaction between two chemicals has taken place?
  • Why did the liquid turn blue without adding any other chemicals?
  • When the container is still, where is the oxygen?
  • What happens to the oxygen when you shake the container up?

What To Do

Preparation

  1. In the container, dissolve the KOH and dextrose in the water. Add the methylene blue and close the lid.
  2. Let it stand for about 30 minutes before the demo takes place so that the “pre-reaction” takes place. Do not leave it for longer than a few hours before you do the demonstration.
  3. Tighten the lid and set aside.

Demonstration

  1. Show the students the container and solution.
  2. Choose a volunteer to shake the container until it turns blue.
  3. Let it sit until it reverts back to clear, then shake it again.

Teacher Tips:

  • ​Dispose of the chemical solution after use. The mixture cannot be re-used from day to day as it oxidizes and turns a yellow colour.
  • ​For a more exciting demo, mix the ingredients in front of class. Use warm/hot water to speed up the reaction.
  • ​Have a volunteer touch the bottom of the container to feel the heat given off by the exothermic reaction.