Feeling Blue

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.