Colorful jar

Procedure

• Prepare the following 3 solutions:
  • Solution A: dissolve 1 g of NaOH in 100 mL of water.
  • Solution B: dissolve 2 g of glucose in 100 mL of water.
  • Solution C: dissolve 0.125 g of methylene blue in 10 mL of water.
• Add solution A, solution B, and a few drops of solution C to the container, then close it.
• Wait for the solution to become colorless and shake the container.
• Observe what happens.

Theoretical explanation

In this experiment, a redox reaction takes place. Glucose in a basic medium readily oxidizes and reacts with oxygen from the air to form gluconic acid.

Glucose + Oxygen   gluconic acid + OH^–

While there is dissolved oxygen in the solution, methylene blue will be in its oxidized form, appearing blue. When all the oxygen is consumed due to the oxidation reaction of glucose, glucose reacts with methylene blue, which then becomes reduced and colorless. Upon shaking the container, dissolved oxygen from the air incorporates into the solution and reacts with methylene blue, oxidizing it. Simultaneously, this oxidized form slowly oxidizes glucose, while glucose itself is reduced, returning to its initial colorless form.