- Half-fill two one-liter Erlenmeyer flasks with tap water.
- Dissolve 2.5 g of glucose in one of the flask (flask A) and 5 g of glucose in the other flask (flask B).
- Dissolve 2.5 g of sodium hydroxide (NaOH) in flask A and 5 g of NaOH in flask B.
- Add ~1 ml of 0.1% methylene blue to each flask.
- Stopper the flasks and shake them to dissolve the dye. The resulting solution will be blue.
- Set the flasks aside (this is a good time to explain the chemistry of the demonstration). The liquid will gradually become colorless as glucose is oxidized by the dissolved dioxygen. The effect of concentration on reaction rate should be obvious. The flask with twice the concentration uses the dissolved oxygen in about half the time as the other solution. A thin blue boundary can be expected to remain at the solution-air interface, since oxygen remains available via diffusion.
- The blue color of the solutions can be restored by swirling or shaking the contents of the flask.
- The reaction can be repeated several times.
Safety & Clean-Up
Avoid skin contact with the solutions, which contain caustic chemicals. The reaction neutralizes the solution, which can be disposed of by pouring it down the drain.
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