NEW ZEALAND TARANAKI

Another magic bottle...

A "magic" colour change that be done over and over again...


MAGIC BOTTLE RECIPE:

SOLUTIONS:

Indigo Carmine Solution: Dissolve 200 mg of indigo carmine in 100 mL distilled water. (Variations: substitute methylene blue or thionin for the indigo carmine).

Dextrose Solution (0.1 M): Dissolve 9 g of dextrose in about 400 mL of distilled and dilute to 500 mL.

NaOH Solution (3 M): Dissolve 24 g of sodium hydroxide in about 100 mL of water and dilute to 200 mL. (Be careful as this solution process is highly exothermic.)

Mixing the solutions:

  • This should be done about 30 minutes before the demonstration is to be used.
  • Pour about 150 mL of 0.1 M dextrose into a bottle.
  • Add 6 mL of 3 M NaOH solution and 4-5 mL of indigo carmine solution
  • The colour of the solution will gradually change and become yellow.

 

WHAT IS HAPPENING HERE?

  • PREDICT: Ask your students "What colour is the ink in this bottle? Will happen if I shake the bottle?"
  • OBSERVE: Shake the bottle (being careful not to drop it!) quite vigorously.
  • EXPLAIN: What does this show us? (seniors might like to consider the chemical reaction involved)

When the solution is shaken gently it will turn from yellow back to green or a light blue. This cycle can be repeated many times. After a while the indicator seems to "wear out". When this happens add more indigo carmine.

WHY THE COLOUR CHANGE? IT"S CHEMISTRY!

The dextrose acts as a reducing agent in a basic solution and reduces the indigo carmine. The progress of this reduction reaction is observed by the colour changes that the indigo carmine goes through. When the bottle is shaken the oxygen in the air mixes with the solution and oxidises the indigo carmine back to its original state.

An extensive article appeared in the February 1994 issue of the Journal of Chemical Education (page 160).


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