WHAT IS HAPPENING HERE?
PREDICT: Ask your
students "What colour is the ink in this bottle? Will happen if I shake
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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