COLOUR CHANGE CHEMISTRY
Chemicals and materials which change colour are fun and useful. In this activity you will see two
colour change chemistry demonstrations.
COLOUR CHANGE REACTIONS – BLUE BOTTLE AND TRAFFIC LIGHT REACTION
Equipment
 Small containers (<30ml) of reaction mixture
a. Methylene blue (<1%) and glucose (3%) in 0.4M sodium hydroxide (IRRITANT)
b. Indigo carmine (<1%) and glucose (3%) in 0.4M sodium hydroxide (IRRITANT)
The activity
1. Make up the bottles and shake the containers.
2. Methylene blue should turn blue on shaking then become colourless again on standing
3. Indigo carmine should turn red, then green, on shaking and return to yellow on standing
Top tips: The reactions can be slow when it is cold. The colour change may become less pronounced
over time – unscrewing the container to allow more oxygen in can help. Make sure you seal it
properly again before the next use. When, after a few hours, the solutions change colour
permanently or fail to change colour they are spent and should be replaced.
Safety:
 Stock solutions must be made fresh and stored out of reach from students
 Solutions used must be kept in closed containers, sealed with
Parafilm and labelled IRRITANT
 Staff must wear safety glasses and gloves when replenishing or disposing of solutions
 Solutions can be disposed of down a domestic sink with plenty of water
Preparing 1.6M NaOH stock solution
1. Weigh out 12.8g sodium hydroxide
2. Place 200ml water in glass bottle
3. Add NaOH solid to the water a little at a time and stir. Heat will be generated.
Preparing reaction mixture – enough for 8-10 demo bottles
1. Weigh 6g glucose into an empty glass bottle
2. Add 50ml 1.6M NaOH stock solution
3. Make up to 200ml with water and mix well
4. Add approx. 3 ml of the appropriate dye solution and mix well
5. Transfer into small sample bottles, cap and seal with Parafilm, label IRRITANT.
The science
Methylene blue has two forms – blue (oxidised) and colourless (reduced). When you shake the
bottle oxygen is mixed into the solution, methylene blue oxidises and the liquid turns blue. When
you stop shaking glucose reduce the methylene blue and it becomes colourless again.
Methylene Blue – oxidised and reduced forms
Indigo carmine has three forms – yellow
(reduced), red (partially oxidised) and
green (fully oxidised). Like in the blue
bottle demonstration, when you shake
the bottle oxygen is mixed into the
solution, oxidising the indigo carmine. It
then reduces again on standing.
Reduced, partially oxidised and fully oxidised
forms of indigo carmine
Methylene blue is used as a redox
indicator and biologists use it for staining
nuclear DNA. Medically it has been used
to treat malaria, cyanide and carbon monoxide poisoning and may even be used as a cancer
treatment in the future. Unfortunately, as a side effect, it turns the whites of the eyes blue.
Indigo carmine is used as a food colouring (E132) and a pH indicator. Doctors also use it to study
kidney and bladder function - they inject the dye into the bloodstream and see how long it takes for
the urine to turn green!