Chemicals and materials which change colour are fun and useful. In this activity, you will see two colour change chemistry demonstrations.

Equipment

  • Small containers (<30ml) of reaction mixture
  • Methylene blue (<1%) and glucose (3%) in 0.4M sodium hydroxide
  • Indigo carmine (<1%) and glucose (3%) in 0.4M sodium hydroxide

Health, safety and technical notes

  • Read our standard health and safety guidance here
  • Wear eye protection if desired
  • 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
  • Sodium hydroxide is an irritant, for more information see CLEAPSS Hazcard HC091a

Procedure

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. 
  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 

What’s the chemistry?

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. 

Notes

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.

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