DIY Light Reactions

Wednesday, March 4th, 2015

This blog post was written by Kelly Zhu, a junior at Whitney Young High School.  She is one of the IIT Boeing Scholars, and with her STEM Ed(You)Cation team, is working on science outreach for family science nights.

Thionin Demonstration

Light reactions are one of many science experiments that are totally illuminating. It is a fun way to expose and engage children to science. The thionin reaction is an example of something that uses light as a reactant.  This intensely purple solution changes color in the presence of white light.  When the light is removed, it changes back to purple, so it is completely reversible!

Note:  This reaction calls for things that really should be handled and stored in a lab.  As a result, you couldn’t and shouldn’t try this at home.


  • Aluminum foil
  • Iron (II) sulfate
  • Sulfuric acid solution (1 M, 100 mL)
  • Thionin solution (0.001 M ,10 mL)
  • Water (500 mL)
  • Glass Beaker (1 L)
  • Balance
  • Graduated Cylinder (10 mL)
  • Stirring rod
  • Overhead projector


Safety Note:  As a safety precaution, be sure to wear goggles, gloves, and chemical resistant apron thought the experiment

  1. Prepare a 0.001 M thionin solution be adding 100 mL of water to 0.023g of thionin.
  2. In a 1-L beaker mix together the following chemical: 50 mL of 0.001 M thionin solution, 100 mL of 1M sulfuric acid, and enough water to make the solution about 600 mL. Mix thoroughly.
  3. Take 2.0 grams of iron(II) sulfate and mix into the solution. Turn off the lights.
  4. Place the beaker on the overhead projector stage. Turn on the overhead projection and observe that the solution changes from purple to clear.
  5. Now turn off the lamp and allow the purple color to return.
  6. To make things more interesting, take the aluminum foil and cover half the surface of the projector. Make sure the foil is several layers thick so no light can pass through.
  7. Place the beaker on the projector stage so that half the beaker is on the aluminum foil.
  8. Turn on the projector lamp, and see what happens. The half of the beaker that is not exposed to the light will remain purple while the other half is clear.
  9. Turning off the light can reverse this reaction. This solution will last several days.

What is Going On?

Thionin is an organic compound that comes in two forms. When thionin is in its oxidized form it will be purple and when it is in its reduced form it will be colorless. A thionin reaction is basically converting light energy to chemical energy and vise versa with the help of light. In this demonstration, by adding a reducing agent such as iron(II) ion (Fe2+), the thionin molecule accepts two hydrogen atoms from iron and is reduced to its colorless form. However, this only happens when there is an intense light source to catalyze the reaction. Therefore, when the light source is removed, the thionin will return to its oxidized state and the purple color will reappear.



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