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TEACHERS: Photochemical Reactions: Light Energy to Chemical Energy

Students will be able to:
  • recognize that light energy can be converted to chemical energy
  • state the role of energy conversion in ozone formation


  • overhead projector
  • Thionin 0.023 grams
  • Sulfuric acid solution, 3 Molar
  • Ferrous sulfate, 6.0g (FeSO4)
  • 10 ml graduated cylinder
  • aluminum foil
  • distilled water, 600 ml
  • glass beaker, 1 or 2 liter
  • balance
  • glass stirring rod
  • glass container 150 ml with cap
  • Fume hood
  • copies of Student Worksheet, pdf
  • copies of vocabulary list, pdf

  • or

  • Flinn Scientific Photochemical Reaction Kit


Ground level ozone is produced by a photochemical reaction. A photochemical reaction is a chemical reaction that requires light (in this case sunlight) to provide the energy for the reaction to proceed. During the summer months, the earth is tilted in such a way that the sun's rays are more direct, and more intense. This combination creates longer and warmer days than in the winter months. Ground level ozone is called the "summertime pollutant" because that is the time of year when the energy from the sun is intense enough to trigger the photochemical reaction necessary to produce ground level ozone from NOx and VOCs.  Cloudless skies create conditions for warmer days to occur. But keep in mind that a completely cloudless sky is not necessary for the photochemical process to start. The more sunlight available, the more likely the ground level ozone photochemical reaction will occur.  The following demonstration will help students understand the abstract concept of photochemical reactions by observing an example of this conversion.

In this demonstration a purple solution of thionin, sulfuric acid, ferrous sulfate and distilled water will turn clear when exposed to the light of an overhead projector.   The light causes the thionin, in an acidic solution, to be photochemically reduced by the iron.  Once prepared, this solution can be used several times in one day, but will not last more than one week.

Preparing the Solution
1.  Make a 100 ml of 0.001 Molar thionin solution.  Do this by adding 100 ml of distilled water to 0.023 grams of thionin in the glass container with the cap.  Cap the container and shake until the thionin is thoroughly mixed.

2.  To get the final solution mix in the 1 or 2 liter beaker 10 ml of thionin solution, 10 ml of the 3 Molar sulfuric acid solution and distilled water to bring the volume to 500 ml.  Then add the 2.0 grams of ferrous sulfate.  Mix well.

Note: Use caution when handling an acid.  The Ferrous sulfate is toxic, wear safety goggles, and chemical-resistant apron and gloves.  Have MSDS sheets on hand.  This solution can be poured down the drain and flushed with a large amount of water.



1.  Turn off the classroom lights.  Place the beaker containing the purple solution on the overhead projector and turn on the projector light.  The solution will turn clear.  When the projector light is turned off, the solution will go back to purple.

Half and Half
1.  Perform this section of the demonstration with the lights off.  Cover half the overhead projector with foil, several layers thick, and position the beaker containing the solution so that half is on the foil and half is on the glass stage.

2.  Turn on the overhead light and the half that is exposed to the overhead light will turn colorless.  The half that is over the foil will remain purple.

3.  Have the students standing where they can observe the vertical line of the different colors.  This will demonstrate that light caused the reaction, not heat.

4.  Have the students predict what will happen if the beaker is gently shaken or stirred.

Explain why the photochemical reaction occurred, the solution noticeably changed color, when exposed to the light of the overhead projector as opposed to the normal lights in the classroom.  What is the difference between the lighting?

How did this demonstration simulate to the formation of ground level ozone in the atmosphere?  When does abundant ground level ozone normally occur and why?

Adapted from a Flinn Scientific Inc. Chem Fax! Sheet.  Publication # 8845.  Chemicals to perform the observation are available on

Copyright all rights reserved 2005. Stevens Institute of Technology.
The Center for Innovation in Engineering and Science Education