How To: Turn Just Half of a Purple Liquid Clear with the Photochemical "Two-Faced" Reaction

Turn Just Half of a Purple Liquid Clear with the Photochemical "Two-Faced" Reaction

We've shown you how to make water change color on command, but how about just half of it? What if I told you that you can split a solution right down the middle and make the color disappear from one side, just by shining light on it?

With the help of a little stain called thionine, you can amaze your friends by creating what's known as the "two-faced reaction."

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Thionine is a dark purple stain that's used in a lot of biology labs to dye specimens. When it's mixed into an acidic solution, "the acid breaks down and allows a lot of positively-charged hydrogen atoms to float around." When it's dark, everything stays separate, but as soon as light hits the solution, the hydrogen atoms combine with the thionine and change it to its reduced form, which is clear. As soon as the light source is removed, it goes back to its original color.

Want to try it for yourself? Here's what you'll need:

  • 2g Ferrous sulfate
  • 10mL Sulfuric Acid Solution 3 M
  • 10mL Thionine solution 0.001 M
  • 500mL Distilled water

As far as equipment, grab a beaker, stirring rod, some aluminum foil, and an overhead projector.

First, mix the thionine solution, sulfuric acid, and water in the beaker. Turn off the light, then add the ferrous sulfate and stir until it's dissolved. Fold a piece of aluminum foil until it's several layers thick, then place it on the overhead projector. Position the beaker on top of the foil so that it's half on and half off. Now, all you have to do is turn on the projector's lamp and watch the magic happen!

You can turn the lamp on and off to your heart's desire and the solution will keep changing colors. You can find more details on the process here, and if you decide to try it out, be sure to upload a video of your reaction in the Inspiration section!

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