How does this chemical vanish when light hits it?

Thionin is a compound that is often used in laboratories to stain specimens in preparation for observation. It is also a two-faced lying vampire of a chemical, that disappears when light is introduced. Want to see a glass split, vertically, between blue and clear water?

Thionin is a strong purple dye that, when placed in an acidic solution, will engage in something that's been nicknamed the "two-faced reaction." In the video above, it's especially two-faced, since the left half of a bottle of liquid is perfectly clear, while the right half is a dark purple. What's make it go? Light.

Thionin is placed in an acidic solution. The acid breaks down and allows a lot of positively-charged hydrogen atoms to float around next to the thionin. As long as it's dark, they stay separate. When the solution gets light, however, the hydrogen atoms make a beeline for the thionin. They combine with the compound, changing the way it absorbs light and making it turn clear. Take the light away, and the whole solution goes dark again. This is so quick, and so complete, that the glass above is split vertically between purple and clear liquid.


Via Elmhurst and SVUSD.

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The chemical doesn't "vanish" at all. It simply changes its properties when exposed to light. Lots of stuff does that, including you; just lie out in the sun for a while.