How does disappearing ink work?

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Disappearing Ink has long been a favorite toy of practical jokers - but how does it work? Much like invisible ink from last week, it operates on the principle of acid/base chemistry. The reason the "ink" is blue to start with is because a particular molecule in the solution, thymophthalein, is blue in solutions that are basic. That same molecule is colorless under neutral conditions. When disappearing ink is sprayed on your clothes, part of the solution evaporates (the part is normally an alcohol of some sort - in our case it is ethanol), and instead of being basic the result is a neutral residue (left-over solid). With the evaporation, the thymophthalein is no longer experiencing basic conditions so it changes colors and "disappears". On camera we were able to speed this process by adding an acid, vinegar, to the solution. That way the color changed without part of it evaporating.
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