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Long story short, I'm planning an in-home escape room and want to create a puzzle where the guests find a vial containing some sort of chemical that they can dump into a water fountain. My intention then is that chemical will react with another chemical that was placed on rocks within the water fountain prior. Then, the reaction causes a code to be spelled out on the rocks within the water fountain.

Obviously, the code on the rocks / in the fountain shouldn't be visible until the other chemical is introduced into the water fountain and the reaction occurs. It also goes without saying that the chemicals should be relatively non toxic or not a major issue if someone accidentally gets some on their hands. My one college chemistry course did not quite equip me for this task! Thanks for any help.

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    $\begingroup$ I really don’t think it is viable to spell out letters in water. Eithe the “chemical placed on rocks” will dissolve in water, or if it os insoluble it will simply disperse out into the medium. $\endgroup$ Sep 8, 2022 at 13:18

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Non-chem suggestion: hide the parts to make a UV light, e.g. a small (5 mw) UV-B LED and a fresh Li coin cell (not for use with small children who might put it in mouth!), as part of the game. Write the text in invisible fluorescent crayon, or use naturally fluorescent pebbles to spell it out.

Hmm... actually, there is chemistry involved: the chemistry of fluorescence and phosphorescence. You could make your own fluorescent paint with activated calcium sulfide, zinc sulfide, or other comparatively safe chemicals.

Note: sources for parts and chemicals are only used as examples; you can likely find them elsewhere.

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    $\begingroup$ (+1) Good answer! There are plenty of naturally fluorescent minerals available quite inexpensively at Gem and Mineral shows, on ebay, etc. Many are mundane looking in daylight or room lighting, but magnificent when exposed to appropriate UV light. Even the “mineral lights” are fairly inexpensive. $\endgroup$
    – Ed V
    Sep 8, 2022 at 21:45
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Acid-base indicators are often used for this purpose, for example phenolphtaleine is common in "secret message" experiments. I am not too sure, however, on how exactly would you ensure that your message is not disperged in the water.

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Hold a gender reveal party with milk of magnesia. Stir some phenolphthalein (girl) or bromothymol blue (boy) into the basic (pH 10) suspension and let the guests watch what happens.

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