Observations and results
When you added a few drops of bleach to the tonic water, did it stop fluorescing under the ultraviolet black light?
You should have clearly seen that the tonic water glowed a brilliant, bright blue color when you put it under the ultraviolet black light (before adding bleach). This is because the (invisible) ultraviolet light from the black light is absorbed by the quinine in the tonic water, and this excites the quinine. When the quinine becomes unexcited, it releases visible blue light that we see. After adding and mixing in a few drops of bleach with the tonic water, however, it should have stopped glowing. Why? Bleach is an oxidizing agent and can disrupt and break certain chemical bonds (specifically carbon–carbon double bonds). It is in these chemical bonds that the quinine absorbs the ultraviolet light, so by adding bleach to the tonic water it makes the quinine unable to absorb ultraviolet light anymore, and so it can no longer emit blue light.
You can pour the very diluted bleach down a drain. Thoroughly clean anything that came in contact with the bleach and rinse your hands.
More to explore
Measure Luminescence in Glow-in-the-Dark Objects, from Science Buddies
Bioluminescence: Investigating Glow-in-the-Dark Dinoflagellates, from Science Buddies
Lightsticks and Luminescence, from ACS ChemClub
Luminescence, from Science Clarified
This activity brought to you in partnership with Science Buddies