It's called the iodine clock reaction. A solution of hydrogen peroxide is mixed with one containing potassium iodide, starch and sodium thiosulfate. After a few seconds the colourless mixture suddenly turns dark blue.
Okay I'm gonna need some more info here. I have the full resources of a university's shitty chemistry department and need to do this with a high speed camera.
E: I am in a chemistry class, but am not very knowledgeable about chemistry, they let us use the lab, the equipment, and some resources as long as we provide valid reason and detailed information.
I am not a chemist, I am a software design major that just happens to be on really good terms with the professor in charge of the department. Sorry for coming off as an idiot, and/or disappointing anybody.
E2: u/Nov52017 commented a link to a cool video showing it in slow motion, just a shame that the shutter syncs poorly with the lights.
The iodine clock! Did this experiment in p-chem. I used to do chemistry "magic"shows in college and did an oscillating version of this. I would mix it up, pretend it didn't work, turn my back to scribble on a board like it didn't work, meanwhile it turned black and back to colorless before I would turn back around to check it again. Kids went nuts. I don't have a vid of me doing it, but [this is what it looks like](https://youtu.be/WasYuiOk5xQ
The only reason I can believe this is real is because the guy stirring visibly flinched when it turned black
I can only imagine it making a subtle cartoony "pop" as it suddenly turns black.
I just yelped at the screen.
I don’t like it!!
At least with that other one being shaken in the flask changed colour gradually.
I don’t know why but that spooked me lol Imagine if people were swimming in a clear pool and this reaction happened in there... Scary to think about🙈