This is one of those fun science experiments that most closely approximates magic, because it's spooky in two different ways. The first is that something happens in, sometimes literally, the blink of an eye. The second is something that seems to hold off for a command from the demonstrator.
Learn the astonishing secrets of the iodine clock!
The iodine clock is a common chemistry experiment — that would also work for anyone who needs to do a cheapo water-to-wine special effect for their movie. It involves mixing two different liquids. Both are clear. When they're mixed... they both remain clear. Sometimes they remain clear for a long, long time. Then in less than a second, the mixed liquid pops to a dark color. This happens so suddenly and completely, it actually looks a little bit fake. With the right calculations, or with the right amount of practice, a person can signal to the glass and make it look like the liquid changed on command.
What's going on? Two different reactions are happening, and one is exhausted more quickly than the other. We start by looking at the two initial liquids. In Liquid A is hydrogen peroxide and sulfuric acid. In Liquid B is potassium iodide, sodium thiosulfate, and starch.
Now those of you who know iodine and starch (and there are so many of you here that I really don't know why you don't form the Iodine-Starch Society), know that when iodine and starch get together, they form a dark solution. And yet even though the potassium iodide and the starch are in the same initial solution, they're clear. That's because the iodine is in a form that doesn't react with the starch yet. The potassium is keeping it close.
When the liquids are mixed, the acid yanks away the potassium, but puts it in a form that the starch doesn't readily react with. The form, three free negatively charged iodine molecules, reacts instead with the hydrogen peroxide.
H2O2 + 3 I− + 2 H+ → I3− + 2 H2O
Now triiodide - three iodine atoms all stuck together - does react with the starch. That should color the liquid right off, but it doesn't. Why? Because the sodium thiosulfate yanks away the triiodide for its own nefarious purposes.
I3− + 2 S2O32− → 3 I− + S4O62−
Just like that, we have those lonely three negatively charged iodine molecules again, and the hydrogen peroxide can react with them again, starting the process over.
This might go forever, but the sodium thiosulfate eventually gets used up. No more can it grab all the triiodide. Suddenly, pretty much all across the glass, the iodine atoms and the starch can react, and the entire glass goes dark.
The timing of the reaction varies with the concentration of chemicals in both liquids, and with the temperature of the liquids, so it would take some practice and consistency to get the timing right, but the sudden pop to color is worth it. Truly, you are a wizard, Harry.
Top Image: Armin Kübelbeck