This black magic reaction is very funny, as the reaction is so quick you can see the colourless water will sudden turn black.
The following two solution are prepared separately and then mixed it together.
Solution A: 0.38g sodium metabisulphite + 0.6g starch to 80ml water
Solution B: 1.4 g potassium iodate to 200ml water.
As starch water are easily to degrade, the starch water is suggested to be prepared less than 1 week. As starch is difficult in dissolved in water, use hot water instead of cold water. After mixing two solutions, it will turn to black after 15s.
1. More the potassium iodate, faster reaction to turn black
2. It is suggested to clean the breaker at once after the experiment
3. There is another recipe using potassium iodide instead potassium iodate, but the procedure is more complex and more difficult to control the reaction timing.