Using food coloring, oil, and water is one way to show the visual effects of fireworks underwater, without actual combustion or flames. This reaction is also a beneficial class exercise that can be used with children. In order to get the desired effect, fill a plastic cup with water. Tap water is acceptable. In another cup, pour 1-2 tablespoons of oil and add a few drops of different food colorings. Stir the food coloring and oil mixture in the second cup, and pour it into the cup of water. This exercise can be useful in educating the concepts of diffusion and density, too.
To get more than just a visual effect of fireworks underwater, but also some underwater explosions, follow this scientific equation: CaC2 (s) + 2 H2O (l) -> Ca(OH)2 (aq ) + C2H2 (g). First react calcium carbide (CaC2) with water (H2) to produce acetylene gas. Then inject chlorine gas into the calcium carbide/water reaction. The contact will cause underwater flames and also hydrochlotic acid and carbon.
Underwater fireworks demonstrations can involve chemistry and self-experimentation with regards to a scientific project. It is therefore a potentially beneficial science class activity, or even a weekend activity between parents and children. Beyond education, though, it can be an impressive trick that displays a familiarity with basic concepts of chemistry, diffusion, and density.
Beyond experimental underwater explosions, many traditional fireworks work under water, too. Most fireworks rely on gun powder, especially the potassium nitrate therein, and potassium nitrate contains the oxygen necessary for an explosion to occur even when there is no oxygen. This reaction is similar to the rocket explosions necessary in space, where there is no oxygen either.