Fireworks: Physics and Chemistry Juxtaposed
Philadelphia’s annual July 4th fireworks display on the Delaware River never fails to impress. The uniquely historic setting helps—in this city the American nation began its emergence onto the world stage. But the sizzling combination of colors and sounds from aerial fireworks, so much more astonishing than when I was a kid, set me thinking about the science of it all.
It starts and ends with chemistry, with a little physics thrown in the middle to provide interdisciplinarity. The chemistry part is simple oxidation/reduction (redox) reactions that manage to involve a huge range of members of the periodic table. First, a fuel is burned (oxidized), usually the elements of gun powder, carbon and sulfur. To make it hotter, burn some aluminum, magnesium, and titanium. Oxygen is provided by oxidizing agents like nitrates, chlorates, and perchlorates. The whole mess of explosives is held together by various binders, generally starches and rubbers.
Then comes the physics: if you initiate a heat producing redox reaction in a confined tube with a hole at one end, the predictable response is propelled liftoff. And since rapid combustion causes the air to expand faster than the speed of sound—BOOM!
Back to chemistry. The production of colors is where most of the innovation has taken place in commercial fireworks displays. The essence is that specific spectral emissions from metals—usually formulated as salts or organic compounds—result from heating to high temperatures. Strontium is red and copper is blue. You can get almost any color you want with mixtures. Barium, magnesium, and aluminum, for example, produce white and completes the patriotic triad of red, white, and blue.
So next time you are enjoying fireworks, also enjoy thinking about the underlying chemistry, and the ingenious scientists who have perfected this chemistry over the centuries. For a nice narrative on the chemistry, check out this article from the Journal of Chemical Education.
Tom Tritton is President and CEO of CHF.