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ireworks are one of the most spec- Each star contains four chemical tacular outdoor shows. They produce ingredients: an oxidizing agent, a fuel, amazing bursts of colors that take a a metal-containing colorant, and a variety of shapes. But how do they binder. In the presence of a flame or work? How do they burn into so many a spark, the oxidizing agent and the Fcolors and patterns? And why, if not handled fuel are involved in chemical reactions properly, can they cause serious injuries or that create intense heat and gas. The even death? metal-containing colorant produces the color, and the binder holds together the What’s inside a oxidizing agent, fuel, and colorants. At the center of the shell is a bursting firework? charge with a fuse on top. Igniting the The source of most fireworks is a small fuse with a flame or a spark triggers the By Kathy De Antonis tube called an aerial shell that contains explosion of the bursting charge and of explosive chemicals. All the lights, colors, the entire aerial shell. and sounds of a firework come from these chemicals. How fireworks An aerial shell is made of gunpowder, which is a well-known explosive, and small globs of explode explosive materials called stars (Fig. 1). The The explosion of a firework happens stars give fireworks in two steps: The aerial shell is shot into their color when they the air, and then it explodes in the air, explode. When we many feet above the ground. watch fireworks, we To propel the aerial shell into the actually see the explo- air, the shell is placed inside a tube, Z sion of the stars. They called a mortar, which is often partially Fireworks! FERNANDE are formed into spheres, buried in sand or dirt. A lifting charge cubes, or cylinders that of gunpowder is present below the shell are usually 3–4 centi- with a fuse attached to it. When this ANTHONY During the explosion, not only meters (1–1½ inch) in fuse, called a fast-acting fuse, is ignited are the gases produced quickly, but diameter. with a flame or a spark, the gunpowder they are also hot, and they expand explodes, creating lots of heat and gas that rapidly, according to Charles’ Law, which Figure 1. Structure of cause a buildup of pressure beneath the shell. states that as the temperature of enclosed an aerial shell. The black balls are the stars, Then, when the pressure is great enough, the gas increases, the volume increases, if the CAMINERO and the gray area is shell shoots up into the sky. pressure is constant (Fig. 1). The loud boom gunpowder. The stars CESAR After a few seconds, when the aerial shell is that accompanies fireworks is actually a sonic and the powder are surrounding a bursting high above the ground, another fuse inside the boom produced by the expansion of the gases charge, which also aerial shell, called a time-delay fuse, ignites, at a rate faster than the speed of sound! contains black powder. causing the bursting charge to explode. This, If the stars are arranged randomly in the in turn, ignites the black powder and the stars, aerial shell, they will spread evenly in the sky which rapidly produce lots of gas and heat, after the shell explodes. But if the stars are causing the shell to burst open, propelling the packed carefully in predetermined patterns, stars in every direction. then the firework has a specific shape—such 8 ChemMatters, OCTOBER 2010 www.acs.org/chemmatters as a willow, a peony, or a spinner—because More often, light from fireworks is produced carefully and have an adult in charge,” says the stars are sent in specific directions during by luminescence. When fireworks explode in John Conkling, an adjunct professor of chem- the explosion. the sky, the gunpowder reactions create a lot istry at Washington College, Chestertown, The timing of the two fuses is important. of heat, causing the metallic substances pres- Md., and former executive director of the The fast-acting fuse ignites first, propelling ent in the stars to absorb energy from the heat American Pyrotechnics Association. the shell into the air, and then the time- and emit light. These metallic substances are delay fuse ignites to cause the aerial shell actually metal salts, which Color Compound to explode when it is high in the sky. If the produce luminescent strontium salts, lithium salts timing of the fuses is not just right, the shell light of different lithium carbonate, Li2CO3 = red ADISON red -M can explode too close to the ground, injuring colors when they strontium carbonate, people nearby. are dispersed in SrCO3 = bright red ISCONSIN W the air. OF calcium salts orange calcium chloride, CaCl2 NIVERSITY , U sodium salts yellow HASHIRI sodium chloride, NaCl K HA . S Z barium compounds + chlorine green ASSAM producer barium chloride, BaCl2 . B ROF copper compounds + chlorine / ; P ORG blue . producer copper(I) chloride, CuCl SCIFUN . mixture of strontium (red) and WWW purple :// copper (blue) compounds HTTP Table 1. Colorant compounds used in fireworks Fireworks! and the colors they produce. This light is produced by electrons inside Knowing the rules and regulations is impor- the metal atoms (Fig. 3). These electrons tant, too. According to Conkling, fireworks absorb energy from the heat, which causes that are publicly available in stores are legally them to move from their allowed in 41 of the 50 U.S. original ground-energy Excited states. So, you may not be electron High- state to an excited state. energy able to purchase fireworks if Z Then, nearly immediately, level your state does not allow it. these electrons go to a Also, regulations require FERNANDE Emission Absorption of light lower energy state and of energy that consumer fireworks ANTHONY emit light with a particular should have no more Figure 2. Schematic illustration of Charles’ Law. Low- When the pressure of a volume of gas is constant, energy and characteristic energy than 50 milligrams (about level an increase in temperature leads to a proportional color. 1/500th of an ounce) of increase in the volume of the gas. The gas The color of the light gunpowder. This may molecules move faster at higher temperatures. emitted by the electrons seem like a relatively small Ground varies depending on the energy amount. But don’t be fooled. Where do fireworks’ level Electron at its lowest STAFF type of metal or com- possible energy Even 50 milligrams of gun- colors come from? ACS bination of metals. So, Figure 3. Principle of luminescence. powder or less can cause What makes fireworks so special is the the colors are specific Heating atoms causes electrons to serious injuries. “You would beautiful colors they produce. These colors to the metals present in move from their ground-energy level be surprised by how power- to a higher energy level (blue arrow). are formed in one of two ways: luminescence the fireworks. The metal- When the excited electrons move to a ful fireworks can be,” says and incandescence. containing colorants for lower energy level (red arrow), they Doug Taylor, president of Incandescent light is produced when a some common fireworks emit light with a specific energy and Zambelli Fireworks, one of characteristic color. substance is heated so much that it begins to are listed in Table 1. the largest fireworks com- glow. Heat causes the substance to become panies in the United States. hot and glow, initially emitting infrared, then Some fireworks contain more than the lim- red, orange, yellow, and white light as it Fireworks’ safety ited amount of 50 milligrams. Although they becomes increasingly hotter. When the tem- Fireworks are a lot of fun to watch, but they are illegal, such fireworks—which include the perature of a firework is controlled, the glow must be handled with great care because they “cherry bombs” and “M-80s”—can be found K of its metallic substances can be manipulated can be dangerous. “When using fireworks, in some stores or on the black market and ISTOC to be a desired color at the proper time. one should follow the label directions very cause even more damage. ChemMatters, OCTOBER 2010 9 Visible Light Spectrum Energies by Element Photon Energy Element (kJ/mol) 181.2 Lithium 228.6 Boron 203.4 Sodium 279.1 Potassium 252.6 Copper 190.5 Strontium .

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