The chemical brilliance of fireworks

On the Fourth of July, many Americans will commemorate Independence Day looking to the sky, enthralled as beautiful explosions light up the night. This tradition began on July 4, 1777, when Philadelphia held its inaugural commemoration of independence with festivities that included a fireworks display.

Fireworks have been used in celebrations as early as the ninth century in China, and over time many scientists, artists and pyrotechnicians have contributed to the development of brighter and more colorful, safer and environmentally friendly fireworks.

Each firework involves a precisely orchestrated series of chemical processes. First, a mixture of fuels and oxidants such as gunpowder (sulfur, charcoal and potassium nitrate) is ignited to initiate a reaction that generates a large amount of gas to launch the missile skyward. A time-delay fuse then initiates a secondary reaction that explodes the firework, sending the loosely packed “stars” radiating outward. The stars contain additional chemicals, particularly metal salts, to produce different effects as they rain down.

One of the most striking elements of fireworks is the brilliant colors that are produced. In many cases, these result from the luminescence (emission of light) from metal salts. The metal salts in the firework stars vaporize from the heat of the chemical reactions, and their electrons are excited to high energy levels. As the electrons relax, the excess energy is converted to light of very precise wavelengths that are unique to each atom. Sodium salts emit a very strong yellow color similar to the yellow glow of sodium street lamps. Strontium, calcium, barium and copper salts are commonly used to generate red, orange, green and blue colors, respectively.

In fact, the unique emission of light from each element can act as a fingerprint. Scientists routinely use various forms of atomic emission to determine the chemical composition of unknown materials.

Environmental scientists can use atomic emission to determine mercury or lead levels in a stream for example, and forensic scientists can determine thallium or arsenic in human liver tissue from poisoning.

So this year, enjoy the fireworks with an appreciation for some of the beautiful chemistry that make them possible.

Schmedake is an associate professor of chemistry at UNC Charlotte. A graduate of Knox College, he completed a Ph.D. from the University of Wisconsin. For several years, as part of UNC Charlotte’s Science and Technology Expo, Schmedake has performed a very popular demonstration on combustion chemistry, which has earned him the nickname “Dr. Boom.”