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Spectators watch the Macy's Fourth of July fireworks explode over the East River in New York, July 4, 2014. New studies sh...

How chemistry lights up the sky for the Fourth of July

Sometime between 600 and 900 CE, two Chinese scientists mistakenly mixed potassium nitrate with sulfur and charcoal, creating a crude gunpowder. Legend has it that they were searching for an immortality elixir. What resulted was the first documented example of a firecracker.

“Fireworks were first just exploding bamboo tubes,” said John Conkling, former executive director and technical director of the American Pyrotechnics Association. “Then they just got more and more ingenious…[and they] realized if you put other minerals in the basic material you could get different colors.”

The basic firework requires three ingredients: an oxidizer, a fuel and a chemical mixture to produce the color. Over the years, scientists have staged various chemical reactions to produce fireworks of different colors.

Happy July 4th weekend! On Monday, millions of Americans will watch fireworks light up the night sky, but have you ever wondered how those explosive displays work?We’re live at the John S. Toll Science Center at Washington College to talk about the chemistry of fireworks. Our guest: John Conkling, Former executive director and technical director of the American Pyrotechnics Association.

Posted by PBS NewsHour on Friday, July 1, 2016

Different elements burn at different visual wavelengths, but the pigments we see all originate from metals and metallic compounds. Lithium and strontium carbonates produce deep reds and purples, copper chlorides produce blue; titanium, aluminum, and magnesium are silvery; calcium chloride is orange; sodium is yellow; and finally, barium burns green. Due to copper’s narrow temperature range, blue is the most difficult color for pyrotechnic experts to create.

“At high temperatures, an electron can get excited and jump to a higher energy level,” said Conkling. “When it cools off, that electron drops back down to its normal energy state. In doing so, that extra energy is emitted as a photon — a particle of light. It’s a very distinct wavelength for the different chemical elements.”

A firework is ignited by lighting a main fuse, which kicks off the chemical reaction inside. The flame travels through the lift charge, the first blast of gun or black powder that sends the firework upward. As it flies through the sky, slow-burning potassium nitrate, charcoal and sulfur combust, heating the main casing of the firework. When the casing explodes, pellets containing the chemical color mixtures are propelled outward. As they fall to the ground, the light energy the pellets release forms the classic firework shape we see.

“In the old days, someone with a torch would walk along the line of the mortars and individually light the fuses for the shells. It certainly limited the artistry [based on] how fast that guy could run,” Conklin said. “Most [fireworks] today fire electrically…you push a switch [and] a current goes through a filament and heats up the powder and gets it to go.”