A Science Lesson For A Fiery Fourth

Fireworks i i

Colorful fireworks explode in the air. iStockphoto.com hide caption

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Fireworks

Colorful fireworks explode in the air.

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It's that time of year when millions of Americans light fuses and run away. Fireworks will explode across the nation this weekend in celebration of Independence Day, but science-wise, just what goes on behind the crackles and pops that give us a night sky ablaze in color?

First, you need two chemicals. Oxygen, for one.

"One of your chemicals has to be oxygen-rich, and give up that oxygen when it's heated up to high temperature," chemistry professor John Conkling says. Then you need a fuel. Gunpowder is popular, but you can use other fuels as well.

"The fireworks that shoot up in the air have a bursting charge, which is usually traditional gunpowder — black powder — in the center of the canister or the sphere that you're shooting up in the air," Conkling says.

Surrounding that bursting charge are pellets of fuel and oxidizers. When the bursting charge explodes, it ignites these pellets and scatters them into briefly burning stars.

"Finding the perfect combination of an oxidizer and a fuel, and then finding the perfect weight ratio of those two chemicals, to give the exact type of burning effect you’re looking for can take a lot of time in the lab to come up with," Conkling says.

And of course, you’ll want to be careful with your colors. If you want yellow, mix some sodium into your fireworks. For green, you'll want barium or boron. Red is strontium, and copper will give you blue.

But now a fireworks secret. Have you ever wondered how a display will burst with one color and then change to another?

Conkling says the secret is to make a pellet with two layers. An inner layer of, say, sodium, and an outer layer with boron. The outer layer burns first, giving green stars, and you get the yellow stars produced by the sodium when the outer layer burns away.

Cool.

And what about those really loud fireworks? Conkling says they usually have a different kind of fuel than the colorful displays. Aluminum powder is common, as mixing it with the right oxidizer makes the explosive burn very fast and very, very hot. Put that in a fireworks casing, ignite it, and you'll get a very loud boom.

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