How 4th of July fireworks explode in midair, and why they trigger a sonic boom

Mark Kolbe/Getty Images

The chemistry of a fireworks show is a tightly timed aerial dance.

First, a base of black powder ignites to shoot the firework into the sky and lights a fuse on fire. Once the firework is at the pinnacle of its trip, that lit fuse sparks a burst charge that acts like gunpowder. That charge exerts tremendous pressure on the dense package of chemicals, fuel, and glue. Inside, colourful “stars” filled with elements that burn into bright-hot colours explode in midair.

To launch fireworks for big shows, according to pyrotechnics expert Mike Tockstein, most professionals don’t use rockets.

“We use mortars and aerial shells,” Tockstein told Business Insider.

Shells are a safer choice, he said, since rockets can change direction once they leave a launch pad, whereas shells are locked into a single trajectory. Nowadays, fireworks are typically shot off electronically for large shows, rather than by hand.

Here’s how the whole process happens:

The loud blast in your ears when the fireworks go off is the sound of a sonic boom that’s produced as the gases packed inside the shell expand quicker than the speed of sound.

This story has been updated. It was originally published on July 4, 2018.

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