The fascinating chemistry that goes into a fireworks display

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  • The fireworks you enjoy every 4th of July are the result of a chain of chemical reactions.
  • When a firework container is lit, its contents of coloured explosives called “stars” light up the sky.
  • The colours that sparkle in the sky are chemical reactions happening right before your eyes.

The spectacle of a fireworks display may leave you wondering what it took to get that colour-changing, dazzling sequence into the sky.

We’re here to help.

Fireworks are the result of a whole bunch of chemical reactions. And it all leads to an explosion.

Simply, a firework is a container – typically a tube or ball shape – that holds explosives hitched up to a time-delay fuse.

There are two explosive parts in a firework – the one that shoots it into the sky, and a set of little balls of explosives called “stars.” These stars are filled with colours that blaze brightly in the sky, but after only a certain amount of time has passed. This is why fireworks can get up high in the sky before exploding into brilliance.

When the fuse gets low enough in the firework, it reacts with a bursting charge, which in turn lights the explosive that will disperse the stars. The ignited explosive creates a high-pressure gas that blows the colourful stars outward. Afterward, the cardboard that enclosed the explosives rains to the ground as charred remains.

Here’s what that looks like:

Chemical reactions create the colours

The colours that sparkle in the sky are chemical reactions happening right before your eyes.

Inside every star is an oxidizing agent, fuel, a metal that acts as the colour, and a binder that holds it all together. The fuel and oxidizing agent are the parts responsible for the intense heat and gas of the explosion, according to the American Chemical Society.

But the coolest part is the metals that act as the colours. The heat makes the atoms inside the wire move faster and faster, causing the atoms to bump into each other more, which gives off light. If you can control the temperature of the firework, then you can pick the exact time you want that firework to be a certain colour.

Some fireworks heat up and cycle through red, orange, yellow, and white, depending on how hot the explosion is. But more commonly, fireworks create light by letting off specific colours that depend on which metals are in the mix.

For a complete display, fireworks often mix different metals and metal salts for vibrant, multicolored effects. Calcium salts will burn orange, while sodium salts will burn yellow. If you burn copper, it will give off light that’s blue-green.

The science behind fireworks’ shapes and sounds

Fascinated by that smiley face or oddly lopsided heart in a firework display? It’s nothing more than some careful organisation of the stars. If they’re just spread randomly, they will expand out evenly through the sky once they explode.

But, because the explosion will push the stars out in a predictable trajectory, it is possible to organise the stars in a pattern on the cardboard cylinder on the outside of the firework. This will create specific shapes.

And of course, no fireworks display would be complete without the ear-shattering booms that freak out dogs and resonate in our chests. Those noises are caused by a sonic boom that happens as the gases inside the firework expand faster than the speed of sound.

In the end you get one bright, loud, beautiful way to celebrate the Fourth of July.

Jennifer Welsh and Mike Nudelman contributed to earlier versions of this article.

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