Before you go celebrate the 4th of July, it's worth brushing up on… the science of fireworks:
Fireworks have gotten a lot more advanced since the discovery of gunpowder in China more than 1,000 years ago. In the video above, John Conkling, a chemist at Washington College, explains how modern-day fireworks are made.
The big aerial fireworks we tend to see on the 4th of July basically look like this:
And there are three broad steps here:
1) This entire aerial shell is first shot out of a mortar by lighting the "black powder propellant" (a mix of potassium nitrate, charcoal, and sulfur — that is, gunpowder).
2) Once the shell is launched, the internal "time fuse" starts burning down until the firework is high in the air. The fuse then sets off the "bursting charge" inside the shell — and the firework explodes.
3) The real fun starts when those "effect pellets" start to burn in the air, producing various colors and effects. These pellets are a combination of fuel and oxygen-rich elements The colors depend on the type of elements used, while the type of fuel determines how hot and how long these pellets burn.
"We produce different colors by using the fact that different chemical elements heated to high temperatures get rid of their energy by emitting very specific wavelengths of light," Conkling explains. Strontium chloride is often used to produce a red color, for instance. Sodium silicate is used to produce yellow. Titanium produces those silver sparks.
So what's the trickiest part here? "Blue is the hardest color to produce with pyrotechnics," Conkling adds. "You need the perfect chemistry." (Copper chloride will burn blue, but it will also decompose if it gets too hot, making it tricky to work with.)
Further reading/watching: Here's a good history of fireworks. Here's a more detailed step-by-step explanation of how aerial fireworks operate. And here's an extremely complex choreographed fireworks show in Sydney to commemorate the 100th anniversary of the Royal Navy's arrival.