 
The Chinese invented fireworks somewhere around 900 - 1200 AD. They shot off fireworks to ward off evil spirits and used them during celebrations, like the Emperor’s birthday and other holidays. Fireworks were used to celebrate independence in the United States in 1776. They were used in England to celebrate the birthdays of kings and queens. Fireworks are used to celebrate American independence each July 4th Designing and building firework displays requires a strong knowledge of chemistry and physics. Fireworks involve a whole lot of chemical reactions happening all at once.  A firework is a container, usually a tube or a ball shape, that holds explosives hooked up to a time-delay fuse. First, the shell must be shot into the air so that the lights explode above the heads of onlookers. Similar to a rocket being shot into the air, you need an explosion on the ground to send the fireworks upwards. To achieve flight, a tube called a mortar is set on the ground, or partially buried in sand. Gunpowder with a fuse attached is placed inside the mortar. The shell is placed on top of this gunpowder, called a lifting charge. When the fuse is lit, the gunpowder explodes, creating enough heat and gas to propel the shell into the sky.   The shell typically contains little balls of coloured explosives called 'stars'. These are filled with colors that blaze brightly, after a time delay. When the timed fuse gets low enough in the firework, it reacts with a burst charge, which in turn lights the black powder explosives that will disperse the stars. The ignited explosive creates a high-pressure gas that blows the colorful stars outward. Inside every star is an oxidizing agent, fuel, a certain 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.  Different metals added to the star give colours to the explosions. Some just heat up and cycle through red, orange, yellow, and white, depending on how hot the explosion is. 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. Fireworks create light by letting off specific colours that depend on what metals you add. For a display, fireworks often mix different metals and metal salts to give vibrant, multicoloured effects. Calcium salts, for example, will burn orange, while sodium salts will burn yellow. If you burn copper, it'll give off light that's blue-green.   Image credit: https://www.stevespanglerscience.com/ 
Shapes or faces in a firework display are the result of careful organization of the stars. If they're just spread randomly, they'll expand out evenly through the sky once they explode. But because the explosion can push the stars out in a predictable trajectory, it's possible to organize the stars in a particular pattern on the cardboard cylinder on the outside of the firework. This will create specific shapes.The noisy booms are caused by sonic booms that happen as the gasses inside the firework expand faster than the speed of sound. |