Happy Fourth of July! Let’s talk about the chemistry of fireworks. 

Fireworks exemplify one of the most visually appealing displays of chemical reactivity. The explosions of bright lights and vibrant colors are the products of two classes of chemical reactions: oxidation and combustion, as well as three fundamental concepts in chemistry: Charles’ Law, incandescence, and luminescence. 

Oxidation is a type of chemical reaction in which an atom loses some of its electrons, the negatively charged, subatomic particles circulating outside of the atom’s nucleus. Chemicals that promote oxidation are called oxidizing agents or oxidizers, and these reagents are key components in combustion reactions. Combustion occurs when a substance (the fuel) reacts rapidly with oxygen (an oxidizer) and generates energy in the form of heat. There are a variety of oxygen sources, the simplest being ambient air. The byproducts of the combustion of simple hydrocarbons (molecules containing only hydrogen and carbon atoms) are water and carbon dioxide gas.

Combustion is the driving force for the explosion of fireworks. 

Fireworks are made up of small tubes containing gunpowder, known as aerial shells, and additional explosive materials called stars, which produce vivid colors upon explosion. The stars contain an oxidizer, a fuel source, a metal-based colorant, and a binding agent (to hold all the components together). When the aerial shell is launched into the sky, a fuse inside is ignited, which ignites the gunpowder and stars. This generates a large volume of gas and heat, which then causes the shell to explode, releasing the stars. The rapid expansion of gas in the hot shell is a tangible example of Charles’ Law, which states that, in a closed container at a constant pressure (in this case, atmospheric pressure), as the temperature of gas particles increases, the volume of the particles also increases. This expansion happens faster than the speed of sound, producing the sonic booms heard during a fireworks display. 

What about the colors?

The colors are the products of incandescence or luminescence. Incandescence occurs when a metal is heated to such an extent that it starts to glow (this is how an incandescent light bulb works!). Luminescence involves the excitation of the electrons in the metal atoms: electrons can move from lower energy states to higher energy states when they absorb energy from an external source (in this case, the heat from the combustion reaction), and this “jump” in energy is known as excitation. Electrons don’t stay in this excited state for very long, and when they “relax” back to their normal, lower energy state, they emit energy in the form of light. This emitted light has a defined energy which corresponds to a specific color, and both properties are characteristic of the type of metal involved. The chemistry of color is a fascinating subject of its own, and you can read more about it here!

In summary, fireworks are the product of strategically timed, expertly designed, and carefully controlled chemical reactions. On this fourth of July, I hope you get the chance to witness the magic of chemistry light up the night sky.