The chemistry behind fireworks

What can you tell us about the chemical reactions that go into a fireworks display? 

Traditionally, three reagents, potassium nitrate, carbon, and sulfur, make gunpowder. You’re doing a combustion reaction out of those types of materials that creates this detonation explosion. Those three reagents react to make solid potassium carbonate, solid potassium sulfate, nitrogen gas, and carbon dioxide gas, so you have solid reagents reacting to make gases.

The explosion spreads out all that material, which is in a super-heated state, and there’s different metal salts that are added to create the colors. The metal salts heat up to become ‘excited’ in that highly energetic situation and emit light as a result. 

What’s the difference between an explosive firework display and other combustion reactions, like burning wood?

For an explosion, the goal is to generate as much gaseous product in as short of a time as possible. 

You could have a relatively slow chemical reaction, like the Pharaoh’s serpent or ‘black snake’ firework, but if you want an explosion then you need the reaction to occur quickly to produce a lot of gas in a short amount of time.

A standard firework has a fuel, oxidizer, and binder. What is the role of each component?

In any kind of explosive, rocket engine, or energetic material that you’re trying to develop to explode or propel something, you need a combination of a fuel and an oxidizer. 

The fuel is a source of electrons, something that stores energy, and it will be burned in the course of the explosion. A chemical reaction, typically combustion, is occurring through reaction of the fuel with an oxidizer. The oxidizer is receiving the electrons; upon reaction with the oxidizer, energy is released, and the electrons are transferred from one to the other.

So you’re creating a mixture of the fuel and the oxidizer, and that’s a lot of stored potential energy that’s ready to be released. You just need a spark under that situation to get the reaction moving to convert all of that fuel, and oxidizer, into the products.

The binder just holds everything together and, ideally, makes the mixture stable so that it doesn’t go off unexpectedly. You want the firework to go off at an appointed time, so by using the binder you can engineer the explosion and timing together with other explosions in the show.

Why do different metals burn in different colors? 

It’s the arrangement of electrons in shells outside of the metal’s nucleus that allows for the absorption of energy and the emission of different wavelengths (colors) of light. 

Each element brings along a specific ‘flavor’ based on their number of electrons, and the electrons have interactions between each other in the shells around the nucleus. That combination of factors gives rise to specific sets of characteristics for a given metal.

As we traverse the periodic table, elements get heavier, and that will also contribute to the relative energy levels that are associated with the distribution of electrons, which will also change the color at which these elements emit light.

Why are the metals in salt form?

The salts are easy to disperse, and they’re less reactive when they are going into the firework. The firework is primarily a combination of compounds to provide the explosion, and the metal salts are additives that give you different colors.

What were your 4th of July traditions growing up? Do you have a favorite place to watch fireworks here in Philadelphia?

I grew up in Michigan, and we always had someone in the neighborhood who would drive out of state at some point before the 4th of July. That was a big deal when I was a kid, to have someone go and get fireworks that were much more intense. Then there would be a big neighborhood demonstration for all the kids in the neighborhood. It was always a lot of fun.

Now my husband and I typically will go and watch them here, along the river for example, or just from a high vantage point. We don’t have a particular site that we go to yet; we’re still feeling it out. We are looking forward to taking our son this year for the first time.

Do you find yourself thinking about the show from a chemist’s perspective? 

A lot of chemists, I think especially inorganic chemists, have an attraction to fire, so I really enjoy the show, but I also appreciate the chemistry when I’m watching them. Strontium, copper, magnesium, calcium, and sodium give such a great show.