Science In Real Life

It’s gonna be a hot time in the old town tonight! Get your fireproof suits on, because we’re about to dive right into the heart of combustion and find out how fire works, what fire is, how to start a fire, how to stop a fire, what you can and can’t do with fire, and why fires come in different colors. However I can’t explain the whole flammable/inflammable thing. The english language, it be weird.

So. The most common way to start a fire is by some sort of friction. Rub two sticks together, et cetera. Why does this start a fire? Well, it very straightforwardly brings together the necessary ingredients. Fire is a chemical reaction, and in order to have any given chemical reaction you need to have all the different things brought together in the right proportion, like a well-made soufflé. For fire, those ingredients are:

1) Fuel. Something that can burn.
2) Oxygen. Chemically, for a substance to burn is for its molecules to combine with oxygen in a reaction that releases heat and light.
3) Heat.

Let’s talk about that second ingredient first: oxygen. The chemical reaction known as fire is also called an oxidation reaction, because we say the fuel is oxidized. To oxidize quite literally means to become an oxide, or a compound containing oxygen. For example, when carbon burns it becomes carbon dioxide, a common byproduct of fires. When hydrogen burns, it becomes dihydrogen monoxide, or water. It is possible to have something that a chemist would call combustion using an oxidizer other than oxygen, like flourine. At the risk of annoying the chemists (do not annoy the chemists, for you are made of carbon and will dissolve in acid), for our intents and purposes oxygen will be sufficient.

But where does all that heat and light come from? The energy that is released in fire comes from the chemical bonds in the fuel. There is tension inside molecules, especially ones that we usually burn, like gasoline or wood. Like an arrangement of rubber bands tautly stretched against itself, holding itself in place through sheer stretchiness, all it takes is oxygen – a pair of scissors in this analogy – to snip one of them and cause them all to flap around wildly as they break. As the atoms in the molecules rearrange themselves into new molecules, electrons jostle. When electric charges move, light is given off. Some of that light is visible, some of it infrared. Infrared radiation is no different from any other electromagnetic radiation, except that it tends to be of the right frequency to vibrate the atoms we’re made of (water), which we perceive as heat. In addition, the atoms themselves jostle around like techno freaks at a rave, creating even more heat.

Consider colors. Different colors of light represent different amounts of energy. Grab some rope, head on out to the white picket fence we all have around our half-acre homes (screw you, the 1950s), and start making wave shapes. You’ll notice, of course, that it takes more energy for you to give the wave more peaks, and less energy for fewer peaks. More peaks per unit length = higher frequency. With light, frequency becomes color once it hits our eyes. Remember our taut rubber band structure of chemical bonds? Not all fuels are created equal, and different fuels release different amounts of energy when they burn. Copper burns green, sodium burns yellow, potassium burns violet, and more. This is how fireworks are made into different colors – by choosing the right mixture of minerals to send up in the rocket, fireworks engineers can get just about any color you can imagine. This is also why copper rusts green: it’s the same chemical reaction as burning copper, but very slowly and without heat.

So the basic picture presents itself: in order to have fire, you need to mix up oxygen and something flammable, get them hot enough, and an exothermic reaction (that means heat and light) makes you wish you had gotten better homeowners’ insurance. Which leads nicely into how you stop a fire. It’s very simple: remove one of the three ingredients. Your average fire extinguisher works by removing the fire’s access to oxygen: smother it in carbon dioxide foam, and the fire goes out. The same principle is at hand when firefighters douse a flame in water, with an added dose of cooling off some of that heat. Wildfires often require removal of the third ingredient: fuel. Firefighters here will use controlled fires to create a gap in the fuel (dry wood and brush) between the fire and the neighborhoods where people live.

There are, of course, cases where throwing water on a fire is a bad idea, like electrical fires. Why? Because the heat in an electrical fire comes from a short in an electrical circuit – imagine the uproar if a baseball player ran from first to third without hitting second, and you’ll have an idea of what happens when current doesn’t follow the path we lay out for it in wires. Remember how water conducts electricity? Dumping water on an electrical short makes it worse, resulting in more heat flowing over more flammable stuff. Most of the fires that cause damage to property and people begin as electrical fires, so please double check your outlets to make sure they’re not being overtaxed. If you do need to plug in a bajillion things in one place, invest in a power strip with a built in fuse. It may save your life.

As for what you can and can’t do with fire … I’ll leave that as an exercise to you, dear readers. Please try not to get arrested.