The chemistry of fire

One of my favorite activities on a chilly winter night is to snuggle on the couch in front of a roaring fire in the fireplace. There’s something so cozy about it! Now that I think about it, fire is kind of a strange thing. It gives warmth and light – but it can also be highly destructive. It can sustain life – or it can destroy it. But what is it? When I have wood burning in my fireplace, what exactly is going on? What is fire?

Fire isn’t a thing. It’s a chemical reaction. Specifically, fire is a chemical reaction that releases heat and light energy as matter changes from one state to another.

Okay, sounds good. But what does it mean?

Let’s deal first with the first part of that definition – it’s a chemical reaction. Chemical reactions are wide and varied, and include everything from nails rusting to the bubbling of alka-seltzer tablets in a glass of water to the metabolism of sugars in your cells to produce energy. A chemical reaction simply means that 2 or more things (molecules, ions or atoms) interact and change to something else. So let’s look at a nail turning rusty. This is an example of an oxidation reaction. When iron is exposed to water and oxygen, the water and carbon dioxide in the air form a weak acid (called carbonic acid). Carbonic acid dissolves the iron, and also breaks more water down into hydrogen and oxygen. Free oxygen (from the water) and dissolved iron (from the nail) combine to form iron oxide – which is rust. The net result is that iron has changed into iron oxide - a completely different molecule than it was when you started.

So what is the chemical reaction that occurs in fire? For fire to occur, you must have 3 things - something to burn, oxygen and an ignition. So let’s look at the burning of wood. The ignition spark – which can come from any number of places (such as a match, a lightning bolt, or friction) - must bring the wood to about 300 degrees Fahrenheit. At this temperature, the cellulose in the wood begins to break apart. This breaking of the cellulose releases three things - volatile gases (namely oxygen, hydrogen and carbon), char (which is pure carbon) and ash (all the rest of the unburnable minerals from the wood). We see the volatile gases as smoke. For the wood to catch in flames, the temperature must now get to 500 degrees Fahrenheit. Now the molecules in the cellulose start to break apart into atoms. The atoms recombine with oxygen, forming water, carbon dioxide, and other stuff. The recombining of those atoms into new molecules produces a lot of energy, which takes the form of heat and light.

So you start with wood, heat and oxygen – and you wind up with water vapor, ash, and smoke. And all of the heat and light energy that is released in the transformation? That’s fire.

So the next time we have a fire in our fireplace, I hope that I can remember that this process of fire is really a great example of some fundamental chemistry. Maybe then I won’t take it for granted quite so much!