Elementary science: playing with fire

Fire is obvious, so it seems. Pretty much every child recognizes the flame of butane lighter is the same as the flames on the stove or on a lit candle.

A child sees that a fire makes solids things smaller. The grown-ups tell children that fire consumes, that the logs burned up, that fire reduces things to ash.

And pretty much every adult who believes this still lives in the world of alchemy, hoping to turn lead into gold.

My September sophomores know what fire is, no surprise, since September sophomores know everything there is to know about anything.

Before I ever say the words respiration or calorie, I ask them about fire—a few look confused (a good sign in science class), but most give me a knowing smile—they know what it is, they “just can’t put it into words” and when they do, they describe the properties of fire. Not a bad start.

I ask them what you need for a fire, and they know that—fuel, oxygen, something to light it—somewhere in elementary school they learned about the fire triangle.

I then pretend to take out a box full of pure oxygen, and ask them what would happen if I lit a match in it.

Most of my sophomores know the photosynthesis/respiration equation before they get to my class:

C6H12O6 + 6O2 => 6CO2 + 6H2O with energy released
Sugar + oxygen combined releases carbon dioxide and water
CO2 + H2O => C6H12O6 + O2 with energy captured

The kids love writing down equations, it gets them feeling all sciency, and now the stupid teacher isn’t asking stupid questions about stupid fire expecting answers that “can’t be put into words.”

The inevitable “Do we have to know this?” comes from the back corner of the classroom—always the same back corner—but I pretend I don’t hear.

I hold up my propane torch—even the back corner crowd notices now. I promise them I will light it in a minute, but they have to answer a simple couple of questions first. What do I need to make it work. (“Well, duh…”), and what is H2O (“Well, duh…” with an advanced eye roll).

I write the equation for the combustion of propane on the board—it’s similarity to the respiration/photosynthesis equations is glaringly obvious, but not a point I care to make at the moment.

C3H8 + 5O2 => 3CO2 + 4H2O

I ask what comes out of the propane torch after the propane as the propane is burned. I consistently get two answers—fire and carbon dioxide. I never get water. I’ve asked hundreds of kids the question, with the equation sitting up on the board, and it’s like H2O is some mysterious stuff stuck to the equation just to make it balanced. The stuff is pretty mysterious when you get down to it.

After our list of stuff that comes out of the torch is made—usually CO2, heat, light, flame, and occasionally propane—I light the torch.

I pass the torch over a cool piece of glass—it could be a large beaker—then pass it over the cool stem of the faucet. The students see the flash of water vapor on the glass. They know it looks like "fog” — but no one wants to say it. It makes no sense. Water from fire? It must be a trick.

To be fair, it pretty much gobsmacks me, too, each time I do this.

And of course, water does not come from fire—it comes from the hydrogen in the propane and the oxygen in the air. Turns out we’re all closet alchemists. We cannot accept the obvious.


Chemistry hit puberty  when Antoine LaVoisier realized that fire consumes nothing—it only transforms. If you figure out the amount of stuff with and compare it to the stuff you end up with, it has the same mass.

Exactly the same mass.

All the heat and light and noise that escaped from the dancing flame took nothing away. Energy has no mass, no inertia, no stuff to it. It's not nothing, but it's not mass, either.

So what do we teach a young child about fire? Let them observe a candle, let them see the water rise from the flame, let them cover it with a glass and see the flame die, let them wonder.

Matchstick photo by Sebastian Ritter via Wikipediaa under CC.

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