Why do things burn?

Things burn because by combining with oxygen, they can move to a lower (and more desirable and stable) energy state.

Burning, technically known as combustion, is a chemical reaction in which a substance combines with oxygen and releases energy in the form of heat and light, which we see as flames. The substance starts off in a higher energy state and, by combining with oxygen, ends up in a lower energy state. In terms of its energy state, matter is like water—it flows downhill. Wherever possible, substances move from a higher to a lower and more stable energy state.

The simplest example of a combustive reaction is when hydrogen burns in oxygen. The two gases combine to produce water, and water is much more stable (that is, harder to move from its current state) than either oxygen or hydrogen—just try setting fire to it and see for yourself. By contrast, both hydrogen and oxygen are highly flammable gases because they exist at much higher energy states than water. The difference between the before and after energy states is the amount of energy that is released during the combustive reaction. A lot of energy is released when hydrogen burns in oxygen, and this makes it a good rocket fuel. The Saturn V rockets that blasted the Apollo moon-landing missions into space used hydrogen and oxygen as fuel—over 960,000 gallons of it! The first-stage tank alone contained enough liquid oxygen to fill 54 railroad tank cars.

Catching fire

If burning moves a substance from a higher to a lower energy state, why doesn’t combustible stuff like wood burst into flames spontaneously? In fact, it is quite hard to set fire to wood, as anyone who has tried to get a campfire going in the rain will tell you. Some substances will catch fire spontaneously, but they tend not to hang around in the environment all that long. Most require a burst of energy to get the combustive reaction going. This is known as activation energy.

The changing energy states of burning wood are like water moving from the top of a slope to the bottom; activation energy is the hump that water has to get over before it can run downhill. Once it gets a little boost to get over the hump, it will continue running downhill until it reaches flat ground. Similarly, once the wood gets a boost from a lit match, it will burn effortlessly. In energetic terms, ash is the equivalent of flat ground. In fact, with wood the picture is more complex, because wood itself doesn’t burn directly; instead, once it reaches ignition temperature at about 500°F, the heat causes it to break up and some of it turns into a gas. It is this gas, rather than the wood itself, that burns —but eventually you still wind up with a pile of ashes.