Imagine yourself sitting in front of a campfire. A smell of smoke wafts through the air as a pot of soup simmers over an open flame. As you watch the flashing orange tongues, you may wonder: Why are the wooden logs burning, but the metal pot isn’t?
The reason some things catch fire and others don’t boils down to their chemical bonds and the energy it would take to change or break those bonds.
But first, here’s an introduction to fire itself. Fire needs a few things to exist: oxygen, heat, and fuel.
Oxygen is a gas that is in the air. Heat can be generated by friction, such as when a match is lit, or it can be generated in other ways, such as by lightning. The fuel is what is burned: In general, this can be anything made up of organic material, Carl Brozek, a chemist at the University of Oregon, told Live Science. In this case, “organic” refers to molecules that are made up primarily of carbon-hydrogen bonds and sometimes include oxygen or other atomsas phosphorus or nitrogen.
Related: What makes something fireproof?
Specifically, burning is a chemical reaction that releases energy from an unstable system with relatively weak chemical bonds. Everything wants to be more stable, especially the organic molecules that contain Coal, oxygen, hydrogen and a few other elements, Brozek said. Materials like wood and paper, which catch fire easily, are made of cellulose, a molecule made up of bonds between carbon, hydrogen, and oxygen.
And when something burns, it ends up “releasing a lot of energy because now you’re moving the system to a lower energy state,” Brozek said. “And that energy has to go somewhere.”
When an object made of wood catches fire, the cellulose that makes up the wood turns into carbon dioxide and water vapor, both very stable molecules with strong bonds. The energy released by this chemical reaction excites the electrons in the gas atoms, which in turn emit visible light. That light appears to us as a flame, Brozek said.
Back to the burning log vs. the pot of hot soup: The difference between a log and a metal pot has to do with how well the material can distribute the energy involved when fire is applied to it, Brozek said, which boils down to what so strong their chemical bonds are. The strong chemical bonds of the metal cannot be easily broken. Meanwhile, a piece of wood lacks those strong bonds, so it doesn’t have the ability to absorb the energy of the flame. Instead of absorbing the energy, the wood releases the energy by burning. But the metal of the pot “has a great capacity to absorb that energy and dissipate it,” so the pot will feel hot to the touch.
Better heat absorption can also prevent wood from catching fire. If a flame were applied to a paper cup filled with water, the cup would not burn, Brozek said. Because the water in the cup can absorb heat, the paper will not catch fire. (Although we don’t recommend trying it yourself.)
However, some metals burn. These “fuel metals,” including potassium and titanium, are used to make Fireworks. The metals in fireworks are in powder form, which provides more surface area so it reacts much faster with heat and oxygen, Brozek said. When those metals are exposed to enough heat to react with oxygen, the amount of energy released causes them to burn different colors.
Originally published on Live Science.