The reason some things catch fire and others don’t comes down to their chemical bonds and the energy required to change or break those bonds.
Additionally, fire needs a few things to exist: oxygen, heat, and fuel. Oxygen is a gas that is in the air. Heat can be created by friction, like when you strike a match, or it can be created in other ways, like lightning.
The fuel is what burns. In general, it can be anything made 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 that sometimes include oxygen or other atoms such as phosphorus or nitrogen.
Combustion, a reaction that releases energy
Specifically, combustion is a chemical reaction that releases energy from an unstable system with relatively weak chemical bonds. Materials such as 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 it’s moving the system into a lower energy state,” Brozek said. “And that energy has to go somewhere.”
When a wooden object catches fire, the cellulose that wood is made of is converted 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. This light appears to us as a flame, Brozek said.
Thus, the difference between wood and metal has to do with how well the material can distribute the energy involved when fire is applied to it, Brozek said, which comes down to how strong its chemical bonds are. Strong chemical bonds in metal cannot be broken easily.
Oxygen, heat and fuel, the ingredients of fire
A piece of wood does not have these strong bonds, so it does not have the ability to absorb the energy from the flame. Instead of absorbing energy, wood releases it by burning. But the metal in the pot “has a tremendous capacity to absorb that energy and dissipate it,” which is why 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 glass can absorb the heat, the paper will not catch fire.
Some metals, however, burn. Such “combustible metals”, including potassium and titanium, are used to make fireworks. The metals in fireworks are in powder form, which provides a larger surface area for it to react much more quickly with oxygen and heat, Brozek said.
When these metals are exposed to enough heat to react with oxygen, the amount of energy released causes them to burn in different colors.