Imagine you are sitting in front of a campfire. A smell of smoke hung in the air as a pot of soup simmered over the open fire. While staring at the flickering orange tongues, you might wonder: Why do the logs burn, but the metal pot doesn’t?
The reason why some things catch fire and others does not depend on their chemical bonds and the energy it would take to change or break those bonds.
But first, this is a self-igniting primer. Fire needs several things to survive: oxygen, heat, and fuel.
Oxygen is a gas found in the air. Heat can be generated by friction, like when you strike a match, or it can be generated in other ways, such as a flash of lightning. Fuel is something that’s combustible: 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 made of mostly carbon-hydrogen bonds and sometimes consisting of oxygen or other elements. atomsuch as phosphorus or nitrogen.
Related: What makes something fireproof?
Specifically, combustion is a chemical reaction that releases energy from an unstable system with relatively weak chemical bonds. Everything wants to be more stable, especially organic molecules that contain carbon, oxygen, hydrogen and several other elements, Brozek said. Materials like wood and paper, which are highly flammable, are made of cellulose – a molecule composed of bonds between carbon, hydrogen and oxygen.
And when something burns, it “releases a lot of energy because now you’re moving the system down to a lower energy state,” Brozek said. “And that energy has to go somewhere.”
When a wooden object catches fire, the cellulose that makes up the wood 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 gaseous atom, thus emitting visible light. Brozek said that light appeared to us like a fire.
Back to a burning log vs a hot soup pot: The difference between a log and a metal pot has to do with how well the material can distribute the energy involved when a flame is applied to it, Brozek speak. Its chemical bonds are. The strong chemical bonds in metals cannot be broken easily. Meanwhile, a piece of wood lacks those strong bonds, so it is incapable of absorbing energy from the flame. Instead of absorbing energy, wood releases energy by catching fire. But the metal in the pot “has a great ability to absorb that energy and dissipate it,” which is why the pot feels hot to the touch.
Better heat absorption can also prevent wood from catching fire. Brozek said: If a flame is lit in a paper cup filled with water, the cup will not burn. Since the water in the cup can absorb heat, the paper will not catch on fire. (Though we don’t recommend trying this yourself.)
However, some metals still burn. Such “combustible metals”, including potassium and titanium, are used in making Fireworks. The metals in fireworks are in powder form, which provides more surface area for it to react with heat and oxygen much more quickly, Brozek said. When those metals are exposed to just enough heat to react with oxygen, an amount of energy is released that causes them to burn into different colors.
Originally published on Live Science.
https://www.livescience.com/why-wood-burns-not-metal Why does wood catch fire, but metal doesn’t?