Why does wood catch fire and metal doesn't?

When lighting a fire, the sticks underneath ignite while the pot of water - made of metal with strong chemical bonds - remains unaffected.

Fire needs several elements to survive, including oxygen, heat, and fuel. The fact that some objects catch fire while others do not is due to the chemical bonds and the energy required to change or break them.

The metal kettle did not burst into flames like the wooden slats below.

Oxygen is a gas present in the air. Heat can be generated from friction such as a match, or in other ways, such as a lightning strike. Fuels are combustibles, which can be anything made of organic material, says Carl Brozek, a chemist at the University of Oregon. In this case, "organic" refers to a molecule made of mainly carbon-hydrogen bonds, sometimes including oxygen or other atoms such as phosphorus or nitrogen.

Combustion is a chemical reaction that releases energy from an unstable system with relatively weak chemical bonds. Everything wants to be more stable, says Brozek, especially organic molecules containing carbon, oxygen, hydrogen and some other elements. Flammable materials like wood and paper are made of cellulose, a molecule that contains bonds between carbon, hydrogen and oxygen.

"When burning, the object releases a lot of energy because the system is moving down to a lower energy state. This energy has to escape somewhere," Brozek said.

When a wooden object catches fire, the cellulose converts to CO2 and water vapor - both very stable molecules with strong bonds. The energy released from this chemical reaction excites electrons in the gaseous atom, emitting visible light. Humans call that light fire.

So why is it that when burning a campfire to boil water, the logs below burn and the metal pot does not? The difference between firewood and a metal pot has to do with the material's ability to distribute energy when exposed to fire, which depends on the strength of the chemical bonds, Brozek explained.

The strong chemical bonds in metals are not easily broken. Wood does not have such strong bonds, so it is not able to absorb energy from fire. Instead of absorbing energy, wood releases energy by catching fire. Meanwhile, the metal of the pot has a strong ability to absorb and disperse energy, so when touching the pot, it will feel hot.

Increased heat absorption can prevent wood from catching fire. Brozek said that if you light a paper cup filled with water, the cup will not burn because the water in the cup can absorb heat.

However, some metals are still combustible, for example potassium and titanium, which are used to make fireworks. The metal in fireworks is in powder form, which creates more surface area to react with heat and oxygen more quickly, Brozek explains. When these metals are exposed to enough heat to react with oxygen, the amount of energy released causes them to burn into different colors.