Burning a substance is a rapid, high-temperature chemical process called combustion. Fire is not a simple disappearance of matter, but a rearrangement of atoms and molecules that releases energy in the form of heat and light. Understanding combustion requires examining the reaction at the molecular level, where the building blocks of matter switch partners.
The Necessary Ingredients for Combustion
Combustion requires two components: fuel and an oxidant. The fuel, such as wood or natural gas, is typically composed of hydrocarbons—molecules consisting of carbon and hydrogen atoms held by chemical bonds. The oxidant is usually oxygen (O₂) gas from the surrounding air, where molecules exist as two double-bonded atoms. These fuel and oxygen molecules are the reactants awaiting the energy required to begin their transformation.
Atomic Reorganization During Burning
Initiating burning requires activation energy, typically supplied by a spark or heat. This energy overcomes the stability of the starting molecules and breaks their existing chemical bonds. The bonds holding the fuel’s carbon and hydrogen atoms, and the double bonds in the oxygen molecules, must first break apart. Once broken, the highly reactive atoms instantly reorganize into new molecular structures. This atomic movement is a chain reaction where the heat generated by the first reactions provides the activation energy for nearby molecules to break their bonds. The energy absorbed to break the initial bonds is significantly less than the energy released when the new, more stable bonds form.
What the Atoms Become
In complete combustion, carbon atoms from the fuel combine with oxygen to form carbon dioxide (CO₂). Simultaneously, hydrogen atoms pair with oxygen to create water vapor (H₂O). These two gaseous compounds are the primary reaction products. The heat and light associated with fire result from this atomic reorganization. The newly formed bonds in CO₂ and H₂O are stronger and more stable than the original bonds in the fuel and oxygen. The excess energy released from forming these stronger bonds provides the heat and the flame’s incandescence. When oxygen is insufficient, incomplete combustion occurs, producing less efficient products like carbon monoxide (CO) and visible soot (unburned carbon particles). Ash consists of mineral components in the original fuel that did not react with oxygen.
The Law of Conservation of Mass
A common perception is that matter disappears when something burns, but this contradicts the Law of Conservation of Mass. This law states that atoms are neither created nor destroyed during a chemical reaction; they are only rearranged. The total number of atoms present before and after the fire remains exactly the same. The apparent loss of mass from a burning log is merely a change in the state of the matter involved. Most of the starting material’s mass transforms into the invisible gases of carbon dioxide and water vapor, which disperse into the atmosphere. If one captured and weighed all the gaseous products, ash, and unreacted oxygen, the total mass would equal the combined mass of the original fuel and the oxygen consumed.