The visible disappearance of a burning object often leads people to assume that fire destroys matter. This intuitive conclusion contradicts a fundamental law of physics. When a log burns down to ash, the original material has not been eliminated. The scientific answer is no; the apparent loss is merely a profound transformation of state.
The Law of Conservation of Mass
The Law of Conservation of Mass governs all chemical reactions, including fire. This law states that matter can change its form but can never be created or destroyed within a closed system. The total mass of all substances present before a chemical change must equal the total mass of all substances remaining afterward.
In essence, the atoms that make up the initial materials are simply reorganized into new molecules. Consider an ice cube melting in a sealed container. The solid water converts into liquid water, and the total mass remains the same. Fire operates under this same principle, involving a dramatic chemical reaction instead of a simple phase change. The mass of the fuel and the oxygen consumed is accounted for in the mass of the resulting smoke, ash, and gases.
The Chemistry of Combustion
Combustion, the process we call fire, is a rapid chemical reaction requiring fuel and an oxidizer, typically oxygen from the air. Fuel, such as wood or paper, is composed primarily of compounds containing carbon and hydrogen atoms. When the fuel reaches its ignition temperature, the heat breaks the existing molecular bonds.
As the bonds break, the liberated carbon and hydrogen atoms quickly bond with oxygen atoms in the surrounding air. This atomic rearrangement produces new molecules. The reaction releases energy, which we perceive as the heat and light of the flame. This energy release makes the reaction self-sustaining once it begins.
Atoms do not vanish during this process. A carbon atom from the fuel remains a carbon atom, and a hydrogen atom remains a hydrogen atom. They simply form new, stable compounds by linking with oxygen. The reaction continues until either the fuel or the oxygen supply is exhausted.
Tracking the Byproducts of Fire
The matter from the original fuel is dispersed into several final products, most of which are invisible. The most significant byproduct by mass is carbon dioxide, a colorless and odorless gas resulting from the combination of carbon atoms with oxygen. Hydrogen atoms in the fuel combine with oxygen to form water vapor, which also dissipates invisibly into the atmosphere.
The small amount of material left behind is the solid residue known as ash. Ash consists mainly of non-combustible minerals, such as calcium and potassium, that were contained in the original fuel. This ash mass is a tiny fraction of the original material because the vast majority of the matter has been converted into gases.
If it were possible to collect and measure the total mass of the gaseous products, the water vapor, and the ash, their combined mass would exactly equal the mass of the original fuel plus the mass of all the oxygen that was consumed from the air. The apparent disappearance of the object is simply the matter changing from a visible, solid form to an invisible, gaseous form.