When a candle burns, the light and heat result from a continuous chemical reaction known as combustion. This process is a sustained interaction between two primary substances: the fuel and the oxidizing agent. Understanding this reaction reveals the fundamental science behind one of the oldest forms of artificial light.
Identifying the Fuel and the Oxidizer
The two substances that react when a candle burns are the vaporized candle wax and the oxygen present in the surrounding air. Candle wax, most commonly paraffin, is composed of long-chain hydrocarbon molecules containing only carbon and hydrogen atoms. These hydrocarbons serve as the fuel.
Oxygen (\(\text{O}_2\)), a component of the atmosphere, acts as the oxidizer, combining with the fuel to release energy. The reaction requires a constant supply of oxygen to maintain the flame. If the oxygen source is removed, combustion immediately stops.
The Steps of Combustion
The solid wax cannot burn directly, so the heat from the flame initiates phase changes to prepare the fuel. The initial heat from lighting the wick melts the solid wax near the wick’s base, creating a pool of liquid fuel. This liquid wax then travels upward through the porous structure of the wick via capillary action.
Once the liquid wax reaches the hottest part of the wick, the intense heat vaporizes it, turning it into a gaseous fuel. This wax vapor combusts, not the solid or liquid wax. The gaseous wax molecules mix with the surrounding oxygen in the flame zone, and the chemical reaction begins.
The heat released by this combustion reaction sustains the entire process. This newly generated heat radiates back downward, melting more solid wax and continuing the cycle of melting, wicking, and vaporization. The entire system is a self-sustaining loop that continues until the fuel is exhausted or the oxygen is depleted.
What the Reaction Produces
The complete combustion of hydrocarbon wax with oxygen yields two primary compounds: carbon dioxide (\(\text{CO}_2\)) and water vapor (\(\text{H}_2\text{O}\)). The reaction rearranges the carbon and hydrogen atoms from the wax and the oxygen atoms from the air into these stable molecules.
This chemical change releases stored chemical energy in the form of heat and light. The heat keeps the reaction going, while the light makes the candle useful as an illuminant. If the oxygen supply is limited, the reaction becomes incomplete, which can produce uncombusted carbon particles, commonly seen as soot.