A candle is a deceptively simple, self-sustaining combustion system. Many people believe the solid wax itself is the fuel that burns, but this is a scientific misunderstanding. The wax, typically a blend of hydrocarbon molecules, must undergo physical and chemical transformations before it can ignite. Understanding the process reveals the science required to maintain a continuous fire.
The Transformation from Solid to Liquid
The burning process begins with a physical change: the melting of the solid fuel. When heat is applied to the wick, the localized heat melts the solid wax immediately surrounding the base of the flame. This forms a small pool of molten wax, preparing the fuel for combustion.
The wick, often made of braided cotton fibers, acts as a sophisticated delivery system for this liquid fuel. Capillary action draws the molten wax upward against gravity, much like a sponge absorbs water. This phenomenon relies on the adhesive forces between the liquid wax molecules and the fine fibers of the wick, pulling the fuel to the hottest part of the system.
This transport mechanism is necessary because solid wax cannot be combusted directly. The heat from the flame maintains the liquid pool, ensuring a continuous supply of fuel. The wick’s design and material are chosen to manage this flow, delivering the liquid wax required to sustain the flame.
Vaporization: The True Candle Fuel
The liquid wax travels up the wick and reaches the zone of intense heat at the flame’s base. It is at this point that the second physical transformation, vaporization, occurs. The high temperature converts the liquid wax into an invisible gaseous state known as wax vapor. This vapor is the actual fuel source that burns in a candle.
For a substance to burn, it must first be in a gaseous state so its molecules can readily mix with oxygen in the air. The wax is composed of long chains of hydrocarbons, which are molecules made of hydrogen and carbon atoms. Once vaporized, these gas molecules break apart and combine with oxygen, initiating the chemical reaction of fire.
If you blow out a candle, the stream of white smoke you see is this wax vapor condensing back into tiny, visible solid particles. If a lit match is quickly passed through this stream of vapor, the candle can be re-ignited without touching the wick. This proves that the gas, not the solid or liquid, is the combustible material.
Combustion and Flame Structure
The moment the wax vapor mixes with oxygen, the chemical reaction known as combustion begins. This high-speed oxidation process releases energy in the form of heat and light. The primary products of complete combustion are carbon dioxide (CO2) and water vapor (H2O), which rise into the air.
The visible flame is not uniform, but a structure composed of distinct zones that reflect different stages of the reaction. At the base is a small, non-luminous blue zone. This area has the highest concentration of oxygen, where complete combustion occurs. This results in the highest temperatures, which can reach approximately 1400 degrees Celsius at the outer edge.
Above this is the large, bright yellow-orange zone that provides the candle’s characteristic light. In this region, less oxygen is available, leading to incomplete combustion of the hydrocarbon molecules. This causes tiny soot particles (pure carbon) to form. These carbon particles become incandescent due to the heat, glowing brightly to produce visible light before they are mostly consumed near the flame’s outer edge. The teardrop shape of the flame is maintained by convection currents, as the hot gases rise and draw in cooler, oxygen-rich air at the base.