Lighting a candle initiates a complex series of physical and chemical transformations that convert solid wax into light and heat. Candle wax is primarily composed of hydrocarbons, which are molecules made of long chains of hydrogen and carbon atoms.
From Solid Wax to Gaseous Fuel
The initial heat source, like a match or a lighter, must first provide the activation energy needed to begin the reaction. This heat melts the solid wax near the wick, creating a small pool of liquid fuel. The liquid wax is then drawn upward through the wick’s fibers by a process called capillary action. Once the liquid wax reaches the hottest part of the flame, it is exposed to temperatures high enough to cause vaporization, turning the liquid into a hot gas. This gaseous wax is the true fuel that combusts; the solid wax itself does not burn.
The Chemistry of Combustion
The burning itself is a rapid chemical reaction known as combustion, which is essentially an oxidation process. The gaseous hydrocarbon molecules react vigorously with oxygen molecules from the surrounding air. This reaction breaks the strong chemical bonds within the wax molecules, combining the liberated hydrogen and carbon atoms with oxygen. For example, a typical wax molecule like Câ‚‚â‚…Hâ‚…â‚‚ reacts with 38 molecules of oxygen to produce 25 molecules of carbon dioxide and 26 molecules of water. This reorganization of atoms results in a significant release of stored chemical potential energy.
Byproducts and Energy Release
The combustion reaction yields several products, with the primary ones being carbon dioxide and water vapor. The reaction is also highly exothermic, meaning it releases a large amount of energy in the form of heat and light.
The visible flame is composed of distinct zones, each with varying temperatures and oxygen concentrations. At the base, a small blue zone exists where the hydrocarbons first break apart and react with sufficient oxygen, leading to a relatively clean burn.
The bright yellow light that characterizes a candle flame comes from the incandescence of tiny, solid carbon particles, commonly called soot. These soot particles are formed in the flame’s central, oxygen-starved region; they become white-hot from the heat before they are completely combusted further up in the flame.
If the combustion process is disturbed or if the oxygen supply is limited, such as when a candle flickers, not all of the carbon particles burn completely. These uncombusted carbon particles escape the flame, becoming visible as a wisp of smoke. Approximately one-quarter of the energy released from the chemical reaction radiates outward as heat, while the rest is emitted as light and other forms of energy.