Candles are a common sight, yet the process by which they produce light and heat involves a scientific transformation. The wax, primarily composed of hydrocarbons, serves as the fuel for this sustained reaction. When a candle is lit, the energy converts the solid wax into gaseous products that power the flame. Understanding this process requires examining the physical and chemical changes the wax undergoes.
The Physical Transformation: Melting and Wicking
The burning process begins when heat causes the solid wax immediately surrounding the wick to melt. This change of state from a solid to a liquid is a physical transformation. The resulting pool of molten wax acts as a reservoir of liquid fuel for the flame.
This liquid wax is then transported upward through the interwoven fibers of the wick via capillary action. This mechanism draws the fuel upwards. The movement is comparable to water being absorbed by a sponge, ensuring a continuous supply reaches the base of the flame. This steady flow of liquid wax is necessary to maintain the combustion process.
The Chemical Transformation: Vaporization and Combustion
Once the liquid wax reaches the top of the wick, the intense heat of the flame causes it to undergo a second physical change: vaporization. The liquid wax is converted into a hot, invisible gaseous state, which is the true fuel that burns. The wax vapor consists of hydrocarbon molecules that must be gaseous to react with the surrounding air.
The actual burning is a chemical reaction known as combustion, where the vaporized hydrocarbon molecules rapidly combine with oxygen from the air. This exothermic reaction breaks the chemical bonds in the wax, creating new molecules and releasing energy in the form of heat and light. The flame exhibits distinct zones that reflect the stages of this chemical process. The inner, non-luminous blue zone near the wick is where the wax vaporizes and begins to break down.
The larger, luminous yellow area generates the majority of the light. Here, tiny, hot solid carbon particles, formed from the incomplete breakdown of hydrocarbons, are heated to incandescence, causing them to glow brightly before they are fully consumed. This energy sustains the entire cycle, as heat produced radiates downward to melt and vaporize the next batch of solid wax.
The Final Products of the Reaction
When the burning process is stable and has a sufficient supply of oxygen, the wax undergoes complete combustion. The main products of this chemical reaction are two invisible gases: carbon dioxide (CO2) and water vapor (H2O). These gaseous end products dissipate into the air, which is why a properly burning candle leaves no ash or solid residue.
If the oxygen supply is insufficient, combustion becomes incomplete. The hydrocarbon molecules do not fully oxidize, leading to the formation of visible soot, which consists of unburned carbon particles. Incomplete combustion can also produce small amounts of carbon monoxide (CO), a colorless and odorless gas.