The burning of a candle appears to make its wax vanish. However, the solid wax undergoes a process where it changes form and composition. Understanding this phenomenon involves examining how the candle operates, the chemical reactions that occur, and the substances that result from the burn.
How a Candle Prepares to Burn
When a candle is lit, the heat from the flame melts the solid wax closest to the wick, creating a small pool of liquid wax. This liquid wax is then drawn upward into the wick through a physical process known as capillary action. The wick, often made of braided cotton, draws up the melted wax, similar to how a plant draws water from the soil.
The wax itself, typically paraffin, is a hydrocarbon, meaning it is composed primarily of hydrogen and carbon atoms. Paraffin wax is solid at room temperature, with a melting point generally ranging between 46 and 68 degrees Celsius (115 and 154 degrees Fahrenheit). The continuous melting and drawing up of liquid wax provides a steady supply of fuel to the flame.
The Chemical Transformation of Wax
Once the liquid wax reaches the flame, the intense heat causes it to vaporize. This wax vapor then undergoes a chemical reaction with oxygen from the surrounding air. This reaction is known as combustion, a process that releases both heat and light, visible as the candle’s flame.
During combustion, the hydrocarbon molecules of the wax break apart. Their hydrogen and carbon atoms then combine with oxygen atoms from the air. This transformation converts the wax into new chemical substances. The flame itself is essentially burning wax vapor, not the liquid wax or the wick directly.
What Remains After the Burn
The primary products of the complete combustion of candle wax are invisible gases: carbon dioxide (CO2) and water vapor (H2O). The water vapor may sometimes be observed as condensation on cool surfaces near a burning candle. This conversion into gaseous forms explains why the solid wax mass diminishes as the candle burns.
However, combustion is not always complete. In instances where there is insufficient oxygen or the flame is disturbed, incomplete combustion can occur. This process can produce visible soot, which consists of tiny unburned carbon particles. For scented candles, the fragrance compounds, which are volatile organic compounds, also vaporize and disperse into the air along with the combustion products, creating the candle’s aroma.