The sight of a burning candle is common, yet the process by which a solid stick of wax slowly shrinks is a sophisticated display of physics and chemistry. A candle is fundamentally a simple system, designed to provide light and heat by consuming the solid fuel source, the wax. This consumption is a continuous, self-sustaining cycle where the heat generated feeds the next stage of the process. The observable change in size—the candle getting smaller—is the physical evidence of the fuel being converted into invisible forms. Understanding this transformation requires looking closely at how the components work together to prepare the fuel and sustain the reaction.
How the Wick Prepares the Fuel
The candle’s solid wax, typically made of long hydrocarbon chains, cannot burn directly in its solid form. The heat from the initial lighting of the wick quickly establishes a small pool of liquid wax at the base of the flame. This melted wax is then drawn upward into the wick’s fibers through a physical process known as capillary action. Just as a sponge absorbs water, the intertwined strands of the cotton wick pull the liquid fuel against gravity toward the heat source.
This continuous upward movement of liquid wax supplies the flame with fuel. The melt pool acts as a reservoir, ensuring a steady supply of liquid wax is available for the wick to draw upon. The wick’s purpose is to transform the bulk solid wax into a manageable stream of liquid fuel. Under normal conditions, this physical process keeps the flame fed.
The Chemistry of Candle Combustion
When the liquid wax is drawn up the wick and reaches the hottest part of the flame, it must first undergo a second physical change: vaporization. The intense heat converts the liquid wax into a gaseous vapor, and it is this vaporized fuel that mixes with the oxygen in the surrounding air. The burning of a candle is not the liquid wax itself igniting, but rather the rapid chemical reaction between the wax vapor and atmospheric oxygen.
This chemical process is an example of combustion, where the hydrocarbon molecules of the wax are broken apart and oxidized. The reaction releases energy in the form of light and heat, which is what we perceive as the flame. The heat produced is significant—about one-fourth of the total energy radiates outward, a portion of which is directed back down to the solid wax to maintain the melt pool and continue the cycle. This self-sustaining loop of melting, wicking, vaporizing, and burning ensures the candle keeps shrinking until the fuel is exhausted.
Why Mass Disappears
The reason a candle visibly shrinks and disappears is directly linked to the chemical products of combustion. The wax, being a hydrocarbon, is composed primarily of carbon and hydrogen atoms. When the wax vapor reacts with oxygen, the carbon atoms combine with oxygen to form carbon dioxide (CO2), and the hydrogen atoms combine with oxygen to form water vapor (H2O).
Both carbon dioxide and water vapor are colorless gases that float away and mix invisibly with the air. The solid wax mass is entirely converted from a solid state into these two gaseous products. The apparent disappearance of the candle is simply a change in form, not a loss of matter, aligning with the principle of conservation of mass.