Observing a candle burn often leads to a curious question: where does the wax go as the candle gets smaller? The solid wax seems to vanish, leaving little residue. This phenomenon is not a magical disappearance but rather a series of physical and chemical transformations that convert the solid wax into invisible gases. Understanding this process reveals the intricate science at work within a simple candle flame.
The Wax’s Path to the Flame
For a candle to burn, the solid wax must first undergo a physical change. When the wick is lit, the heat from the initial flame melts the wax at the base of the wick, creating a small pool of liquid wax. This melting is essential, as solid wax does not burn directly. The wax, typically composed of hydrocarbons, needs to be in a liquid state to be drawn into the flame.
Once melted, this liquid wax is drawn upwards through the wick, which utilizes capillary action. This is the ability of a liquid to flow in narrow spaces, even against gravity, due to the attractive forces between the liquid molecules and the wick material.
As the liquid wax ascends the wick, it moves closer to the flame’s heat. This movement ensures a steady fuel supply. The rate at which the wax melts and is drawn up the wick is balanced by the rate at which it is consumed, allowing the flame to maintain a consistent size and shape.
The Science of Disappearance
Upon reaching the flame, the liquid wax undergoes its most significant transformation. The intense heat causes the molten wax to vaporize, turning it into a hot gas. It is this wax vapor, not the liquid or solid wax, that actually burns.
This gaseous wax, primarily made of hydrogen and carbon atoms (hydrocarbons), mixes with oxygen from the surrounding air. This mixture undergoes a chemical reaction known as combustion. During complete combustion, the hydrocarbon molecules break apart and react with oxygen, releasing energy as heat and light, which is what we perceive as the candle flame.
The primary products of this complete combustion are carbon dioxide (CO2) and water vapor (H2O). These are invisible gases that disperse into the air, explaining why the wax appears to vanish. While complete combustion is the goal, incomplete combustion can occur if the oxygen supply is limited or the flame is disturbed. In such instances, small particles of unburned carbon (soot) may be released, along with trace amounts of carbon monoxide.