When a candle burns, the process involves both a chemical and a physical change happening simultaneously. This common household object illustrates the distinction between a physical change, which alters form but not substance, and a chemical change, which creates entirely new substances. The wax, which serves as the fuel, must first undergo a series of transformations before it can be consumed.
The Physical Transformation: Wax Melting
A physical change occurs when a substance alters its state or form but retains its original chemical identity. When a candle is lit, the heat from the flame causes the solid wax near the wick to transition into a liquid, forming a small pool. This melting is a physical change because the liquid wax is still chemically the same paraffin or hydrocarbon mixture as the solid candle.
The liquid wax is then drawn upward against gravity through the woven fibers of the wick in a process called capillary action. The wick pulls the molten fuel toward the heat source. Once the liquid wax reaches the hottest part of the wick, it absorbs enough energy to vaporize, changing from a liquid state to a gaseous state.
Vaporization is also a physical change, as the wax molecules are simply separating into a gas phase without breaking their internal chemical bonds. If the flame were extinguished, the liquid wax would cool and solidify, and the wax vapor would condense, demonstrating that these initial changes are reversible. This physical process of melting and vaporizing is necessary to prepare the fuel for the actual burning.
The Chemical Transformation: Combustion
The true “burning” of the candle is categorized as a chemical change because it results in the formation of entirely new chemical substances. This process is called combustion, a rapid reaction between the wax vapor and oxygen from the surrounding air. Candle wax, being a hydrocarbon, reacts vigorously with oxygen when subjected to high heat.
The chemical reaction breaks down the large hydrocarbon molecules into new, smaller molecules. The primary outputs of this combustion are water vapor and carbon dioxide, along with the release of energy in the form of heat and light. Small amounts of carbon, often visible as soot, can also be formed if the combustion is incomplete.
This chemical transformation is irreversible; the original wax is permanently consumed and converted into other materials. The continuous production of heat and light from the flame is a clear indication that a chemical reaction is taking place.
Why Both Changes Occur Simultaneously
The physical and chemical changes in a burning candle are mutually dependent processes occurring simultaneously. The physical change of the wax vaporizing must happen before the chemical change of combustion can take place. The flame burns the gaseous wax vapor, not the solid wax.
The chemical reaction, which generates the heat and light, maintains the physical processes that supply the fuel. The energy released by combustion melts the next layer of solid wax, sustaining the liquid pool and the capillary action that draws the fuel up the wick. This creates a self-perpetuating system where the heat from the chemical change drives the physical changes, which in turn feed the chemical reaction.
The candle functions as a dynamic illustration of matter transformation. This constant interplay between the two types of change allows the candle to burn continuously until its fuel source is exhausted.