When you light a candle, the wax slowly melts and seems to disappear. You may wonder what happens to the candle wax as the candle burns down.
How a Candle Burns Wax
When you burn a candle, it changes the physical state of the wax from a solid to a liquid to a gas. The basic burning of a candle happens as follows.
Flame Creates Heat
Candle wax starts as a solid. Lighting the candle wick initiates the physical change to the candle wax. When you light the wick, the flame generates heat.
Heat Melts the Wax
As the heat from the flame continues, it begins to melt the wax surrounding the flame. In chemistry terms, the melting of the wax is a physical change that turns a solid to a liquid. As the heat continues and more wax melts, the liquid wax pooled around the wick slowly increases in volume and generates more thermal mass (heat capacity). In other words, it creates more hot stuff, which further melts even more wax.
Melted Wax Fuels the Flame
The melted wax, now in liquid form, is drawn up through the wick. This fuels the flame to keep it burning steadily, which generates more heat to melt more wax. The melted wax is also hot, which causes even more wax to melt. As more and more solid wax melts and turns into liquid wax, the wick is completely saturated with a steady absorption of liquid wax. This creates a continuous cycle of heat, melted wax, and liquid wax absorption through the wick to fuel the flame.
This process is called capillary action, or, "The movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid." This means the heat shakes up the molecules while the flame attracts the liquid to be absorbed by the wick.
Candle Flame Vaporizes the Wax
With the wax in its liquid state fueling the flame, the heat rises and another physical change takes place as the heat from the flame and melted wax brings small amounts of wax to the boiling point. When it boils, another physical change takes place - vaporization. When this happens, the wax moves from a solid state to a gas state. The heated gas then begins to break down into hydrogen and carbon.
In addition to the hydrogen and carbon being produced, the burning process of the candle creates water. When the air surrounding the candle is heated, it sets up an evaporation process of the water particles released during the melting of the candle wax. This process makes the immediate air around the flame very dry as the moisture from the candle is evaporated.
The result of these changes in the physical state of the wax (caused by heat) is that the candle wax seems to disappear as the candle burns.
Factors That Affect Wax Vaporization
Many variables affect wax vaporization which, in turn, affects burn time. Soy wax and beeswax often have the longest burn times. However, there are other factors that affect the process that can slow vaporization and make candle wax burn longer.
- The ambient temperature where you're burning the candle affects the burn time. Hotter temperatures will speed it up, while cooler temperatures will slow it down.
- Candle additives, such as fragrances, stearic acid, tallow acid, type of wick, and even the candle holder/container are contributing factors to the burn time and how quickly the wax burns and evaporates.
- The wick being either too long or too short to burn the fuel efficiently can also affect how quickly the wax vaporizes.
What Happens to Candle Wax?
When a candle burns, it may appear as though the wax disappears into thin air. It doesn't. The wax evaporates into the atmosphere as the reaction to heat and flame turns it from a solid, to a liquid, to a gas. The candle uses the wax as fuel to keep the flame burning, and when the fuel (wax) has completely evaporated, it will no longer burn.