What Happens When Wax In The Form Of Wax Candle Is Burned?
When a candle burns, it undergoes a complex process that involves both physical and chemical changes. At a basic level, the candle wax melts and vaporizes, the wax vapors combust, and heat and light are produced. The combustion reaction requires oxygen and produces carbon dioxide and water vapor as byproducts. However, many additional processes are occurring that allow the candle to keep burning and determine properties like the flame shape, temperature, and brightness.
Melting Point
Candle wax does not have a single melting point, as it depends on the type of wax used in candle making. According to Nikura (https://nikura.com/blogs/discover/what-temperature-does-wax-melt), paraffin wax melts between 47-65°C while soy wax melts between 48-60°C. Beeswax has a melting point of 62-65°C. The melting point determines the ideal temperature to melt the wax for candle making without burning or scorching it. Heating wax above its melting point allows it to liquefy so it can be poured into candle containers and molded into the desired shape.
Wick
The wick is the piece that goes through the center of a candle. The main purpose of a wick is to provide a continuous surface that pulls wax up above the rest of the candle via capillary action and then burns to help melt more wax (https://candles.org/elements-of-a-candle/wicks/). This capillary action enables the melted wax to travel up the wick where it can be burned and produce a flame.
Wicks are typically made of braided cotton, but can also be crafted from paper, wood, or blends of natural and synthetic fibers (https://smellscandle.com/blogs/news/what-are-candle-wicks-made-of-everything-you-need-to-know). The material needs to be sturdy enough to stand upright when embedded in wax, while also being absorbent so that liquid wax can travel up the fibers. Cotton is commonly used because it meets both criteria well.
The thickness of the wick also matters – a wick that is too thin may not provide enough fuel to the flame, while one that is too thick can create excess soot and smoke. Wick size should be matched to the diameter of the candle. In addition, some wicks have a core material that helps maintain structure when exposed to heat.
Capillary Action
Capillary action is the phenomenon that allows the melted wax to be drawn up the wick as the candle burns. It occurs due to the interaction between the liquid wax, the solid wick, and the air around them. The wick is made up of many fine strands that can absorb liquid via capillary action. Capillary action describes the ability of a liquid to flow against gravity in narrow spaces, which is why it allows the melted wax to travel upwards through the wick.
As the candle burns, the flame melts the wax near the wick into a liquid state. The liquid wax is then absorbed into the spaces between the strands of the wick through capillary action. The small diameter of the strands creates enough surface tension for the liquid to be pulled upwards. The wax continues traveling up the wick through capillary action as long as the flame is burning and melting wax near the wick base. The wax gets vaporized at the flame as it reaches the top, enabling continuous capillary flow.
An everyday example of capillary action is paper towels absorbing spills – the liquid travels through the gaps in the paper towel fibers against gravity. Without capillary action drawing the wax up the wick, the candle would not be able to burn once the initial pool of melted wax is depleted.
Sources:
https://www.cbsnews.com/pittsburgh/news/hey-ray-candles-and-capillary-action/
Combustion
The combustion of a candle happens in a few key steps. First, the heat from the flame melts the solid wax into a liquid form. This liquid wax is then drawn up the wick by capillary action (see Candle Science). As the liquid wax reaches the flame, it vaporizes into a hot gas due to the high temperature. This wax vapor then mixes with oxygen and reacts in an exothermic chemical reaction known as combustion.
Specifically, the hydrocarbons in the wax react with oxygen to produce carbon dioxide, water vapor, light, and heat. The fuel source is the wax vapor, while the oxygen source is the air around the flame. This chemical reaction releases energy in the form of heat and light (ThoughtCo). The heat and wax vapor keep the combustion reaction going as long as there is fuel and oxygen present.
Heat Transfer
The primary method of heat transfer that occurs in a burning candle is conduction. The wick of the candle absorbs heat from the flame through conduction, which then transfers down and melts the solid wax (Analysis of Burning Candle). The liquefied wax moves up the wick via capillary action, where it is vaporized by the flame. Radiative heat transfer also occurs between the flame and surrounding areas. This radiative heating warms the air around the flame, resulting in convection currents. However, conduction through the wick is the main mechanism for melting and vaporizing the wax.
Convection Currents
As the candle flame burns, it heats the surrounding air. This causes the hot air to become less dense and rise. The movement of the hot air upwards creates convection currents. Cooler, denser air then moves in to replace the rising hot air, creating a continuous circulation of air movement known as convection currents. According to a NASA report, “Candle Flames,” convection currents deliver lots of heat to the candle which causes more rapid melting than occurs with small candle flames1. These convection currents ensure a steady supply of oxygen is delivered to the flame so it can keep burning.
The convection currents generated by the candle flame allow oxygen to be continuously delivered to the flame so it does not go out. The currents also transport heat from the flame into the surrounding environment to melt more of the wax.
Soot
When a candle burns, some of the wax vapor produced does not combust completely. This incomplete combustion results in soot, which is composed primarily of carbon (Pagels et al. 2009). Soot particles are very small, often less than 50 nm in diameter, allowing them to stay airborne (Verdugo et al. 2020). As the candle continues to burn, these tiny carbon particles accumulate into larger clumps of soot. The black color of soot comes from the way it absorbs light. The soot eventually rises with the hot air and can deposit on surfaces above the flame.
Wax Vapor
As the candle wax melts and reaches its boiling point, some of the liquid wax evaporates into a gas or vapor. This wax vapor is composed primarily of hydrocarbon molecules like paraffins, which are found in paraffin wax candles. The vaporized paraffin molecules have a low molecular weight and are highly volatile, meaning they evaporate readily at normal temperatures.
According to the Candle Science website, “These vaporized molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapor (H2O) and carbon dioxide (CO2). This chemical reaction, called combustion, allows the candle to keep burning.”
So in summary, the main gaseous compounds released when wax vapor burns are water vapor and carbon dioxide. The vaporization and combustion of wax is what sustains the candle flame and allows it to continue burning.
Conclusion
When a candle is lit, the flame melts the solid wax near the wick and turns it into a liquid. The wick then absorbs this liquid wax through capillary action and draws it up to the flame. The heat of the flame vaporizes the liquid wax, breaking down the hydrocarbons into simpler molecules like carbon and hydrogen. These vapors rise and react with oxygen in the air around the flame, undergoing combustion to produce heat, light, carbon dioxide, and water vapor. The heat released sustains the combustion reaction and melts more of the solid wax to continue the process. As the wax burns, the flame vaporizes more wax toward the edges, leaving a melted wax pool. The candle continues burning until all the wax is used up or the flame is blown out.
