Ben Carter
Candles have been an object of fascination for scientists for hundreds of years. The light and heat produced by a candle are the result of a chemical reaction known as combustion. This combustion is caused by the conversion of hydrocarbons (molecules based on hydrogen and carbon atoms) in wax into carbon dioxide and water vapour. The heat produced by a candle radiates in all directions, and enough heat is created to melt the wax and keep the combustion process going. This process continues until the fuel is used up or the heat source is eliminated.
| Characteristics | Values |
|---|---|
| Chemical energy | The energy transforms from chemical energy to heat and light energy through combustion |
| Thermal energy | Approximately one-fourth of the energy created by a candle's combustion is given off as heat |
| Light energy | The light a candle makes comes from combustion, a chemical reaction in which wax reacts with oxygen to make carbon dioxide and water vapour |
| Conduction | Carries heat down the wick to melt wax at the top of the candle |
| Convection | Draws hot wax vapours out from the wick and sucks oxygen from the surrounding air into the base of the flame |
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What You'll Learn
A candle's combustion process
The combustion process of a candle involves several chemical and physical reactions that transform chemical energy into heat and light energy. Firstly, the heat from the flame melts the wax near the wick of the candle. This process is facilitated by the wick, which draws the liquid wax upwards through capillary action. Once the wax is in a liquid state, it is then vaporized by the heat of the flame, turning it into a hot gas.
At this stage, the hydrocarbon molecules present in the wax begin to break down into hydrogen and carbon atoms. The hydrogen atoms react with oxygen from the surrounding air to form water vapour, while some of the carbon burns to form carbon dioxide. The blue base of the flame is where the oxygen-rich environment enables the breakdown of hydrocarbons and the formation of water vapour and carbon dioxide.
As the vapour rises, it enters the dark orange-brown section of the flame, which has relatively less oxygen. Here, the various forms of carbon continue to break down, and small, hardened carbon particles, or soot, begin to form. These soot particles are heated to approximately 1000 degrees Centigrade as they rise further into the flame.
In the yellow zone of the flame, the formation of soot particles increases. The yellow colour is due to the hot soot particles emitting black-body radiation. The flame may also exhibit other colours due to transient reaction intermediates during combustion, such as the Methylidyne radical (CH) and Diatomic carbon (C2). These molecules emit blue and green visible light through spectral band emission.
The heat generated by the combustion process radiates in all directions, melting more wax to sustain the process. This cycle continues until the fuel (wax) is depleted or the heat source is removed. The combustion process of a candle is a self-perpetuating cycle that converts chemical energy into thermal energy, producing light and heat as by-products.
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The role of the wick
The role of a candle's wick is critical to the candle's function. The primary purpose of the wick is to deliver fuel, typically in the form of liquefied wax, to the flame. This process, known as "wicking," relies on capillary action, where the wick acts as a fuel pump, drawing the melted wax upwards towards the flame. The size of the wick is essential, as it determines the amount of fuel that reaches the flame. A larger wick will result in more fuel being drawn up, while a smaller wick will supply less fuel. Therefore, choosing the right wick size is crucial to achieving a steady and clean burn.
The design of the wick also influences the burning characteristics of a candle. Different types of wicks include twisted, flat, square, and cored wicks. Twisted wicks, often found in birthday candles, burn faster due to their loose construction, allowing more fuel to reach the flame quickly. Flat wicks, commonly used in taper and pillar candles, are typically braided or knitted and provide a consistent burn. They are designed to curl into the flame, making them self-trimming. Square wicks, on the other hand, are more robust and are preferred for beeswax candles to prevent wick clogging.
Cored wicks are designed with a core material to keep them straight and upright while burning. Common core materials include cotton, paper, zinc, or tin. These wicks are often used in jar candles, pillars, and devotional lights. Another type of wick is the ECO series, which is a flat, coreless cotton wick braided with thin paper filaments. This design improves rigidity and burn stability while reducing smoke and "afterglow."
The choice of wick material also plays a crucial role in the burning process. Most wicks are made from braided, plaited, or knitted fibers, usually cotton, to encourage a slow and consistent burn. Wooden wicks have gained popularity for their aesthetic appeal and the soft crackling sound they produce. Additionally, specialty wicks, such as those made from asbestos or plant materials, have been used historically and in specific applications.
The diameter, stiffness, fire resistance, and tethering of the wick are also important characteristics. A larger diameter wick will generally result in a larger flame and faster-burning candle. Stiffeners, such as fine wire or synthetic fibers, are used to keep the wick upright and improve fuel delivery to the flame. Tethering the wick, as seen in tealights, prevents the wick from floating on top of the molten wax and burning prematurely.
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The science of candle flames
Candles have long been a source of fascination for scientists, with the famous physicist Michael Faraday delivering a lecture series on the "Chemical History of a Candle" as far back as 1860. Candle flames remain a topic of scientific interest, with ongoing experiments conducted by scientists and students alike to understand the underlying principles of heat, light, and combustion.
A candle flame is not just a single colour, but rather consists of distinct regions of blue, dark orange-brown, and yellow. The blue zone at the base of the flame is rich in oxygen and serves as the site where hydrocarbon molecules from the wax vaporize and break down into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to produce carbon dioxide.
In the orange-brown region, there is a scarcity of oxygen, and the various forms of carbon continue to break down, leading to the formation of hardened carbon particles. These particles, along with the water vapour and carbon dioxide from the blue zone, are heated to a temperature of approximately 1000°C.
The bottom of the yellow zone experiences an increase in the formation of carbon soot particles. This region is where combustion is incomplete, resulting in the production of smoke. The smoke consists of tiny particles of solid, unburned carbon that escape as a wisp when the flame flickers or flares due to an imbalance in air or fuel supply.
The heat generated by the candle flame plays a crucial role in sustaining the combustion process. Approximately one-fourth of the energy created by the candle is radiated as heat in all directions. This heat melts the wax, which is then drawn up through the wick by capillary action. The wax vapours are released through convection, and oxygen is pulled in from the surrounding air to support combustion. The candle continues to burn until all the wax is consumed or the heat source is removed.
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How wax burns
A candle is indeed a form of thermal energy. When a candle burns, it produces heat and light through a process of combustion. This combustion is a chemical reaction that involves the conversion of chemical energy into heat and light energy.
Wax is a crucial component of candles, and its burning process involves several steps and transformations. Firstly, the heat from the flame melts the solid wax, causing it to become liquid. This liquid wax is then drawn up through the wick by capillary action. The wick plays a vital role in delivering the molten wax to the flame and providing structure to the candle.
As the liquid wax rises through the wick, it reaches the flame, where it vaporizes and combusts. This vaporization occurs due to the high temperature of the flame, which causes the liquid wax to transform into a gaseous state. At this point, the wax molecules, which are primarily hydrocarbons, start to break apart into hydrogen and carbon atoms.
In the presence of oxygen, the hydrogen atoms react with oxygen to form water vapour. Simultaneously, some of the carbon atoms burn to form carbon dioxide. This combustion process releases heat, which radiates in all directions, contributing to the overall thermal energy produced by the candle.
The combustion of wax is not always complete, and there may be incompletely combusted organic compounds present. These compounds contribute to the scent of the candle and can also result in the production of soot, which is essentially unburned carbon particles. The colour of the candle flame, typically yellow, is due to the presence of soot, which forms in the lower temperature regions of the flame.
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The different zones of a candle flame
A candle flame has three main parts, each with distinct characteristics and temperatures: the dark inner zone, the bright yellow middle zone, and the blue outer zone.
The blue zone, located at the base of the flame, is the coolest part, with a temperature of about 800°C. This zone is rich in oxygen, and it is here that hydrocarbon molecules vaporize and begin to break apart into hydrogen and carbon atoms. The hydrogen separates first, reacting with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide.
Above the blue zone is a small dark orange-brown section, with relatively little oxygen. Here, the various forms of carbon continue to break down, and small, hardened carbon particles (soot) begin to form. As they rise, along with the water vapour and carbon dioxide from the blue zone, they are heated to approximately 1000°C.
The yellow zone is the middle zone of the flame, created mainly by the incomplete combustion of wax. This luminous zone has a temperature of about 1200°C and is responsible for generating ashes and soot. The yellow zone is the most luminous part of the flame due to the depletion of oxygen.
The outermost zone of the flame is the hottest part, with a temperature of about 1400°C. This part of the flame is non-luminous and is created by the burning of wax.
The entire candle burning process is exothermic, with approximately one-fourth of the energy created by the combustion given off as heat radiating from the flame in all directions. This heat is enough to melt more wax and keep the combustion process going until the fuel is used up or the heat source is eliminated.
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Frequently asked questions
Yes, a candle is a form of thermal energy. A candle converts the hydrocarbons in wax into carbon dioxide and water through a chemical reaction called combustion, which produces heat and light energy.
Combustion is a chemical reaction that occurs when wax reacts with oxygen in the air to make carbon dioxide and water in the form of steam.
Conduction carries heat down the wick to melt more wax at the top of the candlestick. Convection draws hot wax vapors out from the wick and sucks oxygen from the surrounding air into the base of the flame. The flame also gives off invisible beams of heat in all directions by radiation.
