Brooke Martin
Candles have been the subject of fascination for scientists for hundreds of years. Candle flames are the product of a combustion reaction between the candle's wax and oxygen from the air. The flame's temperature varies across its three distinct regions, with the outer part being the hottest. The colour of the flame also differs across these regions, with the innermost part being black, the middle part being yellow and orange, and the outer part being blue. The blue colour is due to the incandescence of fine soot particles formed inside the flame. The size of the flame and its burning rate are influenced by the candle wick and the type of wax used.
| Characteristics | Values |
|---|---|
| Composition | Heat, light, water vapour, carbon dioxide, unburned soot particles, hydrogen, carbon atoms |
| Fuel | Wax (hydrocarbons) |
| Heat source | Combustion reaction of wax and oxygen |
| Wick function | Draws melted wax to the flame through capillary action |
| Wick material | Cotton, mineral salts, braided cord |
| Wick characteristics | Diameter, stiffness, fire resistance, tethering |
| Flame regions | Blue, dark orange-brown, yellow |
| Flame colours | Black, orange, yellow, white, blue |
| Temperature | 1000°C in the dark orange-brown region, 1200°C in the yellow region, 1400°C in the outer blue edge |
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What You'll Learn
The chemical composition of a candle flame
Waxes are primarily composed of hydrocarbons, which are molecules containing hydrogen and carbon atoms. When a candle is lit, the heat of the flame melts the solid wax, which then moves upward through the wick via capillary action. As the liquid wax reaches the flame, it vaporizes and combines with oxygen, forming a hot gas. This vaporized mixture of hydrogen and carbon reacts further with oxygen, resulting in the production of heat and light, water vapour (H2O), and carbon dioxide (CO2).
The candle flame can be divided into distinct regions, each with its own temperature and colour. The innermost part of the flame, closest to the wick, is the coolest and appears dark due to incomplete combustion. This region contains unburnt particles of carbon, also known as soot. As the carbon particles rise, they heat up to approximately 1000 degrees Celsius, continuing to break down and form hardened carbon particles.
The middle part of the flame is the largest and most luminous, emitting light in various shades of yellow and orange. This region has a limited oxygen supply, which contributes to incomplete combustion and the characteristic orange colour. The outer part of the flame, known as the veil, is the hottest, reaching temperatures of around 1400 degrees Celsius. This outer region has an abundant supply of oxygen, resulting in complete combustion and a blue appearance due to the incandescence of fine soot particles.
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The role of oxygen in a candle flame
The oxygen plays a vital role in sustaining the flame and facilitating the combustion process. The vaporized wax molecules react with the oxygen to produce heat, light, water vapour (H2O), and carbon dioxide (CO2). This combustion reaction releases energy in the form of heat and light, which we perceive as the warm glow of the candle flame.
The blue base of the candle flame, known as the oxygen-rich zone, is where the hydrocarbon molecules of the wax break apart into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide. This zone is characterized by complete combustion, resulting in less soot production and a cleaner burn.
As we move up the flame, the middle section exhibits incomplete combustion due to a limited oxygen supply. This region, with varying shades of yellow and orange, is where the remaining carbon particles continue to break down and form hardened carbon particles. These particles, along with the water vapour and carbon dioxide, are heated to extremely high temperatures, contributing to the overall heat and light produced by the candle.
The outer part of the flame, with an unlimited oxygen supply, is the hottest region, reaching temperatures of approximately 1200-1400°C. This is where the soot particles oxidize, and the flame's temperature peaks. The presence of oxygen in this zone ensures the complete combustion of the fuel, maximizing the energy released and contributing to the self-sustaining nature of the candle flame.
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The different zones of a candle flame
A candle flame consists of three main parts, each with distinct characteristics and temperatures. Understanding these zones helps us comprehend the burning process and the science behind it.
The outer zone, often referred to as the outermost bluish part of the flame, is the hottest among all zones, reaching temperatures of approximately 1400°C. This zone is non-luminous in nature, indicating complete combustion with a sufficient supply of air, resulting in the highest temperature.
The middle zone, identified by its yellow and orange hues, is where partial or incomplete combustion occurs. This zone is responsible for generating ashes and soot. It has a temperature of around 1200°C and is the most luminous due to the depletion of oxygen. The yellow colour is a result of molten wax burning, creating a yellowish luminous flame.
The inner zone, also known as the dark zone, is the coolest segment of the flame, with temperatures ranging from 800°C to 1000°C. This zone is generated by unburnt wax vapours and the oxidation of vaporized wax, producing carbon monoxide. It is black in colour and does not support combustion due to the absence of oxygen.
It is worth noting that the temperature and appearance of these zones can vary slightly depending on external factors, such as airflow and the type of fuel used. Additionally, the innermost part of the luminous zone, a narrow blue layer, is also considered one of the hottest areas of the flame.
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The colour of a candle flame
The innermost part of the flame, closest to the wick, is the dark or black part. This area contains unburnt particles of carbon, or soot, from the wick. It is the least hot part of the flame due to a limited supply of oxygen, which results in incomplete combustion.
The middle part of the flame is the largest and is made up of varying shades of yellow and orange. This is the luminous flame, emitting light and a small amount of heat. The colour is due to the different forms of carbon present, which continue to break down and form small, hardened particles. As these particles rise, they are heated to approximately 1000 degrees Celsius.
The outer part of the flame is the hottest, with a temperature of around 1200 degrees Celsius. This part has an unlimited supply of oxygen, resulting in complete combustion. The colour of this region is blue, which is due to the incandescence of very fine soot particles formed inside the blaze.
The fourth zone, sometimes called the veil, is the faint blue edge that extends from the base of the flame up the sides of the flame cone. It is the hottest part of the flame, reaching temperatures of up to 1400 degrees Celsius. This blue colour is also associated with the hottest part of a candle flame, with the colour becoming more intense as the temperature increases.
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The history of candle research
The study of candle flames has captivated scientists for centuries, with the earliest use of candles traced back to ancient times. Ancient Egyptians crafted rushlights or torches by soaking reed cores in melted animal fat, while the ancient Romans are credited with developing wicked candles by dipping rolled papyrus in tallow or beeswax. The evolution of candle-making continued with various innovations, but the focus of this narrative is the scientific exploration of candle flames.
In the 19th century, Michael Faraday, an influential scientist, delivered a series of lectures titled "The Chemical History of a Candle" in 1848 or 1860. These lectures, intended for young audiences, delved into the chemistry and physics of candle flames. Faraday's enthusiasm and lyrical expression brought the subject to life, inspiring future scientists. He described the zones of combustion, the presence of carbon particles, and the production of gases like hydrogen, oxygen, nitrogen, and carbon dioxide.
Faraday's work laid the foundation for subsequent research. In the late 1990s, NASA took candle experimentation to new heights by conducting space shuttle experiments to understand candle flame behaviour in microgravity. This demonstrates the enduring fascination with candle flames and the pursuit of knowledge beyond Earth's boundaries. University scientists and researchers worldwide continue to explore candle flames, emissions, and combustion processes.
Candles have played a significant role in religious ceremonies and lighting throughout history. The development of candle-making techniques and materials, such as the use of spermaceti in the 18th and 19th centuries and the introduction of paraffin wax in the 1850s, have also contributed to the evolution of candle research. Today, candles are primarily made from paraffin wax, a petroleum byproduct, and continue to be studied by students and professionals alike, shedding light on the intricate science behind their warm glow.
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Frequently asked questions
A candle flame is made up of three distinct regions or zones, each with different temperatures and colours. The innermost part of the flame, closest to the wick, is the least hot and appears black due to unburnt carbon particles. The middle part of the flame is the largest and emits light with varying shades of yellow and orange. This section has a limited oxygen supply, leading to incomplete combustion. The outer part is the hottest, with an unlimited oxygen supply, and appears blue.
The fuel that burns in a candle flame is the candle wax. The wick of the candle draws the melted wax up to the flame, where it vaporises and combusts in the presence of oxygen from the air.
The blue part of a candle flame is the outer region, which is the hottest. It appears blue due to the incandescence of very fine soot particles formed inside the blaze. The increased oxygen supply leads to more complete combustion, resulting in less black body-radiating soot and a blue appearance.
The yellow colour in a candle flame comes from the middle part of the flame, which has a limited oxygen supply. This incomplete combustion results in the emission of light with varying shades of yellow and orange.
