Maddie James
A candle flame is a mesmerizing sight, but there's more to it than meets the eye. The process of combustion, a chemical reaction between oxygen and the candle's fuel (wax), creates the flame. This multi-step process involves the wick drawing up melted wax, which vaporizes and combines with oxygen, forming a self-sustaining flame. The flame's heat melts more wax, continuing the cycle. The flame's shape, movement, and mesmerizing colors result from various chemical reactions and temperature differences within three to five distinct zones. The hottest part, with a blue hue, is small and releases minimal heat. The visible yellow color comes from hot soot particles, and the orange-brown region indicates incomplete combustion. Scientists like Michael Faraday have delved deep into the chemistry and physics of candle flames, revealing their complex nature.
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What You'll Learn
The chemical reaction of combustion
A candle flame is a self-sustaining chemical reaction called combustion. When a candle is lit, the heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporises the liquid wax, turning it into a hot gas.
The vaporised wax molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour (H2O) and carbon dioxide (CO2). This chemical reaction, known as combustion, is what produces the flame. Kerosene oil and molten wax are the two substances responsible for producing the flame during combustion.
The colour of the flame is due to the various temperature zones in a candle flame. The hottest part of the flame is just above the very dull blue part to one side of the flame, at the base. At this point, the flame is about 1,400 °C (2,550 °F). The blue colour is due to chemiluminescence, while the visible yellow colour is due to radiative emission from hot soot particles. The soot is formed through a series of complex chemical reactions, leading from the fuel molecule through molecular growth until multi-carbon ring compounds are formed.
The oxygen-rich blue zone is where the hydrocarbon molecules vaporise and start to break apart into hydrogen and carbon atoms. The hydrogen is the first to separate and reacts with the oxygen to form water vapour. Some of the carbon burns here to form carbon dioxide. The dark orange-brown region has relatively little oxygen. This is where the various forms of carbon continue to break down and small, hardened carbon particles start to form. As they rise, along with the water vapour and carbon dioxide created in the blue zone, they are heated to approximately 1000 °C.
The formation of carbon (soot) particles increases at the bottom of the yellow zone. The yellow colour of the flame is due to soot particles glowing because they are hot (black-body radiation). When the soot particles oxidise near the top of the flame's yellow region, the temperature is approximately 1200 °C.
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How wax vaporizes on burning
When a candle is lit, the heat from the flame melts the wax around the wick, creating a small pool of liquid wax. This liquid wax is then drawn up through the wick via capillary action. The heat of the flame then vaporizes the wax, turning it into a hot gas. This is the second stage of the process, where the wax transforms from a solid to a liquid to a gas.
The vaporized wax molecules are then drawn up into the flame, where they react with oxygen from the air. This reaction produces heat, light, carbon dioxide, and water vapour. This process is known as combustion, and it is what keeps the candle burning. The heat generated by the combustion is enough to melt more wax and keep the process going until the fuel is used up or the heat source is removed.
The blue area at the base of the flame is where the oxygen-rich hydrocarbon molecules vaporize and 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. The dark orange-brown region above the blue zone has a lower oxygen level, and this is where the various forms of carbon continue to break down and form small, hardened carbon particles.
As these particles rise, they heat up to about 1000 degrees Celsius at the bottom of the yellow zone, where the formation of carbon (soot) particles increases. As they continue to rise, they heat up further and eventually ignite, emitting a full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, which is why the human eye perceives the flame as yellowish.
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The multi-zone structure of a candle flame
A candle flame is a multi-step process resulting in combustion, which is a chemical reaction in which oxygen gas reacts with the substance being burned. The candle's fuel, or combustible material, is the wax. When a candle is lit, the heat of the flame melts the wax near the wick, and this liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporises the liquid wax, turning it into a hot gas. The hot gas then reacts with the oxygen in the air and burns, creating the candle flame.
The flame of a candle is made up of three or four distinct regions, or zones, each with different temperatures and colours. The first zone, at the base of the flame, is blue in colour. This is the hottest part of the flame, reaching temperatures of about 1400°C (2550°F or 2552°F). The blue colour is due to chemiluminescence and the presence of oxygen. The second zone is small and dark orange-brown. The third zone is the large yellow region that we typically associate with candle flames. The yellow colour is due to radiative emission from hot soot particles, which form through a series of complex chemical reactions. The fourth zone, sometimes called the veil, is the faint outside blue edge extending from the base of the flame up the sides of the flame cone. This is the second hottest part of the flame, also reaching temperatures of about 1400°C.
The size of the flame and the corresponding burn rate are largely controlled by the candle wick. Commercial wicks are usually made from braided cotton, and their characteristics, such as diameter, stiffness, fire resistance, and tethering, influence how the candle burns. The wick's capillarity, or ability to draw up liquid by capillary action, determines the rate at which the melted wax is conveyed to the flame. If the wick's capillarity is too great, the molten wax will stream down the side of the candle.
The colour of the flame can also provide insight into the combustion process. A blue flame indicates complete combustion, with less black body-radiating soot produced due to an adequate oxygen supply. A flame that is orange, yellow, or white indicates incomplete combustion, with the colour shifting towards white as the temperature rises.
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The role of oxygen in combustion
The combustion of a candle is a self-sustaining process. Once a candle is lit, the heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. This liquid wax is then vaporised by the heat of the flame, turning into a hot gas. The vaporised molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide.
Oxygen is a crucial component of combustion. It is one of the three main ingredients required for fire, the other two being heat and fuel. Combustion is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen. This reaction produces oxidized, often gaseous products, in a mixture termed smoke.
The oxidants for combustion have high oxidation potential and include atmospheric or pure oxygen, chlorine, fluorine, chlorine trifluoride, nitrous oxide, and nitric acid. When a hydrocarbon burns in oxygen, the reaction primarily yields carbon dioxide and water. The oxygen-rich blue zone at the base of the flame is where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. The hydrogen is the first to separate and reacts with the oxygen to form water vapour. Some of the carbon burns here to form carbon dioxide.
The amount of oxygen present in the air affects the combustion process. If a flame gets too little or too much air or fuel, it can flicker or flare, and unburned carbon particles (soot) will escape from the flame before they can fully combust. The blue edge extending from the base of the flame is the hottest part of the flame, typically reaching 1400°C (2552°F). This part of the flame is blue because it directly meets with the oxygen in the air.
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The colour temperature of a candle flame
The colour and temperature of a candle flame are influenced by several factors, with the type of fuel involved in combustion being the most significant determinant. The colour of a flame can be attributed to black-body radiation and spectral band emission, with spectral line emission and spectral line absorption playing minor roles.
Moving up the flame, carbon particles, or soot, increase in concentration and rise until they ignite, emitting a yellow luminescence. This yellow region is the most familiar and distinctive colour associated with candle flames. The yellow colour is due to radiative emission from hot soot particles, which form through complex chemical reactions involving fuel molecules and molecular growth, eventually resulting in multi-carbon ring compounds. The temperature in the lower yellow zone, where soot formation increases, is approximately 1200°C. As the soot particles oxidize near the top of the flame's yellow region, the temperature drops to around 800°C (1070 K).
Extending from the blue base and up the sides of the flame is a faint blue "veil" or outer edge, which is the fourth zone of the candle flame. This thin veil, often invisible, reaches temperatures similar to the blue base, averaging 1400°C (2550°F). The blue colour of this outer zone is a result of direct exposure to oxygen on all sides.
Thus, the colour temperature of a candle flame varies across its different zones, with the blue base and veil reaching the highest temperatures of approximately 1400°C, while the yellow region, produced by soot particles, has a lower temperature of around 1200°C, gradually decreasing to 800°C near the top.
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Frequently asked questions
A candle flame is made of wax vapour. When you light a candle, the heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick and turns into a gas, which combines with oxygen in the air to form a flame.
A candle flame consists of three to five regions or "zones", each with different temperatures and colours. The lowest part of the flame, Zone I, is non-luminous and the coolest, with temperatures of around 600 °C. Zone II, the blue zone, surrounds the base of the flame, where the fuel burns clean and blue. The dark orange-brown Zone III is located directly above the wick, with temperatures of around 1,000 °C. The middle or luminous zone, Zone IV, is yellow/white and is where the formation of carbon (soot) particles increases. The outermost part of the flame is blue and is the hottest part, reaching temperatures of about 1,400 °C.
The vaporised wax molecules in the flame react with oxygen to create heat, light, water vapour and carbon dioxide. Approximately one-quarter of the energy created by a candle's combustion is given off as heat, which radiates in all directions. This heat melts more wax to keep the candle burning.
