Tyler Brooks
Candles have been used for over two millennia, and while they are no longer necessary for illumination, they are still commonly used for functional, symbolic, and aesthetic purposes. The combustion process of a candle involves the release of light, heat, carbon dioxide, and water vapour. When 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. The heat of the flame vaporises the liquid wax, breaking it down into molecules of hydrogen and carbon. These molecules are then drawn into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. The flame of a candle can be divided into several zones based on temperature and oxygen supply, with the hottest part of the flame being the blue edge, where complete combustion occurs.
| Characteristics | Values |
|---|---|
| Heat | The heat of the flame melts the wax near the wick. |
| Capillary Action | Liquid wax is drawn up the wick by capillary action. |
| Vaporization | The heat of the flame vaporizes the liquid wax. |
| Breakdown of Hydrocarbons | The heat of the flame breaks down the hydrocarbons into molecules of hydrogen and carbon. |
| Combustion | The vaporized molecules react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. |
| Fuel | The wax acts as fuel for the combustion process. |
| Flame Shape | The flame maintains a teardrop shape due to the cycle of moving air created by the combustion process. |
| Flame Colour | The flame is light blue in colour, with a yellow zone in the middle. |
| Efficiency | A quietly burning candle flame is a very efficient combustion machine. |
| By-products | If the combustion process is interrupted, the flame produces black soot or wisps of smoke due to incomplete combustion. |
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What You'll Learn
The role of oxygen in combustion
The combustion process of a candle involves the release of light, heat, carbon dioxide, and water vapour to fuel the flame. This process is sustained by the role of oxygen in combustion.
Oxygen is a crucial element in the combustion process. It is the most common oxidizing agent, which means it accepts electrons from the fuel source. In the case of a candle, the fuel is typically a hydrocarbon, composed mainly of hydrogen and carbon atoms. When the candle is lit, the heat of the flame melts the wax near the wick, and this liquid wax is drawn up the wick through capillary action. The flame then vaporizes the liquid wax, breaking down the hydrocarbons into hydrogen and carbon molecules.
These vaporized molecules react with oxygen from the surrounding air, creating heat, light, and by-products such as water vapour and carbon dioxide. The oxygen acts as an oxidizer, accepting electrons from the fuel and facilitating the combustion reaction. This process is similar to the combustion of fossil fuels, where oxygen is also essential for the oxidation of hydrocarbons.
The presence of oxygen in different zones of the candle flame also influences the burning process. Zone I, the lowest part of the flame, has insufficient oxygen for the fuel to burn completely. Zone II, the blue zone, has a plentiful oxygen supply, allowing the fuel to burn cleanly and producing the heat necessary to melt the wax. Zone IV, the middle or luminous zone, is oxygen-depleted, leading to partial combustion of the wax vapour. Finally, Zone V, the outer zone, is where the highest temperatures are reached, and complete combustion occurs due to the abundant oxygen supply.
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How wax melts
When a candle is lit, the heat of the flame melts the wax near the wick. The temperature in this zone, known as Zone II, is around 800°C (1,470°F). This liquid wax is then drawn up the wick by capillary action. The diameter of the wick determines the rate at which the melted wax is conveyed to the flame.
The heat of the flame vaporizes the liquid wax, turning it into a hot gas. This gas rises into the next zone of the candle flame, Zone III, which is directly above the wick and has a temperature of around 1,000°C (1,830°F). Here, pyrolysis takes place, and the hydrocarbons in the wax begin to break down into molecules of hydrogen and carbon.
These vaporized molecules are drawn up further into the flame, where they react with oxygen from the air. This reaction produces heat, light, water vapour, and carbon dioxide. The heat radiates in all directions, melting more wax near the wick to keep the combustion process going.
The melting of wax in a candle can be influenced by the amount of air and fuel reaching the flame. If there is too much or too little of either, the flame may flicker or flare, and unburned carbon particles (soot) may escape from the flame before they can fully combust.
To melt candle wax at home, one can use a stove, oven, or microwave. The wax can be melted in a pot placed inside another pot of simmering water (the double boiler method) or directly in a small pot. Alternatively, the wax can be scooped out and placed in a microwave-safe container before being heated in the microwave. It is important to monitor the temperature of the wax when melting, as different types of wax have different melting points.
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The wick's capillary action
The combustion process of a candle involves the release of light, heat, carbon dioxide, and water vapour. When a candle is lit, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up the wick through capillary action.
The wick of a candle is typically made from braided cotton, which has natural absorbent properties. The wick draws the liquid wax upwards through capillary action, and the heat of the flame vaporises the liquid wax, turning it into a hot gas. This gas is then burnt, creating the flame. The diameter of the wick determines the rate at which the melted wax is conveyed to the flame, with larger wicks resulting in larger flames and faster-burning candles.
The capillary action of the wick is crucial to the combustion process. The liquid wax is drawn up the wick towards the flame, where it is vaporised and combusts. The heat of the flame causes the wax to melt, and this liquid wax is then drawn up the wick, creating a continuous cycle that keeps the candle burning. The wick's capillary action ensures a steady supply of fuel to the flame, allowing the combustion process to continue until the fuel is depleted or the heat source is removed.
The size and material of the wick play a significant role in the combustion process. The wick's diameter, stiffness, fire resistance, and tethering are important characteristics that influence how the candle burns. For example, larger wicks provide more fuel to the flame, resulting in a larger flame and a faster-burning candle. Additionally, some wicks are designed to curl back into the flame as they burn, making them self-consuming.
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The chemical reaction of combustion
The combustion process can be divided into several zones, each with distinct characteristics. Zone I, the non-luminous, coolest part of the flame, surrounds the base of the wick, where oxygen is insufficient for fuel to burn. Zone II, the blue zone, surrounds Zone I, with an ample oxygen supply, causing the fuel to burn clean and blue. The heat from this zone melts the wax. Zone III, the dark zone, is directly above the wick and contains unburnt wax, where pyrolysis occurs. Zone IV, the middle or luminous zone, is yellow/white and is oxygen-depleted, resulting in partial combustion. The outermost zone, Zone V, is non-luminous and light blue, and this is where complete combustion occurs at the highest temperature, around 1400°C.
The colour of the flame is influenced by the presence of soot particles, which glow due to high temperatures, resulting in a yellow flame. The blue and green colours observed in the flame are due to transient reaction intermediates during combustion, such as the Methylidyne radical (CH) and Diatomic carbon (C2). These molecules emit visible light through spectral band emission.
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The different zones of a candle flame
The flame of a candle is a fascinating phenomenon, with multiple zones, each playing a role in the combustion process. These zones are differentiated by their colour, intensity, and heat sensation.
The first zone, Zone I, is the innermost zone, closest to the wick. This zone is non-luminous, the coolest part of the flame, with temperatures around 600 °C. There is insufficient oxygen in this zone for the fuel to burn. The second zone, Zone II, is the blue zone, surrounding the base of the flame. Here, there is an abundance of oxygen, and the fuel burns clean and blue. This zone produces the heat that melts the wax, with temperatures reaching 800 °C.
The third zone, Zone III, is known as the dark zone, located directly above the wick. This zone contains unburnt wax, and pyrolysis takes place here at temperatures of around 1000 °C. The fourth zone, Zone IV, is the middle or luminous zone, with a yellow/white colour. This zone is the brightest, but not the hottest, and it is where the depletion of oxygen occurs, resulting in partial combustion.
The fifth zone, Zone V, is the outermost zone, where the flame reaches its peak temperature of around 1400 °C. This zone is non-luminous, and it is where complete combustion occurs. It is light blue in colour, but mostly invisible to the naked eye.
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Frequently asked questions
A candle is an ignitable wick embedded in wax or another flammable solid substance such as tallow.
The heat of the flame vaporizes the wax molecules and they react with the oxygen in the air. As wax is consumed, capillary action draws more liquid wax along the wick. As long as the wax doesn't melt away from the flame, the flame will consume it completely.
Candle combustion produces light, heat, carbon dioxide, and water vapour.
