Chloe Adams
When a candle starts to melt, the heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. The liquid wax is vaporized by the heat of the flame, breaking down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. The carbon dioxide and water vapour produced cool and mix into the room's air, eventually dispersing into the atmosphere. The cycle of upward-moving air around the flame, known as a convection current, gives the flame its teardrop shape.
| Characteristics | Values |
|---|---|
| Heat from the flame | Melts the wax near the wick |
| Liquid wax | Drawn up the wick by capillary action |
| Heat of the flame | Vaporizes the liquid wax |
| Vaporized wax | Breaks down into hydrogen and carbon |
| Vaporized molecules | Drawn into the flame |
| Vaporized molecules | React with oxygen from the air |
| Products | Heat, light, water vapour, and carbon dioxide |
| Candle flame | Blue zone at the base |
| Blue zone | Oxygen-rich |
| Yellow zone | Relatively low oxygen |
| Yellow zone | Carbon soot particles form |
| Flame shape | Elongated or teardrop |
| Candle additives | Affect burn time and wax burn rate |
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What You'll Learn
The wax melts and travels up the wick
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 molecules of hydrogen and carbon that result from this process are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. This combustion process continues until the fuel is used up or the heat source is eliminated.
The oxygen-rich blue zone at the base of the flame is where the hydrocarbon molecules vaporise and begin to 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. As the small, hardened carbon particles rise, they are heated to approximately 1000 degrees Celsius. At the bottom of the yellow zone, the formation of carbon soot particles increases, and as they continue to rise and heat up, they ignite to emit a full spectrum of visible light.
The continual cycle of upward-moving air around the flame, known as a convection current, gives the flame its teardrop shape. The blue veil that extends from the base of the flame is the hottest part, typically reaching 1400 degrees Celsius. It is blue because it directly meets the oxygen in the air.
The wax acts as fuel for the flame, and when it has completely evaporated, the candle will no longer burn. The wax evaporates into the atmosphere, turning from a solid to a liquid to a gas. While the wax is the primary source of heat, the cotton wick also burns.
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The wax vapour burns, creating heat and light
When a candle is lit, the heat from the flame melts the wax near the wick. The 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. This wax vapour then burns, creating heat and light.
The hot wax vapour is made up of hydrocarbon molecules, which are composed of hydrogen and carbon atoms. As the wax vapour burns, it breaks down into hydrogen and carbon. The hydrogen reacts with oxygen in the air to form water vapour. Some of the carbon burns to form carbon dioxide.
The carbon dioxide and water vapour produced by the burning candle cool and mix into the air in the room, becoming indistinguishable from other molecules of carbon dioxide and water. Over time, as the air in the room is exchanged with outdoor air, the molecules from the candle disperse into the atmosphere.
The burning of the wax vapour also creates light. As the wax vapour burns, the carbon atoms rise and continue to heat up. At around 1000 degrees Celsius, these carbon atoms ignite and emit a full spectrum of visible light. The yellow portion of this spectrum is the most dominant, so the human eye perceives the flame as yellowish.
The heat and light from a candle come primarily from the burning of the wax vapour. The wick, which is usually made of cotton, also burns, but it is the wax that contributes most of the heat.
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The combustion process stabilises
When a candle is lit, the heat from the flame melts the wax near the wick. The liquid wax is then drawn up the wick by capillary action. The combustion process takes a few minutes to stabilize. During this time, the flame may flicker or smoke slightly. Once the process is stabilized, the flame burns steadily in a quiet teardrop shape.
The heat of the flame vaporizes the liquid wax, turning it into a hot gas. This gas rises up into the flame, where it reacts with oxygen from the air. This reaction produces heat, light, water vapour, and carbon dioxide. The water vapour and carbon dioxide mix into the air in the room, becoming indistinguishable from other molecules. Over time, these molecules escape the room and disperse into the atmosphere.
The oxygen-rich blue zone at the base of the flame is where the vaporized wax molecules begin to 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. As the water vapour and carbon dioxide rise, they are heated to approximately 1000 degrees Celsius in the orange/brown region of the flame, which has relatively little oxygen.
The formation of carbon soot particles increases at the bottom of the yellow zone of the flame. As these particles rise, they continue to heat up until they ignite and emit a full spectrum of visible light. The human eye perceives the flame as yellowish because the yellow portion of the spectrum is the most dominant when the carbon ignites.
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Carbon dioxide and water vapour are released
When a candle is lit, the heat of the flame melts the wax near the wick. This liquid wax is drawn up the wick by capillary action. The heat of the flame then vaporises the liquid wax, turning it into a hot gas. The hot gas breaks down into hydrogen and carbon atoms. The hydrogen atoms react with oxygen in the air to form water vapour. Some of the carbon burns to form carbon dioxide.
The blue zone at the base of the flame is oxygen-rich, and this is where the hydrocarbon molecules vaporise and break apart into hydrogen and carbon atoms. The hydrogen atoms react with oxygen to form water vapour. The carbon atoms burn to form carbon dioxide. The dark or 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, they are heated to approximately 1000 degrees Centigrade.
The yellow zone is where the formation of carbon (soot) particles increases. As they rise, they continue to heat up until they ignite to incandescence and emit the 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. When the soot particles oxidise near the top of the flame's yellow region, the temperature is approximately 1200 degrees Centigrade.
The fourth zone of the candle is sometimes called the veil. It is the faint outside blue edge that extends from the blue zone at the base of the flame and up the sides of the flame cone. It is blue because it directly meets with the oxygen in the air and is the hottest part of the flame, typically reaching 1400 degrees Centigrade. The flame heats the nearby air, which starts to rise. As this warm air moves up, cooler air and oxygen rush in at the bottom of the flame to replace it. This creates a continual cycle of upward-moving air around the flame (a convection current), which gives the flame its elongated or teardrop shape.
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The cycle of upward-moving air gives the flame its teardrop shape
When a candle is lit, the heat from the flame melts the wax near the wick. This liquid wax is drawn up the wick by capillary action and then vaporises. The heat breaks down the wax molecules into hydrogen and carbon atoms. These vaporised molecules are 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 is not static but dynamic, constantly moving and interacting with its environment. As the flame heats the surrounding air, it starts to rise. This movement of warm air creates a space at the bottom of the flame, which is quickly filled by cooler air and oxygen rushing in to replace it. As this new air is heated, it too rises, and the cycle repeats. This cycle of upward-moving air, known as a convection current, gives the flame its characteristic teardrop shape.
The teardrop shape of a candle flame is a result of the balance between the heat of the flame and the flow of cooler air. The flame generates heat, which causes the air to become less dense and rise. At the same time, the cooler air, being denser, is drawn towards the flame to replace the rising warm air. This continuous exchange of warm and cool air creates a cycle that shapes the flame.
The flame's teardrop shape is also influenced by the combustion process. As the wax vaporises and burns, it releases heat and light. The combustion process is not always perfect, and sometimes unburned carbon particles, known as soot, escape from the flame. These soot particles contribute to the flickering and flaring of the flame, affecting its shape.
The teardrop shape of a candle flame is a result of the complex interplay between heat, air movement, and combustion. The heat from the flame sets the air in motion, creating a convection current that shapes the flame. The rising warm air and descending cool air create a dynamic system that influences the flame's shape and behaviour. Additionally, the combustion of wax and the release of soot particles also play a role in shaping the flame.
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Frequently asked questions
The heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action and vaporizes.
The flame heats the nearby air, which starts to rise. Cooler air and oxygen then rush in at the bottom of the flame to replace it, creating a cycle of upward-moving air (a convection current) that gives the flame its teardrop shape.
The wax eventually evaporates into the atmosphere as carbon dioxide or water vapour.
This is caused by unburned carbon particles (soot) escaping from the flame before they can fully combust. It can be due to the flame getting too little or too much air or fuel, or the wick being too long and soaking up too much wax.
