Jessie Taylor
Candles have been a source of fascination for scientists for hundreds of years, with Michael Faraday giving a famous lecture series on the Chemical History of a Candle in 1860. Candles are made from wax, which is composed of hydrocarbons, or hydrogen and carbon atoms. When a candle is lit, the heat melts and ignites the wax, which then vaporises and combines with oxygen to form a flame. This flame gives off light and heat, with the hottest parts being the blue, almost invisible area near the base, and the blue/white part of the flame where it meets the oxygen-rich air. The flame gets cooler as you move inwards, with the coolest part of the flame being the tip. As the candle burns, it releases carbon dioxide and water vapour, and when the oxygen supply is cut off, the flame goes out.
| Characteristics | Values |
|---|---|
| Gases given off | Carbon dioxide, water vapour |
| Heat | Three-quarters of the energy given off by the flame is light, and one-quarter is heat |
| Temperature | The hottest part of the flame is the blue, almost invisible area near the base, where oxygen is drawn in. The temperature here is 1000 degrees Centigrade. The flame gets progressively cooler as you move inwards from the outside edge towards the wick. |
| Colour | The flame appears yellow due to the ignition of carbon (soot) particles. |
| Shape | On Earth, a candle flame has an elongated or teardrop shape due to the convection current of upward-moving air. In microgravity, a candle flame is spherical. |
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What You'll Learn
Candles convert hydrocarbons into carbon dioxide and water
Candles are made of wax, which is a hydrocarbon—a molecule composed of hydrogen and carbon atoms. When a candle burns, it undergoes combustion, a chemical reaction that converts hydrocarbons into carbon dioxide and water.
During combustion, oxygen is pulled in at the bottom of the candle, and fuel is drawn up the wick. The heat from the flame melts and ignites the wax, which then vaporizes and combines with the oxygen to form a flame. This flame, in turn, melts the wax at the top of the candle, which moves upward through the wick and is continuously burned, maintaining the flame. As the candle burns, it produces water vapour and carbon dioxide, which rise through the flame.
The carbon in the wax continues to break down in the oxygen-poor region of the flame, forming small, hardened carbon particles that rise and are heated to approximately 1000°C. At the bottom of the yellow region of the flame, the formation of carbon soot particles increases. These particles rise and continue to heat up until they ignite and emit a full spectrum of visible light. As the particles oxidize near the top of the flame, the temperature reaches approximately 1200°C.
The chemical reaction of combustion releases heat, light, and chemical waste products. The heat from the flame is transferred in three ways: conduction, convection, and radiation. Conduction carries heat down the wick to melt more wax, while convection draws hot wax vapours out and pulls oxygen from the surrounding air into the base of the flame. Radiation releases invisible beams of heat in all directions.
The study of candles has a long history, with scientists like Michael Faraday giving lectures on the "'Chemical History of a Candle" as early as 1860. Even NASA has conducted experiments to understand how candle flames behave in microgravity environments.
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Oxygen is pulled in at the bottom of a candle
The combustion of a candle is a complex chemical process that involves the conversion of hydrocarbons (molecules based on hydrogen and carbon atoms) in wax into carbon dioxide and water vapour. This process requires oxygen, which is pulled in at the bottom of the candle.
Oxygen is essential for the combustion process, and it is drawn in at the base of the flame, where the temperature is the hottest. The oxygen combines with the vapourized wax fuel drawn up through the wick to form a flame. This flame then melts the top of the wax, which moves upward through capillary action, ensuring a continuous fuel supply for the flame.
The heat from the flame travels in three directions through conduction, convection, and radiation. Conduction carries heat down the wick to melt more wax, while convection creates upward-moving air around the flame, forming a teardrop shape. This convection current also draws oxygen from the surrounding air into the base of the flame.
The blue zone near the base of the flame has relatively low oxygen levels, and it is where carbon particles begin to form and rise, along with water vapour and carbon dioxide. As these particles rise, they heat up to approximately 1000 degrees Celsius. At the bottom of the yellow zone, the formation of carbon soot particles increases, and they continue to heat up as they rise, eventually igniting and emitting visible light.
The flame receives oxygen from two sources: the oxygen drawn in at the bottom and the oxygen-rich air surrounding the blue/white part of the flame's edge. The flame's heat is concentrated towards the tip, where a large volume of gas burns, and convection sweeps hot gases upwards. Therefore, the hottest parts of the flame is the blue zone at the base and the blue/white part at the edge, where oxygen is most accessible.
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Heat is given off at the top of a candle
The chemistry and physics behind the light and beauty of a candle flame have fascinated scientists for centuries. In fact, Michael Faraday gave a lecture series in 1860 on the "Chemical History of a Candle", in which he demonstrated several scientific principles through his observations of a burning candle.
A candle is a miniature chemical factory that converts hydrocarbons (molecules of hydrogen and carbon) in wax into carbon dioxide and water (steam) through combustion. When the wick of a candle is lit, the heat melts and ignites the wax, which then vaporises and combines with oxygen in the air to form a flame. The flame then melts the top of the wax, which moves upward through the wick and is continually burned, thus maintaining a constant flame.
The heat from the flame travels in three ways: conduction, convection, and radiation. Conduction carries heat down the wick to melt more wax at the top of the candle, while convection draws hot wax vapours out from the wick and pulls oxygen from the surrounding air into the base of the flame. The flame also emits invisible heat beams in all directions through radiation.
The hottest part of a candle flame is the blue, almost invisible area near the base, where oxygen is drawn in, and the blue/white part where the flame meets the oxygen-rich air. The flame gets cooler as you move in from the outside towards the wick, with the cooler areas being darker and coloured orange, red, or brown. Most of the flame's heat is delivered towards the tip, where a large volume of gas is always burning, and convection sweeps hot gases upwards. This is why the top of a candle gives off heat.
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Carbon particles form in the absence of oxygen
The light and beauty of a candle flame are the result of a complex interplay of chemistry and physics. The combustion of candle wax produces carbon dioxide and water vapour, with heat and light as by-products. The wax is drawn up the wick, where it meets oxygen at the base of the flame, and the chemical reaction of combustion occurs.
The base of the flame, where oxygen is drawn in, is the hottest part of the flame. The temperature here is around 1000 degrees Celsius. This is where carbon particles begin to form in the absence of sufficient oxygen for complete combustion. As the carbon particles rise, they continue to heat up and eventually ignite, emitting light across the full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, so the human eye perceives the flame as yellowish.
The formation of carbon particles in the absence of oxygen is a well-studied phenomenon. In the field of materials science, for instance, vacuum heat treatment is used to renew and activate electrodes. Heating the electrode to about 500 degrees Celsius in the absence of oxygen effectively desorbs contaminants and exposes a clean surface. The electrode retains much of the surface oxygen, as carbon-oxygen functional groups do not decompose at significant rates until temperatures exceed 500 degrees Celsius.
In the context of candle combustion, the absence of oxygen in the lower region of the flame allows for the formation of carbon particles, which rise and heat up, eventually igniting and producing light. This process demonstrates the intricate interplay of heat, oxygen levels, and combustion in the behaviour of candle flames.
The study of candle flames has captivated scientists for centuries, with renowned figures like Michael Faraday delivering lectures on the ""Chemical History of a Candle". The behaviour of candle flames in microgravity has also sparked curiosity, with NASA conducting experiments to observe their shape and characteristics in the absence of Earth's gravity.
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Carbon dioxide can be used to extinguish a flame
Candles are made of hydrocarbons, which are molecules based on hydrogen and carbon atoms. When a candle burns, it converts these hydrocarbons into carbon dioxide and water (steam) through a process called combustion.
Carbon dioxide is the most commonly used "inert" gas extinguishing agent. It is particularly useful for tackling electrical fires as it does not conduct electricity, thereby preventing the risk of electrocution. It is also useful in kitchens as it does not spoil food or cooking equipment. However, it is not suitable for chip pan fires as the carbon dioxide will spray the burning oil out of the container, causing the fire to spread.
It is important to note that carbon dioxide extinguishers must not be used in enclosed spaces without proper ventilation afterward. This is because carbon dioxide is harmful to humans in high concentrations, and people need oxygen to breathe.
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Frequently asked questions
Candles give off carbon dioxide.
The heat from a candle flame melts and ignites the wax, which is a hydrocarbon. This vapour combines with oxygen in the air to form a flame, creating carbon dioxide and water (steam) through a process called combustion.
Combustion is a chemical reaction where fuel (in this case, wax) combines with oxygen to produce heat, light, and carbon dioxide.
When a candle is covered with a jar, it eventually goes out because the flame consumes the oxygen inside the jar, and carbon dioxide produced by the flame fills the jar, preventing new oxygen from entering.
The hottest part of a candle flame is the blue, almost invisible area near the base, where oxygen is drawn in, and the blue/white part around the edge, where the flame meets oxygen-rich air.
