Tyler Brooks
Candles have fascinated scientists for hundreds of years. In 1860, Michael Faraday delivered his famous series of lectures on the 'Chemical History of a Candle', exploring the chemical processes that occur when a candle burns. The combustion of a candle involves the breakdown of hydrocarbons into molecules of hydrogen and carbon, which react with oxygen to produce heat, light, water vapour, and carbon dioxide. The colour of the candle flame is due to the presence of soot particles, which emit light in the yellow and red spectral range. Candle chemistry is not only beautiful but also complex, with various chemical reactions and by-products that have been the subject of ongoing research.
| Characteristics | Values |
|---|---|
| Chemical potential energy | Stored within wax's molecular bonds |
| Transformation | Into heat and light through the process of combustion |
| Wax | Hydrocarbons |
| Molecules | Hydrogen and carbon atoms |
| Byproduct | Water vapour and carbon dioxide |
| Wick | Absorbent, like a towel |
| Wax type | Paraffin wax |
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What You'll Learn
Hydrocarbon molecules in wax
Waxes are organic compounds that consist of long aliphatic alkyl chains. They are predominantly composed of long-chain hydrocarbons, which are molecules made up of carbon and hydrogen atoms. The length and branching of these carbon chains vary among different types of wax, giving them distinct physical properties.
Hydrocarbons with the general formula CnH2n+2 are known as alkanes or paraffins in chemistry. Paraffin wax, or petroleum wax, is a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is derived from petroleum, coal, or oil shale and is solid at room temperature, melting above approximately 37°C (99°F). Paraffin wax was first created by Karl von Reichenbach in Germany in 1830, revolutionizing candle-making due to its clean burning and low cost.
The wax in candles is primarily made up of hydrocarbon molecules. When a candle burns, the heat of the flame melts the wax near the wick, and this liquid wax is drawn up the wick by capillary action. The heat then vaporizes the liquid wax, breaking down the hydrocarbons into hydrogen and carbon atoms. These vaporized molecules react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide.
The chemical potential energy stored within the wax's hydrocarbon molecules is converted into other forms of energy during combustion. This energy transformation is observed as the candle's flame, with the released energy causing surrounding molecules to vibrate faster and emit heat and light. Thus, the light and heat of a burning candle are manifestations of the conversion of chemical potential energy into kinetic and thermal energy.
In summary, the hydrocarbon molecules in wax, composed of carbon and hydrogen atoms, play a crucial role in the combustion process of candles, releasing energy and producing heat, light, and by-products through the breakdown and reformation of molecules.
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Conversion of chemical potential energy
A candle is primarily made of wax and a wick. The wax, which is a hydrocarbon, acts as the candle's fuel, while the wick is the conduit through which the fuel is burned. When a candle is lit, the flame from the match warms the wax near the wick, causing it to melt and become a liquid. This liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporizes the liquid wax, turning it into a hot gas, and starts to break down the hydrocarbons into molecules of hydrogen and carbon.
The combustion of a candle is a classic example of an exothermic reaction, a chemical reaction that releases energy in the form of light and heat. In this process, the chemical potential energy stored within the wax's molecular bonds is converted into other forms of energy. This energy is not lost but transformed into heat and light, with the released energy causing surrounding molecules to vibrate faster, resulting in the emission of heat and light. This emitted energy is what we observe as the candle's flame.
The light and heat of a burning candle are related to chemical potential energy through the process of combustion. As the wax undergoes combustion, energy stored in the chemical bonds is released, and the surrounding molecules absorb this energy. This energy is then transformed into kinetic and thermal energy, which we perceive as the warmth and light emitted by the candle. Thus, the light and heat of a burning candle are visible manifestations of the conversion of chemical potential energy into heat and light.
The flame of a candle also creates a convection current, an upward-moving cycle of air around the flame. As the flame heats the nearby air, it starts to rise, and cooler air and oxygen rush in at the bottom of the flame to replace it. This cycle gives the flame its teardrop shape. Interestingly, NASA scientists found that in microgravity, a candle flame takes on a spherical shape instead of the elongated teardrop shape observed on Earth.
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Heat and light emission
The light and heat emitted by a burning candle are related to chemical potential energy through the process of combustion. When a candle burns, the chemical potential energy stored in the wax's hydrocarbon molecules is transformed into heat and light. This transformation occurs as the wax undergoes combustion, releasing energy from the chemical bonds. The released energy causes surrounding molecules to vibrate faster, emitting heat and light, which we observe as the candle's flame.
The light and heat of a burning candle are the visible manifestations of energy transformation, in which chemical potential energy stored in the wax is released due to the process of combustion. This released energy takes the form of heat and light, causing the surrounding air to warm up and the candle to produce a visible flame. The heat produced not only maintains the candle's flame by melting more wax but also warms the surrounding environment.
The colour of a candle's flame is predominantly due to blackbody radiation, which has a continuous emission spectrum. Everything emits electromagnetic radiation as a function of its temperature. At room temperature, this radiation is almost entirely in the infrared region of the spectrum or below—invisible to humans. As the wax in a candle burns, it produces a lot of heat, causing the vaporized wax particles and the air in the immediate vicinity to heat up.
The blue colour of the flame at its base is produced by discrete emission spectra of the molecular radicals produced by the wax particles breaking down. Since there are more of these radicals near the base of the flame, the base appears blue. The yellow/orange colours are due to blackbody radiation, which is more dominant when the carbon ignites, causing the human eye to perceive the flame as yellowish.
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Convection currents
When a candle burns, it releases heat and light energy through the process of combustion. The combustion of a candle involves the breaking and reforming of chemical bonds in its wax, which is composed of hydrocarbon molecules. This process releases energy, which is then transferred into heat and light energy.
In the context of a burning candle, the heat from the flame warms the air directly above it, causing that air to rise. As the warm air moves up, cooler air from the surrounding area moves in to replace it, creating a natural convection current. This cycle of upward-moving air gives the flame its characteristic teardrop shape.
The shape of the flame in a candle is also influenced by gravity. In the absence of gravity, there is no upward direction for warm air to rise and create a convection current. NASA scientists conducted experiments in the late 1990s to observe how candle flames behaved in microgravity. They found that without the influence of gravity, the flame takes on a spherical shape instead of the elongated teardrop shape observed in a gravitational environment.
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Candle flame in microgravity
A candle is primarily made of wax and a wick. The wax, which is a hydrocarbon, acts as the candle's fuel, while the wick is the conduit through which the fuel is burned. When a candle is lit, the heat of the flame melts the wax near the wick, turning it into a liquid. This liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporizes the liquid wax, breaking down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide.
On Earth, a candle flame has an elongated or teardrop shape due to gravity-driven buoyant convection. This convection causes the warm air to rise, creating a continuous cycle of upward-moving air around the flame (a convection current). However, in microgravity, where the pull of gravity is minimal, candle flames behave differently.
In microgravity, candle flames take on a spherical shape instead of the elongated shape typical on Earth. This is because there is no "up" direction for warm air to rise and create a convection current. Without gravity-driven buoyant convection, the flame is also soot-free and blue. The combustion products and oxygen are transported by molecular diffusion, a much slower process than natural convection in Earth's gravity. As a result, the flame in microgravity burns slower and hotter and may appear less vigorous than a flame on Earth.
NASA scientists first observed these differences in the late 1990s during space shuttle experiments investigating the behaviour of candle flames in microgravity. These experiments added to centuries of scientific fascination with candles, including Michael Faraday's famous 1860 lecture series, "The Chemical History of a Candle."
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Frequently asked questions
The chemical bonds in a candle are formed between carbon, hydrogen, and oxygen atoms. The generic chemical formula of wax is C(n) H(2n+2).
When a candle is lit, the heat of the flame vaporizes the liquid wax, turning it into a hot gas. The vaporized molecules are drawn into the flame, where they react with oxygen from the air to create heat, light, water vapour (H2O), and carbon dioxide (CO2).
The blue area at the base of the flame is the oxygen-rich zone, where hydrocarbon molecules break down into hydrogen and carbon atoms. The yellow region is due to soot particles glowing because they are hot, and the dark orange-brown section is where various forms of carbon continue to break down and form hardened carbon particles.
