Jessie Taylor
The chemistry and physics behind a candle's flame have fascinated scientists for hundreds of years. Candle wax, or paraffin, is a hydrocarbon, composed of hydrogen and carbon atoms. When a candle is lit, the heat of the flame melts the wax, which is drawn up the wick. The liquid wax is then vaporized and begins to break down into hydrogen and carbon molecules. These molecules react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide. The colour of the flame depends on the temperature and oxygen levels, with the yellow region being the most dominant.
| Characteristics | Values |
|---|---|
| Atoms in a candle flame | Hydrogen and Carbon |
| Chemical reaction of a burning candle | CH4 + 2O2 → CO2 + 2H2O |
| Generic chemical formula of wax | C(n) H(2n+2) |
| Colour of the flame | Blue, orange, brown, yellow |
| Temperature of the flame | 1000-1400° C |
| By-products of combustion | Carbon dioxide, water vapour, light, heat, soot |
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What You'll Learn
The chemical reaction of a burning candle
When a candle is lit, the heat of the flame melts the solid wax near the wick, turning it into a liquid. This liquid wax is drawn up the wick through a process called capillary action. As the liquid wax rises, it reaches the flame and vaporizes, turning into a hot gas. This gas, composed of hydrocarbon molecules, breaks down into hydrogen and carbon atoms.
The oxygen-rich blue zone at the base of the flame is where the hydrocarbon molecules vaporize and separate into hydrogen and carbon. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide. The dark orange-brown section above the blue zone has a lower oxygen concentration, allowing various forms of carbon to continue breaking down and forming small, hardened carbon particles.
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. The yellow colour is due to the soot particles glowing as they heat up, emitting black body radiation. As the particles continue to rise, they reach incandescence and emit the 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.
The fourth zone of the candle, sometimes called the veil, is the faint blue edge that extends from the base of the flame up the sides of the flame cone. This is the hottest part of the flame, reaching temperatures of about 1400 degrees Celsius. The blue colour is due to the direct contact with oxygen in the air. The flame heats the surrounding air, causing it to rise and create a convection current that gives the flame its teardrop shape.
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How wax becomes a hot gas
A candle flame is a fascinating chemical phenomenon that has captivated scientists and researchers for centuries. The process by which wax becomes a hot gas involves a series of transformations and reactions, resulting in the production of light and heat.
When a candle is lit, the heat from the flame melts the wax near the wick, forming a pool of liquid wax. This molten wax is then drawn up through the wick by a process called capillary action. As the liquid wax rises, it encounters the intense heat of the flame, causing it to vaporize and turn into a hot gas.
The vaporized wax molecules, now in the gaseous state, begin to break down into their constituent atoms. Wax, also known as paraffin, is primarily composed of hydrocarbons, which are molecules made up of carbon (C) and hydrogen (H) atoms. In the presence of the flame's heat, these hydrocarbon molecules dissociate into individual carbon and hydrogen atoms.
The liberated hydrogen atoms react with oxygen from the surrounding air to form water vapour (H2O). Simultaneously, some of the carbon atoms combine with oxygen to produce carbon dioxide (CO2). This process occurs in the blue zone at the base of the flame, where there is an abundant supply of oxygen, resulting in a clean burn.
As the carbon particles rise through the flame, they continue to break down and form small, hardened carbon particles known as soot. In the yellow region of the flame, there is a lower concentration of oxygen, leading to incomplete combustion and the formation of soot. The temperature in this region can reach approximately 1000 to 1200 degrees Centigrade, causing the soot particles to glow and emit a yellowish light.
The hot gas produced from the vaporized wax, along with the water vapour and carbon dioxide, dissipates into the surrounding air, contributing to the overall heat and light emitted by the candle flame. This continuous process of wax vaporization, combustion, and gas formation sustains the candle flame until the fuel (wax) is consumed or the heat source is removed.
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The role of oxygen in combustion
The combustion of a candle is a complex chemical process that involves the interaction of oxygen with other elements. When a candle burns, it releases light and heat energy, and this energy is produced through a chemical reaction between oxygen and the hydrocarbons in the wax. This process is known as combustion, and it is the same process that powers everything from campfires to car engines.
Oxygen plays a vital role in combustion. In fact, combustion cannot occur without it. When a 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 vaporises the liquid wax, breaking it down into molecules of hydrogen and carbon. These molecules react with oxygen from the surrounding air, creating heat, light, water vapour, and carbon dioxide.
The flame of a candle has several distinct regions, each with its own role in the combustion process. The base of the flame is a blue zone, rich in oxygen. This is where the hydrocarbon molecules break apart into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, and some of the carbon burns to form carbon dioxide. The dark orange-brown section above the blue zone has relatively little oxygen. Here, the various forms of carbon continue to break down, and small, hardened carbon particles begin to form.
As these particles rise, they are heated to extremely high temperatures, reaching approximately 1200° C in the yellow region of the flame. This is where the carbon particles, or soot, oxidise, emitting light and heat. The outermost region of the flame is a faint blue edge, where the oxygen of the air combines with the hot gases rising from the flame. This is the hottest part of the flame, typically reaching temperatures of 1400° C.
The combustion process in a candle is a self-sustaining cycle. The heat of the flame melts the wax, which is then vaporised and reacts with oxygen to produce more heat. This heat radiates in all directions, melting more wax and fuelling the combustion process until the fuel is depleted or the heat source is removed. The efficiency of combustion in a candle is evident in the minimal amount of soot produced and the complete disappearance of the wax upon burning.
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The colour of the flame
The colour of a candle flame is influenced by various factors, including the presence of different atoms, molecules, and combustion efficiency. The flame's colour can provide insights into the underlying chemical reactions and processes taking place.
At the base of the flame is a blue zone, which is rich in oxygen. This is where the hydrocarbon molecules from the wax vaporize and break down into hydrogen and carbon atoms. The hydrogen atoms react with oxygen to form water vapour, and some carbon burns to produce carbon dioxide. The blue colour results from the incandescence of fine soot particles formed inside the flame. As the oxygen supply increases, more complete combustion occurs, reducing the amount of black body-radiating soot. This leads to the excitation and ionization of gas molecules, resulting in the blue appearance.
Above the blue zone is a small dark orange-brown or yellow region. This area has a lower oxygen concentration, leading to the partial burning of carbon particles. The formation of carbon soot particles increases at the bottom of this zone. The colour of this region is influenced by the temperature, with orange-brown shifting to yellow as the temperature rises.
The outermost region of the flame is the yellow zone, which is the largest and most recognizable part of the candle flame. The yellow colour is due to the presence of soot particles glowing due to high temperatures, a phenomenon known as black-body radiation. As the soot particles oxidize near the top of the yellow region, the temperature reaches approximately 1200°C.
The fourth zone, sometimes called the veil, is the faint blue edge extending from the base upwards along the sides of the flame cone. This outer blue edge is the hottest part of the flame, typically reaching temperatures of around 1400°C. It appears blue because it directly interacts with the oxygen in the air.
The colour of a candle flame can also be influenced by the presence of other chemicals or additives in the wax. For example, burning copper sulphate can produce different flame colours. Additionally, the shape and movement of the flame can vary depending on factors such as airflow, temperature, and the type of wax used.
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The structure of a candle flame
A candle flame can be divided into three or four distinct regions or zones, each with its own unique characteristics, colours, and temperatures. The first zone, at the base of the flame, is blue in colour and oxygen-rich. This is where the hydrocarbon molecules of the wax 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.
Above the blue zone is a small dark orange-brown section, where the various forms of carbon continue to break down and form hardened carbon particles, or soot. This zone has a limited oxygen supply, resulting in incomplete combustion and the emission of light, giving it a luminous appearance.
The third zone is the large yellow region that is typically associated with candle flames. This is the largest part of the flame, with varying shades of yellow and orange. The temperature at the bottom of this zone can reach approximately 1000 °C, and it is here that the formation of soot particles increases. The yellow colour is due to the radiative emission from hot soot particles, with the yellow portion of the spectrum being the most dominant when the carbon ignites.
The fourth zone, sometimes called the veil, is the faint outside blue edge that extends from the base of the flame up the sides of the flame cone. This is the hottest part of the flame, reaching temperatures of up to 1400 °C. It appears blue due to chemiluminescence and has an unlimited supply of oxygen, resulting in complete combustion.
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Frequently asked questions
A candle flame is made up of hot gases, primarily carbon dioxide, water vapour, oxygen and nitrogen.
A candle flame contains carbon and hydrogen atoms.
A candle flame is typically yellow, but it also has a blue outer edge and an orange-brown section.
The chemical reaction of a burning candle is: CH4 + 2O2 → CO2 + 2H2O.
The generic chemical formula of wax is: C(n) H(2n+2).
