Ben Carter
The colour of a candle flame depends on various factors, including the type of fuel, the presence of certain chemicals, and the level of oxidation of carbon molecules. The typical candle flame is yellow or orange-yellow due to the presence of soot particles, which are formed when there is insufficient oxygen for the carbon to completely turn into carbon dioxide. On the other hand, a flame with sufficient oxygen appears blue as all the carbon atoms are converted into carbon dioxide. Additionally, certain chemicals added to the wax can produce distinct colours, such as red, green, purple, or blue.
| Characteristics | Values |
|---|---|
| Color of a typical candle flame | Orange-yellow |
| Factors that affect the color of a flame | Oxidation level, temperature, presence of soot, oxygen concentration, air pressure, thermal conductivity of air, buoyancy of hot reaction products, wind |
| Color of flame with sufficient oxidation of carbon molecules | Blue |
| Color of flame with insufficient oxidation of carbon molecules | Yellow-orange |
| Flame color of different elements | Copper: blue; lithium and strontium: red; calcium: orange; sodium: yellow; barium: green; magnesium: bright white; potassium: purple |
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What You'll Learn
The presence of oxygen affects the colour of a flame
The colour of a candle flame is influenced by several factors, one of which is the presence of oxygen. The availability of oxygen affects the combustion process and, consequently, the colour of the flame.
A candle flame has different zones, each with varying oxygen levels, contributing to the overall colour of the flame. The blue zone, or the base of the flame, has a surplus of oxygen, resulting in a clean, blue burn. This zone is responsible for melting the wax and allowing it to move upward through the wick. The temperature in this zone reaches approximately 800°C.
Above the blue zone is the dark zone, where oxygen levels are lower. In this zone, pyrolysis or cracking of the fuel occurs due to the shortage of oxygen, leading to the formation of minute carbon particles. These carbon particles, also known as soot, are heated to around 1000°C, causing them to glow with a bright yellow light. This zone is responsible for the characteristic yellow colour typically associated with candle flames.
The yellow zone is where the formation of soot particles is most prominent. As the particles rise, they encounter oxygen and burn out, contributing to the overall combustion process. However, some carbon particles remain unburned, which can be observed by placing a knife or spoon in the flame, resulting in a black coating of carbon.
The amount of oxygen available during combustion plays a crucial role in determining the colour of the flame. When there is an abundance of oxygen, as in the case of a gas-grill flame, the combustion is more efficient and complete, resulting in a blue flame. In contrast, candlelight typically has an insufficient oxygen supply, leading to incomplete combustion and the production of yellow flames.
Additionally, the presence of oxygen can affect the formation of soot, which also influences the flame's colour. When there is sufficient oxygen, as when using a Bunsen burner, less soot is produced, and the flame tends to be yellow due to the incandescence of fine soot particles. However, when the oxygen supply is abundant, as when the air inlet is opened, the flame transitions from yellow to blue as the soot levels decrease, and the blue emissions from excited molecular radicals become dominant.
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Soot and carbon particles produce yellow, orange and red flames
The colour of a candle flame is influenced by the complex structure of soot and carbon particles. When a candle wick is lit, the heat melts and ignites the wax, which then vaporises and combines with oxygen in the air to form a flame. This flame melts the top of the wax, which moves upward through the wick and is continually burned, maintaining a constant flame.
The heat from the flame breaks apart the hydrocarbons in the wax through a process called pyrolysis. This results in smaller, highly reactive radicals that readily undergo chemical reactions, particularly oxidation. Oxygen combines with the carbon and hydrogen radicals to produce carbon dioxide and water, releasing heat in the process. However, some of the radicals may react with each other instead of oxygen, forming rings of carbon called polycyclic aromatic hydrocarbons. These carbon-rich structures grow and agglomerate into long chains that resemble strings of beads, creating soot particles.
As these soot particles travel upward inside the flame, they encounter oxygen molecules, which can break off pieces and cause the particles to incandesce more brightly, emitting light across a wide range of wavelengths. The human eye perceives this as a bright yellow colour, characteristic of hot soot particles. The temperature of the soot also influences its colour, with hotter soot appearing whiter and cooler soot appearing redder.
The presence of soot and carbon particles in the flame gives rise to the yellow, orange, and red hues commonly observed in candle flames. The complex interactions and optical effects produced by these particles contribute to the vibrant colours observed.
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Chemiluminescence causes a blue flame
The blue colour in a candle flame is due to chemiluminescence. Chemiluminescence is the emission of light as a result of a chemical reaction. In the case of a candle flame, the heat of the flame melts the wax, which rises up the candle wick and vaporises. The heat of the flame and reactive molecules (free radicals) in the flame break apart the wax molecules. The blue zone of a candle flame has a surplus of oxygen, and the flame burns clean and blue at around 800°C.
The colour blue is common in bioluminescence, which is chemiluminescence in living organisms. Deep-sea organisms have evolved to produce light to lure and catch prey, as camouflage, or to attract others. The common colours for the light emitted by these animals are blue and green because they have shorter wavelengths than red and can transmit more easily in water.
Chemiluminescence differs from fluorescence or phosphorescence in that the excited state is the product of a chemical reaction rather than the absorption of a photon. Chemiluminescence was first observed with lophine (triphenylimidazole). When in basic solution, this compound converts to the imidazolate, which reacts with oxygen to eventually give a dioxetane. Fragmentation of the dioxetane gives the excited state of an anionic diamide.
In a laboratory setting, chemiluminescence is demonstrated by the luminol test, which indicates the presence of blood through luminescence upon contact with iron in haemoglobin. Chemiluminescence has been applied by forensic scientists to solve crimes. In this case, they use luminol and hydrogen peroxide. The iron from the blood acts as a catalyst and reacts with the luminol and hydrogen peroxide to produce blue light for about 30 seconds.
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Incomplete combustion results in a yellow flame
The colour of a flame depends on the type of fuel used and the level of oxidation the carbon molecules undergo during combustion. When there is sufficient oxygen, all carbon atoms are converted to carbon dioxide, resulting in a blue flame. However, in the case of incomplete combustion, where there is a lack of oxygen, not all carbon is transformed into carbon dioxide. Instead, carbon monoxide and particulate carbon, in the form of soot, are produced. Soot is composed of pure carbon molecules, which burn with a bright yellow-orange flame.
Wax, the fuel in candles, is a complex carbon fuel that cannot undergo complete combustion in atmospheric oxygen. As a result, it produces a yellow flame due to the presence of particulate carbon and soot. The typical yellow colour of a candle flame is primarily caused by the hot soot particles formed during incomplete combustion. These soot particles are solid carbon particles that act as excellent blackbody radiators in the yellow to red spectrum.
The colour of a candle flame is not just limited to yellow. The inner core of the flame, which is the hottest part, appears light blue due to chemiluminescence. As you move away from the centre of the flame, the temperature decreases, and the colour transitions from blue to yellow, orange, and finally, red. This variation in colour within the flame is influenced by both temperature and gas excitations.
The colour of a flame can also be altered by adding certain chemicals to the wax. These chemicals are designed to glow in specific colours, such as red, green, purple, or blue, when heated to a high enough temperature. For example, burning a piece of magnesium produces a bright white flame, while potassium generates a purple flame.
It is worth noting that the combustion process of a candle flame is quite complex, and scientists are still working to fully understand its underlying chemistry and mechanics. However, the factors mentioned above provide a partial explanation for the colour variations observed in candle flames.
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Different elements produce different colours
The colour of a flame is determined by the temperature and the types of fuel being burned. The inner core of a candle flame is light blue, with a temperature of around 1400 °C, making it the hottest part of the flame. The blue colour is due to chemiluminescence. As you move away from the centre of the flame, the temperature decreases, and the flame colour transitions from blue to yellow, orange, and finally red. The yellow colour is due to radiative emission from hot soot particles, which are formed through a series of complex chemical reactions.
The colour of a flame can also be influenced by the presence of certain chemicals or elements. For example, the addition of specific chemicals to the wax of a candle can result in flames of different colours, such as red, green, purple, or blue. Similarly, burning a piece of magnesium produces a bright white flame, while potassium generates a purple flame.
The level of oxidation that the carbon molecules undergo also plays a role in determining flame colour. In the presence of sufficient oxygen, all the carbon atoms are converted into carbon dioxide, resulting in a blue flame. However, when there is insufficient oxygen, not all the carbon is transformed into carbon dioxide, and soot is formed. Soot, composed of pure carbon molecules, burns with a bright yellow-orange flame.
The colour of light emitted during combustion also depends on the energy emitted by electrons as they return to their original state. Each element has a unique line emission spectrum, allowing scientists to identify them by the colour of the flame they produce. For instance, copper produces a blue flame, lithium and strontium produce a red flame, calcium an orange flame, sodium a yellow flame, and barium a green flame.
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Frequently asked questions
The colour of a candle flame depends on the level of oxidation the carbon molecule goes through. In the presence of sufficient oxygen, all carbon atoms turn to carbon dioxide, and the flame appears blue. However, when there is insufficient oxygen, not all carbon forms carbon dioxide. Carbon monoxide and soot are also produced, and the flame appears yellow.
A blue flame indicates complete combustion. LPG, a simple carbon fuel, burns with a blue flame.
A candle flame turns yellow due to the presence of particulate carbon and soot. Organic substances like wood and wax burn with a yellow flame.
Some candles are made by adding certain chemicals to the wax. When these chemicals are heated, they glow in a particular colour like red, green, purple, or blue.
