The flame of a candle appears yellow and orange, but it also emits a faint red glow. This red color is an important indicator of the chemical reactions occurring within the flame. In this article, we will explore why candle flames have a reddish tinge and what this reveals about the combustion process.
The Chemistry of Combustion
A candle flame is a complex chemical reaction involving molecules in the wax and oxygen in the air. As the wax melts, molecules called hydrocarbons vaporize and mix with oxygen. When this fuel-air mixture gets hot enough, combustion occurs and light is produced.
The primary hydrocarbons in candle wax are long chains and rings of carbon and hydrogen atoms, called alkanes and alkenes. During combustion, the hydrocarbon molecules combine with oxygen to produce carbon dioxide and water vapor. This highly exothermic reaction releases energy in the form of heat and light.
The color of the flame depends on multiple variables: the temperature, the concentration of carbon particles, and the types of hydrocarbons present. Hotter regions emit more visible light across the spectrum, appearing white or yellow. Cooler parts of the flame glow red due to the properties of carbon atoms recombining.
The Role of Carbon Atoms
Carbon atoms are the key to the color of a candle flame. As hydrocarbon molecules break apart and react with oxygen, some carbon atoms don’t get attached to oxygen atoms. These lone carbon atoms, called “soot,” get very hot and emit a faint red glow.
When hydrocarbon chains are broken up during combustion, carbon atoms can join together to form small soot particles. The hot soot particles glow red and orange, contributing to the yellow and orange hues in the candle flame.
However, blue and violet light is also emitted by excited carbon particles. Most of this blue light gets absorbed by the cooler outer parts of the flame. The remaining unabsorbed red wavelengths are what give candle flames their distinctive red tinge.
Blackbody Radiation
The carbon particles in the flame behave like a blackbody radiator. A blackbody is an idealized object that absorbs and emits all frequencies of light. According to physics, blackbodies glow with a color that shifts from red to orange to yellow to white to blue as they get hotter.
The soot particles in a candle flame act as miniature blackbody radiators, emitting a faint red glow at temperatures around 1300-1600°C. If the flame got hot enough, these particles would eventually emit orange, yellow, and blue hues as well. But the coolest outer edges of the flame only give off red wavelengths.
Spectral Lines of Excited Atoms
In addition to blackbody radiation from soot, the emissions of excited atoms contribute to the red color. Atoms of carbon, oxygen, nitrogen, and other elements absorb heat energy from collisions and chemical reactions. Excited electrons jump to higher energy levels then emit photons as they decay back to the ground state.
These emission spectral lines from excited atoms tend to be narrow wavelengths. Red spectral lines around 630-650 nm are emitted by lithium, strontium, and neon atoms that mix into the flame. The combination of broad blackbody radiation and atomic spectral lines gives candle flames their red glow.
Role of Temperature Gradients
The temperature inside a candle flame varies dramatically. Near the wick, where combustible gases are produced, temperatures can exceed 1400°C. The hottest regions emit white and yellow light. Further from the wick, temperatures drop off quickly, reaching 500-600°C at the outer edges.
This steep temperature gradient is responsible for the flame’s layered appearance. The hot interior glows yellow and orange, while the cooler exterior portion emits a faint red glow. The blue and violet emissions from excited atoms get absorbed by the intermediate yellow band.
This effect is similar to the way Earth’s atmosphere scatters blue light. If our atmosphere was thinner, the sky would appear violet. The scattered red wavelengths give the candle flame its distinctive coloration.
Effect of Soot Concentration
Higher concentrations of soot particles increase the intensity of the red glow. As more hydrocarbons are vaporized, the flame generates more carbon atoms that can join into soot particles. Candles that produce smokier, sootier flames have a more pronounced reddish tinge.
Conversely, cleaner burning candles have less soot and appear more yellow. Propane and natural gas flames have minimal soot and radiate a bright blue color. The lack of large carbon molecules means no blackbody-emitting soot particles.
Hydrocarbon Composition of the Fuel
The types of hydrocarbons in the candle wax also influence the red emission. Hard paraffin wax tends to have longer hydrocarbon chains than softer beeswax. Longer carbon chains release more soot when broken up during combustion.
Candles with lots of large, complex hydrocarbon molecules will produce more carbon soot particles. The increased blackbody radiation results in a deeper red coloration. Soft waxes produce minimal soot and emit cleaner, yellow flames.
Opacity of the Flame
A more opaque, less transparent flame results in more red glow. Dense, sooty flame interiors absorb more blue and violet light. This greater absorption means less violet and blue reaches your eyes. The flame appears redder when it is thick and opaque.
Conversely, a cleaner flame has less soot and is more transparent. More blue wavelengths travel through the flame unimpeded. This makes the flame appear less red. However, even a clean wax flame still has some faint red emissions.
Conclusion
The red glow from a candle flame comes from multiple processes. Blackbody radiation from hot soot particles accounts for much of the red color. Additionally, spectral lines from excited atoms and the absorption of blue/violet wavelengths gives the flame a faint red tinge.
While the candle flame appears yellow-orange overall, it emits light across the visible spectrum. Temperature gradients and soot concentration accentuate the red glow. Understanding the chemistry and physics behind combustion reveals why candle flames have an inner red layer.
The next time you see a candle burning, look closely at its ghostly red aura. This faint glow is a visible clue to the intricate chemical dance occurring inside the flame. Subtle variations in fuel, temperature, and smoke all determine the exact red hue.
So in summary, the red tips that flicker atop burning candles originate primarily from blackbody radiation of incandescent soot particles. The cooler outer edge of the flame allows this faint red glow to become visible against the brighter yellow emissions. Excited atomic emissions and preferential absorption of blue/violet light also contribute to this diagnostic red hue that reveals the energetic reactions of hydrocarbon combustion.
The candle flame contains secrets waiting to be uncovered by an inquisitive observer. Its aurora of red informs us of the carbon atoms freed during burning. Tiny particles of soot bridge the gap between molecular bonds breaking and new bonds forming. The story of combustion unfolds through the red colors accompanying this elegant, dancing chemical reaction. When we light a candle, we ignite insight into chemistry.
