Candle flame

The elegance and light of a candle flame are the result of a lot of chemistry and physics. For hundreds of years, scientists have been enamoured by candles. Michael Faraday delivered his now-famous series of lectures on the Chemical History of a Candle in 1860, displaying dozens of scientific principles through careful observations of a burning candle. NASA took candle research to new heights in the late 1990s, conducting space shuttle experiments to gain knowledge about the behaviour of candle flames in microgravity. Candle experiments are still being conducted by scientists in universities and research laboratories across the world in order to learn more about candle flames, emissions, and combustion. And, of course, thousands of students each year study the principles of heat, light, and combustion as part of their school science curriculum.

Burning of candles

All waxes are hydrocarbons, which means they are mostly made up of hydrogen (H) and carbon (C) atoms. The intense heat melts the wax near the wick when you light a candle. Capillary action draws the liquid wax up the wick. The heat from the flame vaporises the liquid wax (turns it into a hot gas) and begins to break down the hydrocarbons into hydrogen and carbon molecules. These vaporised molecules are drawn into the flame and react with oxygen from the surrounding air to produce heat, light, water vapour (H2O), and carbon dioxide (CO2). One-fourth of the energy released by the combustion of a candle is given off as heat emanates from the flame in all directions.

The reason behind the candle flame pointing up

Whenever a candle burns, the flame warms the surrounding air and causes it to rise. As this warm air rises, cooler air and oxygen move in at the flame’s base to replace it. When the cooler air is heated, it rises and is supplemented by cooler air at the flame’s base. This results in a continuous cycle of upward airstream around the flame (a convection current), giving the flame its elongated or teardrop shape. Because “up” and “down” are functions of the earth’s gravity, scientists wondered what a candle flame will indeed look like in outer space, where gravity’s pull is minimal and there isn’t really an up or down.

Colours of candle flame

The heat from the flame melts the wax beneath the candle and draws it up through the wick. The same principle applies here about the law of capillary attraction as when you hang a cloth on the edge of a sink and it begins to soak up the water up the length of the cloth. The candle flame heats up sufficiently to vaporise the wax and convert it to a gas of carbon dioxide and water vapour. It’s normal for a candle to flicker and shine for the first few minutes as the combustion process settles. When the flame is properly burning, you’ll notice a little contrast and brightness to the flame.

The blue region

A blue region at the base of a candle flame can be seen if you look closely. Above that is a small dark orange-brown section, followed by the large yellow region associated with candle flames. The oxygen-rich coloured zone is where hydrocarbon molecules vaporise and begin to disintegrate into hydrogen and carbon atoms. The hydrogen separates first and reacts with both the oxygen to form water vapour. Some of the carbon here burns to produce carbon dioxide.

The orange/brown region

There is very little oxygen in the dark or orange/brown region. This is the stage at which the different forms of carbon continue to degrade and small, hardened carbon particles begin to form.A brownish/orange area appears just above the blue area. The crackling sound (Pyrolysis) of a candle occurs here as the fuel loses oxygen. This, in turn, produces minute particles at approximately 1800° Fahrenheit.

The yellow region

Carbon particles enhance and rise upwards as we work our way up the candle flame until they combust/ignite to generate the yellow luminescence light in which carbon particles begin to form around 1400° F. The formation of carbon (soot) molecules increases at the bottom of the yellow zone. As they rise, they raise the temperature up until they kindle to incandescence and emit the entire visible light spectrum. The human eye perceives the flame as yellowish because the yellow part of the spectrum has been the most dominant when the carbon ignites. The temperature is generally 1200⁰ C when the soot particles oxidise near the top of the flame’s yellow region.

The fourth region

The fourth zone of the candle (also known as the veil) is the faint blue edge that stretches from the blue zone at the base of the fire up the sides of the flame cone. It is blue because it comes into direct contact with the oxygen in the atmosphere. It is blue because it comes into direct contact with the oxygen in the air, and it is the warmest part of the flame, generally reaching 1400⁰ C. (2552⁰ F).

Conclusion

A quarter of the energy produced by a candle flame is dissipated as heat, which radiates in all directions. Only 4% of the heat from the candle is used to melt the wax. A candle has three main reaction zones. The primary reaction zone is the area where combustion begins. The burning process comes to an end in the main reaction zone. The luminous zone is the location where free carbon burns and emits light. The process of burning a candle flame is extremely complicated. Capillary action draws liquid wax up the wick and vaporises it with oxygen. The remaining carbon dioxide and water combine to form a variety of complex carbon-rich particles known as soot. Soot rises to the top of the flame, where it is consumed.