Flames are an integral part of burning and combustion, and clearly provide strong evidence that chemical reactions may involve large energy changes. But why is a flame formed? What are its properties? This course extract sets out to provide some answers to these questions. In the process, we discover quite a lot about the way in which many chemical reactions actually occur.

"I propose to bring you, in the course of these lectures, the Chemical History of a Candle … There is no better, there is no more open door by which you can enter into the study of natural philosophy…" - Michael Faraday

These words are taken from a very famous series of Christmas lectures for young people given by Michael Faraday (1791-1867) when he was director of the Royal Institution in London in the mid-19th Century. (Faraday’s lectures were originally published as M. Faraday, A Course of Six Lectures on the Chemical History of a Candle, Royal Institution of Great Britain, London, 1861.) Faraday carried out pioneering experiments in both physics and chemistry during his time at the Institution and is justly recognized today as one of the great scientists of his era. Characteristically, the theme for his lectures was selected on the basis of the simple and familiar; hence the burning candle. This commonplace phenomenon provided the reference point to describe what was then known about the principles of chemistry. The Royal Institution Christmas lectures still continue and are now televised, commanding an audience far larger than Faraday could every have imagined.

The candle still remains familiar, although nowadays it is largely confined to dinner tables and birthday cakes. The flame, which is self-sustaining, always has a certain fascination (you might like to light a candle and look at the flame). In simplified terms, it can be described as a "zone in which chemical reactions between gases are occurring". These reactions produce both heat and light. The temperature of the flame varies from about 600 to 800^oC in the vicinity of the wick to temperatures in the region of 1200^oC at its outer edges. A candle flame, in fact, tells us quite a lot about what we see when a whole variety of fuels burn. It is best described by dividing the ‘volume’ it occupies into regions.

The fuel of a candle is "candle wax" which is largely composed of long-chain hydrocarbon molecules each containing mainly about 30 carbon atoms. The molecular formula for a straight-chain alkane containing 30 carbon atoms is C₃₀H₆₂

The application of a lighted match to the candle melts some of the wax and the liquid formed travels up the wick and is vaporized. (Once alight, the heat from the candle flame sustains this process.) But apart from melting and vaporizing the wax, the heat has a much more dramatic effect. Some of the hydrocarbon molecules in the vapour, and also in the surface layers of the molten wax, are literally torn apart, that is bonds between adjacent carbon atoms, as well as those between carbon and hydrogen atoms, are broken.

The fragments that are formed, as shown here, each contain a carbon atom that is not fully bonded. This means that they will be highly reactive species. Indeed, they will react fairly indiscriminately with any other fragment or molecule they come across, with the result that the number of highly reactive species increases. The overall result is "chemical turmoil" and this spreads out, or diffuses, away from the wick of the candle.

Some of the hydrocarbon fragments that are formed become completely stripped of hydrogen atoms in the reactions that occur and give rise to species - again highly reactive - that contain only carbon atoms. These can combine together, usually towards the centre of the flame where the oxygen supply is limited, to give soot particles consisting of many thousands of carbon atoms. At the temperatures involved, these particles become incandescent and glow, so giving the flame its characteristic yellowish-white appearance.

The outer edges of the flame represent the so-called "main reaction zone". It is here that all of the various species, including soot particles, are eventually oxidized by oxygen in the surrounding air to give carbon dioxide and water vapour as the main products. The heat produced by the many different chemical reactions is responsible for the temperature of the flame. The blue to bluish-green tinge seen in the main reaction zone, but most visible close to the wick, is due to the presence of small molecular fragments which are highly reactive and short-lived: these emit coloured light when they are first formed.

The candle flame is an example of a diffusion flame, that is one in which the rate of combustion is controlled by the rate at which the vaporized fuel and oxygen are transported to the reaction zone. Other types of flame result from the fuel and oxygen being premixed prior to combustion. Good examples are the flames from well-adjusted natural gas burners or a butane gas burner which you may use for camping or plumbing. Soot formation in these flames is minimal because the ample supply of oxygen ensures complete conversion to carbon dioxide and water vapour.

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