Published: 1 August, 2001

Atoms change partners and 'dance'

What material is flame made of?

The things that we most identify with a flame are heat and light. However, these are not really the materials of which the flame is made. In order to answer your question we need to begin by reminding ourselves of the ingredients that are necessary to produce a flame. In principle there are just two: fuel and air, although as we will see there is a bit more to it than that.

As you probably know, the main gases that make up air are nitrogen and oxygen; it is only the oxygen that is required to make a flame. Before the flame starts, the fuel contains molecules in which hydrogen atoms are connected (or bonded, as chemists say) to carbon atoms. The oxygen in the air is also in the form of molecules; each oxygen molecule consists of two oxygen atoms bonded to each other. During the burning of the fuel, the atoms exchange partners. Carbon-carbon, carbon-hydrogen, and oxygen-oxygen bonds are broken, and new carbon-oxygen and hydrogen-oxygen bonds are made. In the end the fuel plus oxygen is converted to just two kinds of molecules containing only carbon-oxygen and hydrogen-oxygen bonds. These molecules are carbon dioxide and water. However the fuel molecules contain lots of carbon-carbon and carbon-hydrogen bonds, and these do not all get broken at once. So in the flame there are fuel molecules that are in various stages of breaking their carbon-carbon and carbon-hydrogen bonds and replacing them with bonds to oxygen from the air. These really are the materials of which the flame is made. The reshuffling of the atoms releases lots of energy in the form of the heat and light that we usually think of as being a flame.

It is common in science for the answer to one question to lead in turn to several new questions. Here are some questions that you might like to explore with one of your teachers:

1) I said that hydrocarbons have carbon-carbon and carbon-hydrogen bonds. They don't have hydrogen-hydrogen bonds. Why not?

2) In addition to fuel and oxygen, you usually need some source of heat to get a flame going (such as a match to light a candle). Why is that?

3) I said that the reshuffling of the atoms in the molecules of fuel and oxygen to give carbon dioxide and water releases lots of energy in the form of heat and light, but why does it?

I hope you have fun finding out the answers.