Expert: George Maxwell - 9/21/2007

Question
I apologize if this is in the wrong category, but I need to know...how is soot formed?

Answer
Hi, and thank you for your question.

Most solid combustibles are carbon-based (e.g. paper, coal, wood etc), and when they burn, the carbon reacts with the oxygen in the air to form carbon dioxide.

However, particularly with solids, there is often not enough air reaching the surface of the burning material to ennsure that all of the carbon in the material is broken down to carbon dioxide. In this case, the heat of burning makes the rest of the material break down in to solid particles of carbon - soot. This phenomenon also occurs in liquid fuels that contain a lot of carbon (e.g. petrol).

We're used to picturing flames as the flickering yellow we see on candles and elswhere: this yellow colour is caused by particles of soot being heated in the flame. If you burn a fuel that contains relatively little carbon (e.g. ethanol), you get a very pale blue flame.

Thanks again for your question.