Blow out the match and examine the underside of the saucer. Follow the same procedure, using candle, lighter, and burning paper. Discuss what was observed and conclude that whenever the yellow part of a flame came in contact with a cool dish, a black substance was deposited. Moisture also was present. Explain to the students that anything that will burn can be called a fuel.
Most fuels contain carbon, hydrogen, or both.
When carbon burns incompletely, which is usually the case, it glows with a yellow color. A flame is made of tiny particles of very hot carbon. When the particles cool quickly, as they did on striking the cool saucer, they were deposited as black carbon. The moisture formed on the saucers, because every flame gives of water vapor. This is because the fuel contains hydrogen, which reacts with oxygen in the air to form water vapor.