Many kinds of fuel are being used at home, industry and automobiles. We all must have seen a burning candle and found that it burns with a flame giving out heat and light. Imagine burning a piece of coal. Coal too burns in air producing carbon dioxide, heat and light. But it does not produce any flame. So why do some substances produce flame while others don’t?
The chemical process in which a substance reacts with oxygen to give off heat is called combustion. Substances that undergo combustion are called combustible substances. It may be a solid, liquid or gas. Along with heat, some produce flame or glow. The substances which vaporize during burning give flames. For example combustion of kerosene oil produces flame. On the other hand certain substances like coal do not vaporize and hence doesn’t produce a flame.
A side effect of the chemical reactions (that takes place while a substance is burning) is a lot of heat. The chemical reactions in a fire generate a lot of new heat that sustains the fire. Many fuels like gasoline burn in one step. Heat vaporizes gasoline and it all burns as a volatile gas. There is no char. A candle on the other hand slowly vaporizes. As they heat up, the rising carbon atoms emit light. This "heat produces light" effect is called incandescence. This is what causes the visible flame. Flame color varies depending on what is burnt and how hot it is.
Imagine burning a piece of wood. Some of the decomposed material is released as volatile gases. These gases form smoke. Smoke is compound of hydrogen, carbon and oxygen. The rest of the material forms charcoal, which is nearly pure carbon, and ash, the unburnable minerals in the wood (calcium, potassium, and so on). Charcoal is wood that has been heated to remove nearly all of the volatile gases and leave behind the carbon. That is why a charcoal fire burns with no smoke.