Whenever wood, coal or some other substance burns, smoke comes out. Smoke is produced due to the incomplete combustion of fuels. If complete combustion of fuels takes place, no smoke will be produced. In fact smoke is a colloid of solid particles in a gaseous form.
Most of the fuels contain carbon, hydrogen, oxygen, nitrogen and small quantities of sulphur. When any fuel burns, we get carbon dioxide, water vapour, nitrogen and traces of sulphur dioxide. For the complete combustion of fuels a large quantity of oxygen is required because burning is a process of oxidation. Due to the lack of oxygen, fuels do not burn completely. As a result of this smoke is produced. Smoke mainly consists of carbon dioxide, water vapour and carbon particles. When the number of carbon particles present in the smoke is large, it appears gray or dark. These very carbon particles get deposited on the walls of the chimneys. This deposit is called soot.
Smoke is the biggest pollutant of air and is very harmful from the health point of view. Today it has become a major problem in the cities. If it does not get scattered by wind, the atmosphere of the cities becomes highly foggy. Smoke is particularly harmful for the heart and lungs and is the cause of many diseases. It is also harmful to the eyes.
However, in certain respects, smoke is useful also. It is used to protect the orchards from cold. In wars, smoke helps in camouflaging. It is also helpful in rain formation by enabling the water vapour to condense on its particles.