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Combustion is a chemical reaction in which a substance combines with oxygen and gives out heat, usually with light as well. Nothing burns without oxygen, which is why oxygen is called the supporter of combustion while the thing that burns is called combustible.
Three conditions must be met together for a fire to start. A combustible substance must be present. A supporter of combustion — oxygen from the air — must be available. And the substance must reach its ignition temperature, the lowest temperature at which it catches fire.
Remove any one and there is no fire, which is also the whole logic of putting one out. Take away the fuel, cut off the oxygen with a blanket or carbon dioxide, or bring the temperature below the ignition point with water. Fire fighting is these three conditions read backwards.
Substances with a very low ignition temperature that catch fire easily are called inflammable — petrol, alcohol and liquefied petroleum gas among them.
SSC CGL and CHSL, RRB NTPC and Group D, BPSC TRE-4 general science and the CTET science paper all draw on this chapter, and it is asked far more often than its size suggests.
1. Which zone of a candle flame is the hottest?
2. Which fuel has the highest calorific value?
3. The lowest temperature at which a substance catches fire is called what?
4. Which variety of coal has the highest carbon content and burns almost without smoke?
5. On which of the following fires should water NOT be used?
6. The yellow luminous colour of a candle flame is caused by what?
7. Calorific value is measured in which unit?
8. Which poisonous gas is produced by the incomplete combustion of a fuel, and can be fatal in a closed room?