Combustion
When a substance burns, it reacts with oxygen. This is known as combustion.
All combustion reactions are exothermic because they release energy, eg heat energy is given out when methane is burned in a bunsen burner.
As combustion is the reaction of a fuel with oxygen, then it is obvious that oxygen must be present for combustion to take place. Normally, the oxygen required in combustion reactions comes from the air.
Only 21 per cent of the air we breathe is oxygen, 78 per cent is nitrogen and 1 per cent other gases.
Fire can be explained using a fire triangle. The triangle has three sides, fuel, oxygen and heat.
Each of these sides represents a key element for fire to occur. If one of them was removed, the triangle would collapse and the fire would go out. Fire-fighting relies on this principle. The fire will extinguish once the fuel runs out, but it is often unsafe to leave fires that long. Water removes the heat, but different types of fires need to be tackled in different ways.
Chip pan fires
Chip pan fires should be extinguished by covering it with a damp cloth. This stops the oxygen getting to the fire. Water must NEVER be put on a chip pan fire.
Petrol fires
There are buckets of sand on petrol forecourts. Adding sand to a petrol fire stops the oxygen getting to the fuel.
Electrical fires
Pouring water on electrical fires to remove the heat is very dangerous. If a carbon dioxide fire extinguisher is used, no oxygen can reach the fuel.
Other fire extinguishers such as powder and foam prevent oxygen from getting to the fuel.
