Lesson 1, Topic 1
In Progress

1.4. Chemistry Of Fire

ryanrori July 16, 2020

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In less than 30 seconds a small flame can get completely out of control and turn into a major fire. It only takes minutes for thick black smoke to fill a house. In minutes, a vehicle/house can be engulfed in flames. Most fires occur in the home when people are asleep. If you wake up to a fire, you won’t have time to grab valuables because fire spreads too quickly and the smoke is too thick. There is only time to escape.

Fire is a chemical reaction known as combustion that occurs when fuel and oxygen are brought together with enough heat to cause ignition.

These elements are required to cause and sustain a fire, should any of the required elements be removed the triangle will collapse therefore combustion will cease.

Each of these elements is explained as follows:

Fuel

Fuel can be any combustible material, solid or gas.  Most solids and liquids need to change into gas before they will burn.

Examples of fuels

Solids Liquids Gasses
Paper, wood, cloth, rubber, plastic, leather, wax and grass Petrol, paraffin, alcohol, paint, oil and varnish LPG, butane, acetylene (found in nail polish remover), propane and hydrogen

Oxygen

The air we breathe consists of 21% oxygen (0²).

An atmosphere of at least 16% oxygen (0²) is needed to sustain a fire.

In the process of burning, a fire consumes oxygen.  It is therefore possible for a fire to use up all the oxygen in a closed space. When all the oxygen is used up – no fire!

BEWARE: a smouldering fire can re-ignite when exposed to air (oxygen).  Do not open doors and windows until it is safe.

Heat / Ignition

Heat is the energy necessary to increase the temperature of the fuel to a point where sufficient vapours are given off for ignition to occur.

A critical temperature must be reached for any fuel to vaporize and ignite.  Once a fire has started, it will generally maintain its own heat supply.  Heat is also called source of ignition.

Heat can be defined as ignition or continuous heat.  Normally ignition heat is from an external source as stated in the example on the next page.

The only time that ignition heat is not from an external source is when spontaneous combustion takes place.  Examples of these heat sources are the mixture of chlorine and brake fluid or chlorine and coke.  After ignition, the external source does not play a role in generating further heat.  An existing fire generates its own heat.

Chemical heat Electrical heat Mechanical heat Solar/radiation heat Nuclear heat
This is generated by mixing chemical substances Overloading of an electrical conductor Friction of two surfaces moving against each other or

Compression of gas in a cylinder

The concentration of heat sends out rays This is generated when atoms are split.  (controlled at power stations.)

Oxygen (o2)

heat / ignition

combustible material (fuel)

Chemical reaction

A chemical reaction can occur when the above three elements: fuel, oxygen and heat, are present under the right conditions and in the proper proportions.  Fire occurs when this rapid oxidation or burning takes place.

Since fires can be prevented by keeping these ingredients (elements) separate, we keep combustible materials (fuels) away from hot equipment and make sure that flames from welding torches do not get into contact with grease or oil.

Extinguishing is an attempt to . . .

  • smother (remove oxygen)
  • quench (interfere with chain reaction)
  • starve (remove fuel)
  • cool (remove heat) a fire.

Some types of extinguishing media make use of a combination of these methods.