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applied science- unit 25 A part 2 - chemistry - distinction level
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Chemistry researchChemistry of combustion
Fire tetrahedral
the fire tetrahedral, also refers to as the fire diamond or the combustion triangle, is a diagram of a
pyramid with each side representing the properties needed for a fire to occur. these properties are;
Fuel
Heat
Chemical reaction
Oxidising agent
What are the properties?
Fuel: if there is no object or material to feed the flames it is impossible for fire to start. Fuel can be
defined as any material which are flammable or combustible. The materials include things such as
wood fabric paper, etc. the fire will extinguish naturally when the fuel is completely consumed.
Heat: heat will cause the fuel s source to lose moisture and raise the temperature to its flashpoint,
the lowest temperature of the material at which ignition occurs. examples of heat source our
matches cigarettes lighter magnifying glass etc. In simply terms, it is the energy required to ignite the
fuel and start combustion
Chemical reaction: The chemical reaction, also refers to as exothermic chemical reaction, is a
reaction cycle used to produce a fire. this cycle begins by having the fuel releases moisture due to
the heat. the fuel vapor reacts with the oxygen (oxidising agent) through combustion causing fire to
spread.
Oxidizing agent: this is any substance that supply oxygen to support the combustion of the materials
to create fire. A typical oxidising agent is the oxygen found in the atmosphere
What happen when you remove one of the properties?
Fuel: when fuel is removed from the fire tetrahedral, the fire will naturally stop through an
extinction method called starvation. Removing the fuel source can be done by simply taking away
any combustible material and placing fire resistant material around the fire or by using a certain type
of fire extinguisher.
Heat: when heat is removed from the fire, the fire will eventually extinguish through an extinction
method called cooling. Removing heat from the fire can be done by cutting of the source of heat by
using a cooling agent, usually water. The cooling agent will absorb the heat faster than the heat
being produced thus lowering the temperature and making it unable to ignite.
Oxygen when an oxidizing agent is removed from the fire, the fire will naturally stop through an
extinction method called starvation. Removing oxygen from the fire can be done by cutting off the
supply of oxygen (oxidizing agent) from the fire. This can be achieved by using a certain type of fire
extinguisher, fire blanket, sand, etc
, Chemical reaction: the chemical reaction is hindered only when one of the 3 elements (fuel, oxygen
and heat) is removed. This is also the reason why it is placed at the bottom of the pyramid because
only when all the properties are present can chemical reaction occur
In the oxygen jar experiment, it shows exactly how important oxygen is needed to be present to
maintain a fire as represented in the fire tetrahedral. this is shown when the largest beaker which
contain a high volume of oxygen can maintain a flame longer than the smallest beaker which has a
low volume of oxygen. The largest beaker can maintain the flame much longer than the smaller
beaker because it had a larger supply of oxygen. This also show that if there is no oxygen precent,
the fire will extinguish
The ignition sources
An ignition source is simply a material or object that causes a fire to start. There are 2 sources of
ignition which are;
Primary source of ignition
Secondary source of ignition
Primary source of ignition:
A primary source of ignition happens when an open flame (the primary source) causes a flammable
or combustible material to catch on fire. For example, if you light a blow torch and used it to set fire
to a chair then the blow torch is the primary source of ignition. examples of primary source of
ignition are cigarettes, matches, candles, bonfire, etc
Secondary source of ignition
A secondary source of ignition happens when a flammable or combustible catches on fire. For
example, if you use a blow torch and set a chair on fire then the chair is considered the secondary
source of ignition. Example of secondary source of ignition are Funiture, carpets, waste bins, etc
Flamming and smouldering combustion
Flaming combustion
Flaming combustion is a type of combustion that produces a flame. flaming combustion is a chemical
reaction in which the fuel reacts with an oxidising agent to produce heat and light which is the flame
we see. The fuel can be solid, liquid or gas but when the fuel is heated up, it produced a highly
combustible vapour called pyrolysis which interact with the oxygen in the air and through
combustion creates flame. This means that the reaction that occurred is a gas-to-gas reaction
because the fuel needs to change it states to gaseous state for it to interact with the oxidising agent.
The flaming combustion is an exothermic reaction which means it releases energy in the form of
heat. Example of flaming combustion are candles, matches cigarettes lighters.
Smothering combustion
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