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Unit 36 - Forensic Fire Investigation Assignment 1 (Pass)
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Assignment 1 – Fire InvestigationThe Chemistry of Combustion and Extinction
What is combustion?
Combustion is known as the scientific term for burning. Essentially, combustion is a chemical reaction whereby
oxygen combines with a substance (usually referred to as a fuel) to produce heat and light in the form of a
flame. Combustion reactions occur at high temperatures. Combustion is an example of an exothermic reaction;
an exothermic reaction is a type of reaction in which energy becomes released into the surroundings. The
energy released is typically thermal energy, however, both light and sound energy can be released. Flames
have a complex structure and a definable composition; flames are deemed to be multiform and have the
ability of existing at rather low temperatures, along with extremely high temperatures. A flames emission light
tends to be the result of the presence of excited particles and generally, of charged atoms, electrons and
molecules. The products of combustion reactions are compounds of oxygen (O), known as oxides. The
products of a complete combustion reaction involve carbon dioxide (CO 2) and water vapor (H2O). If there is
plenty of air, then complete combustion will take place. The typical equation for a complete combustion
reaction Is as follows;
Fuel + Oxygen Carbon Dioxide + Water
Fuel + O2 → CO2 + H2O
An example of this is methane (a natural gas (CH 4)). Below is the equation of its complete combustion:
methane + oxygen → water + carbon dioxide CH 4 + 2O2 → 2H2O + CO2
However, if there isn’t enough oxygen or air for complete combustion to take place, incomplete combustion
will instead take place. Carbon dioxide and water vapour are still produced, but, two other different products
are produced, these are; carbon monoxide (CO) this is a colourless toxic gas and also particles of carbon, these
can appear as smoke and soot which can cause breathing problems.
The Fire Triangle
The fire triangle is one of the simplest method of understanding the elements of fire. Although its called a fire
triangle, occasionally it is called a fire tetrahedron the reason for this will be explained below. This is because
in reality for things MUST BE present at the SAME TIME in order for a fire. The three main elements needed for
a fire is Air, Fuel and heat these are frequently referred to as the “fire triangle”. The occasional fourth is a
chemical chain reaction, this is what creates the “fire tetrahedron.” An important thing to remember is; if one
of just any of these elements get removed, a fire no longer has the ability to burn and will be extinguished.
This is the fire triangle. This is the fire tetrahedron.
Principles of Combustion
1. There must be Fuel to burn.
2. There must be Air to oxygen to be supplied.
3. There must be Heat to start and continue the process of combustion.
1
, Assignment 1 – Fire Investigation
4. There must be a chemical chain reaction to sustain a fire.
Below are the four conditions which are essential for any fire explained in more depth:
1.) Fuel (In the form of a solid, liquid or gas.)
Fuel is any kind of combustible material. This includes; solid combustibles such as; plastics, clothing, paper
and furniture. Flammable liquids such as; oils, paints, petrol, cooking oils/fats, kerosene and solvents.
Flammable gases such as; LPG, natural gas, and acetylene. Fuel is essentially characterised by its size,
quantity, moisture content, shape, as well as the arrangement in which it will spread over a landscape.
Moisture content is an important aspect as this is what determines how easily a fire will burn.
2.) Oxygen
Oxygen is generally readily available. The air we breathe contains about 21% of oxygen and the majority of
fires only require at least 16% oxygen content to burn. Oxygen provides support to the chemical processes
which take place during a fire. When fuel burns, it reacts with oxygen present in the surrounding air, this
releases hear and generates combustion products such as smoke, embers, gases, etc) through a process
which is known as oxidation.
3.) Heat
A heat source is responsible for the initial ignition of fire. Also, heat is required to maintain the fire, and
this is what enables it to spread. Heat is what allows a fire to spread, it does this by drying out and pre-
heating fuel nearby as well as warming the surrounding air. Essentially, the heat which is given off through
the oxidation reaction is what sustains the fire once ignited.
4.) Chemical Chain Reaction
A chemical chain reaction becomes enabled through the reaction between heat, fuel and oxygen. A chain
reaction refers to the self-perpetuating capability of combustion. Due to the continuous reactions occurring
between fuel and oxygen it creates a generate surplus amounts of heat energy, however, the flame will always
be hot enough to keep the fuel at its ignition temperature. So, as long as there is enough oxygen and fuel the
fire will continue to burn. Flames chemically progress through branched chain reactions; whereby constant
free radicals are being formed and consumed.
Chemical Reactions Involving fuel, oxygen and heat
All mater consists of energy therefore, when a chemical reaction takes place bond-bonding and bond-making
processes change the energy of a system when reactants become converted into products. For example;
energy is required for bonds to be broken in reactants. However, energy becomes evolved when bonds are
formed in the products. Usually these energies aren’t equal and due to this the reaction becomes
accompanied by an energy change, this is manifested by either an increase or a decrease in the surrounding
temperatures.
Energy Profile for an Exothermic Reaction:
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