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Unit 10: Forensic Fire Inves ga on
Aim A - Explore the chemistry of combus on and methods for ex nc on and heat transfer
Assignment report: Inves ga ng the chemistry of re
Introduc on to the re tetrahedron and combus on
Combus on refers to the chemical reac on that allows a substance to burn. In this inves ga on, it was
important to consider the factors making up and a ec ng combus on.
Fire needs a chain reac on in order to stay lit. This requires the interac on of four factors, commonly
referred to as the re tetrahedron:
This is an improvement on the previously Consists of:
learned re triangle, which consisted of
only heat, fuel and oxygen. However, as
- Heat
discovered in this unit of the course, it is
- Oxygen (O2)
essen al to consider the chemical
- Fuel
reac on that ignites a ame and allows
- A chain chemical
for it to stay lit for any amount of me. reac on
(Shown by gure 1)
All four elements must be present for the occurrence of re. If
any of these essen al elements of combus on, the re will not
be able to be sustained and will be ex nguished.
Figure 1 - Fire Tetrahedron
The general equa on for complete combus on is
as follows: h ps://www.highspeedtraining.co.uk/hub/ re-triangle-tetrahedron-combus on/
Fuel + Oxygen ——> Water + Carbon dioxide
(Heat)
The products of complete combus on tested for using various equipment. One method (in a drawn
diagram) can be seen below:
CO2 (g)
U-tube
H2O (g)
Tube to
Inverted funnel
vacuum pump
Cobalt
Methane gas
chloride
burning
paper CO2 (g)
Limewater,
H2O (l) milky in the
Bunsen burner on low
Ice presence of
CO2
ame Ice water Test tube
Water condenses
here
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Figure 2 - annotated diagram of the products of combus on
(Animated diagram - h p://www.learnaboutair.com/science/sec on_03.html)
In this demonstra on, the equa on for combus on is as follows:
Methane + Oxygen ——> Carbon dioxide + Water
(Heat)
CH4 + 2O2 ——> CO2 + 2H2O
(Heat)
The apparatus was set up as shown. The vacuum pump was necessary to draw air through the tubes,
carrying with it the products of combus on through the apparatus.
- Fuel: the fuel used in this demonstra on is methane (CH4)
- Heat: the heat needed for combus on is provided by the ame of the bunsen burner (orange ame)
- Oxygen: present in environment (O2)
- This all allows for chain reac ons to occur and complete combus on
The products of combus on
The rst of the products of complete combus on is water (H2O). In this demonstra on, there is evidence of
this shown in two ways:
- H2O is condensed in the tube that sits in the iced water; the cold temperature of the iced water around
the tube allows for the gas to condense, and so water is formed here.
- The cobalt chloride paper turns from blue to pink in the presence of water.
The second product of complete combus on is carbon dioxide (CO2). Limewater (calcium hydroxide) turns
“milky” in the presence of carbon dioxide as it forms a precipitate (calcium carbonate). The equa on for this
is as follows:
Calcium Hydroxide + Carbon Dioxide ——> Calcium Carbonate + Water
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Ca(OH)2 + CO2 ——> CaCO3 + H2O
Alterna vely, carbon dioxide cane tested for by using a hydrogen carbonate indicator, which turns from red
to yellow in the presence of CO2.
Inves ga ng combus ble materials
A combus ble material, under the condi ons an cipated, will ignite, burn, support combus on or release
ammable vapours when subjected to re or heat. A ammable material is a material that ignites easily at
ambient temperatures. In this next sec on, some examples of combus ble materials were inves gated,
considering their ini al ammability (ease of burning or igni ng), the amount of heat given o , the length
of me the ame lasted, and whether the combus on was complete or incomplete.
Incomplete combus on (general equa on):
Fuel + Oxygen ——> Carbon Monoxide + Carbon + Water
(Heat)
EXAMPLE: the incomplete combus on of propane
2C3H8 + 9O2 —-> 4CO2 + 2CO + 8H2O
Propane + oxygen —-> carbon dioxide + carbon monoxide + water
(Although carbon dioxide is produced here, it is incomplete because carbon monoxide is also formed, being
evidence of a lack of oxygen)
Incomplete combus on occurs when there is not enough oxygen present (for example the environment
does not contain su cient oxygen). As a result, carbon is not completely oxidised, and so carbon monoxide
(CO) is produced (carbon dioxide contains two oxygens, carbon monoxide contains only one oxygen). Water
and carbon is also produced. During combus on the carbon can be seen as a ame containing soot or giving
o smoke. However, carbon monoxide is a colourless and odourless gas, and so can’t be detected.
This is a problem as carbon
monoxide is toxic; it is
poisonous to humans if it
reaches a certain level in
the atmosphere. At 70
PPM (parts per million),
no ceable symptoms of
poisoning can develop
including headaches and
nausea. At sustained
concentra ons above 150
to 200 PPM,
disorienta on,
unconsciousness and
death can occur. This is
due to the fact that carbon
monoxide has a 210 mes
larger a nity for
haemoglobin (a protein in Figure 3 - annotated dissocia on curve a ected by CO poisoning
the blood) than oxygen. This means that carbon h ps://slideplayer.com/amp/4463177/
monoxide binds to the haemoglobin in the blood
instead of oxygen, displacing it and forming
carboxyhaemoglobin (COHb). As a result of the
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