Chemistry of Fire

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Chemistry of Fire
What is fire?
• Answer : Fire is heat and light from
rapid combination of oxygen and
other materials.
• For fire to exist, a combustible
substance must be present, the
temperature must be high enough to
cause combustion, and enough oxygen
must be present to sustain rapid
combustion.
Chemistry of Fire
• Oxygen combines with other
substances to produce new products –
oxidation
• CH4 + 2O2  CO2 + 2H2O
• Stored chemical energy converted to
energy in the form of heat and light.
• Energy in chemical reaction comes
from the breaking and formation of
bonds.
• Heat of combustion – exothermic
• Activation energy needed to start the
combustion – ignition temperature
• Ignition Temperature: Minimum
temperature at which a fuel will
spontaneously ignite.
• Fire continues to burn until the supply
of oxygen is exhausted or fuel is gone
• Activation energy –the minimum
energy necessary for a specific
chemical reaction to occur i.e.
matches, electrical discharges,
sparks, and chemicals.
• Fuel needs to be in gaseous state to
ignite – temperature must be high
enough to vaporize the fuel.
• Spontaneous Combustion – is a type of
combustion which occurs without an
external ignition source.
• Heat builds up to the point of ignition
• Use chemicals to supply oxygen –
oxidizing agents
Requirements for
Combustion
• A fuel must be present
• Oxygen must be available in sufficient
quantity
• Heat must be applied to initiate
combustion, and sufficient heat must be
generated to sustain the reaction.
• Rate of oxidation (reaction): Rate of
oxidation of a fuel must be capable of
sustaining a flaming fire.
• Flash Point: Minimum temperature at
which a liquid fuel will produce enough
vapor to burn.
• Flammable Range: The (range of)
composition of fuel-air mixture
required for combustion.
• Flammable range of gasoline is 1.36%.
Detonation
• The speed at which explosives decompose
(detonation) varies greatly and permits the
classification of explosives as high and low
explosives.
• Low Explosives: Low explosives have velocities
of detonation less than 1,000 m/s.
• Black powder: (KNO3:Carbon:Sulfur::75:15:10):
Merely burns when unconfined, used as safety
fuse because of its low rate of detonation (rate
of deflagration)
• High Explosives: classify high explosives as
primary and secondary explosives .
• Primary explosives are very sensitive to
heat and shock and will detonate violently
(not burn).
• Lead azide and diazodinitrophenol are
examples of Primary explosives.
• TNT is an example of a secondary
explosive.
Difficult to Analyze Arson
(i) These crimes are carried out at the
convenience of the perpetrator and are
often "well-planned" to hide crucial
evidences
(ii) Inability to collect crucial/useful
evidence due to the accompanied
destruction of the crime scene
(iii) volatile evidences are hard to collect and
preserve.
Searching the Fire Scence
• Must be done soon after the suspected fire.
•
Determine fire's origin (use of
streamers to ignite multiple points at the
same time)
• Normally, a fire has a tendency to move in an
upward direction. The origin of the fire may be
closest to the lowest point that show the most
intense characteristic of burning. (weather
condition will alter the normal progression of
fire)
• GC-MS is the analytical method of choice

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