Chemistry of Fire
Fire Investigation Terms
• Fire - Produced when a substance undergoes rapid oxidation
involving heat and light.
• Fire Triangle – Shows the three elements needed to produce
and sustain a fire.
• Flash Point – The lowest temperature to which a substance
must be heated in order for the substance to give off vapors
which will burn when exposed to a flame or ignition source.
• Point of Origin – The location where the fire started.
• Burn patterns –Noticeable patterns created by the fire as it
burns.
• Accelerants – Substances, such as gasoline, paint thinner, and
alcohol, that accelerate the burning process.
• Arson – A fire started deliberately.
The Chemistry of Fire
• Chemically, fire is a type of oxidation, which is the
combination of oxygen with other substances to
produce new substances.
• To start fire, the minimum temperature needed to
spontaneously ignite fuel, known as ignition
temperature, must be reached.
• The heat evolved when a substance burns is
known as heat of combustion.
The Chemistry of Fire
• An additional factor, besides the liberation of
energy, needed to explain fire is the rate or speed
at which the oxidation reaction takes place.
• A fuel will achieve a reaction rate with oxygen
sufficient to produce a flame only when it is in the
gaseous state.
The Chemistry of Fire
• A liquid burns when the temperature is high
enough to vaporize it (flash point), while a solid
must be hot enough to decompose into gaseous
products (pyrolysis).
• Glowing combustion or smoldering is burning at
the fuel-air interface, such as a cigarette.
• Spontaneous combustion, which is rare, is the
result of a natural heat-producing process in
poorly ventilated containers or areas.
The Combustion Reaction
Flaming and Glowing Combustion
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Combustion is a rapid oxidation reaction, the
combination of fuel and oxygen to form
carbon dioxide, water, and heat
Chemical reactions that give off heat are
called exothermic reactions
Incomplete combustion reactions produce
poisonous carbon monoxide
The Combustion Reaction
Flaming and Glowing Combustion
1. Necessary Components for Combustion:
• Combustion requires a number of components as shown by
the fire triangle, tetrahedron, and pentagon
– The fire triangle shows the essential components as: fuel, oxygen, &
heat
– The fire tetrahedron adds free radical reactions (chain reaction)
– The fire pentagon
adds an ignition
source
The Combustion Reaction
Flaming and Glowing Combustion
• There are two ways to interrupt a combustion
reaction: adding water to absorb heat or adding fire
retardants to interrupt the chain reaction process
• There are two major types of combustion:
– Flaming combustion - both the fuel and oxygen are in the
gaseous phase
– Glowing combustion - the fuel
is solid and only oxygen is in
the gaseous phase
The Combustion Reaction
Flaming and Glowing Combustion
2. Nature of Fuels:
• Common fuels can be classified as solids, liquids or
gases
– Gases – fuels include hydrogen gas, natural gas, methane,
and propane
– Liquids – fuels include
gasoline, fuel oil, kerosene,
and ethanol
– Solids – fuels include wood,
coal, charcoal
The Combustion Reaction
Flaming and Glowing Combustion
3. Characteristics of Fuels:
• Flash point – is the lowest temperature at which a liquid
produces enough vapor to be ignited by a small flame
• Fire point (self - ignition temperature) – is the temperature at
which there is enough heat to cause combustion even in the
absence of a source of ignition
• Flammable Range – is a measure of the percentage of fuel
that, when mixed with air, is needed to sustain combustion
The Combustion Reaction
Flaming and Glowing Combustion
• Relative Vapor Density – a property of compounds
relating vapor density to molecular weight. Most
materials when vaporized are much heavier than air
• Pyrolysis of Solid Fuels – the process by which solid
materials are decomposed by heat, forming smaller
molecules that can support flaming combustion