Chemistry and Physics of Fire
Firefighter I
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Fire/Combustion
Defined
• Fire – rapid, self-sustaining oxidation
accompanied by heat and light in varying
intensities
• Combustion – a chemical reaction
that releases energy as heat and usually
light
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Fire Triangle
Fire Triangle
Fuel
Heat (energy)
Oxidizer (air)
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Fire Tetrahedron

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Fire Tetrahedron
• Fuel
• Heat (energy)
• Oxidizer (air)
• Chemical Chain Reaction
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Oxidizers
• Oxygen is the most common
• Occurs as 21% of air
• Increasing the amount of
oxidizer may increase the
intensity of the fire.
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Oxidizers
Other oxidizers:
• fluorine
• chlorine
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Fuels
• Fuel
• Occurs in the three
states of matter:
• Solid
• Liquid
• Gas
• State is often temperature-dependent
• To combine with oxidizer both MUST be in gaseous
states
• Is vaporized by input heat in a process called pyrolysis
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Fuels
• The most common fuels contain
• Carbon
• Hydrogen
• Complete combustion yields
• H20
• CO2
• Most combustion is incomplete producing
• Smoke
• CO2
• Other fire gases
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Solid fuels
• As heat is added to solid fuels, the molecules are
broken down into smaller components that
vaporize and recombine with the oxidizer.
• When the fuel is hot enough to self-sustain
combustion, it is at its ignition temperature.
• The size, arrangement, continuity, and moisture
content of the solid fuel can affect the rate of
pyrolysis.
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Pyrolysis
Pyrolysis – a chemical
change brought about by heat
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Liquid Fuels
 Factors affecting liquid fuels
• Flow like water but do NOT readily
separate
• Specific Gravity – the weight of the
liquid compared to the weight of an equal
volume of water
• Boiling Point – when the vapor pressure
equals atmospheric pressure
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Liquid Fuels
• Volatility – the ease at which the liquid
gives off vapors at ambient temperatures
• Flash Point – the minimum temperature at
which a liquid gives off vapors sufficient to
form an ignitable mixture with air
• Miscibility – the ability of the liquid to mix
with water
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Gaseous Fuels
• Tend to expand indefinitely
• Flammable Limits/Flammable Range –
the fuel mixture can be too rich or too lean
to burn
• Classification is flammable or nonflammable
• Some nonflammable gases support
combustion (oxygen is an example)
• Flammable vapors are NOT always visible
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Heat and Temperature
 Sources of heat energy
• Chemical
• Mechanical
• Electrical
• Nuclear
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Heat and Temperature
 British Thermal Unit –
• The amount of heat needed to raise the
temperature of 1 pound of water by 1 degree
Fahrenheit
 Calorie –
• The amount of heat needed to raise the
temperature of 1 gram of water by 1 degree
Celsius
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Heat Transfer
 Three methods of heat transfer:
• Conduction – through a medium without visible
motion
• Convection – through a circulating medium
(liquid or gas)
• Radiation – by wavelengths of energy
• Direct Flame Contact – a combination of the
three methods of heat transfer, as objects are
bathed in flames
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Classification of Fires
 Class A: ordinary combustibles
 Class B: flammable liquids (gases)
 Class C: energized electrical equipment
 Class D: flammable metals
 Class K: cooking materials
 (cooking oils/fats/grease)
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Stages/Phases of Fire
• Incipient stage
• Oxygen at 21%
• Flame temperature 180 degrees F - 220 degrees F
• Heat has not spread to other fuels nearby
• Growth stage
• Early stage of a fire
• Availability of fuel and oxygen is unlimited
• Characterized by rapid increase of heat
(temperature)
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Stages/Phases of Fire
• Fully Developed
• Energy release is at a maximum rate and limited
only by the availability of fuel and/or oxygen
• Decay
• When much of the fuel has been consumed
• Energy being released has diminished
• Temperature decreases
• Fire goes from ventilation-controlled to fuel-
controlled
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Stages/Phases of Fire
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Related Terminology
 Flashover
• Phenomena in which all surfaces and objects in a fire have
been heated to their ignition temperature and ignite
 Rollover/Flameover
• Phenomena where unburned gases accumulated at the top
of a compartment ignite and flames spread across the
ceiling
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Related Terminology
 Backdraft
• An instantaneous explosion or rapid burning of
superheated gases that occurs when oxygen is introduced
into an oxygen-depleted space
• Signs of a possible backdraft
• Smoke stained window
• Yellow-grey smoke
• Little or no flame
• Smoke issuing from small cracks in puffs
• High heat levels
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Resources
 1418001775, Introduction to Fire Protection (3rd
Edition), Klinoff, Robert
 0879392886, Essentials of Firefighting (5th
Edition), IFSTA
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