Chapter 4
Chemistry and Physics of Fire
Introduction
• To appreciate how fire is controlled, we must first
understand the chemical and physical properties of
fire itself
• Information can be used to predict what the fire will
do with the available fuel and where it is headed
• With this knowledge, we are able to choose the
proper extinguishing agent and its method of
application
Learning Objective 1
Fire Triangle and Fire Tetrahedron
FIRE DEFINED
• Rapid, self-sustaining oxidation process
accompanied by the evolution of heat and light in
varying intensity
• Combustion described as a chemical reaction that
releases energy as heat and usually as light
RUSTING OF IRON
• Not considered combustion
• Proceeds at a slow rate
Learning Objective 1
Fire Triangle and Fire Tetrahedron
FIRE TRIANGLE
• Originally based on three elements



Fuel
Air
Heat
FIRE TETRAHEDRON
• Fourth component called chemical chain reaction
• Produces free radicals
Learning Objective 2
Describe What Constitutes an Oxidizer
CHEMISTRY OF FIRE
• Two basic necessary components


Oxidizer
Fuel
OXIDIZER
• Evolves or generates oxygen
• Oxygen is most common oxidizer
• Most fires are burning at 21% oxygen
Learning Objective 2
Describe What Constitutes an Oxidizer
FLOURINE AND CHLORINE
• Listed under halogen family on periodic table
• Both naturally occur as gases
• Flourine is a much stronger oxidizer than oxygen
• Fire in a flourine atmosphere will burn more rapidly
than one occurring in air
Learning Objective 3
Describe What Constitutes a Fuel
FUEL
• Described as anything that will burn
 Carbon and hydrogen are two most common
• Hydrocarbons
 Main element in organic fuels, fuels that were at
one time living things
• Other elements as fuels, including metals
 Sodium, aluminum, magnesium
Learning Objective 3
Describe What Constitutes a Fuel
FIRES INVOLVING CARBON, HYDROGEN,
AND OXYGEN
• Two byproducts of complete combustion
 Water vapor
 Carbon dioxide
• Byproducts of incomplete combustion
 Smoke
 Carbon monoxide (CO)
 Carbon dioxide (CO²)
 Other fire gases
Learning Objective 4
Illustrate the States of Matter
FUEL MAY OCCUR IN THREE STATES OF MATTER
• Solid
• Liquid
• Gas
MOLECULES AND TEMPERATURE
• Molecules in liquids vibrate faster than those in solids,
and gas molecules vibrate the fastest of the three
• As this vibration increases, solids become liquids and
liquids become gases
Learning Objective 5
Explain the Process of Pyrolysis
PYROLYSIS
• Chemical decomposition of matter through heat
• Combustion occurs when fuel has been converted
to vapor or gas
• Oxidizer and fuel must be gaseous for
recombination
• Solid and liquid fuels are converted to gaseous
state by the application of energy
Learning Objective 5
Explain the Process of Pyrolysis
IGNITION TEMPERATURE
• Combustion continues without
external input of heat
FREE RADICALS
• Longer molecules break into shorter molecules
• Byproduct of the fuel that directly combines with
the oxidizer
Learning Objective 6
Properties Affecting Solid Fuels
FACTORS AFFECTING SOLID FUEL
PYROLIZATION
• Mass
• Arrangement
• Continuity
• Moisture content
Learning Objective 6
Properties Affecting Solid Fuels
MEASURING FLAME SPREAD
• Steiner tunnel



Test consists of a 25’ vented tunnel
A fan draws the flame across the surface of the
material being tested
Flame spread is determined visually through
windows built into the tunnel
Learning Objective 7
Properties Affecting Liquid Fuels
LIQUID FUELS
• Molecules flow freely but do not readily separate
• Will assume the shape of their container
• Physical properties make them difficult to
extinguish
• They increase the hazards to persons and property
• If spilled, will flow and increase in size
Learning Objective 7
Properties Affecting Liquid Fuels
OTHER IMPORTANT PROPERTIES OF
LIQUIDS
•
•
•
•
•
•
•
Specific gravity
Volatility
Vapor pressure
Boiling point
Vapor density
Flash point
Miscibility
Learning Objective 8
Properties Affecting Gas Fuels
GAS AND VAPOR FUEL CHARACTERISTICS
• Defined as a fluid that has no shape or volume and
tends to expand indefinitely
• Always fill the container in which they are stored
FLAMMABLE RANGES
• Upper flammable limit
• Lower flammable limit
• Flammable range
Learning Objective 9
Differentiate Heat and Temperature
SOURCES OF HEAT (FORM OF ENERGY)
•
•
•
•
Chemical
Mechanical
Electrical
Nuclear
MEASUREMENTS OF HEAT
• British thermal unit (BTU)
• Calorie (metric term)
• Joule (international system of units)
Learning Objective 9
Differentiate Heat and Temperature
MEASUREMENTS OF TEMPERATURE
• The measure of the hotness or coldness of an
object

Fahrenheit

Celsius

Kelvin

Rankine
Learning Objective 10
Illustrate the Four Methods of Heat
Transfer
ENERGY TRANSFERS FROM HEAT SOURCE
TO FUEL
• Conduction
• Convection
• Radiation
• Direct flame impingement
or autoexposure
Learning Objective 10
Illustrate the Four Methods of Heat
Transfer
STRUCTURE FIRE AND HEAT TRANSFER
METHODS
• Fire starts in a stack of boxes in a warehouse,
convection catches the ceiling on fire
• Fire spreads up the stack due to direct flame
impingement
• Radiated heat spreads the fire to piled stock
several feet away
• A pipe running through the wall heats to the point
that stock on the other side of the wall ignites
Learning Objective 11
Illustrate the Five Classifications of Fire
FIRE HAS BEEN DIVIDED INTO BASIC
CLASSIFICATIONS
•
•
•
•
•
Class A: Ordinary combustibles
Class B: Flammable liquids
Class C: Energized electrical
Class D: Combustible metals
Class K: Cooking materials
Learning Objective 12
Describe the Four Stages of Fire
FOUR STAGES OF FIRE DEVELOPMENT
• Ignition
 Fire ignites and reaches a point where it no
longer needs input heat from outside sources to
keep burning
• Growth
 Fire releases heat, bringing more fuel to its
ignition temperature
 Heat is transferred to nearby surfaces though
radiation
Cont.
Learning Objective 12
Describe the Four Stages of Fire
FOUR STAGES OF FIRE DEVELOPMENT
• Fully developed

All available fuels in the fire’s perimeter are
burning
• Decay
 Occurs when the fire has run out of available
fuel

Suppression action has reduced the fire to
smoldering embers
Learning Objective 12
Describe the Four Stages of Fire
FLASHOVER
• The contents in the room are brought to their
ignition temperature and if sufficient oxygen is
present, flashover can occur
• If a flashover were to take place, temperatures,
even at floor level, would rise dramatically
• Firefighters cannot survive in a room with a
flashover, even wearing full PPE and SCBA
Learning Objective 12
Describe the Four Stages of Fire
BACK DRAFT
• Flame may die out and glowing combustion takes
place
• Pyrolysis continues to occur, with amounts of
combustible gases produced
• Room is now superheated and charged with
smoke and combustible fire gases
• Fire gases and smoke are alternately forced out
and sucked back into the structure
Summary
• This chapter introduces the chemical and physical
properties of the elements found in fires and what
actually occurs during the combustion reaction
• A solid understanding of the combustion process
and the stages of fire must be in place before we
discuss the selection, application, and tactics of
extinguishing agents