This 1-hour presentation discusses the differences and similarities between solids and gases as they move to equilibrium through the combustion process.

1. MOVING TO EQUILIBRIUM
BURNING SOLIDS AND GASES: A COMPARISON
Abdur-Raheem As-Siddiq
Fire Behavior and Combustion
SOLID LIQUID LIQUID GAS GAS

2. DEFINITIONS
• Combustion – the process of energy release when one item
reacts completely with oxygen (Gann & Friedman, 2015, p.
79)
• Chemical Thermodynamics – the flow of enthalpy or
energy associated with reactions leading to a state of
chemical equilibrium (Gann & Friedman, 2015, p. 279)
• Chemical Equilibrium – the state in which all chemical
species are present at concentrations that have no further
tendency to change with time. All molecular compounds seek
this state. (Gann & Friedman, 2015, p. 279)
• Gas – A physical state of matter possessing perfect
molecular mobility, has low density, easily compressed, and
the property of indefinite expansion (Dictionary.com &
General Chemistry)

3. DEFINITIONS
 Liquid – a physical state of material that is composed of
molecules that move freely amongst themselves with a
constant volume and consistency (Dictionary.com & General
Chemistry)
 Molecule – The smallest particle of a substance that retains
all the properties of the substance and is composed of one or
more atoms (Gann & Friedman, 2015, p. 283)
 Solid – a relatively dense, rigid state of matter, with a definite
volume and shape. (General Chemistry)

4. PROCESS OF COMBUSTION
 Before we discuss the differences between the burning of a
solid versus the burning of a gas, we have to set a foundation
of what happens during combustion.
 Anytime something “burns” it is a combustion reaction.
 We will use the term “combustion” to describe the process
and “burn” to describe the energy that is released in the form
of heat and light as a result of “combustion.”

5. PROCESS OF COMBUSTION
 Combustion is the culmination of four conditions (fuel, heat,
oxygen, chemical reaction) coming together simultaneously to
produce a change in molecular configuration. Commonly
referred to as the Fire Tetrahedron. (Gann & Friedman, 2015)
 In simple terms, combustion is just breaking apart of
molecular compounds to form other molecular compounds.
 This process is an exothermic reaction therefore it produces
energy. That energy is often experienced as heat and light.

6. PROCESS OF COMBUSTION
THE FIRE TETRAHEDRON
When these
four
conditions
reach their
respective
appropriate
levels, they
begin the
process of
combustion

7. PROCESS OF COMBUSTION
As an easy demonstrative example of combustion, let’s look at the
combustion of methanol (rubbing alcohol):
CH3OH + O2 CO2 + 2H2O + Heat
1. We begin with the molecular compound as methanol (CH3OH)
2. Oxygen is introduced (O2)
3. Heat is generated and a chemical reaction ensues.
4. The result is a breakdown of methane and the formulation of two
different molecular compounds, Carbon Dioxide and Water. (CO2),
(2H2O)
5. The energy generated from the reaction (combustion) is more heat.
Combustion - breaking apart of molecular compounds to form other
molecular compounds
(Combustion Reactions)

8. COMBUSTION OF A SOLID
 In the most simplest of terms, the solid itself does not
“burn”.
 The solid goes through a physical change process once
a sufficient amount of heat is applied to begin
combustion.
 Combustion starts the molecular compound’s journey to
reach chemical equilibrium.

9. COMBUSTION OF A SOLID
Gann & Friedman discuss the physical and
chemical change of solids in their book
Principles of Fire Behavior and Combustion
(2015). They conclude that when a solid
changes state due to combustion, it goes
through five thermochemical steps. Those
steps are (p. 34):
1. The temperature of the solid rises
2. The solid melts, or fuses, becoming a liquid
3. The temperature of the liquid rises
4. The liquid vaporizes, or gasifies
5. The gas temperature rises

10. HEAT
CHEMICAL
TRANSFORMATION
HEAT
CHEMICAL
TRANSFORMATION
HEAT
STAGE
The temperature of the solid rises
The solid melts, or fuses, becoming a
liquid
The temperature of the liquid rises
The liquid vaporizes, or gasifies
The gas temperature Rises
SOLID LIQUID LIQUID GAS GAS
COMBUSTION OF A SOLID

11. • The stages of change incorporate both physical and chemical
change depending on what is happening at any given time.
• Whenever heat is being absorbed, this stage is considered purely
a physical change.
• Whenever the matter changes density, is an indication of
chemical change (i.e. the molecular compounds shifted in
configuration). (Gann & Friedman, p. 34)
STAGE TYPE OF CHANGE
The temperature of the solid rises physical change
The solid melts, or fuses, becoming a liquid chemical change
The temperature of the liquid rises physical change
The liquid vaporizes, or gasifies chemical change
The gas temperature Rises physical change
COMBUSTION OF A SOLID

12. COMBUSTION OF A SOLID
• We will use a sugar molecule to demonstrate the combustion
process for a solid
• As we showed in the methanol combustion, the result will be that
sugar will cease to exist.
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
OXYGEN
OXYGEN
OXYGEN
OXYGEN
OXYGENCARBON
CARBON
CARBON
CARBON
CARBON
CARBON
SUGAR
C6H12O6
OXYGEN

13. C6H12O6 6 O2
6 CO2 6 H2O
COMBUSTION OF A SOLID
SUGAR OXYGEN
CARBON
DIOXIDE
WATER
1. Sugar is exposed to heat
2. The heat causes the molecular compounds in
sugar to destabilize and react with oxygen
3. A chemical reaction ensues
4. The molecular compound of sugar is disrupted
and form new compounds
5. Carbon Dioxide and Water are produced
6. Sugar ceases to exist (Solid ceases to exist)
7. The process continues through physical and
chemical changes until chemical equilibrium is
achieved.
(The Combustion Reaction)

14. COMBUSTION OF A GAS
• As we used sugar as a demonstrative example for solid
combustion, we will use methane as an example of the combustion
process for gases.
• Methane is a natural gas that is commonly used as fuel and is the
major component of gas used in kitchens and heaters.
• The molecular formula for Methane is CH4 – One (1) Carbon
molecule and 4 Hydrogen atoms.HYDROGEN
HYDROGEN
HYDROGEN
HYDROGEN
CARBON
(The Combustion Reaction)

15. COMBUSTION OF A GAS
METHANE OXYGEN
CARBON DIOXIDE WATER
O2CH4
CO2 H2O
1. Methane is exposed to heat
2. The heat causes the molecular compounds in
methane to destabilize and react with oxygen
3. A chemical reaction ensues
4. The molecular compound of methane is
disrupted and forms new compounds
5. Carbon Dioxide and Water are produced
6. Methane ceases to exist
7. The process continues through physical and
chemical changes until chemical equilibrium is
achieved.
(The Combustion Reaction)

16. SOLIDS AND GASES
 The process of combustion is fundamentally the same
whether it is a solid or vapor (gases).
 The difference between them is what occurs after
combustion.
 Solids are at one end of the matter spectrum while
gases are on the opposite end.
 During exothermic combustion, the molecules that
comprise solids continuously move through the chain of
events until they form molecules of gas.
 Gases are the last stage before chemical equilibrium is
achieved.

17. SOLIDS AND GASES
HEAT
CHEMICAL
TRANSFORMATION
HEAT
CHEMICAL
TRANSFORMATION
HEAT
SOLID LIQUID LIQUID GAS GAS
• Exothermic combustion always moves in one direction – From Solid – to
Liquid – to Gas – to Equilibrium
• So you can say that in this process, Gas is an evolutionary product of a
solid in terms of combustion
• The reverse order, whereas the gas becomes a solid is a separate subject
that will be discussed in another class

18. SOLIDS AND GASES
Similarities between “burning” of solids and
gases:
• The combustion process (i.e. The Fire tetrahedron)
• The molecular compounds become increasingly unstable when heat
is applied
• The molecular compounds change to form new substances
METHANE
CH4C6H12O6
SUGAR

19. SOLIDS AND GASES
Differences between “burning” of solids and
gases:
• Solids don’t burn whereas gases do
• To reach equilibrium, solids pass through at least two stages of
chemical transformation
• Gases are only one stage away from reaching equilibrium
METHANE
CH4C6H12O6
SUGAR

20. CONCLUSION
 Solids are at the beginning of the evolutionary scale
while gases are towards the end of the scale in the
combustion process
 Solids don’t “burn” but gases do
 In simple terms, “burning” or the process of combustion
is merely a physical and chemical change taking place
by means of energy transfer
 Once combustion begins, its goal is to move toward
chemical equilibrium
 The “combustion process” is the same between a solid
and a gas.

21. REFERENCES
Gann, R. & Friedman, R. (2015). Principles of fire behavior and
combustion (4th ed.). Burlington, MA: Jones and Bartlett.
Gas. (n.d.). Dictionary.com Unabridged. Retrieved February 28,
2017 from Dictionary.com
website http://www.dictionary.com/browse/gas
Gas. (n.d.). General chemistry online. Retrieved February 28,
2017 from General Chemistry Online! website
http://antoine.frostburg.edu/chem/senese/101/glossary/g.shtml)
Liquid. (n.d.). Dictionary.com Unabridged. Retrieved February
28, 2017 from Dictionary.com
website http://www.dictionary.com/browse/liquid Liquid. (n.d.).
General chemistry online. Retrieved February 28, 2017 from
General Chemistry Online! website
http://antoine.frostburg.edu/chem/senese/101/glossary/l.shtml)

22. REFERENCES
Solid. (n.d.). General chemistry online. Retrieved February 28,
2017 from General Chemistry Online! website
http://antoine.frostburg.edu/chem/senese/101/glossary/s.shtml)
Combustion Reactions. (n.d.) Indiana University Northwest.
Retrieved February 28, 2017 from
http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactio
ns/combustion.html
The Combustion Reaction. (n.d.). University Federico II of
Naples, Italy, European Chemistry Thematic Network. Retrieved
February 27, 2017 from
http://www.whatischemistry.unina.it/en/burn.html
The Combustion Reaction. (n.d.). University Federico II of
Naples, Italy, European Chemistry Thematic Network. Retrieved
February 27, 2017 from
http://www.whatischemistry.unina.it/en/combust.html