1. Arson and Fire Investigation
Chapter 4
Arson and Fire Investigation

2. Chemistry of Fire
Fire = rapid oxidation reaction that involves a combustible material
Oxidation reaction = complete or partial loss of electrons or the gain of oxygen
- 2Na + Cl2  2NaCl Na loses e- , so Na is oxidized

3. Combustion reactions – oxidation rxns that involve oxygen and produce flames
- release energy in form of heat & light
= exothermic rxns

4. Ingredients for Fire Oxygen Fuel – material that is burning
- accelerant – used to start or maintain a fire
Heat – temp. at which fuel reacts with oxygen (burn) = ignition temperature
Chain reaction - continue to burn until all oxygen or fuel is used or removed

5. Accelerants Hydrocarbons – made of only H & C
ex. Gasoline, kerosene, lighter fluid
In gas state – has weaker molecular bonding
So, liquid will NOT burn, but gas vapors will
Vaporization – when liquid changes to gas state

6. For liquid to burn, need enough heat to vaporize liquid
Lowest temp for this = flash point
At flash point, fire burns only with a heat source
Above flash point, liquid becomes vapor & can ignite
Accelerant
Flash Point (oC)
Ignition Temperature (oC)
Gasoline
-46
257
Kerosene
52-96

7. Pyrolysis Solid fuels – not flammable
Vapors from solid fuels are flammable
As wood decomposes in fire, flammable vapors are given off
process of decomposition caused by heat in the absence of oxygen = pyrolysis