Chapter 13 Forensic Aspects of Fire Investigation Flashcards
What is oxidation?
The combination of oxygen with other substances to produce new substances
Molecules must absorb energy to __________, and they liberate energy with ___________.
Break apart their chemical bonds; their bonds are reformed
What is combustion? Why is rusting not accompanied by combustion?
Rapid combination of oxygen with another substance, accompanied by the production off noticeable heat and light
Rusting is not accompanied because iron combines wit oxygen too slowly to permit combustion
How does an exothermic reaction differ from an endothermic reaction?
In an exothermic reaction, heat energy is liberated. In an endothermic reaction, heat energy is absorbed from the surroundings.
A fire will burn until either of which two components is exhausted?
Oxygen or the fuel is exhausted
Name two factors that influence the speed of reaction of a fire.
Physical state of the fuel and the temperature
In what physical state must a fuel exist in order to produce combustion when a reacts with oxygen? Why must it be in this state?
A fuel achieves a reaction rate with oxygen sufficient to produce combustion only when it is in the gaseous state, for only in this state molecules can collide frequently enough to support a flaming fire.
How high must the temperature of a liquid fuel be before the fuel will burn? What is the term for the lowest temperature at which this occurs?
In order to burn, the temperature of a liquid fuel must be high enough to vaporize the fuel. The flash point is the lowest temperature at which a liquid gives off sufficient vapor to form a mixture with arie that will support combustion.
What is pyrolysis? How does pyrolysis produce fire?
Pyrolysis is the chemical breakdown of solid organic matter by heat
The gaseous products of pyrolysis combine with oxygen to produce fire
How does the fuel-air mix affect combustion?
If the fuel concentration in the fuel- air mix is too low (lean) or too great (rich), combustion does not occur.
Define and give two examples of glowing combustion.
Combustion on the surface of a solid fuel in the absence of heat high enough to pyrolysis the fuel; cigarette and red glow of hot charcoal
Define spontaneous combustion and give two examples of conditions under which it may occur.
Spontaneous combustion is fire caused by a natural heat-producing process in the presence of sufficient air and fuel. Two examples of conditions under which it might occur are hay stored in an improperly ventilated barn and improperly ventilated containers containing rags soaked with highly unsaturated oils.
What three requirements must be satisfied to initiate and sustain combustion?
a. Fuel must be present
b. Oxygen must be available in sufficient quantity to combine with the fuel
c. Heat must be applied to initiate the combustion and sufficient heat must be generated to sustain the reaction
List and define the three mechanism of heat transfer.
a. Conduction- movement of heat through a solid object
b. Radiation- transfer of heat from a heated surface to a cooler surface by electromagnetic radiation
c. Convection- transfer of heat energy by movement of molecules within a liquid or gas
What is an insulator? Give an example of a good insulator.
Insulators are materials that are poor conductors of heat. Wood is an example of a good insulator.
List three common signs of arson at a fire scene.
Presence of accelerants, the discovery of an ignition device, irregularly shaped burn patterns on the floor or ground, streamers, and separate and unconnected fires.
Name three reasons why arson investigator must work quickly to collect evidence at fire scene.
a. Accelerant residue evaporate within days/hours
b. Safety-clean up needs to take place quickly
c. Accelerants can quickly be degraded by bacterial action
What are streamers? What does evidence of their presence at a fire scene suggest?
Trails of flammable material, such as gasoline or paper, spreading outward from a fire’s point or origin in order to cause the fire to move rapidly from one room to another
They suggest arson
Why should fire evidence be packaged in airtight containers?
So possible residues are not lost through evaporation
What instrument do most criminalists consider the most sensitive and reliable for detecting a characterizing flammable residue?
Gas chromatograph
What is headspace? How is it recovered?
Headspace is vapor containing volatile residues recovered from debris at a fire scene. It is recovered by heating the airtight container containing the debris, which drives off any volatile residue present and traps it in the container’s enclosed airspace. The headspace is then removed with a syringe.
What is the main advantage of vapor concentration over the headspace technique?
Using vapor concentration, a forensic analyst can increase the sensitivity of accelerant detection at least a hundredfold over the conventional headspace technique.
