Chapter 14 Flashcards
Oxidation
the combination of oxygen with other substances to produce new substances; rust is also oxidation, but does not produce fire because of energy levels
Energy
the ability or potential of a system or material to do work
Energy Forms
heat energy, electrical energy, mechanical energy, nuclear energy, light energy, chemical energy
Heat transfer: conduction
electrons of atoms within a heated object collide with one another (metals are best conductors)
Insulators: poor conductors of heat (wood)
Heat transfer: radiation
heat energy from a heated surface transfers to a cooler surface by radiation, exposes surfaces to radiant heat, which burst into flame at ignition temperature
Heat transfer: convection
transfer of heat energy by movement of molecules within a liquid or gas; heated molecules spread apart and move upwards, cooler molecules migrate to the bottom
Combustion
the rapid combination of oxygen with another substance, accompanied by production of noticeable heat and light
Heat of Combustion
the heat liberated during combustion, excess energy from exothermic reaction
Exothermic reaction
more energy is liberated than is required to break the chemical bonds
Endothermic reaction
reaction that requires more energy than it liberates
Requirement for initiating combustion
Energy (heat), temperature raised to exceed energy barrier
Ignition temperature
minimum temperature at which fuel spontaneously ignites
Requirement for sustaining combustion
enough heat is liberated to keep reaction going, burns until either oxygen or fuel is exhausted
Headspace technique
way to recover accelerant residues: heat airtight container & remove vapor (or headspace) is removed with a syringe and put into gas chromatograph; disadvantage is size of syrings
Vapor Concentration
Charcoal-coated strip is placed within container holding debris collected from the scene, then heated, it absorbs accelerant vapor, washed with acetone and inserted into chromatograph; advantage is sensitivity
GC/MS
usually GC is enough but if accelerants are combined with other things, MC may be needed
Searching the scene
locating fire’s origin is main focus, can show signs of arson by separate, unconnected fires and burns on floors as opposed to ceilings
Searching for accelerants
liquid seeps into porous surfaces: should be collected; can be found with portable vapor detector
Packaging and preservation
2-3 quarts of ash and soot debris must be collected at point of origin, all porous materials must be collected, all materials must be packaged in airtight containers (new, clean paint cans filled ½-2/3 full), plastic is not suitable for packaging evidence
