Fire and Explosives Flashcards
Combustion
Chemical reaction that results in fire and/or explosions
Reaction of fuel and oxygen
Products of complete combustion
Carbon dioxide, water, energy
Activation energy
Required energy to start a chemical reaction
Arson
Fires that are deliberately set with criminal intent
4 components of the fire tetrahedron
Oxygen, fuel, heat, chain reaction
Ignition temperature
Temperature needed to start a fire- dependent on the fuel
Flash point
Temperature with enough energy to convert enough of the liquid fuel to a vapor to support combustion
Flame point
A higher temperature than the flash point that will sustain combustion
Incomplete combustion
When there are limited reactants in the combustion equation (usually oxygen)
Results in smoke
Smoke
Product of incomplete combustion
Made up of carbon particles (soot), unburnt and partially burnt gases
Flashback
If there is not enough oxygen present, ventilation of the space (opening a door or window) will increase oxygen and result in an explosive fire
Accelerants
Fuels that are easily vaporized and support combustion, highly exothermic
Incendiary fire
Intentionally set fire (not necessarily arson)
Three types of fires (forensic standpoint)
Natural, accidental, deliberate
Natural fire
Occur without human input (ex. lightning strike fires)
Accidental fires
Fires that were probably preventable, occur without criminal intent (ex. dryer lint fire)
Characteristics present at a point of origin in a fire
Low burning (arson/deliberate) V patterns of smoke around the point of origin Wood charring will be greatest near it Spalling of plaster or concrete Material distortion Soot and smoke staining
Indications of an arson fire
The presence of an accelerant
Elimination of natural or accidental causes
Fire trails (from accelerant)
Multiple points of origin
Fire scene evidence packing
Airtight containers
Henry’s law in regards to headspace analysis
At equilibrium, depending on the vapor pressure, there will be a certain amount of the volatile substance in vapor form and a certain amount in the residue (evidence)
Absorption methods in headspace analysis
Passive and active
Passive- charcoal strip is added and either left overnight or heated. The charcoal strip is then put into a solvent to extract the accelerant
Active- two holes in the paint can with charcoal tubes, one pushing in air. This upsets the equilibrium and more accelerant will vaporize and be pushed out the second carbon tube
SPME
Solid-phase micro extraction
A absorbent needle/fiber is inserted into the fire residue container. The fiber can then be injected directly into the GC because the heat at the inlet will rapidly elute the accelerant into the mobile phase stream
Weathering
Describes the degradation of an accelerant due to heat or other environmental factors
Conflagration
Very rapid combustion (think spark plug in engine)
Instantaneous combustion or detonation
More rapid than conflagration, occurs when the oxygen and fuel are chemically combined into a single molecule
How are explosives categorized?
Low, high
By detonation velocity
Three primary effects of an explosion
Blast pressure, fragmentation, thermal/heat effects
Blast pressure
Escaping gases travel at high speeds, compressing gases and surrounding air, exerting pressure, causes vacuum
Negative pressure phase- “reverse” blast effect, once the blast wave dissipates, the vacuum must be filled in and the air will rush back towards the bomb seat
Bomb seat
Origin of the explosion
Fragmentation effects
Bomb casing can shatter and the pieces will be propelled away with force, shrapnel may cause fragmentation, blast may break objects
Thermal effects
Least damaging, heat produced in explosion will travel outwards
Low explosives
Oxygen is physically mixed with the fuel, speed of explosion is slower, push rather than shatter
Examples- smokeless powder, nitrocellulose
High explosives
Faster detonation rates, shatter objects and destroy them
Initiating/Primary high explosives
Very powerful and very sensitive
Ex- nitroglycerine
Non initiating/Secondary high explosives
Not sensitive, take a large effort for detonation, easily transported
Ex- C4
Low- and High-order explosions
Nothing to do with the type of explosive, but rather the efficiency of the explosion
