Study Notes on Arson and Explosives
Ch.17: Arson and Explosives
Arson
Definition:
Arson is the criminal act of deliberately setting fire to property.
Complexity of Investigations:
Arson investigations often present complex and difficult circumstances to investigate.
Perpetrator typically has thoroughly planned the act and is usually not present during the crime.
Destruction caused by arson is often extensive, making it difficult to gather evidence.
Proving the commission of the offense is challenging due to these complexities.
Investigative Process
Challenges:
The investigative process in an arson case is intricate and requires extensive efforts.
Proving that a fire was set intentionally can be particularly difficult.
Key clues in arson investigations include:
Presence of intentional ignitable liquids.
Presence of an igniter mechanism.
Main Investigation Goals:
Establish the cause of the fire.
Determine if the fire was accidental or deliberate (e.g., due to faulty wiring, overheated electrical motors, improperly maintained systems).
Understand the specific circumstances surrounding the ignition and propagation of the fire.
Role of Forensic Scientists
Forensic scientists provide essential information to fire investigators for fire reconstruction.
The ultimate cause of the fire is usually determined by a fire investigator, not by the forensic scientist alone.
Investigators must consider various factors and conduct extensive on-site investigations to ascertain the fire's origin.
Chemistry of Fire
Definition of Fire:
Chemically, fire is a type of oxidation reaction, which involves a combination of oxygen with another substance to produce new substances.
When the reaction happens rapidly enough, it produces heat and light as byproducts.
Examples of Oxidation Reactions:
Methane gas combusts as follows:
Iron rusting, represented as:
Energy in Chemical Reactions
Energy Definition:
Energy is the ability of a system to do work.
Types of Energy Transitions:
Energy can come in various forms:
Heat and light.
Electrical energy.
Mechanical energy.
Nuclear and chemical energy.
Chemical Reactions:
Breaking bonds in reactants leads to the formation of new bonds in products, utilizing and liberating energy in a process that conserves atoms.
Notable chemical reaction example:
Reaction Energetics:
Exothermic Reactions: More energy is released than needed to break bonds.
Endothermic Reactions: Require more energy than released.
All reactions need an initial input of energy to overcome an energy barrier to proceed.
Combustion
Fire Tetrahedron Components:
For combustion to occur, four elements are essential:
Fuel.
Oxygen.
Heat.
Chemical chain reaction.
Ignition Temperature:
Minimum temperature required to spontaneously ignite fuel.
Can be reached through various means such as a lighted match or electrical sparks.
Sustaining Fire:
Once combustion starts, enough energy is released in heat and light forms, which sustains the fire.
The fire acts as a chain reaction, continuing until reactants are consumed (fuel and oxygen).
Factors Affecting Reaction Rate:
Reaction rates depend on molecular collisions, which increase with higher temperatures.
A 10°C increase can double or triple the reaction rate.
Fuel Temperature Considerations:
The flash point is the lowest temperature at which a liquid produces enough vapor to ignite in the air.
Flash point is always lower than ignition temperature.
Solid fuels require sufficient heat for thermal decomposition into vapors for combustion.
Fuel-Air Mixture
The combustible fuel-air mixture must lie within specific limits for combustion to occur, termed the flammable range.
Example: Flammable range of gasoline is 1.3% to 6.0% concentration in air.
Mixtures too lean (low fuel concentration) or too rich (high fuel concentration) will not ignite.
Types of Combustion
Glowing Combustion:
Burning without an open flame or smoldering condition.
Spontaneous Combustion:
Rare phenomenon often cited as a cause for arson; occurs with sufficient fuel and ventilation leading to natural heat and ignition.
Heat Transfer Mechanism:
Heat from a fire moves from high to low temperature regions, vital in understanding fire behavior in structures.
Types of Heat Transfer
Conduction: Movement of heat through solid objects, effectively executed by metals due to loosely held electrons.
Radiation: Transfer of heat via electromagnetic radiation; hot surfaces radiate heat to cooler surroundings.
Convection: Transfer of heat via molecular movement in fluids; in fires, hot gases rise causing pyrolysis on surfaces.
Flashover
Definition:
Flashover is when all combustible fuels in an area ignite simultaneously, creating a dangerous situation often described as an explosion.
Fire Scene Investigation
Timing:
Examination of the fire scene should begin promptly due to the presence of volatile accelerants and the time-sensitive nature of evidence preservation.
Indicators of Arson:
Evidence such as containers for accelerants, chemical indicators, irregular burn patterns, and signs of forced entry can suggest arson.
Indicators of Arson
Burn Patterns:
Normally, fire moves upward; if intense burn patterns are found at low points, it may indicate accelerants.
Presence of unconsumed flammable liquids in porous surfaces can also indicate arson.
Evidence Collection and Analysis
Evidence related to arson requires careful collection procedures to prevent contamination.
New paint cans are preferred containers, while plastic should be avoided as it reacts with hydrocarbons.
Evidence analyses include sampling headspace vapors, using gas chromatography for residue detection, and employing sniffer technology to detect ignitable liquids.
Explosives
Definition:
Explosives are substances that undergo rapid oxidation reactions releasing large quantities of gases, which creates explosive effects.
Classification of Explosives:
They can be categorized as low explosives (subsonic deflagration) or high explosives (supersonic detonation).
Examples of Low Explosives:
Black powder, smokeless powder, and chlorate mixtures.
Examples of High Explosives:
Dynamite (containing nitroglycerin), TNT (Trinitrotoluene), and RDX (Cyclonite).
Detonation Mechanism:
High explosives require an initiating explosion for detonation, often from a blasting cap.
Conclusion
Comprehensive understanding of arson and explosives is critical for effective scene investigation and forensic analysis, requiring detailed knowledge of chemical reactions, fire behavior, and explosive mechanisms.