11.1 N OTES Arson & Explosion. C RIMINALISTS ’ ROLE IS TO A. Establish the motive B. Establish the modus operandi C. Establish the suspect D. Detect and.

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Presentation transcript:

11.1 N OTES Arson & Explosion

C RIMINALISTS ’ ROLE IS TO A. Establish the motive B. Establish the modus operandi C. Establish the suspect D. Detect and identify relevant chemical materials collected at the scene E. Reconstruct and identify igniters or detonating materials

F IRE Can be caused by many things that leave no chemical trace The fascination of fire Four basic elements to the Greek Alchemist saw it as a means of transformation Some saw it as having mystical powers (page 297)

T ODAY FIRE IS A RESULT OF : Oxidation – the process in which oxygen is combined with other substances to produce new substances Fire: Methane + oxygen  carbon dioxide + water Rust: Iron + oxygen  iron oxide

C HEMICAL R EACTIONS Occur when bonds are broken and new ones form. These involve energy. Some energies are absorbed and some are given off.

Oxidation reactions, such as combustion, release more energy than is absorbed and is therefore called exothermic. The energy released is in the form of heat (heat of combustion) and light When reactions require more energy than they give off they are called endothermic

Heat energy needed for combustion of methane and gasoline is high. The temperature must reach a certain level to cross the energy barrier – ignition temperature Another factor is the rate of the reaction: fire and oxidation involves the physical state of the fuel and the temperature.

F LASH POINT The lowest temperature at which a liquid gives off sufficient vapor to form a mixture in the air that will support combustion. The ignition temperature is always higher than the flash point. Solid materials have to be broken down to form a gas.

P YROLYSIS Chemical breakdown of a solid material Gas-fuel-air mix must fall within the flammable range Requires a certain fuel concentration Flammable range for gasoline is 1.3 to 6.0%

G LOWING COMBUSTION Fuel that burns without the flame (smoldering) Rate of reaction is increased with the increase in temperature

S PONTANEOUS COMBUSTION A fire caused by a natural heat producing process in the presence of sufficient air and fuel Bacterial metabolism in hay Rag soaking in containers

E XPLOSIVE DEVICES Contain their own oxidizing agents (chemicals that supply oxygen) i.e.: black powder – potassium nitrate(contains the oxygen), charcoal, sulfur. Nitroglycerin – carbon, hydrogen, nitrogen, oxygen (dynamite)