11.1 NOTESArson & Explosion

CRIMINALISTS’ ROLE IS TOA. Establish the motiveB. Establish the modus operandiC. Establish the suspectD. Detect and identify relevant

chemical materials collected at the scene

E. Reconstruct and identify igniters or detonating materials

FIRECan be caused by many things

that leave no chemical trace

The fascination of fireFour basic elements to the Greek Alchemist saw it as a means of

transformation Some saw it as having mystical

powers(page 297)

TODAY 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

CHEMICAL REACTIONSOccur 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.

FLASH POINTThe 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.

PYROLYSISChemical breakdown of a solid

materialGas-fuel-air mix must fall within

the flammable range Requires a certain fuel

concentrationFlammable range for gasoline is

1.3 to 6.0%

GLOWING COMBUSTIONFuel that burns without the flame (smoldering)

Rate of reaction is increased with the increase in temperature

SPONTANEOUS COMBUSTION A fire caused by a natural heat producing process in the presence of sufficient air and fuelBacterial metabolism in hay Rag soaking in containers

EXPLOSIVE 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)