Lecture 3: Arson Investigation Flashcards
Define molecules.
A molecule is formed when two or more atoms join together by chemical bonds, e.g. O2 (diatomic molecule).
Examples include air (a mixture) and water (a compound).
Define compounds
A compound is a molecule that contains at least two different elements, e.g. H2O.
Define mixtures
A mixture is made from two or more substances, e.g. air (78% N2, 21% O2, 0.033% CO2).
What is a combustion reaction?
- Rapid oxidation
- Produces carbon dioxide and water
- Exothermic: Releases excess energy as heat
- Example: CH4 + 2O2 → CO2 + 2H2O.
Example: CH4 + 2O2 → CO2 + 2H2O.
Complete combustion VS Incomplete combustion
Complete
- Enough oxygen
- Non-toxic products
- E.g. natural gas in stove: intense blue, no smoke
Incomplete
- Limited oxygen
- Toxic products (Carbon monoxide)
What is the fire triangle?
Fuel, heat, and oxygen.
Removing any one of these elements can extinguish a fire.
How can fire be extinguished?
- Removing any element in the fire triangle
- Remove fuel
- Remove heat: cooling with water
- Remove oxygen: add inert gas (CO2 fire extinguisher), separation (fire blanket)
This is a common method used by firefighters.
Where is fuel obtained?
Combustible material
- Solid fuels e.g., wood, coal, charcoal.
- Liquid fuels e.g., gasoline, diesel
- Gaseous fuels e.g., natural gas, hydrogen
Where is oxygen obtained?
- Air (21% oxygen)
- Oxidizers, e.g. nitrates, peroxides
What factors indicate the point of origin of a fire?
- Intense location of burning
- Low point in the building.
- Smoke/damage shows V pattern
How do arsonists tamper with the fire triangle?
- More accelerants to increase fuel load
- Use match/lighter as heat source
- Open windows to increase oxygen
What are common motives for arson?
- Revenge
- Vandalism
- Profit
- Crime concealment
- Excitement
- Extremism
What are indicators of arson?
- Use of accelerants
- Multiple points of origin
- Disabled fire protection devices
- Forced entry
What are the four stages of a fire?
- Ignition: Elements in suitable ratios
- Growth: Short stage, fire spreads (flashover)
- Fully Developed: Combustible materials ignited, hottest
- Decay: Decrease in oxygen or fuel (backdraft)
What is flashover?
- During Growth stage
- All combustible materials heated to their ignition temperatures
- Simultaneous ignition, rapid fire spread.
It poses a severe risk during fire incidents.
What is backdraft?
- During decay stage
- Explosive event
- Rapid re-introduction of oxygen to combustion in an oxygen-depleted environment.
This can occur when doors or windows are opened.
How to find, recover, and identify accelerants?
- Use sniffers/dogs
- Identify the chemical nature of the accelerant
- Collect fire debris
- Extract and analyze samples using gas chromatography
True or False: All compounds are molecules, but not all molecules are compounds.
True
What is an accelerant?
- Substances used to enhance the speed of fire spread
- E.g. pentane, hexane, octane, and toluene.
What are the two main types of chromatography?
- Gas chromatography (Gas as mobile phase)
- Liquid chromatography (Liquid as mobile phase)
What is a chromatogram?
- Record of separation in chromatographic system
- Retention Time: Time for component to pass through the system
- Peak area: Amount of component in the sample
What does retention time indicate in chromatography?
The characteristic time for a component to pass through the chromatographic system
How is the amount of a component in a sample determined?
By the peak area on the chromatogram
What type of samples does gas chromatography analyze?
- Volatile and thermally stable samples
- Heated and vaporized as gaseous forms
- e.g. gasoline, essential oils
