1. Chapter 5 Explosions
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2. Introduction—Objectives
Identify the characteristics of gases.
Compare and contrast the categories of explosives.
Differentiate between components of the various types of explosives.
Describe the methods of detecting, collecting, and processing explosion evidence.
List some common analytical techniques used for explosives and explosive residue.
Define terrorism.
Forensic Science II: Explosions, Chapter 5
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3. Introduction—Vocabulary
explosion - the sudden release of chemical or mechanical energy caused by an oxidation or decomposition reaction that produces heat and a rapid expansion of gases
high explosives - chemicals that oxidize extremely rapidly,producing heat, light, and a shock wave; will explode even when not confined
kinetic molecular theory - a theory that states that the behavior of gases is predictable and explainable based on certain assumptions
Forensic Science II: Explosions, Chapter 5
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4. Introduction—Vocabulary
low explosives - chemicals that oxidize rapidly, producing heat,light, and a pressure wave; will explode only when confined
reagent - a substance used to produce a chemical reaction to detect, measure, or produce other substances
terrorism - the intentional use of force or violence to coerce or intimidate governments or other large organized groups
Forensic Science II: Explosions, Chapter 5
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5. Timothy McVeigh
April 19, 1995—Timothy McVeigh loaded a rental truck with ammonioum-nitrate fertilizer and other chemicals.
When detonated, the explosion demolished one-third of the 7-story Murrah Federal Bldg.
70 people died, including 19 children.
The FBI created a criminal profile.
How was Timothy found and convicted? 5
Forensic Science II: Explosions, Chapter 5
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6. Introduction to Explosions
Like fire
Caused by a chemical reaction—either by
Oxidation—two substances combine to form a new substance
Decomposition—a single compound breaks down
A large amount of gas and energy is released very quickly
Forensic Science II: Explosions, Chapter 5
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7. Properties of Gas (Obj 5.1)
Kinetic Molecular Theory —the behavior of gases is predictable
Gases are made up of rapidly and randomly moving particles
Most of the volume of a gas is empty space. Why?
When a gas particle collides with the container or with other particles, there is no net loss of energy.
The average kinetic energy is directly proportional to the temperature of the gas.
There is no force of attraction between gas particles or between the particles and the walls of the container.
Forensic Science II: Explosions, Chapter 5
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8. Properties of Gas Forensic Science II: Explosions, Chapter 5
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9. Properties of Gas
Combined Gas Law—an equation that describes the relationship among volume, temperature, and pressure in confined gases.
Forensic Science II: Explosions, Chapter 5
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10. Properties of Gas
Figure 5-5 A
If the temperature of the enclosed gas increases and volume is held constant, the pressure increases.
Forensic Science II: Explosions, Chapter 5
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11. Properties of Gas
Figure 5-5 B
If the volume of the enclosed gas increases and temperature remains constant, the pressure decreases.
Forensic Science II: Explosions, Chapter 5
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12. Characteristics of an Explosion (Obj 5.2)
Wave—a disturbance in matter that carries energy
Longitudinal wave—a pressure wave
Compressions
Rarefactions
Shrapnel
Forensic Science II: Explosions, Chapter 5
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13. Low Explosives (Obj 5.2, 5.3)
Materials that burn rapidly but explode only when confined in a container
Deflagration—the resulting combustion reaction
Rapid, intense burning
Pressure waves at less than 340 meters / sec.
Often used as propellants, such as a bullet
Forensic Science II: Explosions, Chapter 5
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14. High Explosives (Obj 5.2, 5.3) Materials that
Detonates easily
Produces pressure waves to 8500 meters / sec.
Shock wave moves faster than speed of sound
Detonation—an explosion that causes violent disruption to the surrounding area
Forensic Science II: Explosions, Chapter 5
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15. High Explosives Primary High Explosives Secondary High Explosives
Extremely sensitive to heat, pressure, and movement
Usually too volatile for most uses
Nitroglycerin
Secondary High Explosives
Less sensitive
Often used for military and commercial purposes
TNT and dynamite
Forensic Science II: Explosions, Chapter 5
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16. Fireworks Forensic Science II: Explosions, Chapter 5
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17. Collecting Evidence (Obj 5.4)
Soil samples
Debris from inside the crater
Traces of explosive residue
Detonating devices
Other pieces of the explosive device
Forensic Science II: Explosions, Chapter 5
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18. Microscopic Examination (Obj 5.5)
Low-power stereomicroscopy
Examine the debris for unexploded material
Forensic Science II: Explosions, Chapter 5
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19. Color Spot Tests (Obj 5.5)
Reagent—a substance used to produce a chemical reaction to detect, measure, or produce other substances
Modified Griess Test—detects presence of nitrate compounds
Diphenylamine Test—detects most explosive residues, but not TNT
Alcoholic potassium hydroxide (KOH)— detects TNT
Forensic Science II: Explosions, Chapter 5
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20. Color Spot Tests Forensic Science II: Explosions, Chapter 5
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21. Chromatography (Obj 5.5) Thin Layer Chromatography (TLC)
Presumptive test for explosives
Rf values from TLC provide tentative identification of the components of the mixture
A cellulose or silica gel plate as the stationary phase
Forensic Science II: Explosions, Chapter 5
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22. Chromatography Forensic Science II: Explosions, Chapter 5
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23. Confirmatory Tests (Obj 5.5)
Infrared spectroscopy
A beam of infrared light passes through a sample
Measure how much energy is absorbed
Gas chromatography-mass spectroscopy
Gas chromatography separates a substance into its compounds
Mass spectroscopy breaks down the compounds into smaller charged particles and separates them based on their mass and charge
Forensic Science II: Explosions, Chapter 5
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24. Examining Physical Evidence (Obj 5.5)
Examples:
Pieces of wire
Pieces of pipe
Detonation devices
Fingerprints
Residue on clothes
Timing devices
Forensic Science II: Explosions, Chapter 5
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25. Terrorism
The intentional us of force or violence to coerce or intimidate governments or other organized groups.
Groups or individuals
Common reasons
Political
Environmental
Religious
Forensic Science II: Explosions, Chapter 5
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26. Chapter Summary
An explosion is an oxidation or decomposition reaction that releases a lot of gas and energy very quickly.
Knowledge of the kinetic molecular theory is important in the investigation of explosions.
Low explosives burn rapidly but explode only when confined. The burning of a low explosive is called deflagration.
Forensic Science II: Explosions, Chapter 5
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27. Chapter Summary
High explosives can explode even when they are not confined, causing a violent disruption.
Primary high explosives, such as nitroglycerin, are extremely sensitive to heat, pressure, and movement.
Secondary high explosives are not as sensitive to heat or shock, but are capable of violent explosions. These are typically used by the military or for mining or construction.
Forensic Science II: Explosions, Chapter 5
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28. Chapter Summary
Investigators collect soil samples and debris from an explosion’s point of origin. They search for explosive residue, pieces of the explosive device, or detonating devices.
The crime lab examines evidence under a stereomicroscope.
The crime lab can also wash the debris with acetone to dissolve the explosives and then analyze it to identify the type of explosive.
Forensic Science II: Explosions, Chapter 5
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29. Chapter Summary
Thin layer chromatography is a presumptive test to provide a preliminary identification.
Forensic scientists then perform confirmatory tests, such as infrared spectroscopy and gas chromatography–mass spectroscopy.
Explosions are often caused by acts of terrorism. Terrorism may be politically, environmentally, or religiously motivated.
Forensic Science II: Explosions, Chapter 5
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