Chapter 5 Explosions © 2012 Cengage Learning. All Rights Reserved

Chapter 5

Explosions

© 2012 Cengage Learning. All Rights Reserved

2 Forensic Science II: Explosions, Chapter 5


Introduction—Objectives

1. Identify the characteristics of gases.2. Compare and contrast the categories of explosives.3. Differentiate between components of the various

types of explosives.4. Describe the methods of detecting, collecting, and

processing explosion evidence.5. List some common analytical techniques used for

explosives and explosive residue.6. Define terrorism.



Introduction—Vocabulary

o 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

o high explosives - chemicals that oxidize extremely rapidly,producing heat, light, and a shock wave; will explode even when not confined

o kinetic molecular theory - a theory that states that the behavior of gases is predictable and explainable based on certain assumptions



Introduction—Vocabulary

o low explosives - chemicals that oxidize rapidly, producing heat,light, and a pressure wave; will explode only when confined

o reagent - a substance used to produce a chemical reaction to detect, measure, or produce other substances

o terrorism - the intentional use of force or violence to coerce or intimidate governments or other large organized groups


© 2012 Cengage Learning. All Rights Reserved5

Timothy McVeigh

o April 19, 1995—Timothy McVeigh loaded a rental truck with ammonioum-nitrate fertilizer and other chemicals.

o When detonated, the explosion demolished one-third of the 7-story Murrah Federal Bldg.

o 70 people died, including 19 children.o The FBI created a criminal profile.o How was Timothy found and convicted?



Introduction to Explosions

o Like fireo Caused by a chemical reaction—either by

• Oxidation—two substances combine to form a new substance

• Decomposition—a single compound breaks downo A large amount of gas and energy is

released very quickly



Properties of Gas (Obj 5.1)

Kinetic Molecular Theory—the behavior of gases is predictable

o Gases are made up of rapidly and randomly moving particleso Most of the volume of a gas is empty space. Why?o When a gas particle collides with the container or with other

particles, there is no net loss of energy. o The average kinetic energy is directly proportional to the

temperature of the gas. o There is no force of attraction between gas particles or

between the particles and the walls of the container.



Properties of Gas



Properties of Gas

o Combined Gas Law—an equation that describes the relationship among volume, temperature, and pressure in confined gases.



Properties of Gas

Figure 5-5 A

If the temperature of the enclosed gas increases and volume is held constant, the pressure increases.



Properties of Gas

Figure 5-5 B

If the volume of the enclosed gas increases and temperature remains constant, the pressure decreases.



Characteristics of an Explosion (Obj 5.2)

o Wave—a disturbance in matter that carries energy

o Longitudinal wave—a pressure wave• Compressions• Rarefactions

o Shrapnel



Low Explosives (Obj 5.2, 5.3)

o Materials that burn rapidly but explode only when confined in a container

o Deflagration—the resulting combustion reaction• Rapid, intense burning• Pressure waves at less than 340 meters / sec.

o Often used as propellants, such as a bullet



High Explosives (Obj 5.2, 5.3)

o Materials that • Detonates easily• Produces pressure waves to 8500 meters / sec.• Shock wave moves faster than speed of sound

o Detonation—an explosion that causes violent disruption to the surrounding area



High Explosives

o Primary High Explosives• Extremely sensitive to heat, pressure, and

movement• Usually too volatile for most uses• Nitroglycerin

o Secondary High Explosives• Less sensitive• Often used for military and commercial purposes• TNT and dynamite



Fireworks



Collecting Evidence (Obj 5.4)

o Collect• Soil samples• Debris from inside

the crater• Traces of explosive

residue• Detonating devices• Other pieces of the

explosive device



Microscopic Examination (Obj 5.5)

o Low-power stereomicroscopyo Examine the debris for unexploded material



Color Spot Tests (Obj 5.5)

o Reagent—a substance used to produce a chemical reaction to detect, measure, or produce other substances

o Modified Griess Test—detects presence of nitrate compounds

o Diphenylamine Test—detects most explosive residues, but not TNT

o Alcoholic potassium hydroxide (KOH)— detects TNT



Color Spot Tests



Chromatography (Obj 5.5)

o 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



Chromatography



Confirmatory Tests (Obj 5.5)

o Infrared spectroscopy• A beam of infrared light passes through a sample• Measure how much energy is absorbed

o 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



Examining Physical Evidence (Obj 5.5)

Examples:o Pieces of wireo Pieces of pipeo Detonation deviceso Fingerprintso Residue on clotheso Timing devices



Terrorism

o The intentional us of force or violence to coerce or intimidate governments or other organized groups.

o Groups or individualso Common reasons

• Political• Environmental• Religious



Chapter Summary

o An explosion is an oxidation or decomposition reaction that releases a lot of gas and energy very quickly.

o Knowledge of the kinetic molecular theory is important in the investigation of explosions.

o Low explosives burn rapidly but explode only when confined. The burning of a low explosive is called deflagration.



Chapter Summary

o High explosives can explode even when they are not confined, causing a violent disruption.

o Primary high explosives, such as nitroglycerin, are extremely sensitive to heat, pressure, and movement.

o 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.



Chapter Summary

o 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.

o The crime lab examines evidence under a stereomicroscope.

o The crime lab can also wash the debris with acetone to dissolve the explosives and then analyze it to identify the type of explosive.



Chapter Summary

o Thin layer chromatography is a presumptive test to provide a preliminary identification.

o Forensic scientists then perform confirmatory tests, such as infrared spectroscopy and gas chromatography–mass spectroscopy.

o Explosions are often caused by acts of terrorism. Terrorism may be politically, environmentally, or religiously motivated.

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Chapter 5 Explosions © 2012 Cengage Learning. All Rights Reserved