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Fire is a chemical chain reaction which takes place with the evolution of
heat and light
In order for a fire to take place there are 3 main ingredients that must be present
Oxygen, Heat and Fuel.
In chemistry we call the type of reaction that produces fire a
combustion reaction
Combustion is a
high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.
Whenever we complete a combustion reaction a hydrocarbon (compound of C and H) there are generally the same products formed:
CO2 and H2O
Combustion cannot take place in an atmosphere devoid of
oxygen
When heat is produced in the process of a chemical reaction this is known as an
Exothermic Reaction
When heat is absorbed from the reacting substances this is known as an
Endothermic Reaction
There are 4 main methods for stopping a combustion reaction (putting out a fire):
Smothering
Starvation
Cooling
Breaking the Chain Reaction
Smothering
placing an object over a fire that allows no oxygen to enter the combustion area, the fire will quickly run out of fuel and die.
Starving
removing any flammable materials from its path
Cooling
essentially extinguishes the fire by removing the "spark" necessary to keep the reaction going.
Breaking the Chain Reaction
chemicals in a fire extinguisher work by creating a non-flammable coating in the surface of the area on fire.
another way of starving the fire since the coating essentially removes the fuel from the path of the fire
Arson is
the criminal setting of a fire to commit at least vandalism and at worst murder or even mass murder.
Arson is difficult to investigate for three main reasons:
The arsonist can plan out the arson well in advance and bring all the tools needed to commit the act with him/her.
The arsonist does not need to be present at the time of the act.
The fire itself destroys evidence tying the arsonist to the crime.
Looking for accelerants is the
first step to determining if a fire is arson
The presence of residues in the soot left by petroleum based accelerants can be a dead giveaway that
an arson has been committed
The search of the fire scene must focus on finding the
fire's origin
How the fire started gives the best indication as to whether the fire was
accidental or intended
Some common signs of arson include:
Evidence of multiple sites of ignition
Lines of accelerant residue indicating it was poured from space to space in the structure
The majority of the burning taking place at the floor rather than the ceiling. (Heat rises so naturally fire does too but if there is a lot of accelerant on the floor the majority of the burning will take place there)
The presence of unburned combustible liquids (these are rarely completely consumed in the fire)
Starting at the location suspectedto be the origin of the fire, the ash, soot and any other porous materials should be collected and stored in
airtight containers
A vapor detector (sniffer) can be used to
collect and identify vapors from the burned areas of the fire.
Finally, the investigators look for obvious ignition devices like
matches, electronic ignitors or even the glass of a "Molotov cocktail".
Once collected the most common method for identifying the accelerants at a fire is by the use of a
GC (Gas Chromatograph) or GCMS (Gas Chromatography Mass Spectrometer).
The gas chromatograph is the most
sensitive and reliable instrument for detecting and characterizing flammable residues
Most arsons are started by accelerants such as
gasoline and kerosene
Gas chromatography can separate the hydrocarbon components and
assign a unique patterned graphic for each product type.
The pressure exerted by the vapor of the liquid at any given temperature is called the
Vapor Pressure
Liquids with high vapor pressures have lower boiling points and therefore
lower activation energies
The boiling point is the temperature at which the vapor pressure equals
atmospheric pressure
This means that liquids with lower boiling points and higher vapor pressures are easier to
burn
The Flash Point of a liquid is
the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid
flash point is dependent on
the boiling point and consequent vapor pressure of the liquid.
any liquid with a flashpoint less than 100°F is considered to be a
flammable liquid
Any liquid with a flashpoint between 100°F - 200°F is considered
combustible
the relative hazard of a flammable liquid increases as the flashpoint
decreases
Explosives are substances that undergo a rapid oxidation reaction with the production of large quantities of
gases
TNT is explosive for two reasons:
1. TNT is composed of the elements carbon, oxygen and nitrogen. When TNT explodes it forms several covalent gases: CO, CO2 and N2 that are very stable. The production of these very low energy (stable) bonds means that a great deal of energy is released. It should be noted that most explosives contain these same elements.
2. TNT itself is high energy and unstable. The structure of TNT (above) shows three fairly large nitro groups (NO2) bound to Toluene. Because these groups are fairly large and in close proximity to each other they cause strain on the structure of the Toluene. Remember that groups of electrons repulse each other. Other compounds under similar conformational strain are also explosive for this same reason.
There are 3 major classes of explosives:
Low
Primary High
Secondary High
Low explosives are those that burn only at their
surface
Gun powder and fireworks are the most common
low explosives
primary explosives are those that
ultrasensitive to heat, shock, or friction and provide the major ingredients found in blasting caps or primers used to detonate other explosives.
Secondary high explosives are those chemicals that
do not have to be contained to explode and are relatively stable and safe to handle
require an electrical spark, fuse, intense heat, or sharp blow to initiate their explosion.
The process for examining evidence at the site of an explosion:
Search systematically the entire scene and try to locate the epicenter (origin) of the explosion.
Objects located at or near the origin of the explosion must be collected for laboratory examination. Each object should be placed individually into a sealed airtight metal canister. Some explosive residues are known to be able to seep through plastic containment and contaminate other evidence.
Often a crater is located at the origin and loose soil and other debris must be preserved from its interior for laboratory analysis.
One approach for screening objects for the presence of explosive residues in the field or laboratory is the ion mobility spectrometer (IMS).
An Ion-mobility spectrometer (IMS) is an instrument that detects and separates ions in the
gas phase
Chemical Kinetics is the study of
reaction rates, how reaction rates change under varying conditions and by which mechanism the reaction proceeds.
There are five general properties that can affect the rate of a reaction:
The concentration of the reactants. The more concentrated the faster the rate.
Temperature. Usually reactions speed up with increasing temperature.
Physical state of reactants. Powders react faster than blocks - greater surface area and since the reaction occurs at the surface we get a faster rate.
The presence (and concentration/physical form) of a catalyst (or inhibitor). A catalyst speeds up a reaction, an inhibitor slows it down.
Light. Light of a particular wavelength may also speed up a reaction
Increasing temperature also means the molecules are
moving around faster and will therefore "bump" into each other more often.
Catalysts speed up
chemical reactions
increasing the concentration of the reactants will increase
the frequency of collisions between the two reactants.
Higher concentrations mean more
collisions and more opportunities for reaction.
By increasing the pressure,
you squeeze the molecules together so you will increase the frequency of collisions between them.
the larger the surface area of the solid,
the faster the reaction will be
rate=
change in concentration/change in time
There are a couple of rules to writing rate expressions:
Expressions for reactants are given a negative sign. This is because the reactant is being used up or decreasing.
Expressions for products are positive. This is because they are increasing.
All of the rate expressions for the various reactants and products must equal each other to be correct. (This means that the stoichiometry of the reaction must be compensated for in the expression)
There are two general methods by which investigators determine the time of death:
The Rate Method: in this method the time of death is estimated by evaluating the presence/absence of an indicator in a deceased in conjunction with the known behavior of such indicators.
The Concurrence Method: in this method the time of death is estimated by evaluating events which happen at or near the time of death, or offer information suggesting a time period for the death event.
The first steps in determining time of death are
1) collection of the body
, 2) collection of any evidence around the body and
3) collection of information about the deceased.
As an aqueous fluid, blood will dry at a predictable rate if we know the environmental parameters:
temperature, humidity, airflow, surface area etc.
Rigor Mortis
If the body feels warm and no rigor is present, death occurred under 3 hours before.
if the body feels warm and stiff, death occurred 3-8 hours earlier.
If the body feels cold and stiff, death occurred 8-36 hours earlier.
If the body is cold and not stiff, death occurred more than 36 hours earlier.
There are also several factors that can severely impact the onset and timeline of Rigor:
temperature
illness
activity before death
physical conditions where the body are found
Livor Mortis (Lividity) is
the settling of blood in body due to gravity
Livor Mortis starts to develop
2-4 hours after death, becomes non-fixed or blanchable up to 8-12 hours after death and fixed or non-blanchable after 8-12 hours from the time of death.
Blanching is what occurs when you
press your finger on your skin and you see a white spot for a few seconds
The pooling of blood is a
physical process based on the loss of blood pressure when the heart stops beating and will therefore occur at the same rate whether the temperature is cold or not, so it is less susceptible to atmosphere than rigor
Algor Mortis is
The cooling of the body after death
There are two main ways the body decomposes:
Autolysis and Putrefaction
Autolysis
is the process by which digestive enzymes within the body cells break down carbohydrates and proteins.
Putrefaction
is the predominant cause of tissue degradation and is due to bacterial activity. Putrefaction starts 4 to 10 days after death.
Most of the appearance of a dead body over time is due to putrefaction:
Bloating
Green discoloration of abdomen
Marbling along blood vessels-a brown black discoloration in blood vessels caused by hydrogen sulfide gas
Blisters and skin slippage
Loss of hair and nails
There are four general stages of putrefaction:
Putrefaction (4-10 days after death) - Autolysis occurs and gases (odor) and discoloration starts.
Black putrefaction (10-20 days after death) - exposed skin turns black, bloating collapses and fluids are released from the body.
Butyric fermentation (20-50 days after death) - the remaining flesh is removed, butyric acid is formed "fermenting" the remains and the body begins to mold if in contact with the ground.
Dry Decay (50-365 days after death) - decay is very slow now due to lack of fluids, hair and fingernails fall out.
The food found in a victim's stomach can give approximate time of death based on
the degree of digestion that has taken place
One method for approximating time of death not listed in the table is the determination of
Vitreous (eye fluid) Potassium concentration.
In a human being, the signs of death are:
No breathing
No heartbeat
No pupillary response to light
No response to pain stimuli
This process of blood seeping into the peripheral tissues of the body is called
livor mortis
Between 3 to 6 hours after the heart stops, a chemical in the body called
ATP (Adenosine Triphosphate) runs out.
ATP requires
the intake of oxygen to be produced so its production ceases with death.
When ATP is no longer available, the chemical myosin becomes irreversibly locked onto the muscle tissue and the muscle "locks" into place. This is called
rigor mortis
As bacteria inside the body continue to multiply and consume tissue, the product gases of their digestion: hydrogen sulfide, carbon dioxide and methane (H2S, CO2, CH4) cause two very well-known features of death:
The smell and the swelling.
The smell is predominantly caused by
Putrescine, or tetramethylenediamine, and Cadaverine or 1,5-pentanediaminethe
The bloating or swelling of the body is due to the pressure
(CO2, H2S) build up as gases are released from the autolysis of cells as well as the bacterial digestion of tissues.
Time line of Decomposition
2 days
Cells autolysis
Greenish purple staining occurs, blood decomposing
Skin takes on Marbled Appearance
4 days
Skin blisters
Abdomen swells with carbon dioxide due to bacteria in intestines
6-10 days
Corpse bloats with CO2
Corpse eventually bursts
Fluid begins leaking from openings as cell membranes rupture
Eyeballs liquefy
Skin sloughs off
The simplest and most efficient way to stop decomposition is
freezing, called cryopreservation
The material that composes most weapons is
metal
physical properties of metals include
shininess, malleability, ductility, and conductivity.
A malleable (mal ee uh bul) material is one
that can be hammered or rolled into flat sheets and other shapes.
A ductile material is one that
can be pulled out, or drawn, into a long wire
Metal atoms are arranged around each other into what is called a
cubic cell
There are three main types of packing in the transition metals:
Face-Centred Cubic (FCC),
Body-Centred Cubic (BCC) and Hexagonal Close Packed (HCP).
Grains are
regions of irregular structure that exist in the overall crystalline structure of the metal.
Grain size can be controlled by the time in which the metal is allowed to
cool
The faster the cooling process the smaller the grains that form and
the harder the metal becomes.
Another common material used to make weapons is
wood
Wood is a mixture of compounds:
cellulose, hemicelluloses, lignin, and extractives
The cellulose in wood makes it strong and resistant to
breakage
the lignin in wood makes it resistant to
compression
Polymers are
long chains of interlinked molecules
A ligature is
a piece of rope or wire that is used to suffocate the victim
Guns are dangerous because they
fire projectiles at a high rate of speed that cause cellular damage when they strike the body.
Bullets are cylinders that contain a packing of black powder and a primer to create a spark to start the
reaction
The rate at which a projectile leaves a gun is called its
Muzzle Velocity.