Transcript Physical Properties

Physical Properties:
Glass and Soil
Criminalistics
Chapter 4
What is a property?
• A property is an identifying characteristic
of a substance.
• There are two groups of properties that are
important to forensic scientists when
identifying substances:
• Physical properties
• Chemical properties
Physical Properties
• A physical property describes a substance
without reference to any other substance.
• Physical properties can be measured
without altering a substance’s chemical
composition.
• Physical properties are only associated with
the physical existence of the substance
• Examples of physical properties:
• Weight, Volume, Color, Boiling Points, and
Melting Points
Chemical Properties
• A chemical property describes the
behavior of a substance when it reacts or
combines with another substance.
• Examples of chemical properties:
• Wood combining with oxygen in the air to burn
or combust
• Suspect specimen of heroin mixed with
Marquis reagentColor change to purple
indicates positive result for heroin
The Metric System
• A system of measurement that uses a
decimal relationship so that a unit of
length, mass, or volume can be converted
into a subunit by multiplying or dividing by
multiples of ten.
• Basic units of measurement in the metric
system are as follows:
• Length Unit: Meter
• Mass Unit: Gram
• Volume Unit: Liter
Metric System Conversion Values
PREFIX
deci
centi
milli
micro
nano
kilo
mega
EQUIVALENT
VALUES
1/10 or 0.1
1/100 or 0.01
1/1000 or 0.001
1/1,000,000 or
0.000001
1/1,000,000,000 or
0.000000001
1000
1,000,000
Metric-to-English Conversion Values
ENGLISH VALUE
1 inch
39.37 inches
1 pound
1.06 quarts
2.2 pounds
METRIC
VALUE
2.54 cm
1 meter
453.6 grams
1 liter
1 kilogram
Example: Convert 12” into centimeters.
12 inches X 2.54 cm = 30.48 cm
1 inch
Physical Properties
• Temperature: a measure of heat intensity,
or the hotness or coldness of a substance
• Two common temperature scales:
• Fahrenheit scale:
– Freezing point of water is 32º
– Boiling point of water is 212º
• Celsius scale:
– Freezing point of water is 0º
– Boiling point of water is 100º
Physical Properties
• The Difference between Mass and Weight
• Weight: The force in which gravity attracts a
body to the earth
• Mass: the amount of matter an object contains
• Independent of its location on earth or any
other place in the universe
• Weight and Mass have a mathematical relationship
W=mg
We measure mass with a balance
Physical Properties
• Density: mass per unit volume
• Density is an intensive property of matter—
this means that it is the same no matter the
size of the sample
• Mathematical relationship:
D= m
V
The fact that objects will either sink, float, or remain
suspended in solution depends on the density of the
object and the density of the solution.
Physical Properties
• Light can have the property of a wave. It
travels in air at a constant velocity of nearly
300 million meters per second. It will
continue to travel at this speed until it meets
another object or medium.
• Refraction: The bending of a light wave as
it passes from one medium to another
Physical Properties
• Refractive Index: The ratio of the velocity of
light in a vacuum to the ratio of light in a medium
• Refractive Index = Velocity of light in vacuum
Velocity of light in medium
Refractive index is an intensive property of matter
and can characterize a substance.
Crystalline Solids
• Crystalline solids have TWO refractive
indexes
• Crystalline solids have definite geometric
forms because of orderly arrangement of the
fundamental particle of the solid—the atom.
Amorphous Solids
• Amorphous solids: will only have ONE
refractive index
• Amorphous solids: have atoms randomly
arranged throughout the sample
• Example: glass
Birefringence
• Most crystals that have cubic configuration
refract light at two angles and therefore
have TWO refractive indexes
• The difference between the two refractive
indexes characterize the crystal based on the
substance’s birefringence.
Light Dispersion
• Dispersion: the separation of light into its
component wavelengths
• Light is often separated by a prism
• The ability of a prism to disperse light into its
different colors is due to dispersion
• Each color of light, in passing through the glass
prism, slows to a speed slightly different than
the others and bends at different angles as it
emerges from the prism.
• The different wavelengths of light makes the
colors
Glass Fragments
• Glass: hard, brittle, amorphous substance
that is composed of silicon oxide (sand) and
other various metal oxides
• By adding soda, Na2CO3, soda-lime glass is
formed, which will not dissolve in water.
• Most glass that is examined by forensic
scientists is soda-lime glass, which makes
up most automotive and bottle glass.
Typical Types of Glass
• Pyrex: A borosilicate— boron oxide is added to
the glass to increase resistance to heat
• Tempered Glass: glass to which strength is added
by introducing stress through rapid heating and
cooling of the glass surfaces
• When tempered glass breaks, it does not shatter
• Usually found in car windows because of safety issues
Laminated Glass: two sheets of ordinary glass
bonded together with plastic film
• Usually used in auto windshields in the United States
Glass Comparison
• Difficult because of prevalence of glass in
our society
• Often matched by piecing fragments
together like a puzzle
Glass Comparison
• If glass fragments are too small to be pieced
together, they are identified by their
densities and refractive indexes.
• Only identifies glass to a common source:
Class Characteristic
• Density: compared by a flotation method
• Reference sample is suspended in solution
• Glass sample is added
• When the glass sample is at a equal point in
solution as the reference sample, then the
density of the glass equals the reference sample.
Glass Comparison
• Refractive index is compared by an immersion
method.
• Glass particles are immersed in a liquid whose refractive
index is varied until it is equal to that of the glass
particles.
• At this point, called the match point, the observer will
note the disappearance of the Becke Line and the glass
will seem to disappear.
• Becke Line: a bright halo that is observed near the border of a
particle immersed in a liquid of a different refractive index.
Becke Line Disappearance
Glass Fractures
• Glass bends in response to any force exerted
on it.
• When its’ elasticity is reached, the glass
fractures.
• Fractured glass reveals information that is
useful for forensic scientists in
reconstructing the force and direction of the
impact.
Projectile Glass Fractures
• Glass fractures based on a projectile,
whether a stone or a bullet.
• This type of fracture results in two basic
patterns:
• Concentric fractures: circular cracks in the
glass
• Radial fractures: straight cracks which radiate
outward from the center of the fracture
Bullet Glass Fractures
• Evidence of gunshot residue on the glass
• Often leaves a round, crater-shaped hole
that is surrounded by a nearly symmetrical
pattern of radial and concentric cracks
• The hole is wider on the exit side of the
glass
• When the velocity decreases, the fracture
pattern increases and provides less information
Bullet Glass Fractures
Upper side is the exit side
Radial &
Concentric
Fractures
How Does Glass Fracture?
• Once glass reaches its’ elasticity point, it
begins to crack.
• Radial fractures develop first on the side
opposite of the applied force
• The continued motion of the force places
tension on the front surface of the glass and
results in concentric fractures
• Examination of stress marks on the edges of the
fractures reveals information related to the side
on which the glass first cracked
Stress Markings On Fractures
• Stress marks are shaped like arches that are
perpendicular to one glass surface and
curved nearly parallel to the opposite
surface and are found on radial cracks.
• The perpendicular end of the arch is always
on the side opposite of the impact.
• 3R Rule: Radial cracks form Right angles
on the Reverse side of the force
Stress Markings On Fractures
Stress Markings:
Force was applied to
the side opposite the
RIGHT angle
Radial Cracks make
Right Angles on the
Reverse Side of Force
Radial then
Concentric Fractures
Which fracture came first?
• You can determine the sequence of impacts by
observing the existing fracture lines and their
points of termination.
• A fracture always terminates at an existing line of
fracture.
Collection and Preservation of Glass
• If glass evidence is to be pieced together, all
glass must be collected.
• If no fit can be completed, glass fragments must
be submitted with reference samples found at
scene.
• When direction of impact need be determined,
all broken glass must be recovered and
submitted to the crime lab.
Soil Evidence
• Soil: any disintegrated surface material,
both natural and artificial, that lies on or
near the earth’s surface.
• Soil evidence may include not only rocks,
minerals, vegetation, and animal matter but
also glass, paint chips, asphalt, that when
mixed, may be particular to a specific
location.
Soil Evidence is Important
• Particular soils at crime scenes may be
found on the victim and/or the suspect.
• Soils may lead investigators to particular
locations where a crime has been committed
or may link a criminal to a particular area
based on geology.
Identification of Soil
• Most soils can be differentiated by their
gross appearances.
• Low-power microscopic observation may
reveal the presence of plant and animal
material, as well as artificial debris.
Identification of Soil
• A soil sample can also be identified by its’
mineral composition.
• A mineral is a naturally occurring crystal whose
physical properties are known.
• Minerals make up rocks and therefore, can help
identify soil samples to a particular rock
formation.
Identification of Soil
• Forensic scientists also rely on a DensityGradient Tube to compare soil samples.
• Density-Gradient Tube: a glass tube fill from
top to bottom with liquids of successfully
lighter densities
• Used to determine the density distribution of
the soil sample
Identification of Soil
• Sometimes soil samples in a particular area
are basically identical because of the rock or
mineral composition.
• Soil samples are basically useless to
forensic scientists unless there is something
in the soil that is different from adjacent
areas of the crime scene.
Preservation of Soil Evidence
• Reference soil samples must be collected
within a 100-yd radius of the crime scene
and must be the top layer of the soil
• Soil found on a suspect must be carefully
preserved and not removed from garments.
• Soil samples found under cars must be
collected intact and can be valuable if soil is
layered.