Download PHYSICAL PROPERTIES * GLASS AND SOIL

Honors Forensic Science
 A.
Our understanding of the nature of
properties can be made easier by classifying
them into two categories

i. Physical properties = describe a substance
without reference to any other substance

1. ex. Weight, color, boiling point, and melting point
 Ii.
Chemical property = describes the
behavior of a substance when it reacts or
combines with another substance


1. ex. When wood burns, it chemically
combines with oxygen in the air to form new
substances
2. ex. Marquis reagent turns purple in the
presence of heroin
 A.
Has basic units of measurement for
length (meter), mass (gram), and volume
(liter)
 B.
In the metric system, volume can be
defined in terms of length
 C.
At times it may be necessary to convert
units from the metric system into the English
system or vice versa
 D.





Some of the more useful equivalents:
i. 1 inch = 2.54 cm
Ii. I meter = 39.37 inches
Iii. 1 pound = 453.6 grams
Iv. 1 liter = 1.06 quarts
V. 1 kilogram = 2.2 pounds
 A.
Temperature

i. The determination of the physical properties
of any material will often require the
measurement of temperature

Ii. Temperature is the measure of heat intensity,
or the hotness or coldness of a substance
 Iii.
The two most common
temperature scales used
are the Fahrenheit and
Celsius scales
 1.
In Fahrenheit, water boils at 212 degrees
F and freezes at 32 degrees F
 2.
In Celsius, water boils at 100 degrees C
and freezes at 0 degrees C
 i.
The force with which gravity attracts a
body is called weight
 Ii.
Mass differs from weight because it refers
to the amount of matter an object contains
and is independent to its location on earth or
any other place in the universe
 Iii.
In the metric system, the basic unit of
mass is the gram
 Iv.
The mass of an object is determined by
comparing it against the know mass of
standard objects
 V. The comparison is performed on a balance
 i.
 Ii.
Is defined as mass per unit volume
Is an intensive property of matter – is the
same regardless of the size of a substance
and is a characteristic property of a
substance and can be used as an aid in
identification
 Iii.
Solids are more dense than liquids, and
liquids more dense than gases
 Iv.
The volume of gases and liquids vary
considerably with temperature so when
determining density, it is important to
control and record temperature at which the
measurements are made
 i.
Light waves travel in air at a constant
velocity of nearly 300 million meters per
second until they penetrate another medium
 Ii.
When light waves hit another medium,
they slow, causing the rays to bend
 Iii.
The bending of light wave because of a
change in velocity is called refraction
 Iv.
The ratio of the velocity of light in a
vacuum to that in any medium determines
the refractive index
 V.
Like density, the refractive index is an
intensive property of matter and can serve to
characterize a substance
 Vi.
Tests used to determine a substance’s
refractive index must be performed under
carefully controlled temperature and lighting
conditions
 Vii.
Normally a solid or a liquid would be
expected to exhibit only one refractive index
value, however, many solids that are
crystalline in nature will have two refractive
indices whose values in part depend on the
direction in which the light enters the crystal
with respect to the crystal axis
 1.
crystalline solids – have definite
geometric forms because of the orderly
arrangement of the fundamental particle of
a solid, the atom
 Viii.
Not all solids are crystalline in nature,
some are amorphous

1. amorphous solids – a solid in which the
constituent atoms or molecules are arranged in
random or disordered patterns

2. Glass is an amorphous solid
 Ix.
Other characteristics of glass

1. birefringence – a difference in the two indices
of refraction exhibited by most crystalline
materials

2. dispersion – the separation of light into its
component wavelengths
 A.
Glass that is broken and shattered into
fragments and minute particles during the
commission of a crime can be used to place a
suspect at the crime scene
 B.
Glass is a hard, brittle, amorphous
substance that is composed of silicon oxides
mixed with various metal oxides
 C.
Tempered glass = glass to which strength
is added by introducing stress through the
rapid heating and cooling of the glass surface
 D.
Laminated glass = two sheets of ordinary
glass bonded together with a plastic film

i. Most car windshields
 E.
Glass will possess its greatest evidential
value when it can be individualized to one
source. This can only be accomplished when
pieces can be assembled and physically fitted
together
 F.
At this time, the physical properties of
density and refractive index are used most
successfully for characterizing glass particles
 G.
Glass can be distinguished by a density
determination and refractive indices.
However, even glass fragments removed from
a single sheet of glass may not have a
uniform refractive index value
 H.
A significant difference in either density
or refractive index proves that the glasses
examined do not have a common origin
 i.
FBI has a database of density and
refractive indices of different glasses
collected
 A.
Glass bends in response to any force that
is exerted on any one of its surfaces; when
the limit of its elasticity is reached, the glass
fractures
 B.
Fractured glass may reveal information
regarding force and direction of an impact
 i.
Radial – a crack in a glass that extends
outward like the spoke of a wheel from the
point at which the glass was struck
 Ii. Concentric – a crack in a glass that forms
a rough circle around the point of impact
 D.
Stress marks – are shaped like arches that
are perpendicular to one glass surface and
curved nearly parallel to the opposite
surface
 E.
Radial cracks form a right angle on the
reverse side of the force
 F.
When there have been successive
penetrations of glass, it is frequently possible
to determine the sequence of impact by
observing the existing fracture lines and their
points of termination. A fracture always
terminates at an existing line of fracture.
 A.
Soil = includes any disintegrated surface
material, both natural and artificial, that lies
on or near the earth’s surface

i. Encompasses analysis of naturally occurring
material such as rocks, minerals, vegetation and
animal matter, as well as, glass, paint chips.
Asphalt, brick fragments, cinders, etc.
 B.
Value of soil as evidence rests with its
prevalence at crime scenes and its
transferability between the scene and the
criminal
 C.
Soil evidence is comparative in nature
 D.
Most soils can be differentiated and
distinguished by their gross appearance.
Samples must be dried before comparison.
 E.
Low power microscopic examination of
soil will reveal the presence of plant and
animal materials as well as of artificial
debris.
 F.
Minerals = a naturally occurring crystal
and like any other crystal, its physical
properties are most useful for identification.
 G.
Some forensic laboratories currently rely
on the density-gradient tube technique to
compare soil specimens.
 i.
Density – gradient tube = glass tube filled
from bottom to top with liquids of
successively lighter densities, used to
determine the density distribution of soil.
 H.
The ultimate forensic value of soil
evidence depends on its variation at the
crime scene
 I.
Up to this time, there have been no
statistically valid forensic studies on the
variability of soil evidence.