Transcript PROPERTIES OF MATTER AND THE ANALYSIS OF GLASS

Chapter 4
PROPERTIES OF MATTER
AND THE ANALYSIS OF
GLASS
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-1
Physical vs. Chemical Properties
• The forensic scientist must constantly determine those
properties that impart distinguishing characteristics to
matter, giving it a unique identity.
• Physical properties such as weight, volume, color,
boiling point, and melting point describe a substance
without reference to any other substance.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-2
Physical vs. Chemical Properties
• A chemical property describes the behavior of a
substance when it reacts or combines with
another substance.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-3
Measurement System
• Scientists throughout
the world use the metric
system of measurement.
• The metric system has
basic units of
measurement for length,
mass, and volume; they
are the meter, gram, and
liter, respectively.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-4
Measurement System
• The following are common prefixes used in the
metric system: deci, centi, milli, micro, nano,
kilo, and mega.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-5
The Nature of Matter
• An element is the simplest substance known
and provides the building block from which all
matter is composed.
• Matter is anything that has a mass and
occupies space.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-6
The Nature of Matter
• All of the elements are listed by name and
symbol in the periodic table.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-7
The Nature of Matter
• Two or more elements combine to form a
compound.
• An atom is the basic particle of an element and
a molecule is the smallest unit of a compound.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-8
The States of Matter
• Matter can be classified according to the
physical form it takes.
– Solid-definite shape and volume
– Liquid-specific volume, takes the shape of its
container
– Gas/vapor-neither a definite shape nor
volume
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-9
The States of Matter
• Substances can change from one phase to
another without forming a new chemical
species, matter is simply being changed from
physical state to another.
• Whenever a situation exists in which a
substance can be distinguished by a visible
boundary, different phases exist.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-10
Theory of Light
• Two models describe the behavior of light.
– Light is described as a continuous wave.
– Light is depicted as a stream of discrete energy
particles.
• When white light passes though a prism, it is
dispersed into a continuous spectrum of colors.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-11
Theory of Light
• Visible light ranges in color from red to violet
in the electromagnetic spectrum.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-12
Theory of Light
• Waves are described in terms such as:
– Wavelength, the distance between two
successive crests (or one trough to the next
trough).
– Frequency, the number of crests (or troughs)
passing any one given point per unit of time.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-13
Theory of Light
• Frequency and wavelength are inversely
proportional to one another.
• The electromagnetic spectrum is the entire
range of radiation energy from the most
energetic cosmic rays to the least energetic
radio waves.
– Visible light is only a small part of the
electromagnetic spectrum.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-14
Theory of Light
• As electromagnetic radiation moves through
space, its behavior can be described as that of a
continuous wave; however, once radiation is
absorbed by a substance, it is best described as
discrete particles of light known as photons.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-15
Important Physical Properties
• Temperature is a measure of heat intensity, or
the hotness or coldness of a substance.
– In science, the most
commonly used temperature
scale is the Celsius scale.
This scale is derived by
assigning the freezing
point of water a value
of 0°C and its boiling
point a value of 100°C.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-16
Important Physical Properties
• Weight is the force with which gravity
attracts a body.
• Mass refers to the amount of matter an object
contains independent of gravity.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-17
Important Physical Properties
– The mass of an object is determined by
comparison to the known mass of standard
objects.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-18
Important Physical Properties
• Density is defined as the mass per unit
volume. (D = M/V)
– Density is an intensive property of matter,
meaning it remains the same regardless of
sample size.
– It is considered a characteristic property of a
substance and can be used as an aid in
identification.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-19
Important Physical Properties
• Light waves travel in air at a constant velocity
until they penetrate another medium, such as
glass or water, at which point they are suddenly
slowed, causing the rays to bend.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-20
Important Physical Properties
• The bending of light waves because of a change
in velocity is called refraction.
• Refractive index is the ratio of the velocity of
light in a vacuum to that in the medium under
examination.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-21
Important Physical Properties
• For example, at 25oC the refractive index of
water is 1.333.
• This means that light travels 1.333 times faster
in a vacuum than it does in water.
• Like density, refractive index is an intensive
property and will serve to characterize a
substance.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-22
Important Physical Properties
• Crystalline solids have definite geometric forms
because of the orderly arrangement of their
atoms.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-23
Important Physical Properties
• These solids refract a beam of light in two
different light-ray components.
• This results in double refraction.
• Birefringence is the numerical difference
between these two refractive indices.
• Not all solids are crystalline in nature. For
example, glass has a random arrangement of
atoms to form an amorphous or noncrystalline
solid.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-24
Glass Fragments
• Glass is a hard, brittle, amorphous substance
that is composed of silicon oxides mixed with
various metal oxides.
• Amorphous solids have their atoms arranged
randomly, unlike crystals.
• Tempered glass is stronger than normal glass
due to rapid heating and cooling.
• Laminated glass found in car windshields has a
layer of plastic between two pieces of ordinary
window glass.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-25
Glass Fragments
• For the forensic scientist, the problem of glass
comparison is one that depends on the need to
find and measure those properties that will
associate one glass fragment with another while
minimizing or eliminating other sources.
• To compare glass fragments, a forensic scientist
evaluates two important physical properties:
density and refractive index.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-26
Flotation Method
• The flotation method is a rather precise and
rapid method for comparing glass densities.
• In the flotation method, a glass particle is
immersed in a liquid.
• The density of the liquid is carefully adjusted
by the addition of small amounts of an
appropriate liquid until the glass chip remains
suspended in the liquid medium.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-27
Flotation Method
• At this point, the glass will have the same
density as the liquid medium and can be
compared to other relevant pieces of glass
which will remain suspended, sink, or float.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-28
Immersion Method
• The flotation and the immersion methods are
best used to determine a glass fragment’s
density and refractive index, respectively.
• The latter involves immersing a glass particle
in a liquid medium whose refractive index is
varied until it is equal to that of the glass
particle.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-29
Immersion Method
• At this point, known as the match point, the
Becke line disappears and minimum contrast
between liquid and particle is observed.
• The Becke line is a bright halo near the boarder
of a particle that is immersed in a liquid of a
different refractive index.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-30
Analyzing Cracks
• The penetration of window glass by a
projectile, whether it is a bullet or a stone,
produces cracks which radiate outward (radial
fractures) and encircle the hole (concentric
fractures).
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-31
Analyzing Cracks
• By analyzing the radial and concentric fracture
patterns in glass, the forensic scientist can
determine the direction of impact.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-32
Analyzing Cracks
• A high-velocity projectile such as a bullet often
leaves a hole that is wider at the exit side, and
hence its examination is important in
determining the direction of impact.
• The direction of impact can also be
accomplished by applying the 3R Rule: Radial
cracks form a Right angle on the Reverse side
of the force.
• The sequence of impacts when there have been
successive penetrations of glass, is frequently
possible to determine because a fracture always
terminates at an existing line of fracture.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-33
Collection of Glass
• If even the remotest possibility exists that glass
fragments may be pieced together, every effort
must be made to collect all the glass found.
• When an individual fit is thought improbable,
the evidence collector must submit all glass
evidence found in the possession of the suspect
along with a representative sample of broken
glass remaining at the crime scene.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-34
Collection of Glass
• The glass fragments should be packaged in
solid containers to avoid further breakage.
• If the suspect’s shoes and/or clothing are to be
examined for the presence of glass fragments,
they should be individually wrapped in paper
and transmitted to the laboratory.
FORENSIC SCIENCE: An Introduction, 2nd ed.
By Richard Saferstein
©2011, 2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-35