Transcript properties of matter and the analysis of glass

Chapter 4
PROPERTIES OF MATTER
AND THE ANALYSIS OF
GLASS
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©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.
• A chemical property describes the behavior of
a substance when it reacts or combines with
another substance.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-2
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.
• The following are common prefixes used
in the metric system: deci, centi, milli,
micro, nano, kilo, and mega.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-3
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.
• All of the elements are listed by name and
symbol in the periodic table.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-4
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
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-5
Figure 4–1 The periodic table.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-6
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.
• Visible light ranges in color from red to violet in the
electromagnetic spectrum.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-7
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.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-8
Figure 4-2 The frequency of the lower wave is
twice that of the upper wave.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-9
Figure 4–3 Representation of the dispersion of light by a
glass prism.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-10
Figure 4-4 The electromagnetic spectrum.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-11
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.
• Weight is the force with which gravity attracts a
body.
• Mass refers to the amount of matter an object
contains independent of gravity.
– The mass of an object is determined by
comparison to the known mass of standard
objects.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-12
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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-13
Figure 1 Volume equivalencies in the metric system.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-14
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.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
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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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-16
Important Physical Properties
• Crystalline solids have definite geometric forms
because of the orderly arrangement of their
atoms.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-17
Figure 4–6 Comparison of the Celsius and Fahrenheit temperature scales.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-18
Figure 4–7 The measurement of mass.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-19
Figure 4–10 Light is refracted when it travels obliquely
from one medium to another.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
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Figure 4–11 Diagram
of a sodium chloride
crystal. Sodium is
represented by the
darker spheres,
chlorine by the lighter
spheres.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
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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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-22
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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-23
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.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-24
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.
• 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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-25
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).
• By analyzing the radial and concentric
fracture patterns in glass, the forensic
scientist can determine the direction of
impact.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-26
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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-27
Figure 4–19 Production of radial and concentric fractures in glass. (a)
Radial cracks are formed first, commencing on the side of the glass
opposite to the destructive force. (b) Concentric cracks occur
afterward, starting on the same side as the force.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-28
Figure 4–21 Two bullet holes in a piece of glass. The left
hole preceded the right hole.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-29
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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-30
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
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
4-31