Physical Properties: Glass and Soil

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Physical Properties Glass and Soil
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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

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Physical Properties

• A physical property describes a substance without
  reference to any other substance.
• Physical properties can be measured without
  altering a substances 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

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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

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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

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Metric System Conversion Values
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Metric-to-English Conversion Values
Example Convert 12 into centimeters. 12 inches
X 2.54 cm 30.48 cm 1 inch
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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

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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
• Wmg
• We measure mass with a balance

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Physical Properties

• Density mass per unit volume
• Density is an intensive property of matterthis
  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.

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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

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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.

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Crystalline Solids

• Crystalline solids have TWO refractive indexes
• Crystalline solids have definite geometric forms
  because of orderly arrangement of the fundamental
  particle of the solidthe atom.

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Amorphous Solids

• Amorphous solids will only have ONE refractive
  index
• Amorphous solids have atoms randomly arranged
  throughout the sample
• Example glass

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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 substances
  birefringence.

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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

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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.

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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

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Glass Comparison

• Difficult because of prevalence of glass in our
  society
• Often matched by piecing fragments together like
  a puzzle

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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.

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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.

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Becke Line Disappearance
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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.

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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

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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

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Bullet Glass Fractures
Upper side is the exit side
Radial Concentric Fractures
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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

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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

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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
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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.

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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.

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Soil Evidence

• Soil any disintegrated surface material, both
  natural and artificial, that lies on or near the
  earths 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.

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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.

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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.
• 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.

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Identification of Soil

• Forensic scientists also rely on a
  Density-Gradient 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
• 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.

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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.