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Physical Properties: Glass and Soil

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A property is an identifying characteristic of a substance. ... Radial then Concentric Fractures ... Used to determine the density distribution of the soil sample ... – PowerPoint PPT presentation

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Title: Physical Properties: Glass and Soil


1
Physical Properties Glass and Soil
2
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

3
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

4
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

5
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

6
Metric System Conversion Values
7
Metric-to-English Conversion Values
Example Convert 12 into centimeters. 12 inches
X 2.54 cm 30.48 cm 1 inch
8
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

9
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

10
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.

11
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

12
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.

13
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.

14
Amorphous Solids
  • Amorphous solids will only have ONE refractive
    index
  • Amorphous solids have atoms randomly arranged
    throughout the sample
  • Example glass

15
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.

16
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

17
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.

18
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

19
Glass Comparison
  • Difficult because of prevalence of glass in our
    society
  • Often matched by piecing fragments together like
    a puzzle

20
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.

21
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.

22
Becke Line Disappearance
23
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.

24
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

25
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

26
Bullet Glass Fractures
Upper side is the exit side
Radial Concentric Fractures
27
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

28
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

29
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
30
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.

31
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.

32
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.

33
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.

34
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.

35
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.

36
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.
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