1. Chapter 5 PROPERTIES OF MATTER AND THE ANALYSIS OF GLASS
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2. 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.
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3. Physical vs. Chemical Properties
A chemical property describes the behavior of a substance when it reacts or combines with another substance.
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4. 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.
These units are the meter, gram, and liter, respectively.
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5. Measurement System
The following are common prefixes used in the metric system:
Deci
Centi
Milli
Micro
Nano
Kilo
Mega
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6. 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.
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7. The Nature of Matter
All of the elements are listed by name and symbol in the periodic table.
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8. 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.
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9. 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
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10. 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.
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11. 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.
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12. Theory of Light
Visible light ranges in color from red to violet in the electromagnetic spectrum.
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13. 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.
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14. 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.
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15. 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.
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16. 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.
The Celsius scale is derived by assigning the freezing point of water a value of 0°C and its boiling point a value of 100°C.
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17. 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.
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18. Important Physical Properties
The mass of an object is determined by comparison to the known mass of standard objects.
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19. Important Physical Properties
Density is defined as the mass per unit volume. (D = M/V)
Density is an intensive property of matter, meaning that it remains the same regardless of sample size.
Density is considered a characteristic property of a substance and can be used as an aid in identification.
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20. 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.
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21. 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.
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22. Important Physical Properties
For example, at 25C the refractive index of water is
This means that light travels 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.
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23. Important Physical Properties
Crystalline solids have definite geometric forms because of the orderly arrangement of their atoms.
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24. 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.
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25. 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.
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26. 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.
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27. 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.
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28. 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.
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29. 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.
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30. 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.
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31. 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).
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32. Analyzing Cracks
By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact.
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33. 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.
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34. 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.
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35. Collection of Glass
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.
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