Physical Properties:Glass and Soil

Chapter 4

Properties

• Physical - describes a substance without reference to any other substance.– Ie. Mass, volume, color, BP, MP– Intensive - retains property regardless of how

much material is there• ie. Density, MP, BP

– Extensive - dependent upon the amount of material

• ie. Mass, length

Properties

• Chemical - describes the behavior of a substance when it reacts or combines with another substance; a new substance formed– ie. Wood burning, digestion,– Heroin test - the presence of heroin is tested by

reacting the specimen with a chemical reagent called Marquis reagent. If heroin in present the reagent turns purple; hence the color change indicates a chemical property of heroin

Properties and Forensics• Which physical and chemical properties the

forensic scientist chooses to observe and measure will depend on the type of material that is being examined.

• Logic requires the fact that if the property can be assigned a numerical value, it must be one that relates to a standard system of measurement accepted by the scientific community.

The Metric System

• 1791 - The French Academy of Science

• Base units for length, mass, and volume are meter, gram, and liter respectively.

• All other subunits are multiples of 10 for the base unit.

• Subunits are represented by a prefix.

Common Conversions

• 1 mL = 1 cm3

• 1 inch = 2.54 cm

• 1 meter = 39.37 inches

• 1 m = 453.6 grams

• 1 liter = 1.06 quarts

• 1 kilogram = 2.2 pounds

Temperature

• Fahrenheit and Celsius Scales– C = (1.8*F) + 32– F = (C/1.8) - 32

• 0 C = 32 F

• 100 C = 212 F

Weight and Mass

• Weight - the force with which gravity attracts a body; dependent upon its location

• Mass - the amount of matter an object contains; independent of its location on earth

• W = m*g

• Weight and mass are used interchangeably

Density

• The most important physical property of matter with respect to the analysis of certain kinds of physical evidence.

• Density = mass/volume

• Intensive property

• Solids - volume measured by submersion; independent of temperature

Density

• Liquids and gases - very dependent on temperature so temperature must be recorded at time of analysis

• Observation of whether objects sink, float, or suspend is an indication of density.

Refractive Index

• Refraction - the bending of a light wave as it passes from one medium to another.

• The refraction is caused by the change in velocity of the wave.

• The ratio of the velocity of light in a vacuum to its speed in a given substance determines the refractive index of that substance.

Refractive Index

• Refractive Index = velocity of light in vacuum

velocity of light in medium

ie. 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 at 25 degree Celsius.

• Refractive index is an intensive physical property.

Refractive Index

• Procedures used to determine a substance’s refractive index must be performed under carefully controlled temperature and lighting conditions, because the index of a substance varies with its temperature and the frequency wavelength of light passing through it.

• Nearly all indices are determined at the wavelength commonly emitted by sodium light; 589.3 nm; known as Sodium D light

Crystalline Solids• Crystalline solids have definite geometric

forms because of the orderly arrangement of the fundamental particle of a solid; the atom.

• Atom - the smallest unit of an element; not divisible by ordinary chemical means.

• In any type of crystal, the relative location and distance between its atoms are repetitive throughout the solid.

Amorphous Solids

• Solids in which the constituent atoms or molecules are arranged in random or disordered positions. There is no regular order in amorphous solids.

• ie. glass, plastic, gel

Double Refraction• Crystals that refract a light beam into two different

light-ray components.

• The two light rays are refracted at different angles which in turn gives two different refractive index values.

• The difference of the two indices is known as the birefringence.

• Thus, the optical properties of crystals provide points of identification that will help characterize a crystal.

Dispersion• The separation of light into its component

wavelengths.• The ability of a prism to disperse light into its

component colors is explained by the property of refraction.

• Each color component of light, on passing through the glass, will be slowed to a speed slightly different from the others, thus causing each component to bend at a different angle as it emerges from the prism.

Comparing Glass Fragments

• Glass that is broken and shattered into fragments and minute particles during the commission of a crime can be used to place a suspect at the crime scene.

• Chips of glass may be found imbedded in the shoe or clinging to the clothing; headlight glass can be matched to the suspect vehicle.

Glass

• Defined: a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides.– Main ingredient - silicon dioxide (SiO2)

– Adding Na2CO3 lowers the melting point and viscosity making the substance easier to work.

– Common metal oxides: sodium, calcium, magnesium, and aluminum

Specialty Glass • Pyrex - Replacing some of the silica, sodium or calcium

oxides with boron oxide allows for a more heat resistant glass.

• A.k.a. - borosilicates

• Tempered glass - made stronger by introducing stress through rapid heating and cooling of glass surfaces. Prevents glass from shattering, instead it breaks into small square fragments. ie…back and side car windows.

• Laminated glass - gains strength by sandwiching a layer of plastic between two layers of glass. Windshields.

Evidential Value - Glass• Greatest value when it can be

individualized to one source.

• To do this the glass from the suspect and the crime scene must be assembled and physically fitted together.

• Not only do the irregular edges match but also the glass ream patterns.

Characterizing Glass

• Physical properties of density and refractive index are used.

• Density - flotation method; a standard/reference sample is suspended in a liquid mixture of known density, then a similar piece of suspect glass is immersed in the same liquid. If it behaves the same then the densities of the two pieces are similar.

Characterizing Glass• Similar density results also required additional comparison

of refractive indices.

• Refractive Index - Immersion method; glass particles are immersed in liquid medium whose refractive index is varied until it is equal to that of the glass particles.

• When the indices are equal the glass particle will not be visible and the Becke line will also disappear.

• Becke line - a bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index.

• Match Point - the point at which the refractive indices are equal.

Probabilities• The FBI has started to collect density and

refractive indices values for glass submitted to the lab for examination.

• This means that a criminalist may correlate the density and refractive index values to their frequency of occurrence and can make a meaningful assessment as to the probability that the fragments were at one time from the same source.

Glass Fractures• Fracture patterns tell whether the breakage

was due to a high velocity bullet, low velocity bullet, or another type of projectile or object.

• The direction of impact may also be determined from the close examination of the glass breakage.

• It is also possible to determine the sequence of fractures when more than one is present.

Fracture Patterns• Conchoidal - shell-like fracture patterns in

glass are a result of stress on the glass and breakage due to application of a force.– Radial - extend outward in a line from the point

on the glass where the force originated.• Radial cracks form a Right angle on the Reverse side

of the force (3R rule).

– Concentric - make circular patterns around the point of origin of the force.

• Concentric cracks form right angle on the same side of the force.

Fracture Patterns• An object traveling at high velocity will

cause a small hole in the impact side and a larger hole on the exit side.

• The slower an object is traveling the less symetrical the breakage and the less likely to contain usable data to determine direction of impact.

• More than one impact - the fracture always terminates at an existing line of fracture.

Collection and Preservation• If there is even the remotest possibility that

the fragments may be pieced together, every effort must be made to collect ALL glass.

• Improbable for putting together, then all the pieces from the suspect plus standard/reference samples must be collected from the scene for comparison.

Soil Characteristics• Soil - including any disintegrated surface material,

both natural and artificial, that lies on or near the earth’s surface.

• Forensic examination includes not onlynaturally occurring rocks, minerals, vegetation, and animal matter, but also the detection of such manufactured objects as glass, paint chips, asphalt, brick fragments, and cinders.

• The presence of the above items may impart soil with characteristics that will make it unique to a particular location.

Soil• Most soils can be differentiated and distinguished

by their gross appearance.• Side-by-side visual comparison of the color and

texture of soil specimens is easy to perform and provides a sensitive property for distinguishing soils that originate from different locations.

• All soils must be dried under the same laboratory conditions to assure color comparison.

• Approximately 1,100 distinguishable soil colors, hence, color comparison is a logical first step.

Soil• A mineral is a naturally occurring crystal that

is commonly found in soils.

• More than 2,200 minerals are known, however, only approx. 40 are common to soils.

• Rocks are characterized by their mineral content and grain size.

• Rocks and minerals are also used to manufacture a large variety of industrial and commercial products.

Soil Analyses• Density-gradient tube is used to compare soil samples.

– A glass tube filled with liquids of varying densities; soil separates out so that similar soil particles suspend in the liquid with the similar density.

• Because many soils may have similar density gradients the density-gradient test results must be used in combination with other tests.

• Forensic geologists try to individualize soil to any one location by determining the variation in the soil at the crime scene.

• The more the soil varies the more individual the characteristics become.

Collection and Preservation• Standard/reference samples must be taken

at various intervals within a 100-yard radius of the crime scene, as well as the crime site.

• Soil samples are also taken at all possible alibi locations that the suspect claims.

• Generally speaking, only the top-surface soil would be picked up by the suspect, therefore that is the only type of soil that should be collected for the comparison.

Collection and Preservation• Soil found on different clothing or locations

on the suspect must be packed separately.

• Lumps of soil are to be kept intact. The lumps tend to gather soil layers from numerous locations over the period of time, this could enable the scientist to individualize the soil and in turn give the soil more evidential value.

Vocabulary

Vocabulary• Amorphous solid

• atom

• Becke line

• birefringence

• Celsius scale

• chemical property

• concentric fracture

• crystalline solid

• density

• density-gradient tube

• dispersion

• Fahrenheit scale• intensive property• extensive property• laminated glass• mass• mineral• physical property• radial fracture• refraction• refractive index• tempered glass• weight