Physical Properties: Glass and Soil

Criminalistics

Chapter 4

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

Physical Properties

• A physical property describes a substance without reference to any other substance.

• Physical properties can be measured without altering a substance’s 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

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 reagentColor change to purple indicates positive result for heroin

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

Metric System Conversion ValuesPREFIX EQUIVALENT

VALUES

deci 1/10 or 0.1

centi 1/100 or 0.01

milli 1/1000 or 0.001

micro 1/1,000,000 or 0.000001

nano 1/1,000,000,000 or 0.000000001

kilo 1000

mega 1,000,000

Metric-to-English Conversion ValuesENGLISH

VALUEMETRIC VALUE

1 inch 2.54 cm

39.37 inches 1 meter

1 pound 453.6 grams

1.06 quarts 1 liter

2.2 pounds 1 kilogram

Example: Convert 12” into centimeters.

12 inches X 2.54 cm = 30.48 cm

1 inch

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

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

W=mg

We measure mass with a balance

Physical Properties• Density: mass per unit volume

• Density is an intensive property of matter—this 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.

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

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.

Crystalline Solids • Crystalline solids have TWO refractive

indexes

• Crystalline solids have definite geometric forms because of orderly arrangement of the fundamental particle of the solid—the atom.

Amorphous Solids

• Amorphous solids: will only have ONE refractive index

• Amorphous solids: have atoms randomly arranged throughout the sample

• Example: glass

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 substance’s birefringence.

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

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.

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

Glass Comparison• Difficult because of prevalence of glass in

our society

• Often matched by piecing fragments together like a puzzle

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.

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.

Becke Line Disappearance

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.

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

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

Bullet Glass Fractures

Upper side is the exit side

Radial & Concentric

Fractures

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

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

Stress Markings On Fractures

Radial then Concentric Fractures

Stress Markings: Force was applied to the side opposite the

RIGHT angle

Radial Cracks make Right Angles on the Reverse Side of Force

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.

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.

Soil Evidence• Soil: any disintegrated surface material,

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

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.

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.

Identification of Soil

• 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.

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

Identification of Soil

• 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.

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.