Chapter 10 Forensics

Physical properties

Describe a substance without reference to any other substance

• weight, volume, color, boiling point, melting point

Chemical property

describes the behavior of a substance when it reacts or combines with another substance

• color transformation of heroin when chemical reagent is applied

Matter

anything that has a mass and occupies space

• can be classified accoridng to the physical form it takes

Element

simplest substance known and provides the building block from which all matter is composed

• 118 elements

Compounds

Two or more elements combine to form a compound

• atom is the basic particle of an element

• molecule is the smallest unit of a compound

Physical states

solid, liquid, gas/vapor

Solid

definite shape and volume

• molecules are held closely together in rigid state

Liquid

specific volume, takes the shape of its conatiner

• molecules are in contact with one another but not rigidly held in place

Gas

neither a definite shape nor volume

• attractive forces between molecules are small enough to permit them to move with complete freedom

Measurement system

metric system

• basic unit of measurement: meter, gram, liter

• common prefixes used in metric system: deci, centi, milli, nano, kilo, mega

Metric conversion

2.54 cm= 1 inch

Temperature

measure of heat intensity of the hotness/coldness of a substance

Derived by assigning the freezing point of water a value of 0°Celsius and its boiling point a value of 100°Celsius.

Weight

force with which gravity attracts a body

Mass

refers to the amount of matter an object contains independent of gravity

Density

defined as the mass per unit volume

D=M/V

• density is an intensive property of matter, meaning it remains the same regardless of sample size

• the observation that a solid object either sinks, floats, or remains suspended when immersed is accounted for by the property of density.

  
  

Refractive index

ratio of the velocity of light in a vacuum to that in the medium under examination

• intensive property and will serve to characterize a substance

Refraction

bending of light waves because of a change in velocity

Birefringence

numerical difference between these two refractive indices

Refractive light method

Crystalline solids have definite geometric forms because of the orderly arrangement of their atoms.

These solids refract a beam of light in two different light-ray components.

• This results in double refraction.

Two models describing behavior of light

• continuous wave- when light is moving through space

• stream of discrete energy particles- when light is absorbed by a substance

Wavelength

the distance between two successive crests

Frequency

the number of crests passing any one given point per unit of time

Relationship between wavelength and frequency

Frequency and wavelength are inversely proportional to one another.

Electromagnetic spectrum

entire range of radiation energy from the most energetic cosmic rays to the least energetic radio waves

• consists of several wavelengths each corresponding to a different color

Define the terms of physical property and chemical property

physical property- behavior of a substance without alteration of the substances composition through a chemical reaction

chemical property- behavior of a substance when it reacts or combines with a substance

What is weight/mass? Explain the difference.

Weight is a property of matter that depends on both the mass of a substance and the effects of gravity on that mass.

Mass is a constant property of matter that reflects the amount of material present.

• Mass differs from weight because it remains the same regardless of location.

Name the 3 forms or states of matter and explain how the shape and volume of matter are expressed in each state

The three states of matter are:

solid- has both a specific shape and volume.

liquid- Ahas a specific volume, but takes the shape of the container in which it resides.

gas- has neither a specific shape nor a specific volume.

What two models do scientists use to explain the nature of light? Under what conditions does each model best describe the behavior of light?

continuous wave- When light is moving through space

discrete energy particles- Light as a stream of discrete particles best describes its behavior when light is absorbed by a substance

Glass

hard, brittle substance that is composed of silicon oxides (sand) mixed with various metal oxides

• sodium carbonate (soda) normally added to the sand to lower its melting point

• calcium carbonate (lime) added to sand mix to prevent the glass from dissolving in water

Types of glass

float glass

tempered glass

laminated glass

Float glass

when molten glass is cooled on a bath of molten tin; typically used for windows

Tempered Glass

Stronger than normal glass due to rapid heating and cooling of the glass surfaces (does not shatter, just cracks)- rear windows of cars

Laminated glass

a layer of plastic between two pieces of ordinary window glass (used in windshield)

Two important physical properties in comparing glass fragments

Density

Refractive index

• both are class characteristics

Flotation method

A precise and rapid method for comparing glass densities

• glass particle immersed in liquid

• add in small amounts of liquid until the glass chip remains suspended in the liquid medium

Glass immersion method

used to determine a glass fragment’s refractive index

• temperature of the liquid is increased or decreased until the glass fragments disappear

Becke line

bright halo near the border of a particle that is immersed in a liquid of a different refractive index

• When the refractive index of the liquid medium is equal to the glass particle, the Becke line disappears and minimum contrast between liquid and particle is observed.

  • liquid is usually an oil that is heated at 0.2 degrees C per minute until a match point is reached

    
    

Individualization of glass fragments

can only happen when the suspect and crime-scene fragments are assembled and physically fitted together

physical fit= individual characteristic

Analyzing crack features

•The penetration of window glass by a projectile produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures).

• help determine direction of impact

3R rule

• used to determine direction if impact

• Radial cracks form Right angle on the Reverse side of the force

Analyzing cracks- sequence

a fracture always terminates at an existing line of fracture

Collection of glass

• must collect all glass found to see if they can be pieced together

• When an individual fit is thought improbable, the evidence collector must submit all glass evidence found with a representative sample of broken glass remaining at the crime scene.

• label inside/outside

• package in solid containers

• shoes/clothing should be individually wrapeed if looking for presence of glass fragments