Transcript Chapter 4 Physical properties of Glass and Soil

Glass and Soil
Physical vs. Chemical Properties
Physical properties: describes
substances without reference to other
substances.
– Mass, density, color, weight, volume,
boiling point, and melting point
Physical vs. Chemical Properties
Chemical properties: describes the
behavior of substances when it reacts or
combines with another substance.
– Ex. Wood burning, Oxidation, reaction to
indicators
• Starch: iodine, brown  purplish black
• Heroin: Marquis reagent, turns purple
Properties Assessment
 The property we choose to observe and
measure will depend on the type of material
that is being examined.
 Properties must:
– be assigned a numerical value
– Relate to a standard system of measurement
accepted throughout the scientific community
Physical Properties: Temperature
Melting and boiling point.
Temperature: the measure of heat
intensity
–Fahrenheit
–32o F freezing point, 212o F Boiling point
–Celsius (centigrade)
–0o C freezing point, 100o C Boiling point
Temperature
freezing point
Celsius
(0o C)
0
Fahrenheit
(o F)
32
Boiling point
100
212
Body
temperature
37
98.6
Room
Temperature
23
69
Density
 The relationship of Mass per unit volume
– D=m/v
Water at 20o C 0.998g/mL = 1g/1.002 mL
 NOT dependent on the size of an object.
– Same regardless of the size of the substance.
– Solids (more dense) liquids gases
Volume
The amount of occupied space
Regular Shaped Objects
=LxWxH (cm3)
Irregular Shaped Objects
– Volume of displaced fluid in a graduated
cylinder.
Physical Properties: Density
Silver
10.5 (g/mL)
11.5
2.47-2.54
13.6
Lead
Window glass
Mercury Liquid
Water at 4 C
Ice
1.00
0.92
VII. Physical properties of glass
 Refraction: the bending of a light
wave as it passes from one medium
to another.(a change in velocity,
slows down)
 Index =
velocity of light in vacuum
velocity of light in medium
 water at 25C =1.333 (1.333 times
faster in a vacuum then in water at
that temp.
Refractive Index
Ratio of speeds in a vacuum vs a medium
– At a specific temperature
– And Wavelength Frequency
V of light in Vacuum
RI=
V of light in medium
Refractive Index
Water at 25C =1.333 (1.333 times faster
in a vacuum then in water at that temp.
Dependent on temperature and the
frequency of the wavelength of light
Sodium D light: STANDARD wavelength
– 589.3 nanometers
Refractive Index
Transparent solids immersed in a
liquid having a similar RI, light will
not be refracted as it passes from
liquidsolid.
Reason why the eye unable to
distinguish between the solidliquid
boundary.
Becke line:
http://www.hfni.gsehd.gwu.edu/~forchem/BeckeLine/BeckeLinePage.htm
Becke line:
nglass >nmedium
nmedium = 1.525
nglass = 1.60
nglass < nmedium
nmedium = 1.525
nglass = 1.34
List the four major types of
glass and describe how they
are made.
Glass Objectives
Types
Matching
Fractures
Preservation
Comparing Glass Fragments
Composed of silicon oxides mixed with metal oxides
Soda (NaCo3)
Lime (CaO)
use Boron oxide,
Pyrex
Can with stand
Borosilicates
HIGH heats
Tempered Glass: Rapid heating and
cooling does not
shatter
Soda-lime glass
windows
bottles
Test tubes
Headlights
Shower doors
Side + rear
windows
Laminated Glass Plastic or Glass and windshields
glues and sandwich
Matching glass fragments
 Suspect and crime scene fragments must fit
together to be from same source
 Physical properties of density and
refractive index are used most successfully
for characterizing glass particles.
1. Flotation test in density column!! 0.0001
2. Immersion Method
3. GRIM 2: Glass RI measurement (automated)
Flotation test in density column!!
Control glass added to liquid
Density of liquid adjusted
until control glass suspended
Unknown is then added to
see if it floats or sinks
Matching glass fragments:
Immersion method:
– Glass put into liquid
– RI of liquid adjusted by temperature until
a match point is reached.
• Point when Becke line disappears
because both liquid and glass have same
RI.
Becke line: a bright halo that is
observed near the border of a particle
immersed in a liquid of a different RI
Matching glass fragments:
Becke line: a bright halo that is observed near
the border of a particle immersed in a liquid
of a different RI
What If?
Density and RI values do not match:
• Then not from the same source
Density and RI values Match!
• Still cannot ID from same source
• Must compare results to frequencies in
society
• Must develop meaningful assessment as
to probability that fragments were at
one time from one source.
Glass Fractures
 If the force is greater then glass’s elasticity, it
fractures
 Fractures provide valuable information at
the crime scene.
– Ex:
• Force
• Direction
• Object used
Information Obtained
 Small hole: small stone thrown hard, small bullet
 Shattered glass: close range shot, large stone
(obvious gunpowder deposits)
 Hole from projectile can determine direction
 Hole is wider at exit side
 As projectile decreases irregularity of shape and
cracks increase
Fracture patterns
Radial fractures: cracks extend outward
like spokes of a wheel from point of
impact
Concentric Fractures: forms rough circle
around point of impact
Glass Fractures Sequence:
Counting the bullet holes
Collecting and preservation of glass
evidence.
All glass must be found to include odds
of matching pieces from suspect to C.S.
Must Obtain Controls: suspect, C.S.
– Control from area closest to point of break
– Glass placed in solid containers
– Individually wrap garments from suspect.
• To be later check for glass fragments!!!