PHYSICAL PROPERTIES:
Glass and Soil
Chapter 4
Physical Properties
Physical properties are those that describe a substance
without changing it through a chemical reaction. In other
words, physical properties describe this substance
without reference to any other substance. These are
some examples of physical properties:
•
•
•
•
•
•
•
Color
Volume
Boiling/Melting points
Temperature
Weight and Mass
Density
Refractive Index
Chemical Properties
Chemical properties describe the behavior
of a substance when it reacts or combines
with another substance.
• One chemical reacting with another and
producing a color, odor, etc.
• A substance combusting – like wood- in
the presence of oxygen
Intensive vs. Extensive Properties
– Intensive - Properties that do not depend on the amount
of the matter present.
•
•
•
•
•
•
•
•
•
•
Color
Odor
Luster - How shiny a substance is.
Malleability - The ability of a substance to be beaten into thin
sheets.
Ductility - The ability of a substance to be drawn into thin wires.
Conductivity - The ability of a substance to allow the flow of
energy or electricity.
Hardness - How easily a substance can be scratched.
Melting/Freezing Point - The temperature at which the solid and
liquid phases of a substance are in equilibrium at atmospheric
pressure.
Boiling Point - The temperature at which the vapor pressure of a
liquid is equal to the pressure on the liquid (generally atmospheric
pressure).
Density - The mass of a substance divided by its volume
Intensive Vs. Extensive properties
-Extensive - Properties that do depend on the
amount of matter present.
• Mass - A measurement of the amount of matter
in a object (grams).
• Weight - A measurement of the gravitational
force of attraction of the earth acting on an
object.
• Volume - A measurement of the amount of
space a substance occupies.
• Length
Mass vs. Weight
• Mass is the amount of matter an object is made
of.
• Weight is the force with which gravity attracts an
object
– Therefore, the mass of an object remains constant,
– But the weight of an object changes depending on the
gravitational force
• So your weight on Earth may be 60 Kg, but on the moon, it will
be 10 Kg, since the gravitational pull of the moon is 1/6th that
of the Earth.
• The amount of matter you’re made up though – your mass,
remains the same in both places.
• W = mg
Density
• Mass of an object, per unit of volume
• Density is an intensive property – it does
not change, even if the size of the object
increases or decreases
D = mass
volume
Refraction
• Refraction is the change in
direction of a wave due to a
change in its speed. This is
most commonly seen when a
wave passes from one
medium to another.
• Refraction of light is the most
commonly seen example, but
any type of wave can refract
when it interacts with a
medium,
– for example when sound
waves pass from one
medium into another –
from air to water.
Refractive Index
• The refractive index (or index of refraction) of a
medium is a measure for how much the speed of light (or
other waves such as sound waves) is reduced inside the
medium. For example, typical glass has a refractive
index of 1.5, which means that light travels 1.5 times
faster in air or a vacuum than it does in glass.
• Two common properties of glass and other transparent
materials are directly related to their refractive index.
– First, light rays change direction when they cross the interface
from air to the material.
– Second, light reflects partially from surfaces that have a
refractive index different from that of their surroundings.
Calculating Refractive Index
• Refractive Index= velocity of light in vacuum
velocity of light in medium
Refractive Index, cont’d.
• The refractive index of common glass is 1.50,
but other types of glass can have slightly
different numbers.
• The refractive index of water at 25 degrees
Celsius is 1.33
• So the refractive index of common glass is
greater than water
– Meaning: Light travels 1.5 times faster in vacuum
than it does in glass
– And light travels 1.33 times faster in vacuum than it
does in water
• So light travels faster in water than in glass
The Becke Line Method
• The Becke line is a bright halo of light that appears around the
perimeter of a particle when:
1. The indices of refraction of the
particle and surrounding medium are
different.
2. The microscope is defocused.
• By convention, the microscope is defocused by increasing the
distance between objective and sample. The Becke line results
from the concentration of light either inside or outside of the image
of the particle, depending on whether the particle or the medium has
the larger index of refraction. This refraction of light at the
boundaries creates an optical halo perceived as the Becke line. This
halo is caused by the concentration of refracted light rays along the
edge of the particle . The Becke line will move toward the region
with higher index of refraction.
nglass >nmedium
nmedium = 1.525
nglass = 1.60
•Glass has higher refractive index
•Becke line seen inside
•Rays converge
nglass < nmedium
nmedium = 1.525
nglass = 1.34
•Glass has lower refractive index
•Becke line seen outside
•Rays diverge
Becke Lines
In (a) the transparent
specimen has a higher
refractive index than the
surrounding medium.
•When the objective is
raised above focus point,
a bright Becke line
appears inside the
specimen.
In (b) the transparent
specimen has a lower
refractive index than that of the
medium.
•When the objective is
raised above focus point,
a bright Becke line
surrounds the specimen.
In either case, light is converged into the medium having a higher refractive index.
When the refractive indices are matched the Becke line disappears.
What would you see if the
particle being observed has the
same refractive index as the
medium it is in?
• The Becke Lines would disappear!
THE METRIC SYSTEM
Countries that have not adopted the metric
system:
USA, Liberia and Myanmar (formerly: Burma)
How to Convert
Celsius vs.
Fahrenheit
TEMPERATURE SCALES
• Kelvin
• Celsius
• Fahrenheit
THE CONVERSION
• F = (C x 1.8) + 32
• C = (F - 32)
1.8
• K = ºC + 273
Main properties of glass
• Solid and hard material
• Disordered and amorphous structure (not
crystalline like ice or salt for example)
• Fragile and easily breakable into sharp
pieces
• Transparent to visible light
• Inert and biologically inactive material
• Glass is 100% recyclable and one of the
safest packaging materials due to its
composition and properties
COMPOSITION OF GLASS
• Depending on the final use and application the composition of
the glass and cooling rate will vary to achieve the adequate
properties for the specific application. These are the common
ingredients to obtain glass:
1. Sand (SiO2 silica)
In its pure form it exists as a polymer, (SiO2)n.
2. Soda ash (sodium carbonate Na2CO3)
Normally SiO2 softens up to 2000°C, where it starts to degrade
(at 1713°C most of the molecules can already move freely).
Adding soda will lower the melting point to 1000°C making it
more manageable.
3. Limestone (calcium carbonate or CaCo3) or dolomite (MgCO3)
Also known as lime, calcium carbonate is found naturally as
limestone, marble, or chalk.
The soda makes the glass water-soluble, soft and not very
durable. Therefore lime is added increasing the hardness and
chemical durability and providing insolubility of the materials.
Types of Glass
1.
Commercial glass or Soda-lime glass:
This is the most common commercial glass and less
expensive. The composition of soda-lime glass is
normally 60-75% silica, 12-18% soda, and 5-12% lime.
A low percentage of other materials can be added for
specific properties such as coloring.
•
Soda-lime glass is primarily used for bottles, jars,
everyday drinking glasses, and window glass.
•
The disadvantages of soda-lime glass is that is not
resistant to high temperatures and sudden thermal
changes. For example, everybody has experienced a
glass breaking down when pouring liquid at high
temperature, for example to make tea.
Soda-Lime Glass
Types of Glass
2.
Lead glass:
This type of glass is composed of 54-65% SiO2, 1838% lead oxide (PbO), 13-15% soda (Na2O) or potash
(K2), and various other oxides. When the content of
PbO is less than 18% is known as crystal glass.
•
Lead glass is usually used for art glass, wine and
champagne glass, etc. Glass with high lead oxide
contents (i.e. 65%) may be used as radiation shielding
glass because lead absorb gamma rays and other
forms of harmful radiation, for example, for nuclear
industry.
•
As with soda-lime glass, lead glass will not withstand
high temperatures or sudden changes in temperature.
Lead Glass
Types of Glass
3.
Borosilicate glass:
This is glass mainly composed of silica (70-80%), boric oxide
B2O3 (7-13%) and smaller amounts of the alkalis (sodium and
potassium oxides) such as 4-8% of Na2O and K2O, and 2-7%
aluminum oxide (Al2O3). Boron gives greater resistance to
thermal changes and chemical corrosion.
•
It is suitable for industrial chemical process plants, in
laboratories, in the pharmaceutical industry, in bulbs for highpowered lamps, etc. Borosilicate glass is also used in the
home for cooking plates and other heat-resistant products. It
is used for domestic kitchens and chemistry laboratories, this
is because it has greater resistance to thermal shock and
allows for greater accuracy in laboratory measurements when
heating and cooling experiments.
•
More expensive than regular commercial glass
Borosilicate Glass
Types of Glass
4. Laminated glass (Safety Glass):
This is a type of safety glass that holds together when
shattered. It typically has 2 layers of thick commercial
glass with a layer of resin called PVB (Polyvinyl Butaryl),
between them. In the event of breaking, the interlayer
keeps the layers of glass bonded even when broken,
and its high strength prevents the glass from breaking
up into large sharp pieces. This produces a
characteristic "spider web" cracking pattern when the
impact is not enough to completely pierce the glass.
• Laminated glass is normally used when there is a
possibility of human impact or where the glass could fall
if shattered. Shop-front glazing and windshields are
typically laminated glasses. The PVB interlayer also
gives the glass a much higher sound insulation rating,
due to the damping effect, and also blocks 99% of
transmitted UV light.
Laminated Glass
Types of Glass
5.
Tempered Glass (Safety glass):
Tempered glass is four to five times stronger than standard glass
and does not break into sharp shards when it fails. Tempered glass
is manufactured through a process of extreme heating and rapid
cooling, making it harder than normal glass.
•
The brittle nature of tempered glass causes it to shatter into small
oval-shaped pebbles when broken. This eliminates the danger of
sharp edges. Due to this property, along with its strength, tempered
glass is often referred to as safety glass.
•
The thermal process that cures tempered glass also makes it heat
resistant. Tempered glass is used to make the carafes in automatic
coffee makers and the windows in ovens. Computer screens,
skylights, door windows, tub enclosures and shower doors are
more examples of places you will find tempered glass. Building
codes also require the windows of many public structures to be
made of tempered glass.
Tempered Glass
Types of crack (Break!) patterns
How Glass Breaks
1. Radial Cracks – Look like lines, radiating
out from the point of impact.
These cracks form first, and appear on the opposite
side of the glass (NOT where the force was first
applied)
2. Concentric cracks – Look circular rings,
each larger than the other.
These cracks form later and appear on the same
side of the glass (Where the force was applied.
HUH?
(appears first)
Where was the force applied?
You can figure out where the force was applied if the glass sheet is
still intact, by looking at the radial and concentric cracks. But what
if all you have to work with is a piece of glass from a crime scene?
Remember the 3R rule:
RADIAL cracks form a
RIGHT angle, on the
REVERSE side of the
force.
“Ribs” or Heckle marks are stress marks
In other words, parallel
lines become
perpendicular, moving
away from the force.
Any other way to determine from
which direction did the bullet
come?
The exit hole made by a bullet is usually wider, so by measuring the
entry and exit holes, one can determine from which direction a bullet
was shot.
The problem with this technique is that it works best with really thick
glass.
Which bullet-hole was made first?
The Hole on the Right
• The hole on the right was created first.
Cracks radiating out from the hole will stop
when they encounter another crack.
Stress placed on the glass (causing it to
crack) will be transferred along the existing
crack rather than across it. Basic
principles of physical science allow
criminalists to reconstruct events in time
order.
Collection of Glass Evidence
• Must be thorough
• Packaged in solid containers to avoid further breakage
• Suspect’s clothing can be collected to see if it contains glass
fragments – packed in paper
• Fragments can be pieced together to get “the whole picture”
• Or fragment pieces found at a hit-n-run can be matched to
the remaining broken glass of a suspect vehicle
PHYSICAL PROPERTIES: SOIL
• Forensics definition of “Soil” : any
disintegrated materials – either natural or
man-made that lies on the surface of the
Earth.
• Therefore, soil can contain
– natural clay, minerals, animal waste,
vegetation etc. but also asphalt and brick
fragments, concrete, paint chips and manmade chemicals.
Basic soil types:Loam Soil
Loam is soil composed
of sand, silt, manure,
and clay in relatively
even concentration –
best for gardening
Basic soil types: Sandy Soil
Basic soil types: Clay Soil
Soil Types
• But there are huge variations even in
these 3 basic soil types
– Mineral content of the soils can vary, causing
a change in color, refractive index, density,
etc.
– Animal and plant matter amounts can vary
(humus)
Soil Analysis
1. Visual comparison – observe differences in
color, particle content and size (side-by-side
analysis)
2. Low power microscope – will reveal animal
and plant debris, arificial fibers and particles
3. High power microscope – will help identify
certain minerals in the soil
4. Density-gradient tube – used to separate soil
components of different densities and to
compare the layers with other soils.
What is wrong with this photo? Why
can't it be entered into evidence?
The photo has been altered
• The criminalist holding the gun has a
wedding ring and wrist watch. These are
typically worn on the left hand. When the
photo was flipped (as if the negative was
reversed) these items appear to move to
the right hand.
THE END