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PHYSICAL PROPERTIES: Glass and Soil. Chapter 4. Physical Properties.
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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.
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)
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: • Sand (SiO2 silica)In its pure form it exists as a polymer, (SiO2)n. • 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 • 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.
Types of Glass • Lead glass: This type of glass is composed of 54-65% SiO2, 18-38% 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.
Types of Glass • 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 high-powered 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
Types of Glass • 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.
Types of Glass • 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.
How Glass Breaks • 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) • 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.
(appears first) HUH?
“Ribs” or Heckle marks are stress marks 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. 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.
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 man-made chemicals.
Basic soil types:Loam Soil Loam is soil composed of sand, silt, manure, and clay in relatively even concentration – best for gardening
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 • Visual comparison – observe differences in color, particle content and size (side-by-side analysis) • Low power microscope – will reveal animal and plant debris, arificial fibers and particles • High power microscope – will help identify certain minerals in the soil • 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.
