Physical Properties: Glass & Soil Chapter 4

Physical Properties Identifying characteristics of substances Describes a substance without reference to any other substance Example: Mass, Volume, Color, Boiling & Melting points

Chemical Properties Describes the behavior of a substance when it reacts or combines with another substance Transformed

Measurement The type of material will influence which physical or chemical properties the forensic scientist will ultimately chose to observe and measure. Making assessments and comparisons against a known measurement standard, may require conversions between the “English system” and the metric system. 1 inch = 2.54 centimeters 1 meter = 39.37 inches 1 pound = 453.6 grams 1 liter = 1.06 quarts 1 kilograms = 2.2 pounds

Physical Properties: Temperature Scale requires two reference points Freezing point of water Fahrenheit - 32°F Celsius - 0°C Boiling point of water Fahrenheit - 212°F Celsius - 100°C

Physical Properties: Weight & Mass Weight – a property of matter that depends on both the mass of a substance and the effect of gravity on that mass. W = m x g Mass – the amount of matter an object contains

Solids Crystalline solids have definite geometric forms because of the orderly arrangement of the fundamental particle of a solid, the atom. Most crystals refract a beam of light into two different light-ray components known as double refraction. Birefringence is the difference in the two indices of refraction exhibited by most crystalline materials. Amorphous solids are solids in which the constituent atoms or molecules are arranged in random or disordered positions. There is no regular order in amorphous solids

Physical Properties: Density An intensive property of matter that is equivalent to the mass-per-unit volume of a substance. Density (D) = Mass (g) Volume (cm3 or ml) Volume = Length x Width x Height for regular sided objects = displacement is used for irregular shaped objects Intensive properties are not dependent on the size of an object.

Physical Properties: Density Volume of gases and liquids vary according to temperature Water most dense at 4°C = 1ml= 1cm3 = 1g Water is unique in that: as temperature increases from 4°C, vol. expands and density decreases. Density can be affected by heat and pressure. as temperature decreases from 4°C, vol. expands and density decreases

The Effects of Temperature and Pressure on Density As the temperature of an substance increases, the substance expands, volume increases and density decreases. As the temperature of an substance decreases, the substance contracts, volume decreases and density increases. As the pressure on a substance increases, the substance contracts, volume decreases and density increases. As the pressure on a substance decreases, the substance expands, volume increases and density decreases.

Physical Properties: Density (Buoyancy) Buoyancy – the ability to float in a liquid Object with greater density than the liquid it is placed in will sink Object with a density less than the liquid it is placed in will float Object with a density equal to the liquid it is placed in will remain suspended within the liquid

Physical Properties: Refractive Index Refraction – bending of light waves at it passes from one medium to another Air water Air translucent solid Refractive Index – an intensive property ratio of the speed of light in a vacuum to its speed in a given substance Refractive Index = velocity of light in a vacuum velocity of light in a medium

Physical Properties: Refractive Index Water at 25°C – refractive index is 1.333 This means that light in a vacuum travels 1.333 times faster than water at this temperature. Must control temperature and lighting conditions when determining a substances refractive index since refractive index varies with temperature and frequency wavelength of light Standard wavelength for tabulated refractive indices is normally 589.3 nm – the predominant wavelength emitted by sodium light. This is referred to the sodium D light.

Physical Properties: Refractive Index When a transparent solid is immersed in a liquid that has a similar refractive index it will appear to disappear. Many solids (crystalline) will have two refractive indices whose values will depend on the direction in which light enters the crystal with respect to the crystal axis.

Physical Properties: Refractive Index Amorphous solids – random arrangement of atoms in a solid (glass) Calcite – double refraction (double image) has two refractive indices: 1.486 and 1.658. The difference is .172

Physical Properties: Birefringence & Dispersion Birefringence is the difference in the two indices of refraction exhibited by most crystalline materials. The optical properties of crystals provide points of identification that will help characterize a crystal. Dispersion is the separation of light into its component wavelengths

Comparing Glass Fragments Window glass – a hard, brittle, amorphous substance composed of silicon oxides (sand) mixed with various metal oxides such as sodium, calcium, magnesium, and aluminum –Tempered glass – strength added by introducing stress through the rapid heating and cooling of the glass surfaces. Laminated glass – two sheets of ordinary glass bonded together with a plastic film.

Comparing Glass Fragments Glass is very common – can be used as corroborative evidence. As analytical techniques develop, trace elements in glass may provide distinctive and measurable characteristics Best when fragments can be individualized to one source i.e. fragments assembled and physically fit together. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration. Currently the physical properties of density and refractive index are used most successfully for characterizing glass particles (class evidence)

Comparing Glass Fragments Using density Floatation – a rapid and precise method A standard/reference glass particle is immersed in a liquid (a mixture of bromoform and bromobenzene) Density of a sheet of window glass is not consistent throughout the sheet. Can vary as much as 0.0003 g/ml Comparable density results require the comparison of refractive indices using the immersion method. Glass particle is immersed in a liquid medium whose refractive index is varied until it is equal to that of the glass particles. Match point is reached when the Becke line disappears.

Becke Line 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 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.

Glass has higher refractive index Becke line seen inside Rays converge nglass >nmedium nglass < nmedium          nmedium  = 1.525      nglass    = 1.60         nmedium = 1.525 nglass    = 1.34 Glass has higher refractive index Becke line seen inside Rays converge Glass has lower refractive index Becke line seen outside Rays diverge

How Windows Break When struck, flat glass breaks in a very specific way. Glass is weaker under tension than under compression so the window will break on the opposite side to the strike. Radial fractures form initially. These run out from the point of impact on the opposite surface to the applied force. Then concentric fractures form. These run between the radial fractures but on the same side of the glass as the impact.

Fractures formed following impact onto a piece of glass 1 – Radial fracture 2 – Concentric fracture

Which bullet hole was made first?

                           

Soil May provide associative evidence due to its prevalence at a crime scene and its transferability between the scene and the criminal Most soils can be differentiated and distinguished by their gross appearance. Comparison of color (can be affected by degree of moisture content so examine only dry samples) & texture

Soil Comparison of mineral and rock content. Mineral – a naturally occurring inorganic crystalline solid. Utilize a density-gradient tube A glass tube filled from bottom to top with liquids of successively lighter densities; used to determine the density distribution of soil

basalt (A), anorthosite (B), breccia (C). round glass particles (D)

Available Water Capacity

> 200 mm 60 - 200 mm Coarse soils G GRAVEL coarse 20 - 60 mm medium Very coarse soils BOULDERS > 200 mm COBBLES 60 - 200 mm Coarse soils G GRAVEL coarse 20 - 60 mm medium 6 - 20 mm fine 2 - 6 mm S SAND 0.6 - 2.0 mm 0.2 - 0.6 mm 0.06 - 0.2 mm Fine soils M SILT 0.02 - 0.06 mm 0.006 - 0.02 mm 0.002 - 0.006 mm C  CLAY < 0.002 mm

Sand

                                                    Sands

Silt

Clay

Clay Particles

Porosity: the % open space between rock particles able to hold water

Factors which affect Porosity: Particle Shape Sorting Packing

The ability of water to flow through rock material

Factors Affecting Permeability Porosity Particle Size