Physical Properties of Glass and Soil

1. Physical Properties of Glass and Soil Forensics: Chapter 4

2. Objectives: • Describe process of identifying physical properties of soil, water, plastic, and glass. • Use laws of physics to explain glass fracture patterns and refraction index of glass.

3. Types of Properties: • Nature of Properties: divided into 2 categories (physical & chemical). • Physical properties- describes a substance without reference to any other substances. ex: weight, volume, color, boiling & melting point. • Chemical properties – describes the behavior of a substance when it reacts or combines with another substance. ex: burning, color transformation (Marquis reagent)

4. Systems of Measure: • Two Basic Types: English System & the Metric System (SI) • Metric system is based on units of tens. FYI: (U.S. system of currency is analogous to the metric system because it can be divided into units of tens.) • Common measurements involve distance (meter), volume (liter) and mass (kilogram).

5. Seven SI Base Units: 1. Kilogram (kg) – mass 2. Meter (m) – distance 3. Kelvin (K) – temperature 4. Second (s) – time 5. Mole (mol) – amount of a substance 6. Candela (cd) – light intensity 7. Ampere (A) – electric current

6. Common Metric System Prefixes • Giga Mega (Kilo Hecto Deka Unit Deci Centi Milli) Micro Nano 109----106------103----102-----101------1------10-1-----10-2-----10-3------10-6-----10-9

7. Metric Conversion Practice:

8. Good To Know! • Some useful equivalents: 1 mL = 1 cm3 1 pound = 453.6 g 1 inch = 2.54 cm 1 L = 1.06 quarts 1 m = 39.37 inches 1 kg = 2.2 pounds

9. Dimensional Analysis Practice:

10. Temperature: • The determination of the physical properties of any material will often require the measurement of temperature. • Temperature: - measure of the average kinetic energy of the particles in a substance - indicates intensity of hotness or coldness of a substance. - intensive property (same regardless of size of substance) • The construction of temperature scales requires 2 reference points (usually the freezing & boiling points of water). Freezing of water = 0 °C or 32 °F Boiling of water = 100 °C or 212 °F

11. 3 Temperature Scales: • Three Temperature Scales: 1. Fahrenheit F = 9/5 C + 32 2. Celsius (formerly centigrade) C = 5/9 (F-32) 3. Kelvin K = C + 273 • Scientists in most countries use the Celsius scale to measure reference points.

12. Weight: • Weight and mass are not the same!!! • Weight: - force of gravity acting on an object. - dependent on location - the weight of a body is directly proportional to its mass • Equal arm balances are used for weighing an object against a known standard (pg 94).

13. Mass: • Mass: - amount of matter in an object - independent of location • The relationship between weight and mass is shown in the equation W = mg , where g represents acceleration due to gravity (9.8 m/s2 near the earth’s surface). FYI: The force of gravity on the moon is one-sixth that of earth.

14. Density: • Density: - mass per unit volume - intensive property (same regardless of size of substance). - solids tend to be more dense than liquids & liquids tend to be more dense than gases. (exception: water) - see table 4.1 (pg 96) - formula for density is D = M/V - varies depending on temperature of substance. • Float vs. Sink (using density) • Volumes of fluids (gases & liquids) vary considerably depending on temperature.

15. Refraction: • Refraction – the bending of light as it passes from one medium to another. • The speed of light in air is approximately 3.0 x 10 8 m/s. • Refractive Index: - the ratio of the speed of light in a vacuum to its speed in a given substance. - intensive property (same regardless of size of substance) - varies with the temperature, color, frequency & wavelength of light passing through it. • Standard wavelength is usually 589.3 nm (sodium D light)

16. Refractive Index: • Refractive Index = Velocity of light in vacuum Velocity of light in medium • Solids will have 2 refractive indices because most are crystalline in nature (birefrigence)

17. Two Types of Solids: • Crystalline solids - solid in which the constituent atoms have a regular arrangement. • Amorphous solids – solid in which the constituent atoms or molecules are arranged in random or disordered positions.

18. Stuff To Know: • Birefringence – the difference in the two indices of refraction exhibited by most crystalline materials. • Most crystals, excluding those that have cubic configurations, will refract a beam of light into two different light-ray components (double refraction). • Dispersion – separation of light into its component wavelengths….ROY G BIV

19. Glass Analysis: • Glass can become evidence when it breaks, scattering fragments that may attach to weapons, shoes, clothing, skin or hair. It can pick up fingerprints or other impressions as well as cause bleeding. • Glass analysis rarely provides individual characteristic evidence, but can offer class characteristic analysis (same color, shape, surface characteristics, optical & chemical properties….cannot positively show common source). • When WILL glass analysis provide individual characteristic evidence????

20. Did You Know? • Glass will possess its greatest evidential value when it can be individualized to one source (jig-saw fit). • Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of jig-saw fit. At this time, density & refractive index measurements are most successfully used.

21. How To Make Glass: • Glass – made by heating a mixture of sand (silica or silicon dioxide), limestone (calcium carbonate), and soda (sodium carbonate). • Various other chemicals are added along the way to alter the characteristics of the glass.

22. Types of Glass: • 1. Soda-lime glass – soda + sand + lime - (used in manufacturing of most windows and bottles) • 2. Borosilicates - boron oxide + metal oxides - (automobile headlights, heat resistant glass, Pyrex) • 3. Tempered glass – heated & cooled glass to produce stress (will break but not shatter) - (used in side & rear windows of US made cars) • 4. Laminated glass - plastic between 2 pieces of glass provide strength - (windshields of all US made cars)

23. Determining Density of Glass: • Flotation method - method by which a glass particle is immersed in a liquid of known density. • liquid most commonly used is a mixture of bromoform & bromobenzene. • Density of a single sheet of window glass is not completely homogeneous throughout. It has a range of values.

24. Determining Refractive Index of Glass: • Immersion method – glass particles are immersed in a liquid medium whose refractive index is varied until it is equal to that of the glass particle……..(this is the match point). • The refractive index of an immersion fluid is best determined by adjusting the temperature of the liquid (may use hot stage microscope and GRIM 2).

25. Other Stuff: • Becke Line – bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index. • The FBI laboratory has collected density and refractive indices values from glass submitted to it for examination, producing a data bank correlating these values to their frequency of occurrence in the glass population of the U.S.

26. Glass Fractures: • Glass bends in response to any force exerted on any of its surfaces. • Upon reaching its limit of elasticity, the glass will fracture. • Glass fractures can reveal the force & direction of impact of the glass.

27. Types of Glass Fractures: • (as produced by projectiles): 1. Radial – cracks that extend outward like the spokes of a wheel from the point of impact. - appear first, starting on opposite side of causative force. 2. Concentric – cracks that form a rough circle around the point of impact. - occur after radial cracks, starting on same side as causative force.

28. More About Fractures: • Holes produced by high velocity projectiles leave round, crater-shaped holes surrounded by both radial and concentric fractures. The hole will be wider on the exit side (indicating direction of impact). • Stress marks – (along radial cracks) parallel to the side on which the force was applied. • (3 R rule): Radial cracks form a Right angle on the Reverse side of the force. (figure 4-19). • Low velocity projectiles tend to shatter the glass completely instead of producing fracture patterns.

29. Fracture Information: • When there are more than one projectile penetrations of glass, it is generally easy to determine the sequence of impact by observing the existing fracture lines and their points of termination. • A fracture always terminates at an existing line of fracture. (figure 4-20)

30. Characteristics of Soil: Soil – complex mixture of minerals, plant & animal matter as well as tiny particles of man-made products such as glass, paint, asphalt, concrete, and other materials.

31. More About Soil: • The contents of soil vary greatly from region to region. • Soil analysis – begins with visual & microscopic inspections and progresses to determinations of color, consistency, pH & mineral content. (soil should be dry). • Mineral – naturally occurring crystalline solid. - color, geometric shape, density & refractive index are useful for identification