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Chapter 3

Chapter 3. Physical Properties: Forensic Characterization of Soil. Objectives. Students should gain an understanding of: The difference between physical and chemical properties Conversions between the English system of measurements and the metric system The forensic characteristics of soil

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Chapter 3

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  1. Chapter 3 Physical Properties: Forensic Characterization of Soil

  2. Objectives • Students should gain an understanding of: • The difference between physical and chemical properties • Conversions between the English system of measurements and the metric system • The forensic characteristics of soil • Ways to collect and preserve soil evidence

  3. Introduction • Forensic laboratories examine common items at crime scenes for two reasons: • To identify properties that may place an object within a particular class • To discover additional characteristics that will allow the object to be individualized

  4. Physical and Chemical Properties • Properties: distinguishing characteristics that are used to identify different objects • Physical properties: properties that can be observed and recorded without referring to another substance • Intensive physical properties: depend on the identity of the substance • Extensive physical properties: depend on the amount of substance present • Chemical properties: properties that can be observed when the substance reacts or combines with another substance

  5. The Metric System • International System of Units (SI) base units • Length: meter • Mass: kilogram • Time: second • Electric current: ampere • Thermodynamic temperature: kelvin • Amount of substance: mole • Luminous intensity: candela • Units differ by factors of 10

  6. Conversions from SI to English System (1 of 3) • Mass and weight • Matter: stuff that makes up all things; occupies space and has mass • Mass: measure of the quantity of matter that an object contains • Weight: force exerted on an object by the pull of gravity

  7. Conversions from SI to English System (2 of 3) • Temperature • Most scientists use Celsius scale (SI scale is Kelvin) • Freezing point of water = 0 °C; boiling point of water = 100 °C • United States primarily uses Fahrenheit scale • 5 °C = 9 °F • Temperature at crime scenes is measured with either electronic or optical thermometers

  8. Conversions from SI to English System (3 of 3) • Density • May help establish the composition of an object • Density = sample mass/sample volume • Density characteristics: • Is the same regardless of the sample size • Often measured by water displacement • Generally decreases as temperature increases

  9. Soils (1 of 2) • Soil: a complex mixture of inorganic and organic materials • Inorganic: remnants of rock fragments formed over thousands of years by weathering bedrock • Organic: decayed remains of plants

  10. Soils (2 of 2) • Soil layers: • O: heavily decomposed organic matter • A: topsoil • B: lighter-colored, humus-poor, more compacted materials • C: layer of fragmented bedrock mixed with clay

  11. Forensic Characteristics of Soils(1 of 5) • Soil evidence is often found at a crime scene and transferred onto the criminal. • Soil evidence must be carefully collected and compared to soil samples found at the scene.

  12. Forensic Characteristics of Soils(2 of 5) • Examiners begin with a visual comparison of the color of the soils, using the Munsell soil color notation. • Soils are passed through sieves to separate components by size. • The composition of soils can vary greatly, even over short distances.

  13. Forensic Characteristics of Soils(3 of 5) • Forensic geologists encounter about 50 minerals on a routine basis. • Rocks are a combination of minerals. • The combination of minerals and human-made components in soil creates a unique signature that can be used to compare soil samples.

  14. Forensic Characteristics of Soils(4 of 5) • Gradient tube separation • Technique uses two glass tubes of liquids • Soil samples are placed on top of liquids • Components fall through the layers, with elements floating at different layers of density • Distribution of particles within the liquid can be used to tell whether the samples have a common origin • Technique is useful for comparing soils based on density

  15. Forensic Characteristics of Soils(5 of 5) • Collection and preservation of evidence • Collect soil samples as soon as possible • Take specimens at crime scene, within a 100-yard radius, and from paths into and out of scene • Take samples at alibi locations • Need specimens only from the top surface • Package in individual containers • Preserve lumps of soil • Process soil on tools before fingerprints

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