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Objectives. Students should gain an understanding of:The difference between physical and chemical propertiesConversions between the English system of measurements and the metric systemThe forensic characteristics of soilWays to collect and preserve soil evidence. Introduction. Forensic laboratories examine common items at crime scenes for two reasons:To identify properties that may place an object within a particular classTo discover additional characteristics that will allow the object to197
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1. 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