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 –Ways 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 class –To discover additional characteristics that will allow the object to be individualized

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

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

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

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

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

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

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

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.

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.

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.

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

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