Background: Often, forensic scientists need to determine the characteristics of items found at a crime scene and
compare these items to ones found on a suspect or at the suspect's home or car or boat or anything connected to the suspect.
Physical Properties of these trace substances can be used to describe and compare these items of interest.
The Locard Principle states that if one surface touches another, there will be an exchange of some physical material, which
can be identified. Physical properties that describe substances can include: weight, volume, color, boiling or melting
points, density, conductivity, and pH.
Examples of physical properties that can be used to compare soil samples are the fluorescence, pH, sedimentation rate, and
conductivity. In general different soil samples will have different properties.
Knowledge and Skills:
Students should know how to determine pH
Students should know how use an ultraviolet light.
Students should know how use CBL or Labpro with sensors.
Fundamental Understanding:
Different soils will have different physical characteristics e.g. pH, fluorescence, sedimentation rates,
conductivity, and color
Amount of electrolytes and color
Essential Questions:
What are some possible components of soil and how can they be analyzed?
Purpose:
To determine characteristics of soils in order to compare soil samples
Materials:
pH paper
10.0 mL graduated cylinder
100 mL beakers or paper cups
Petri dish
Distilled water
Sieves to remove larger particles
(Optional)
UV black light
Vernier CBL's or Labpro's with conductivity probes and/or pH probes
Activity 1 - Physical Characteristics
Procedure:
Place about 2.0 cm3 of soil into a Petri dish.
Observe physical characteristics, color, types of particles, coarseness.
Determine the presence of fluorescence in the soil by using a UV black light.
Activity 2 - pH of Soil
Procedure:
Dissolve a small amount of soil in 1.0 cm3 into 20-30 ml of distilled water.
Measure pH of each soil sample with pH paper or Verneir pH probe
Repeat for each soil sample.
Activity 3 - Sedimentation Rate
Procedure:
Fill 10.0 ml graduated cylinder with H2O (or any other solvent e.g. methanol or acetic acid)
Record how long it takes for most of the soil sample to sediment (or fall to the bottom of each graduated cylinder).
Repeat for each soil sample.
Activity 4 - Conductivity of Soils
Procedure:
Place ~2.0 g of soil into beaker of 50.0 mL H2O. (It is important to weigh the same amount of soil for each soil
sample.)
Using a conductivity probe connected to CBL or Labpro, record the conductivity level of each sample.
Repeat for each sample
Questions and Conclusions:
What is pH?
What is fluorescence?
What is conductivity? How can it be measured?
Resources:
"Criminalistics" Lab Manual, seventh edition by Clifton E. Meloan, Richard E. James and Richard Saperstein, Prentice Hall
2001. (p.55, Soil Analysis)
Teacher Notes:
Medium sized sieves can be used to remove larger soil particles from the soil samples.
