Materials Included In Kit

Additional Materials Required

Prelab Preparation

Part III of the soil profile is designed to be done as an all-class demonstration. Set up the density gradients in advance of the first class session and set the gradient tubes in a place where they can remain undisturbed overnight. Place the soil samples into the gradients and allow time for student observations during the first day’s class period. Let the gradients set overnight and allow time for students to record observations on the second day.

The actual gradients can be tricky to set up. Practice pouring the thickest density gradient solution (3%) ahead of time. It must be poured very slowly, right down the center of the graduated cylinder without touching the sides. This will be the most difficult with the thickest solution (3%). Pouring down a stirring rod can help direct the flow of the material.

Pour approximately 25 mL of 3% gradient solution into each of four 100-mL graduated cylinders or very large test tubes. Allow the solution to settle into the bottom of the cylinders. Next, slowly pour approximately 25 mL of the 2% gradient solution on top of the 3% again being careful to pour it right down the center of the cylinder without touching the sides. When these two solutions have settled into the cylinders, pour approximately 25 mL of 1% gradient solution on top of the 2% solution. Next, slowly pour 25 mL of distilled water on top of the 1% solution. Do all the pouring very slowly and deliberately with as little mixing as possible. The final density gradients should be as follows:

Grind a sample of each soil type in a mortar and pestle being sure not to cross contaminate the samples. Pour the ground samples through a fine wire screen and throw away the particles that do not go through the screen. Save the fine particles from each soil type “CS,” “SA,” “SB” and “SC.”

For the demonstration, sprinkle equal size samples (2–3 g) of each soil type on top of a labeled gradient. The soil particles will immediately start to settle down through the gradients. Students should observe as you add each sample. Let the gradient stand overnight for additional observations on the second day.

Safety Precautions

TesTabs^® contain chemicals which may irritate skin or be harmful if swallowed. The TesTab reagents used in this kit were designed with safety in mind. The single-use, foiled-packaged TesTabs are easy to dispense. Store TesTabs in a cool, dry place and only open when ready to use the tablet. A single tablet, either alone or reacted with a sample, is a very low health hazard; however, TesTabs should not be ingested. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Have students wash thoroughly after working with soil samples. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Dispose of reacted samples following Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Part III of the laboratory is designed to be done as a class demonstration activity.
• Both parts of the activity can be completed in two 50-minute class periods. It is suggested that Part I be completed on Day 1 along with the set-up of the demonstration in Part III. Part II should be completed on Day 2 along with observations of the density demonstration.
• Be sure to shake the density gradient solutions just prior to making the density gradients.
• The phosphate test often needs to settle for 5–10 minutes before the color of the liquid in the tubes becomes unmasked by the dirt. Be sure students let the tubes sit undisturbed while waiting. If no color change occurs, allow the test tubes to settle for a longer period of time.
• TesTabs are a vendor product of the LaMotte Company. MSDS are available through the manufacturer website.

Sample Data

The actual data from this laboratory will vary considerably. The following data represents only one possible interpretation.

Part 1. Appearance

Observations

Microscopic Examination Part 2. Soil Tests Part 3. Soil Density Profile

Draw a sketch of the density profile for each soil sample.

Analysis

Study all of the data for the four soil types. Does one of the soil profiles “match” the crime scene profile better than the others? Explain why this soil profile might be incriminating for the suspect. Would a jury be influenced by your profile data?

Crime scene profile best matches SA.

Introduction

Mud collected from the bottom of a suspect’s boot might link the suspect to the crime scene. How do forensic scientists match soil samples? What exactly is dirt or soil?

Concepts

• Density gradients
• Soil testing
• pH
• Soil profile

Background

Soil is not a simply defined substance. Farmers consider soil to be the top 6–12 inches of the Earth’s crust where they grow crops. Geologists, who study the Earth’s surface, define soil as the organic and mineral matter composing the Earth. Forensic geologists (your role in this lab) consider soil as Earth material that has been collected in a particular investigation. All natural and artificial objects on or near the surface of the Earth are considered part of the soil. This might include rocks, minerals, vegetation, glass, paint, asphalt, concrete and other particles. The presence of these items makes soil in one place different from soil in another.

Most agree that no two places on Earth have precisely the same soil. Soil from one area is identifiably different from soil in another area. Many variables, such as depth of collection and weather conditions, can affect the nature of the soil collected. For comparison studies it is critical to treat all soil samples identically in the laboratory.

In a forensics lab, soil samples are dried to the same temperature and for the same length of time. This prevents decomposition of the soil components and creates identical treatments for controlled chemical experiments. Once in the crime lab, scientists use a variety of tests and observations to create a “profile” of the soil in question. Profiles of other soils, such as those from the crime scene, are then compared. When soil profiles match to high degrees, speculations about the soil relative to the crime are often made and the soil profile might be added to the “evidence” for a particular case. Soil profiles are not considered 100% proof, i.e., soil profiles alone are not usually the only evidence used for a conviction.

Density is a characteristic property of a material and pure substances can be identified by their density. Density is defined as the mass of a substance per unit volume.

Almost four hundred years ago, Galileo found that a ball of wax placed between a layer of salt water and a layer of fresh water will float. He observed that objects float at the level of their density. An application of this principle is seen in the use of “density columns” to test the density of an object. A density column can be created by placing very dense liquids on the bottom of a column and “floating” less dense liquids carefully on top of the more dense liquids. When an object is dropped into a density column of liquids of different densities, the object will sink to the place where its density just equals the density of the surrounding liquid. If the object sinks completely, it is more dense than the most dense liquid in the column. If the object floats on the surface, it is less dense than the least dense liquid in the column.

If different soil types (different particles of different densities) are placed into a density column, they will have different settling patterns. When one soil is compared to another, identical soils will have nearly identical settling patterns. These patterns can be used as an analysis tool in profiling soil samples.

In addition to density, other soil tests can be conducted to contribute to the soil’s total profile. In this experiment, three other variables will be tested—pH, soil nitrogen and soil phosphorus.

pH is a measure of how acidic or basic things are. Recall that the pH scale runs from 0 to 14 with pH 7 being neutral, greater than pH 7 being basic and less than pH 7 being acidic. When we measure the pH of soil, we actually measure the pH of the soil in solution. pH refers to the relative abundance of hydrogen ions in solution.

As water becomes more acidic, the pH values decrease from 7 to 6 to 5 to 4 and so on. As the solution becomes more basic, the pH values increase from 7 to 8 to 9, etc. (see Figure 1). A normal pH scale has a range of 0 to 14. Most aquatic organisms require a pH range between 6.5 and 8.2. At pH levels below 5 larval stages of insects and other small aquatic organisms may die off rapidly. Water with abundant algae and vegetation growth usually has a significantly high pH. This is due to the fact that rapidly growing algae and vegetation remove carbon dioxide from the water during photosynthesis. At pH levels above 9, fish may have a difficult time excreting ammonia from their bodies. The pH of the soil solution affects how much soil nutrients are available to plants. When soil is too acidic or too basic, important soil nutrients, like nitrogen and phosphorus, are not available to plants. The pH of soil can be altered by adding chemicals, such as limestone (to make it more basic) or alum (to make it more acidic).

Phosphorus is a vital element of life and is usually found naturally in water as phosphate ions. Phosphate originates from fertilizers, wastewater of domestic origin, such as human, animal and plant residue and from wastewater of industrial origin. Phosphates are also added to farm and city water systems to control water hardness. Phosphates from detergents can result in overgrowth of algae (also know as algae blooms), which in turn will cause the algae to die at a high rate and undergo decomposition. This decomposition process depletes oxygen from the water and results in increased fish kill.

Phosphorus is necessary in plants for root growth and development. It helps plants grow strong and helps in the production of flowers and fruit. Phosphorus is especially important in crops where we eat the root part of the plant (e.g., beets, potatoes, carrots, radishes). Testing for phosphorus usually requires dilution since the phosphorus test is very sensitive.

In this activity, we will investigate soil samples that have been collected from the scene of a robbery. A house was robbed and the police discovered a cut window screen and a broken glass window where the burglar was believed to have entered the house. Since it was very wet the night of the robbery, police collected a soil sample beneath the window. They also collected mud samples from the shoes and boots of three suspected burglars. Does the mud from any of the three suspects match the profile of the soil from beneath the window?

Materials

Safety Precautions

Do not handle soil samples with bare hands. TesTabs^® contain chemicals which may irritate skin or be harmful if swallowed. TesTabs were designed with safety in mind and a single tablet is a very low health hazard; however, TesTabs should be handled with care. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water upon completion of the lab work.

Procedure

Part 1. Appearance

1. Use a plastic spoon to obtain exactly one spoonful of crime scene soil. Place the soil in a small plastic cup and label it “CS.”
2. Clean the spoon thoroughly with a paper towel. Be sure to dry the spoon if it gets wet.
3. Use the clean spoon to obtain a sample of soil from Suspect A. Place Suspect A soil in a plastic cup and label it “SA.”
4. Repeat steps 2–3 for soil samples from Suspect B and Suspect C, labeling them “SB” and “SC, respectively.”
5. Examine the four soil samples carefully. How would you describe and characterize each of the four soil types? Use the chart for Part I on the Soil Analysis Worksheet as a guide to organize and record your observations.
6. Place each cup of soil under a stereoscope microscope and examine each soil sample carefully. Record your microscopic observations on the Soil Analysis Worksheet for Part I.
7. Save the soil samples for use in Part II.

Part 2. Soil Tests

pH

1. Label four plastic soil test tubes—CS, SA, SB and SC.
2. Use a spoon to transfer a very small amount of crime scene soil to the “CS” test tube. Add soil up to the 1-mL mark on the test tube.
3. Similarly add SA, SB, and SC soil to three other test tubes. Add soil up to the 1-mL mark on the test tube in each case.
4. Add distilled water to each of the tubes up to the 10-mL mark on each test tube.
5. Add a pH TesTab^® tablet to each test tube.
6. Screw a top onto each test tube and shake each tube vigorously for 30 seconds.
7. Replace each tube in the test tube rack and let it sit undisturbed for one minute.
8. Observe the color of the liquid in each tube. Compare the color to the Color Comparator Chart for pH and record the pH on Part II of the Soil Analysis Worksheet.
9. Rinse thoroughly and dry the four test tubes for the next tests.

Phosphates

1. Label four test tubes and add the appropriate soil sample up to the 1-mL mark on each test tube.
2. Usine a Beral pipet, add vinegar up to the 2-mL mark on each test tube.
3. Add distilled water up to the 10-mL mark on each test tube.
4. Cap the test tubes and shake for one minute.
5. Let the tubes sit for at lease five minutes to let the soil particles settle.
6. Decant 5 mL of the solution from each tube into clean test tubes.
7. Add one Phosphorus TesTab tablet to each test tube.
8. Screw a top onto each test tube and shake each tube vigorously for at least one minute or until the TesTab disintegrates.
9. Stand the tubes in a rack and let them sit undisturbed for at least 5–10 minutes.
10. Observe the color of the liquid in each tube and compare the color to the Color Comparator Chart for phosphate.
11. Record the phosphate concentration on the Soil Analysis Worksheet.

Part 3. Soil Density Profile

1. Your instructor will set up a soil density gradient for each of the four soil samples “CS,” “SA,” “SB” and “SC.”
2. Do not touch or disturb the density gradients. Observe the gradients and draw a sketch of each gradient on the Soil Analysis Worksheet Part III.
3. Complete the analysis question on the Soil Analysis Worksheet.

Student Worksheet PDF

13382_Student1.pdf