Materials Included In Kit

Additional Materials Required

Prelab Preparation

Congo Red Solution

1. Dilute 140 mL of 0.1% congo red solution with 260 mL of distilled or deionized water in a 1-L beaker.
2. Add 4 g of sodium sulfate decahydrate (Na₂SO₄•10H₂O) and 3 g of anhydrous sodium carbonate (Na₂CO₃) and stir to dissolve.
3. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
4. Carefully pour the resulting solution into three 250-mL beakers to ease congestion for student use. Wear heat-resistant gloves or use a hot vessel gripping device.

1. Cut the multifiber test fabric into 6-cm strips and distribute.

Safety Precautions

Exercise caution when heating fibers over the Bunsen burner. If any fiber begins to burn dangerously, quickly dip it in water to extinguish the flame. Congo red will stain skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. 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. The congo red solution may be rinsed down the drain with an excess of water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period.
• Other multifiber test fabrics containing 8 or 13 different fabrics are available from Testfabrics, Inc. See their website at www.testfabrics.com.

Teacher Tips

• The “Color of Chemistry” student lab kit is an excellent extension to this activity which examines fabric dying using several different types of fabric and dyes.

Further Extensions

Optional Microscopic Investigation Test

1. Obtain the test fabric strip and pull it apart slightly by grabbing the wool section and the acrylic section. Once loose enough, pull out a wool strand.
2. Place the strand on a microscope slide. Add a few drops of immersion oil to the strand and place a cover slip over the specimen.
3. Examine the specimen underneath a compound microscope. Record observations at 100X on the Forensics of Fibers Worksheet.
4. Repeat steps 1–3 with the five remaining fibers.
   {11102_Extensions_Table_1}

Answers to Prelab Questions

1. Why is it important to not rely solely on a visual inspection of a fiber to determine its identity?

   The appearance of synthetic fabrics can have such a wide range that it is not a definitive test to simply look at its appearance. Visual inspection combined with additional tests, such as burning properties and dye adhesion, leads to a more definitive conclusion.

2. When an unknown fiber is burnt it smells like burning paper. What type of fiber does this odor indicate?

   This indicates that the fiber is likely cotton, rayon or linen.

Sample Data

Part A. Burn Analysis

Answers to Questions

1. Which of the fabric samples when burned produced an odor similar to burning hair in the flame test? Why would this occur?

   The wool produced an odor of burning hair because it is hair—from an animal (sheep).

2. Mrs. Nelson’s house was robbed on the evening of Friday, May 13, 2011. The crime lab obtained a fiber from a broken screen where the suspect entered. The fiber was tested and found to twist and turn brown when heated but did not melt. When the fiber was ignited it burned extremely quickly with no smoke produced. When it was dyed with congo red it turned dark red. What type of fiber is it?

   Based on the results of the heat test, burn test and congo red dye it indicates the fiber is cotton.

3. Which type of fabric would be an ideal material to make a blanket used to extinguish fires? Explain.

   Wool would be an ideal fabric to use to make a fire blanket because it burns at the slowest rate and does not melt.

4. Which type of fabric would you be unlikely to find in firefighter uniforms? Why might this be the case?

   It would be very unlikely to find nylon in firefighter uniforms because it readily melts. When worn in a fire it would melt to the firefighter’s skin and not serve as a heat barrier.

5. Rank the six fabrics tested in order of their affinity for dyes, starting with the greatest affinity.

   Cotton, acetate and wool (about equal), nylon, acrylic and polyester.

References

Collins, David. Investigating Chemistry in the Laboratory; W. H. Freeman & Company: New York, NY; 2006; p. 131.

Kubic, T., Petraco, N. Forensic Science—Laboratory Experiment Manual and Workbook; CRC Press: Boca Raton, FL; 2003; p. 93.

Introduction

Crime scene investigators obtain many different types of evidence at a crime scene, including samples of fibers. With this kit you will identify the type of fiber left at a crime scene by performing a series of three tests on the obtained evidence.

Concepts

• Physical and chemical properties
• Chemical bonding
• Forensics

Background

Fibers are the smallest component of a textile material. They are used to form fabric, rope, carpet, etc. Fibers gathered as evidence at a crime scene may arise from numerous scenarios. They may be transferred via personal contact between suspect and victim and they can also be transferred to other items at the crime scene, such as furniture, weapons or flooring, due to physical struggle.

Fibers may be analyzed by visual inspection, based on their appearance and comparison with known samples. Identifying fibers based on appearance requires the use of a microscope to view miniscule details. Natural fibers are easier to distinguish under a microscope. Synthetic (manmade) fibers are traditionally less descriptive under a microscope because they can consist of practically any strand diameter or color. Synthetic or man-made fibers typically have a more uniform diameter and appearance than natural fibers. Therefore, microscopic analysis is not the main determining test used to identify fibers but it can be helpful as an ancillary test.

In most cases, fibers are analyzed based on their physical and chemical properties. The burn test is a primary identification technique used to determine fiber identity. When held near a flame different fibers will exhibit different behaviors. Some will begin to melt and others will twist or curl up. When ignited they will also react differently. Some fibers burn slowly, others burn quickly. Some will melt and drop off before the flame reaches the rest of the fiber. Odor is also a powerful tool in identifying an unknown burnt fiber. If the smell is similar to that of burning hair, the fiber is most likely silk or wool. If the smell is similar to burning paper or burning wood, the fiber is probably cotton, rayon or linen. If the fiber melts, it is likely a synthetic material such as nylon or polyester.

Fabrics are also identified based on how chemical dyes bond to them. How well a dye is attracted to a piece of cloth (its affinity) depends on both the fabric and the dye molecules. Chemistry thus plays an important role in how and why dyes work. Dyes are water-soluble compounds that are charged. Animal fibers such as wool, which are composed of protein molecules, are usually easier to dye than plant fibers such as cotton, which are composed of cellulose. Wool fibers have many dye sites—groups of molecules that have positive or negative charges and thus attract the charged dye molecules. Dye sites may be ionic, that is, fully charged, or polar, that is, partially charged. In general, dyes have a greater affinity for natural fibers like wool and cotton than for most synthetic fabrics. Many synthetic fabrics such as acrylic and polyester are hydrophobic (water-fearing) and have fewer dye sites, making them more difficult to dye. One exception is nylon, the first completely synthetic fiber developed in the 1930s from petrochemicals. Nylon dyes more easily than many other synthetic fabrics because it has polar dye sites. Acetate, another synthetic fiber, is chemically similar to cotton, but has fewer dye sites.

Experiment Overview

The purpose of this experiment is to examine six different types of fibers using a burn test and dye analysis to observe their unique physical and chemical properties.

Materials

Prelab Questions

1. Why is it important to not rely solely on visual inspection of a fiber to determine its identity?
2. When an unknown fiber is heated in a flame, it smells like burning paper. What type of fiber does this odor indicate?

Safety Precautions

Exercise caution when heating fibers over the Bunsen burner. If any fiber begins to burn dangerously, quickly dip it in water to extinguish the flame. Congo red will stain skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Note: The multifiber test fabric consists of six different fibers in the following order starting at the cream colored end: wool, acrylic, polyester, nylon, cotton and acetate. Cut the multifiber test fabric into two 3-cm strips.

Part A. Burn Analysis

1. Fill a 50-mL beaker with tap water. This will be used to extinguish any stubborn flames resulting from the burn analysis test.
2. Obtain the multifiber test fabric strip and pull it apart slightly by grabbing the wool section and the acrylic section. Once the fabric is loose enough, pull out a single wool strand (see Figure 1).
   {11102_Procedure_Figure_1}
3. Light the Bunsen burner. Using forceps, hold the single fiber over the heat but not within the flame. Record observations on the Forensics of Fibers Worksheet.
4. Place the fiber into the flame. Does it ignite quickly or slowly?
5. Remove the fiber from the flame of the Bunsen burner. Does it extinguish quickly? Does it melt or drip? Record observations on the worksheet.
6. Repeat steps 2–5 to test single strands of all the different fibers in the multifiber fabric. Note: With some fibers the reactions in steps 2–5 may occur very quickly. Repeat tests as needed in order to make accurate observations.

7. Obtain a second multifiber test fabric strip. Mark one end of the strip with pencil so that the wool side is differentiated from the acetate side.
8. Using forceps or tongs, immerse the test strip into the congo red dye bath. Caution: The dye bath is very hot. Exercise caution to avoid burns.
9. After 5–10 minutes, remove the dyed test strip from the bath using forceps. Hold the fabric above the dye bath for approximately one minute to allow excess dye to drain back into the dye bath.
10. Pat the test strip with paper towels and rinse the dyed test strip under running water from the faucet or use a wash bottle. Continue rinsing the test strip until all of the excess dye has been removed and the rinse water is colorless.
11. Place the test fabric on a small piece of aluminum foil or weighing dish and allow it to dry overnight.
12. Record observations on the Forensics of Fibers Worksheet the following day.

Student Worksheet PDF

11102_Student1.pdf