Materials Included In Kit

Additional Materials Required

Prelab Preparation

Activity B. Dyes, Dyeing and Chemical Bonding

Preparation of Dye Solutions
Directions are given for preparing 200 mL of each solution.

Congo Red: Dilute 70 mL of 0.1% congo red solution with 130 mL of distilled or deionized water in a 400-mL beaker. Add 2 g of sodium sulfate decahydrate (Na₂SO₄•10H₂O) and 1.5 g of anhydrous sodium carbonate (Na₂CO₃) and stir to dissolve. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.

Crystal Violet: Dilute 10 mL of 1% crystal violet solution with 190 mL of distilled or deionized water in a 400-mL beaker. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.

Malachite Green: Dilute 10 mL of 1% malachite green solution with 190 mL of distilled or deionized water. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.

Safety Precautions

Hexane is a flammable organic solvent and a dangerous fire risk. Keep away from flames, heat and other sources of ignition. Cap the solvent bottle and work with hexane in a fume hood or designated work area well away from any Bunsen burners used in the lab. All of the dyes are strong stains and will stain skin and clothing. Crystal violet and malachite green are toxic by ingestion and irritating to body tissue. The dye baths are very hot, near boiling. Exercise care to avoid scalding and skin burns. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

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 hexane solutions should be collected in a flammable organic waste container and allowed to evaporate according to Flinn Suggested Disposal Method #18a. All other solids and solutions from activity A may be disposed of in the trash according to Flinn Suggested Disposal Methods #26a and b, respectively. The dye solutions may be washed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Common solids with a wide range of physical properties were deliberately chosen for Activity A. There is enough overlap to be able to identify patterns in the relationship between the properties of a material and its structure. The challenge in this experiment comes as students try to use their observations to “see inside” the world of atoms and bonds. Using common household materials removes one (unnecessary) stumbling block in this process.
• Many other common solids may also be used. Any metal may be used instead of aluminum and many different ionic compounds may be substituted for sodium chloride. Suitable nonpolar organic solids that may be used instead of or in addition to stearic acid include lauric acid or paraffin wax.
• For Activity A, borosilicate glass test tubes are provided in this kit for use in both step 5, testing the solubility of the solids in hexane, and step 11, testing the melting points of the solids in a Bunsen burner flame. Have students dispose of the hexane from step 6 in a flammable, organic waste container, then clean and dry the test tubes. When performing the melting points of the solids in the Bunsen burner flame, make sure students use borosilicate test tubes.
• Low-voltage conductivity meters are available from Flinn Scientific (Catalog No. AP1493) for individual student use. The copper wire electrodes are about 2 cm long and are easily inserted into the wells on a microscale reaction plate. Two LEDs make it possible to compare the conductivity of strong versus weak electrolytes. The green LED requires more voltage than the red LED. A weak electrolyte will cause only the red LED to glow. A strong electrolyte will cause both the red and green LEDs to glow. Because the meter uses only a 9-volt battery, the conductivity tester is convenient, portable, and safe. Conductivity tests may also be done using conductivity sensors with a LabPro or CBL-2 computer interface system. Using a conventional 110-V “lightbulb-type” conductivity tester will require larger sample sizes. It is recommended that the teacher perform the conductivity tests as a demonstration if 110-V conductivity testers will be used.
• Remind students not to use flammable organic solvents around or near a heat source.
• Caution students to use the proper technique to detect the odor of a substance. Place the open container about 6 inches away from the nose and use your hand to waft the vapors toward the nose. While the chemicals in step 3 post no risk from inhalation, every opportunity should be taken to develop safe laboratory techniques.
• In Activity B, for best results, allow the fabrics to dry overnight before recording the final results. Students may test the colorfastness of the dyed fabrics at home, if desired. Make sure excess dye has been completely rinsed out of the fabric strips before allowing students to take the samples home.
• In Activity B, place lots of paper towels, absorbent lab mats or newspaper all around the dye baths. This will help keep the room clean. Instruct students to store books, bags, and other personal items away from the lab area to avoid staining them.
• Congo red is an acid–base indicator. The red color of fabrics dyed with congo red will turn blue when placed in a mild acid solution, such as 0.1 M HCl. The blue color disappears and the red color returns when the Congo red–dyed fabric is placed in a washing soda (sodium carbonate) bath.

Teacher Tips

• See the experiment “It’s in Their Nature” in Solubility and Solutions, Volume 12 in the Flinn ChemTopic™ Labs series, for a detailed investigation into the solubility of ionic, polar, and nonpolar compounds in a variety of solvents. Students classify compounds and learn about the different types of attractive forces that exist between molecules.
• It is hard to convey the principles of bonding and structure using only two-dimensional drawings or pictures. We strongly encourage the use of three-dimensional models to help students recognize and understand the relationship between structure and bonding. Consult your current Flinn Scientific Catalog/Reference Manual for a complete selection of models, including diamond (AP6176), graphite (AP6175), ice (AP6178) and sodium chloride (AP6179).
• Many new terms and definitions are introduced in this activity, which provides an overview of all types of chemical bonding. Encourage students to make a list of all the new terminology and write out their definitions. Remind students also to consult their textbooks for additional examples, models, and illustrations that may help explain the concepts.
• One of the most famous dyes is indigo, which is used to dye blue jeans. Indigo is a so-called vat dye—the dye is first reduced to a colorless, water-soluble form, which is then applied to a fabric. The ingrained dye is re-oxidized back to its colored form when the fabric is exposed to air. The history and chemistry of dyeing with indigo are investigated in “Dyeing with Indigo,” a student laboratory kit available from Flinn Scientific (Catalog No. AP6166).

Answers to Prelab Questions

Activity A. Properties of Solids

1. A student wanted to illustrate the structure of magnesium chloride and decided simply to replace the Na⁺ ions in Figure 1 with Mg²⁺ ions. What would be wrong with the resulting picture?

   The picture would show the wrong ratio of ions in the crystal structure. The formula of magnesium chloride is MgCl₂—there are two chloride ions for every magnesium ion. The ratio of positive and negative ions in the sodium chloride crystal structure is 1:1.

2. Covalent bonds may be classified as polar or nonpolar based on the difference in electronegativity between two atoms. Look up electronegativity values in your textbook:
   1. Why are C—H bonds considered nonpolar?

      The electronegativity values of carbon and hydrogen are similar (2.1 and 2.5, respectively.) Both atoms in a C—H bond have similar attractions for the bonding electrons and the bond is nonpolar.

   2. Which is more polar, an O—H or N—H bond?

      The electronegativity difference between O and H is greater (3.5–2.1) than that between N and H (3.0–2.1). An O—H bond is more polar than an N—H bond.

Activity B. Dyes, Dyeing and Chemical Bonding

1. Redraw the structure of malachite green (see Figure 4) and identify the groups in the dye that will bind to ionic and polar sites in a fabric.
   {12265_Answers_Figure_6}
2. Complete the following “If/then” hypothesis to explain how the structure of a fabric will influence the relative color intensity produced by malachite green.

   If a fabric contains more ionic and polar groups in its structure, then the intensity of the dye color due to malachite green should increase, because there will be more sites on the fabric for the dye molecules to bind to.

3. Using this hypothesis, predict the relative color intensity that will be produced by malachite green on the six fibers in the multifiber test fabric. Rank the fabrics from 1 = lightest color to 6 = darkest color.
   {12265_Answers_Figure_7}

Sample Data

Activity A. Properties of Solids

*The average temperature of a Bunsen burner flame is greater than 1000 °C.
† The melting point of sodium chloride (801 °C) is greater than that of pure aluminum metal (660 °C). Sodium chloride is observed to melt in a test tube placed in a Bunsen burner flame, while aluminum granules generally do not melt under these conditions. This is probably due to the invisible oxide coating which is always present on aluminum. The melting point of aluminum oxide is about 2000 °C.

Activity B. Dyes, Dyeing and Chemical Bonding

Answers to Questions

Activity A. Properties of Solids

1. Compare the volatility and odor of stearic acid and sucrose. Which is more volatile? Why? Is it possible for a compound to be volatile but have no odor? Explain.

   Stearic acid has an odor and seems to be more volatile than sucrose. In order for a substance to have an odor, some molecules must enter the gas phase and diffuse in air to reach the nose. Some volatile substances, however, may not have an odor, because the nose lacks the appropriate receptors to “detect” the odor.

2. Both stearic acid and sucrose are molecular substances, but one is polar and the other is nonpolar. Compare the solubility of the two compounds in water and in hexane to determine which is which.

   Stearic acid dissolved in hexane, not in water. Sucrose dissolved in water, not in hexane. This suggests that stearic acid is nonpolar (like hexane) while sucrose is polar (like water). Note: Stearic acid consists of a very long (C₁₇H₃₄—), nonpolar hydrocarbon “tail” attached to a small polar carboxylic acid (—CO₂H) group. The nonpolar hydro¬carbon tail dominates the physical properties of the solid (e.g., solubility, melting point).

3. Based on the answers to Questions 1 and 2, predict whether the intermolecular forces (forces between molecules) are stronger in polar or nonpolar substances.

   Polar substances have stronger intermolecular forces—it takes more energy to pull polar molecules apart and have molecules enter the gas phase.

4. In order for a substance to conduct electricity, it must have free-moving charged particles.
   1. Explain the conductivity results observed for sodium chloride in the solid state and in aqueous solution.

      Sodium chloride does not conduct electricity in the solid state. It has charged particles (ions) but the ions are “locked” into position in the crystal structure and are not able to move freely. A solution of sodium chloride in water does conduct electricity because the ions are no longer fixed into position. (The solute particles in a liquid are able to move freely.)

   2. Would you expect molten sodium chloride to conduct electricity? Why or why not?

      Molten sodium chloride should conduct electricity because the particles in a liquid are able to move freely.

   3. Use the model of metallic bonding described in the Background section to explain why metals conduct electricity.

      Metals conduct electricity because the valence electrons of the metal are not “attached” to any one metal atom. The electrons are delocalized among all of the metal cations in the crystal structure and are able to move freely throughout the crystal.

5. Complete the following table (some of the entries have been filled in for you). Note that sand (silicon dioxide) is a covalent-network solid.
   {12265_Answers_Table_3}

   Note: Stress to students that these are general properties—there are many exceptions. The melting points of metals, for example, range from –39 °C (for mercury) to 3407 °C (for tungsten). Many low-melting metals (e.g., lithium, sodium, potassium, gallium) are also soft enough that they can be cut with a knife. Finally, not all ionic compounds are water-soluble.

Activity B. Dyes, Dyeing and Chemical Bonding

1. Compare the general ease of dyeing the six different fabrics in the multifiber test fabric. Which fabric(s) consistently developed the most intense colors, regardless of the type of dye used? Which fabric was the most difficult to dye?

   Wool consistently developed the most intense colors with all of the dyes except congo red. Even with congo red, however, wool was only a shade paler than cotton, which gave the most intense color. Nylon, cotton and acetate were also relatively easy to dye. They gave fairly intense colors with at least two out of the three dyes tested. Polyester was the most difficult fabric to dye.

2. Consult Figure 3: What feature stands out as unique in the structure of the fabric that was the easiest to dye? What feature stands out as unique in the structure of the fabric that was hardest to dye?

   Wool contains many charged groups in its structure. None of the other fabrics show any charged groups in their normal repeating units. Polyester is unique in that it appears to be the least polar of all the fabrics. Polyester has no –X–H (where X = O or N) groups capable of forming hydrogen bonds with electron donor sites in dye molecules. Note: Students may notice that acrylic fiber is similar to polyester in that is lacks polar groups capable of hydrogen bonding to electron donor sites in dye molecules. The dyeability of acrylic is improved commercially by incorporating small amounts of charged monomers such as AMPS (see below) into the growing polymer.

   {12265_Procedure_Figure_8}
3. Consult Figure 4: Which two dyes have very similar structures? Compare the relative color intensities produced by these dyes on the different fabrics in the multifiber test fabric. Are the color patterns (from lightest to darkest) similar for these two dyes? Explain.

   Crystal violet and malachite green have similar structures and produced similar color patterns with the six fabrics in the multifiber test fabric. The observed color intensity produced by crystal violet and malachite green was:

   Wool > cotton, acrylic and acetate > nylon >> polyester.

4. Compare the color patterns produced on the different types of fabrics by crystal violet (a direct dye) and congo red (a substantive dye). Suggest a possible reason for any differences based on the chemical bonding interactions of direct versus substantive dyes (see the Background section).

   Congo red dyed every fabric! It gave nice bright reds of almost equal color intensity with four of the fabrics (wool, nylon, cotton and acetate) and light pink colors with acrylic and polyester. Congo red binds to fabrics via hydrogen bonding. More fabrics are capable of hydrogen bonding than ionic bonding.

5. Show by means of a diagram one hydrogen bond that might form between a glucose unit in cotton and congo red. Hint: Hydrogen bonds have the general form X—H --- :Y, where X and Y are highly electronegative atoms, such as N, O, F, and Y has an unshared pair of electrons.
   {12265_Answers_Figure_9}

Introduction

Chemical bonding describes interactions among atoms. What kinds of forces hold atoms together in a molecule or compound? How does the nature of the forces holding atoms together influence the properties of a material? Looking for patterns in the properties of solids and liquids can help us see inside the hidden world of atoms and molecules, to visualize and describe the forces holding atoms together. Use these two activities to investigate ionic, covalent and metallic bonding and to explore the relationships between the properties of a material, its structure and chemical bonding.

Activity A. Properties of Solids
Looking for patterns in the properties of different substances can help us understand how and why atoms join together to form compounds. What kinds of forces hold atoms together? How does the nature of the forces holding atoms together influence the properties of a material?

Activity B. Dyes, Dyeing and Chemical Bonding
The art of dyeing dates back thousands of years to the use of natural dyes extracted from plants and animals. Dyes are organic compounds that can be used to impart bright, permanent colors to fabrics. The affinity of a dye for a fabric depends on the chemical structure of the dye and fabric molecules and on the interactions between them. Chemical bonding thus plays an important role in how and why dyes work.

Concepts

• Chemical bonds
• Metallic bonding
• Ionic bonding
• Polar vs. nonpolar bonds
• Covalent bonding
• Hydrogen bonding

Background

Activity A. Properties of Solids

Groups of atoms are held together by attractive forces that are called chemical bonds. The origin of chemical bonds is reflected in the relationship between force and energy in the physical world. Think about the force of gravity—in order to overcome the force of attraction between an object and the Earth, we have to supply energy. Whether we climb a mountain or throw a ball high into the air, we have to supply energy. Similarly, in order to break a bond between two atoms, energy must be added to the system, usually in the form of heat, light or electricity. The opposite is also true: whenever a bond is formed, energy is released.

The term ionic bonding is used to describe the attractive forces between oppositely charged ions in an ionic compound. An ionic compound is formed when a metal reacts with a nonmetal to form positively charged cations and negatively charged anions, respectively. The oppositely charged ions arrange themselves in an extended, tightly packed, three-dimensional structure called a crystal lattice (see Figure 1). The net attractive forces between oppositely charged ions in the crystal structure are called ionic bonds.

Covalent bonding represents another type of attractive force between atoms. Covalent bonds are defined as the net attractive forces resulting from pairs of electrons that are shared between atoms (the shared electrons are attracted to the nuclei of both atoms in the bond). A group of atoms held together by covalent bonds is called a molecule. Atoms may share one, two or three pairs of electrons between them to form single, double, and triple bonds, respectively. Substances held together by covalent bonds are usually divided into two groups based on whether individual (distinct) molecules exist or not. In a molecular solid, individual molecules in the solid state are attracted to each other by relatively weak intermolecular forces between the molecules.

Covalent-network solids, on the other hand, consist of atoms forming covalent bonds with each other in all directions. The result is an almost infinite network of strong covalent bonds—there are no individual molecules.

Covalent bonds may be classified as polar or nonpolar. The element chlorine, for example, exists as a diatomic molecule, Cl₂. The two chlorine atoms are held together by a single covalent bond, with the two electrons in the bond equally shared between the two identical chlorine atoms. This type of bond is called a nonpolar covalent bond. The compound hydrogen chloride (HCl) consists of a hydrogen atom and a chlorine atom that also share a pair of electrons between them. Because the two atoms are different, however, the electrons in the bond are not equally shared between the atoms. Chlorine has a greater electronegativity than hydrogen—it attracts the bonding electrons more strongly than hydrogen. The covalent bond between hydrogen and chlorine is an example of a polar bond. The distribution of bonding electrons in a nonpolar versus polar bond is shown in Figure 2. Notice that the chlorine atom in HCl has a partial negative charge (δ^–) while the hydrogen atom has a partial positive charge (δ⁺). The special properties of metals compared to nonmetals reflect their unique structure and bonding. Metals typically have a small number of valence electrons available for bonding. The valence electrons appear to be free to move among all of the metal atoms, which must exist therefore as positively charged cations. Metallic bonding describes the attractive forces that exist between closely packed metal cations and free-floating valence electrons in an extended three-dimensional structure.

Activity B. Dyes, Dyeing and Chemical Bonding

The chemical structures of six common fabrics—wool, acrylic, polyester, nylon, cotton and acetate—are shown in Figure 3. Cotton and wool are natural fibers obtained from plants and animals, while acrylic, polyester and nylon are synthetic fibers made from petrochemicals. Acetate, also called cellulose acetate, is prepared by chemical modification of natural cellulose. All of the fabrics, whether natural or synthetic, are polymers. These are high molecular weight, long chain molecules made up of multiple repeating units of small molecules. The structures of the repeating units are enclosed in brackets in Figure 3. The number of repeating units (n) varies depending on the fiber and how it is prepared.

Wool is a protein, a naturally occurring polymer made up of amino acid repeating units. Many of the amino acid units have acidic or basic side chains that are ionized (charged). The presence of many charged groups in the structure of wool provides excellent binding sites for dye molecules, most of which are also charged. Cotton is a polysaccharide (cellulose fibers) composed of glucose units attached to one other in a very rigid structure. The presence of three polar hydroxyl (—OH) groups per glucose repeating unit provides multiple sites for hydrogen bonding to ionic and polar groups in dye molecules. Acetate is cellulose in which some of the −OH groups have been replaced by acetate groups (—OCOCH₃). The presence of acetate side chains makes acetate softer and easier to work with than cotton but also provides fewer binding sites for dye molecules.

Nylon was the first completely synthetic polymer fiber. It is a polyamide, made up of hydrocarbon repeating units joined together by highly polar amide (—CONH—) functional groups. The amide groups provide sites for hydrogen bonding to dye molecules. The repeating units in polyester are joined together by ester (—COO—) functional groups. Finally, acrylic fiber is poly(acrylonitrile). Each repeating unit contains one nitrile (—C≡N) functional group. Dyes are classified based on both the structure of the dye and the way in which the dye is applied to the fabric.

• Direct dyes are charged, water-soluble organic compounds that bind to ionic and polar sites on fabric molecules. Direct dye molecules contain both positively and negatively charged groups and are easily adsorbed by fabrics in aqueous solution. Simple salts such as sodium chloride and sodium sulfate may be added to the solution to increase the concentration of dye molecules on the fiber.
• Substantive dyes interact with fabrics primarily via hydrogen bonding between electron-donating nitrogen atoms (–N:) in the dye and polar —OH or —CONH— groups in the fabric.

Experiment Overview

The purpose of this activity-stations lab is to investigate the relationship between the properties of material and its bonding and study the forces of attraction between different substances. There are two separate activities—each activity focuses on different properties and is a self-contained unit.

Activity A. Properties of Solids
The purpose of this activity is to study the physical properties of common solids and to investigate the relationship between the type of bonding in a substance and its properties. The following physical properties will be studied:

• Volatility and Odor: Volatile substances evaporate easily and may have an odor.
• Melting Point: The temperature at which a solid turns into a liquid.
• Solubility: Ability of one substance to dissolve in another. Water is a highly polar solvent. Hexane is a nonpolar solvent.
• Conductivity: Ability to conduct electricity.

Activity B. Dyes, Dyeing and Chemical Bonding
The purpose of this activity is to investigate the interaction of dyes with different fabrics. The dyes are malachite green, and crystal violet (a direct dye) and congo red (a substantive dye). See Figure 4 for the structures of the dye molecules. The dyes will be tested on a multifiber test fabric that contains strips of six different fibers—wool, acrylic, polyester, nylon, cotton and acetate.

Materials

Prelab Questions

Read the Background material and Procedure for each activity A and B. Answer a brief set of PreLab Questions before starting each activity.

Activity A. Properties of Solids

1. A student wanted to illustrate the structure of magnesium chloride and decided simply to replace the Na⁺ ions in Figure 1 with Mg²⁺ ions. What would be wrong with the resulting picture?
2. Covalent bonds may be classified as polar or nonpolar based on the difference in electronegativity between two atoms. Look up electronegativity values in your textbook:
   1. Why are C—H bonds considered nonpolar?
   2. Which is more polar, an O—H or N—H bond?

Activity B. Dyes, Dyeing and Chemical Bonding

1. Circle and identify the groups in the structure of malachite green (Figure 4 in the Experiment Overview) that will bind to ionic and polar sites in a fabric.
2. Complete the following “If/then” hypothesis to explain how the structure of a fabric will influence the relative color intensity produced by malachite green. “If a fabric contains more ionic and polar groups in its structure, then the intensity of the dye color due to malachite green should (increase/decrease), because __________________________________________________________________________________________________.
3. Using this hypothesis, predict the relative color intensity that will be produced by malachite green on the six fiber swatches in the multifiber test fabric. Rank the fabrics from 1 = lightest color to 6 = darkest color.
   {12265_PreLab_Figure_5}

Safety Precautions

Read the entire Procedure before beginning each experiment. Work carefully to avoid scalding skin with hot water. Exercise care when working with hot metals. Hexane is a flammable organic solvent and a dangerous fire risk. Keep away from flames, heat and other sources of ignition. Cap the solvent bottle and work with hexane in a fume hood or designated work area well away from the Bunsen burner used in step 12. All of the dyes are strong stains and will stain skin and clothing. Crystal violet and malachite green are toxic by ingestion and irritating to body tissue. The dye baths are very hot, near boiling. Exercise care to avoid scalding and skin burns. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water when you have finished this activity.

Procedure

Activity A. Properties of Solids

1. Prepare a boiling water bath for use in step 10: Half-fill a 150-mL beaker with water, add a boiling stone and heat the beaker on a hot plate or Bunsen burner setup at a medium setting.
2. Label five weighing dishes for the five solid samples and obtain 0.2–0.3 g samples of each solid in the appropriate weighing dish. Record the color and appearance of each solid in the data table. The five samples are: aluminum, silicon dioxide, sodium chloride, stearic acid and sucrose.
3. Test the volatility and odor of each solid by wafting any vapors to your nose with your hand. Record all observations in the data table. Note: To detect the odor of a substance, place the open container about 6 inches away from the nose and use your hand to waft the vapors toward the nose.
4. Test the conductivity of each solid by touching the wires of the conductivity tester directly to the solid. Record the conductivity of each sample in the data table.
5. Place five small test tubes in the test tube rack. Label the five small test tubes for the five solid samples and add a small amount of each solid, about the size of a grain of rice, to its labeled test tube.
6. Add about 20 drops of hexane to each test tube. Stir each mixture and observe whether the solid dissolves in hexane. Record the results in the data table. Dispose of the hexane as directed by your instructor, then clean and dry the test tubes.
7. Add about 20 drops of water to each weighing dish. Stir each mixture and observe whether the solid dissolves in water. Record the solubility (soluble, partially soluble or insoluble) in the data table.
8. For water-soluble substances only: Determine the conductivity of the aqueous solution by placing the wires of the conductivity tester directly into the liquid. Record the results in the data table.
9. Obtain a weighing dish and place a small, pea-sized amount of each solid in separate locations on the dish.
10. Using the test tube clamp, set the dish on top of the boiling water bath and heat the solids for 1–2 minutes. Observe whether any of the solids melt and record the observations in the data table.
11. For solids that did not melt at the boiling water bath temperature: Place a small, pea-size amount of each solid in a clean and dry test tube. Using a test tube holder, heat the test tube in a burner flame for 1–2 minutes. Record observations in the data table.
12. Dispose of all waste liquids and solids as directed by your instructor.

Activity B. Dyes, Dyeing and Chemical Bonding

1. Cut the multifiber test fabric crosswise to obtain three 2-cm multifiber strips. Each test strip should contain all six fabric samples. Notice that the wool fabric is cream-colored, not white. Use a pencil to mark the wool ends with a “W.” Label the strips with your initials.
2. Obtain a 12-inch square piece of aluminum foil. Using a permanent marker, write the names of the dyes to be tested (see the “Dye baths” in the Materials section) in separate locations on the aluminum foil.
3. All of the dyes are strong stains. Avoid getting any dye solution on your skin, clothes or books. To avoid contamination, rinse tongs or forceps with water before inserting them into a new dye bath.

Part A. Direct Dyes

4. Fold one multifiber test strip in half. Using forceps or tongs, immerse the test strip into the crystal violet dye bath. Caution: The dye baths are very hot. Exercise care to avoid scalding or skin burns.
5. After 5–10 minutes, remove the dyed test strip from the bath using forceps. Hold the fabric above the dye bath to allow excess dye solution to drain back into the dye bath.
6. Pat the test strip with paper towels and rinse the dyed test strip under running water from the faucet or a wash bottle. Continue rinsing the test strip until all of the excess dye has been removed and the rinse water is colorless.
7. Place the rinsed test strip in the appropriately labeled section on the aluminum foil and allow it to air dry.
8. When the fabric is dry, record the dye color produced by each direct dye on each type of fiber. See the data table.
9. Repeat steps 4–8 with a new multifiber test strip in the malachite green dye bath. Exercise care and caution to avoid staining or scalding.

Part B. Substantive Dye

10. Repeat steps 4–8 with a third multifiber test strip in the congo red dye bath.

Student Worksheet PDF

12265_Student1.pdf