Materials Included In Kit

Additional Materials Required

Prelab Preparation

Splatter Test Demonstration

1. Draw the structures of acetone, ethyl alcohol and water on the chalkboard (see Figure 9).
   {14109_Preparation_Figure_9}
2. Enlist some volunteers to help with the demonstration. Steps 3–5 should be done simultaneously.
3. Squirt several milliliters of water onto the chalkboard under the structure of water.
4. Squirt several milliliters of ethyl alcohol onto the chalkboard under the structure of ethyl alcohol.
5. Squirt several milliliters of acetone onto the chalkboard under the structure of acetone.
6. Observe.

Preparation of Dyes

• Enough supplies are provided for two baths of each dye.
• Cut 3 strips of fabric for each student group. Strips should be cut into approximately 1 inch pieces. Help students identify the unknown fabric types.
• 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.
• Methyl orange: Dissolve 0.7 g of methyl orange in 200 mL of distilled or deionized water. Add 2.5 g of sodium sulfate decahydrate (Na₂SO₄•10H₂O) and 5 mL of 1 M sulfuric acid and stir to dissolve. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
• Alizarin red: Dilute 25 mL of 1% alizarin red solution with 175 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.

Unknowns for Chemical Bonding Portion

• Transfer the nine “unknown” solids into unmarked bottles and label them 1–9. Students can obtain four of their samples from these bottles. Label the container with potassium nitrate “Do not heat in burner flame.” Provide at least one of each bond type: Ionic, polar covalent, nonpolar covalent or metallic.

Safety Precautions

Acetone and ethyl alcohol are flammable organic solvents and dangerous fire risks. Keep away from flames, heat and other sources of ignition. Cap the solvent bottles. Addition of a denaturant makes ethyl alcohol poisonous; it cannot be made nonpoisonous. Copper(II) sulfate is a skin and respiratory tract irritant and is toxic by ingestion. Potassium nitrate is a strong oxidant and a fire and explosion risk when heated or in contact with organic material. It is also a skin irritant. Salicylic acid is moderately toxic by ingestion. Acid solutions are irritating to skin and eyes. All of the dyes are strong stains and will stain skin and clothing. Methyl orange is toxic by ingestion and irritating to body tissue. Sulfuric acid is corrosive and toxic by ingestion. Alizarin red is a body tissue irritant. The dye baths are very hot, near boiling. Exercise care to avoid scalding and skin burns. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Avoid contact of all chemicals with eyes and skin. 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 lab. 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. Hydrochloric acid may be neutralized with base according to Flinn Suggested Disposal Method #24b. The remaining solid samples may be stored for future use or placed in the trash according to Flinn Suggested Disposal Method #26a. Acetone and ethyl alcohol may be saved for future demos, otherwise follow Flinn Suggested Disposal Method #18a.

Lab Hints

• Common solids with a wide range of physical properties were deliberately chosen for this study. 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.
• Many other common solids may also be used. Any metal may be used and many different ionic compounds may be substituted for those in the kit. Suitable nonpolar organic solids that may be used include stearic acid or lauric acid.
• 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 samples.
• Dodecyl alcohol has a very low melting point of 24 °C. Therefore, in warm environments, it is very likely that this chemical will be in a liquid state. If it cools below 24 °C, it will reform as a solid.
• 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.
• Other multifiber test fabrics containing 8 or 13 different fabrics are available from Testfabrics, Inc. See www.testfabrics.com.

Teacher Tips

• As an extension, students can test the mordant dye, alizarin, by treating with alum to observe the advantages of adding metal ions to the fabric for dye affinity. Note, this extension might exceed the 50-minute lab class period. To make the alum bath: Dissolve 0.7 g of alum [AlK(SO₄)₂•12H₂O] in 200 mL of distilled or deionized water in a 400-mL beaker. Add 0.4 g of calcium oxide and stir to dissolve. Place a boiling stone in the solution and heat to near boiling on a hot plate. Students submerge fabric in the alum bath for 15–20 minutes, then wring out over bath to remove excess liquid, and then immerse the fabric in the alizarin dye bath for 5–10 minutes.
• Most students should predict that wool will show the greatest affinity for the dye, and thus the most intense color with methyl orange. The results for nylon may be a surprise, since there are no charged groups shown in the structure of nylon. Students may notice, however, that both nylon and wool contain amide-linking groups in their repeating units—maybe the polar amide groups interact very strongly with the dye via hydrogen bonding.
• 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.
• The Sample Data section presents examples of possible student results. Students will create a variety of flow charts and classification schemes.
• The identity of solids A, B, C and D in Table 1 are copper(II) sulfate, dextrose, paraffin wax and zinc. These are included in the kit (except zinc) for used as possible unknowns on lab day.
• The kit contains enough variety of unknowns (9 possible). Provide different groups with different unknowns for a cooperative class study to collectively identify all.

Further Extensions

Alignment to the Curriculum Framework for AP^® Chemistry 

Forces of attraction between particles (including the noble gases and also different parts of some large molecules) are important in determining many macroscopic properties of a substance, including how the observable physical state changes with temperature. (2B)
2B1: London dispersion forces are attractive forces present between all atoms and molecules. London dispersion forces are often the strongest net intermolecular force between large molecules.
2B3: Intermolecular forces play a key role in determining the properties of substances, including biological structures and interactions.

The strong electrostatic forces of attraction holding atoms together in a unit are called chemical bonds. (2C)
2C1: In covalent bonding, electrons are shared between the nuclei of two atoms to form a molecule or polyatomic ion. Electronegativity differences between the two atoms account for the distribution of the shared electrons and the polarity of the bond.
2C2: Ionic bonding results from the net attraction between oppositely charged ions, closely packed together in a crystal lattice.
2C3: Metallic bonding describes an array of positively charged metal cores surrounded by a sea of mobile valence electrons.

The type of bonding in the solid state can be deduced from the properties of the solid state. (2D)
2D1: Ionic solids have high melting points, are brittle, and conduct electricity only when molten or in solution.
2D2: Metallic solids are good conductors of heat and electricity, have a wide range of melting points, and are shiny, malleable, ductile, and readily alloyed.
2D3: Covalent network solids generally have extremely high melting points, are hard, and are thermal insulators. Some conduct electricity.
2D4: Molecular solids with low molecular weight usually have low melting points and are not expected to conduct electricity as solids, in solution, or when molten.

Learning Objectives
2.11 The student is able to explain the trends in properties and/or predict properties of samples consisting of particles with no permanent dipole on the basis of London dispersion forces.
2.13 The student is able to describe the relationship between the structural features of polar molecules and the forces of attraction between the particles.
2.15 The student is able to explain observations regarding the solubility of ionic solids and molecules in water and other solvents on the basis of particle views that include intermolecular interactions and entropic effects.
2.16 The student is able to explain the properties (phase, vapor pressure, viscosity, etc.) of small and large molecular compounds in terms of the strengths and types of intermolecular forces.
2.19 The student can create visual representations of ionic substances that connect the microscopic structure to macroscopic properties, and/or use representations to connect the microscopic structure to macroscopic properties (e.g., boiling point, solubility, hardness, brittleness, low volatility, lack of malleability, ductility, or conductivity).
2.20 The student is able to explain how a bonding model involving delocalized electrons is consistent with macroscopic properties of metals (e.g., conductivity, malleability, ductility, and low volatility) and the shell model of the atom.
2.22 The student is able to design or evaluate a plan to collect and/or interpret data needed to deduce the type of bonding in a sample of a solid.

Science Practices
1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
6.4 The student can make claims or predictions about natural phenomena based on scientific theories and models.
7.1 The student can connect phenomena and models across spatial and temporal scales.

Answers to Prelab Questions

1. Rate the evaporation of the three liquids from the Splatter Test demonstration from increasing to decreasing order. Explain.

   Acetone > ethyl alcohol > water
   The slow rate of evaporation of water molecules reflects the strong hydrogen bonding in water—only the water molecules at the liquid surface that have enough kinetic energy to break the hydrogen bonds will escape into the gas phase. Hydrogen bonding is also present in ethyl alcohol, although to a lesser extent than in water. Acetone is a polar compound—the molecules are held together in the liquid phase by dipole–dipole forces, which are weaker than hydrogen bonds.

2. Choose the property that is not typically used to determine the strengths and types of intermolecular forces present in solids and liquids. Circle your answer.
   • Surface tension
   • Capillary action
   • Vapor pressure
   • Boiling point
   • Odor
3. Match the intermolecular force to its correct diagram. Select all that apply.
   {14109_PreLabAnswers_Figure_10}
   1. Dipole-dipole
   2. Hydrogen bonding
   3. Dipole induced dipole
   4. London dispersion
4. Show by means of a diagram one hydrogen bond that might form between a glucose unit in cotton and congo red. Hint: look at Figures 3 and 4 in the Background section.
   {14109_PreLabAnswers_Figure_11}
5. Bonds can be formed between atoms that come from the same or different regions of the periodic table. For each bond type below, shade the region(s) of the periodic table where atoms might come from for bond formation.
   {14109_PreLabAnswers_Figure_12}
6. Look up the electronegativity values for each atom in F₂, NaI and Na.
   1. Define electronegativity. What is its periodic trend?

      Electronegativity is a measurement of how strongly an atom attracts shared electrons in a chemical bond. Electronegativity increases from left to right on the periodic table and from the bottom, up. F₂ is the most electronegative element.

   2. Calculate the change in electronegativity (ΔEN) for each.

      See table for Queston 5b answer.

   3. Use the calculated change in electronegativity (ΔEN) value as evidence to determine the bonding type, ionic, covalent or metallic for each.

      See table for Question 5c answer.

      {14109_PreLabAnswers_Table_3}

      In ionic bonding, one atom draws on the electron, while the other atom draws on it weakly. The AvgEN will be between the extremes. The ΔEN, hence, will be large.
      
      In metallic bonding, both atoms attract electrons weakly. The AvgEN is the average of two low numbers. The ΔEN is low since both of the atoms have electronegativities that are similar.
      
      In covalent bonding, both atoms attract electrons strongly. The AvgEN will be the average of two high numbers. The ΔEN is low since both of the atoms have electronegativities that are similar.

7. Answer the following questions for these molecules: methane, iodine, formaldehyde, ammonia and water.
   1. What type of bonding do all of these molecules have in common?

      They all contain a type of covalent bond.

   2. Use Lewis diagrams and VSEPR to draw the molecular structure for each molecule.
      {14109_PreLabAnswers_Figure_13}
   3. Draw a shaded area of the electron cloud for iodine and water. Hint: Look at the Background section.
      {14109_PreLabAnswers_Figure_14}
   4. Indicate whether there are polar or nonpolar bonds present in each.

      Methane: the C—H bond is nonpolar.
      Iodine: nonpolar bond.
      Formaldehyde: the C=O bond is polar.
      Ammonia: the N—H bond is polar
      Water: the O—H bond is polar.

   5. Look at the methane molecule. 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.

   6. Compare the N—H and O—H bonds in ammonia and water, respectively. Which bond is more polar?

      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.

8. A student determines the unique bond types of four test samples in lab from a series of tests (located in Table 2). Table 1 shows the student’s observations and results. Use Table 1 to answer Questions a–d.
   1. Fill in the blank spaces for bond type for solids B and D in Table 2. Solids A and C are already determined.

      Solid B is polar covalent and solid D is metallic.

   2. Which of the following best describes your answer for the bond type in solid D? Circle your answer.
      • Net attractive forces result from pairs of electrons that are shared between atoms.
      • Attractive forces exist between oppositely charged ions.
      • Attractive forces do not exist.
      • Attractive forces that exist between closely packed cations and free-floating valence electrons in a 3D structure.
   3. Why does solid A have a much higher melting point than solid C?

      The forces holding solid C together are London dispersion forces and induced dipole–induced dipole attractions, which are very weak. Those holding ionic compounds together, such as solid A, are strong attractive forces between positive and negative ions.

   4. Explain why solid C is insoluble in water, while solid B is soluble.

      Water, being a polar covalent molecule, dissolves the polar covalent solid B to a much greater extent than with nonpolar covalent solid C. Dipole–dipole interactions and hydrogen bonds between the water molecules and —OH groups in solid B help stabilize the solution.

9. Create a flowchart based on the student’s data from Table 1. You will use this flowchart on lab day to determine four solid unknowns. Start by determining if the solid is ionic, polar covalent, non-polar covalent or metallic. Note: Utilize the procedures in Table 2 to create the flowchart.

   See sample flowchart. Student flowcharts may vary.

   {14109_PreLabAnswers_Figure_15}
10. Write a detailed step-by-step procedure to determine the identity of four unknown solids and two fabric types. Include your flowchart from step 9 in your lab notebook organized with your selected procedures from Table 2 to be used on the day of lab.

    Student procedures will vary, helpful tips were provided. Students use the procedures in Table 2 to test the various properties. Students organize their data in a data table drawn in their lab notebook. The flow chart created prior to lab helps the students identify bond-type in the solids. Both the data table and flowchart help the student identify the unknowns. See sample data table.

    {14109_PreLabAnswers_Table_4}
    • Specifically for the unknown fabrics: When fabrics were dyed with methyl orange, alizarin and congo red, the resulting colors ranged from dark in intensity on unknown fabric 1 to essentially colorless (white) on unknown fabric 2.
    • Wool is unknown fabric 1 and acrylic is unknown fabric 2.
    • If a fabric contains more ionic and polar groups in its structure, as in wool, then the intensity of the dye color should increase, because there will be more sites on the fabric for the dye molecules to bind to. Acrylic does not have available sites for the dye molecules to readily bind; slight dyeing occured due to London forces.
    • 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—a very strong form of dipole–dipole interaction.
    • Methyl orange showed a much greater variability in the colors that it produced on different fabrics. Binding of methyl orange may depend on its ability to form ionic bonds with fabric molecules. More fabrics are capable of hydrogen bonding than ionic bonding.
    • Alizarin dye bonds to fabric molecules by hydrogen bonding and also shows greatest affinity to wool. Since alizarin is a mordant dye, you may add the alum step (as an extension) and observe the equal color intensity on all fabric types. See the Teaching Tips section for the procedure.

References

AP^® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013. 

Chemical Bonding, Volume 5, Flinn ChemTopic™ Labs; Cesa, I., Editor; Flinn Scientific: Batavia IL, 2004.

Solids and Liquids, Volume 5, Flinn ChemTopic™ Labs, Cesa, I., Editor; Flinn Scientific, Inc., Batavia IL (2004).

Introduction

Experience and learn the concepts you need to help you succeed on the AP^® Chemistry exam with this guided-inquiry activity! Witness a simple demonstration—the Splatter Test—to gain understanding of the intermolecular forces of three liquids. Then complete a homework set to solidify your understanding of types of bonds, their stregths, and how they differ from typical intermolecular forces. The homework set will guide you to develop your own flowchart to use on the day of lab to aid in determining four unknown solids. An exciting and colorful inquiry extension allows you to use your acquired knowledge of chemical bonding and intermolecular forces to determine the identity of two fabric types based on the intensity of the resulting fabric color!

Concepts

• Bonding
• Physical and chemical properties
• Intermolecular forces
• Electronegativity

Background

Chemical Bonds

Groups of atoms are held together by attractive forces that we call 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 describes 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 are arranged in a tightly packed, extended 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 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, some of which must exist or act 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.

Vibrant Colors in Dyed Fabrics—Intermolecular Forces

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 and intermolecular forces thus play an important role in how and why dyes work.

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 fabrics, both natural and synthetic, are polymers. Polymers 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 on the next page. 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). Cotton is a polysaccharide composed of glucose units attached to one other in a very rigid structure. There are three polar hydroxyl (–OH) groups per glucose repeating unit. Acetate is cellulose in which some of the –OH groups have been replaced by acetate groups (–OCOCH₃).

Nylon was the first completely synthetic fiber. It is a polyamide, made up of hydrocarbon repeating units joined together by highly polar amide (–CONH–) functional groups. 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. Can you identify the sites where the dye molecules will bind? By which forces or interactions? Dyes are classified based on both the structure of the dye and the way in which the dye is applied to the fabric (see Figure 4). Direct dyes are charged, water-soluble organic compounds. Direct dye molecules contain both positively and negatively charged groups. 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 contain nitrogen atoms (–N:). The ability of a dye to bond to a fabric may be improved by using an additive called a mordant. Mordant dyes are used in combination with salts of metal ions, typically aluminum, chromium, iron and tin.

Experiment Overview

The purpose of this advanced inquiry lab is to determine the identity of four unknown solids and two dyed unknown fabric types. The investigation begins with an introductory homework assignment, where you will be asked to answer, observe and form conclusions based on a series of questions pertaining to types of bonds, intermolecular forces and laboratory data. You will use your answers to these questions and your interpretation of the sample laboratory data to develop a procedure for identifying the type of bonding in four unknown solids and the identities of two unknown fabric based on their ability to bind dyes. Completion of the homework assignment will promote success in the lab!

Prelab Questions

Complete the following homework set and write a lab procedure to be approved by your instructor prior to performing the lab. Along with your procedure, you will turn in any graphs or figures you were asked to create in this homework set and answers to the questions.

1. Rate the evaporation of the three liquids from the Splatter Test demonstration from increasing to decreasing order. Explain.
2. Choose the property that is not typically used to determine the strengths and types of intermolecular forces present in solids and liquids. Circle your answer.
   • Surface tension
   • Capillary action
   • Vapor pressure
   • Boiling point
   • Odor
3. Match the intermolecular force to its correct diagram. Select all that apply.
   {14109_PreLab_Figure_5}
   1. Dipole-dipole
   2. Hydrogen bonding
   3. Dipole induced dipole
   4. London dispersion
4. Show by means of a diagram one hydrogen bond that might form between a glucose unit in cotton and congo red. Hint: look at Figures 3 and 4 in the Background section.
5. Bonds can be formed between atoms that come from the same or different regions of the periodic table. For each bond type below, shade the region(s) of the periodic table where atoms might come from for bond formation.
   {14109_PreLab_Figure_6}
6. Look up the electronegativity values for each atom in F₂, NaI and Na.
   1. Define electronegativity. What is its periodic trend?
   2. Calculate change in electronegativity (ΔEN) for each.
   3. Use the calculated the change in electronegativity (ΔEN) value as evidence to determine the bonding type, ionic, covalent or metallic, for each
7. Answer the following questions for these molecules: methane, iodine, formaldehyde, ammonia and water.
   1. What type of bonding do all of these molecules have in common?
   2. Use Lewis diagrams and VSEPR to draw the molecular structure for each molecule.
   3. Draw a shaded area of the electron cloud for iodine and water. Hint: Look at the Background section.
   4. Indicate whether there are polar or nonpolar bonds present in each.
   5. Look at the methane molecule. Why are C—H bonds considered nonpolar?
   6. Compare the N—H and O—H bonds in ammonia and water, respectively. Which bond is more polar?
8. A student determines the unique bond types of four test samples in lab from a series of tests (located in Table 2). Table 1 shows the student’s observations and results. Use Table 1 to answer Questions a–d.
   {14109_PreLab_Table_1}
   1. Fill in the blank spaces for bond type for solids B and D in Table 1. Solids A and C are already determined.
   2. Which of the following best describes your answer for the bond type in solid D? Circle your answer.
      • Net attractive forces result from pairs of electrons that are shared between atoms.
      • Attractive forces exist between oppositely charged ions.
      • Attractive forces do not exist.
      • Attractive forces that exist between closely packed cations and free-floating valence electrons in a 3D structure.
   3. Why does solid A have a much higher melting point than solid C?
   4. Explain why solid C is insoluble in water, while solid B is soluble.
9. Create a flowchart based on the student’s data from Table 1. You will use this flowchart on lab day to determine the identity of four solid unknowns. Start by determining if the solid is ionic, polar covalent, nonpolar covalent or metallic. Note: Utilize the procedures in Table 2 to create the flowchart.

   A flowchart is a powerful tool that uses boxes, ovals, circles, and arrows to organize your experiment in a visual, step-by-step fashion. See Figure 7 for an example flowchart.

   • Using yes−no logic, create a flowchart that can be used to characterize an unknown solid as ionic, polar covalent, nonpolar covalent or metallic.
   • If given a white solid, what testing results would help you to identify the solid as polar covalent?
   • Carry out the flowchart tests on the four unknowns and record the results of each test in an appropriate data table.
     {14109_PreLab_Figure_7_Example flowchart}
10. Write a detailed step-by-step procedure to determine the identity of four unknown solids and two fabric types. Include your flowchart from step 9 in your lab notebook organized with your selected procedures from Table 2 to be used on the day of lab.

    Helpful tips:

    1. Think safety, first. Make sure you have the proper PPE available to perform this lab (i.e. goggles, apron and gloves).
    2. Make a list of the equipment and glassware needed for this lab.
    3. Number the steps in your procedure. Remember to be as detailed as possible, from set-up to clean-up.
    4. Draw any necessary data tables in your notebook for data collection during the lab.

Safety Precautions

Acetone and ethyl alcohol are flammable organic solvents and dangerous fire risks. Keep away from flames, heat and other sources of ignition. Cap the solvent bottles. Addition of a denaturant makes ethyl alcohol poisonous; it cannot be made nonpoisonous. All of the dyes are strong stains and will stain skin and clothing. Methyl orange is toxic by ingestion and irritating to body tissue. Sulfuric acid is corrosive and toxic by ingestion. Alizarin red is a body tissue irritant. The dye baths are very hot, near boiling. Exercise care to avoid scalding and skin burns. Copper(II) sulfate is a skin and respiratory tract irritant and is toxic by ingestion. 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 before leaving the lab. Please follow all laboratory safety guidelines.

Procedure

Part A. Determine the types of unknown fabrics after dyeing in three dye baths, methyl orange, congo red and alizarin. To save time, dye the fabrics at the beginning of lab and allow the fabric to dry before analysis. You must correctly identify which unknown is wool and which is acrylic. While fabrics are drying, continue to Part B.

1. Obtain 3 strips. Each should be around 1 inch in width. Lay out aluminum foil to set wet dyed fabrics. Using forceps or tongs, immerse the test strip into the dye bath. Caution: The dye baths are very hot. Exercise care to avoid scalding or skin burns.
2. After 5–10 minutes, remove the dyed test strip from the bath using forceps. Hold the fabric above the dye bath for a few seconds to allow excess dye solution to drain back into the dye bath.
3. 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.
4. Place the rinsed test strip in the appropriately labeled aluminum foil and allow it to air dry. See example in step 5.
5. When the fabric is dry, record the dye color produced by each direct dye on each type of fiber.
   {14109_Procedure_Figure_8}

Part B. Determine the identity of four unknown solids. Using the flowchart as a guide, carry out the procedures for testing the solids and identifying the type of bonding in each. Possible unknowns are: aluminum, calcium carbonate, dextrose, dodecyl alcohol, potassium nitrate, copper(II) sulfate, salicylic acid, paraffin wax and glycine.