Materials Included In Kit

Additional Materials Required

Safety Precautions

Ethyl alcohol, hexane and toluene are flammable organic solvents and dangerous fire risks. Keep away from flames and other sources of ignition. Addition of denaturant makes ethyl alcohol poisonous. Toluene is moderately toxic by ingestion, inhalation and skin absorption. Work with these solvents in a well-ventilated lab only and avoid breathing their vapors. Iodine is toxic by ingestion or inhalation. It is a skin and eye irritant and will stain skin and clothing. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Please consult 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. Waste solutions containing iodine (Part A) may be disposed of by reduction with sodium thiosulfate according to Flinn Suggested Disposal Method #12. Waste solutions containing volatile organic solvents may be disposed of according to Flinn Suggested Disposal Method #18a.

Lab Hints

• The laboratory work for this experiment can reasonably be completed in one 50-minute lab period. To cut down on the amount of aqueous organic waste generated using organic solvents, consider assigning this experiment as a collaborative classroom activity with different groups doing Parts A, B and C individually and then sharing their results with each other.
• For best results, set up an “Iodine Waste” container and an “Organic Waste” container in central locations in the lab (the hood is ideal) for immediate collection of waste solutions. Label one 600-mL beaker “Organic Waste” and another “Iodine Waste.” Add the 100 g of sodium thiosulfate to 200 mL of distilled or deionized water in the “Iodine Waste” beaker and mix to make a 50% aqueous sodium thiosulfate solution. The solution will be ready to dispose of down the drain with plenty of excess water when the lab is done.
• The solvent “hexanes” is a mixture of n-hexane and other isomers, and 95% denatured ethyl alcohol.
• To avoid long lines in front of reagent bottles, assign some groups to start with Part B or Part C first.

Teacher Tips

• Organic chemistry is an interesting and important part of chemistry education, even at the high school level. Don’t wait until the end of the year to introduce some organic chemistry into the curriculum—you won’t have time! Let the organic chemistry come naturally, as it does in this experiment, by using organic chemicals as examples to teach the principles of general chemistry. Use this lab as a springboard to highlight and discuss the applications of organic compounds in our daily lives.

Answers to Prelab Questions

1. Is the iodine molecule polar or nonpolar? Explain.

   The two iodine atoms in the iodine molecule share electrons equally in the covalent bond joining them. Because the electrons are equally shared between the two atoms, the bond and the molecule itself are nonpolar.

2. The formula of hexane is CH₃—CH₂—CH₂—CH₂—CH₂—CH₃. Based on its structural formula, is hexane a polar or nonpolar compound? Explain.

   Hexane is a hydrocarbon molecule consisting of only C—H and C—C bonds. Because carbon and hydrogen atoms have similar electronegativity values, the covalent bonds between these atoms have equally shared electrons. The molecule is nonpolar.

3. Draw the structure of a water molecule and explain why it is polar. Show by means of a diagram the types of attractive forces acting between water molecules and also between water molecules and dissolved ions such as Na⁺ and Cl^– ions.
   {13967_PreLabAnswers_Figure_1}
4. Why is it important to work with iodine crystals in a well-ventilated lab? What are the hazards associated with hexane and toluene?

   Iodine creates vapors that are toxic by inhalation. Both hexane and toluene are flammable solvents and dangerous fire risks. Toluene vapors are moderately toxic by inhalation and skin absorption.

Sample Data

Part A. Solubility of Iodine

*May be difficult to determine slightly soluble.

Answers to Questions

1. In which solvents is iodine soluble? In which solvents is iodine insoluble?

   Iodine is soluble in hexane and toluene, partially or slightly soluble in ethyl alcohol, and insoluble in water.

2. Define the term miscibility, then circle the correct choice in each statement to summarize the miscibility of the solvent pairs tested in Part B:

   Miscibility is the term used to describe the mutual solubility of liquids in one another. Two liquids are said to be miscible if they dissolve freely in each other in all proportions to form a single liquid phase when mixed.

   Water and ethyl alcohol are (miscible/immiscible).

   Water and hexane are (miscible/immiscible).

   Water and toluene are (miscible/immiscible).

   Hexane and ethyl alcohol are (miscible/immiscible).

   Hexane and toluene are (miscible/immiscible).

   Toluene and ethyl alcohol are (miscible/immiscible).

3. Rank the four solvents tested in Parts A and B in order from most polar to least polar (nonpolar). Which two solvents are most alike in their polarity? Explain your reasoning.

   From most polar to nonpolar: Water > ethyl alcohol > hexane and toluene. Ethyl alcohol is intermediate in polarity between water (which is highly polar) and hexane or toluene (which are nonpolar). Thus, ethyl alcohol is miscible with all of the solvents tested. Hexane and toluene are most alike in their polarity—they are miscible with each other, and both are immiscible with water.

4. Write a general statement describing the solubility of nonpolar solutes in different solvents and suggest a reason for this pattern.

   Nonpolar solutes dissolve in nonpolar solvents but do not dissolve in water, a highly polar solvent. The solubility of nonpolar solutes in other polar solvents varies—they may be partially soluble. Nonpolar solutes and solvents have similar attractive forces.

5. Potassium nitrate (Part C) is an ionic compound. Write a general statement describing the solubility of ionic compounds in different solvents.

   Ionic compounds dissolve in water, a highly polar solvent. They do not dissolve at all in nonpolar solvents.

6. Dextrose, cholesterol, and benzoic acid are molecular (organic) compounds. Based on their solubility patterns in Part C, arrange these three solutes in order from most polar to least polar. Explain your reasoning.

   From most polar to least polar: Dextrose > benzoic acid > cholesterol.

   The fact that dextrose is soluble in water and slightly soluble in ethyl alcohol suggests that it is a highly polar compound (its solubility pattern is the same as that of potassium nitrate, an ionic compound). Cholesterol is soluble only in hexane, a nonpolar solvent, and is thus a nonpolar compound. Benzoic acid is slightly soluble in water, soluble in ethyl alcohol, and insoluble in hexane, suggesting that it is intermediate in polarity. Note to teachers: Benzoic acid is soluble in hot water.

7. Based on its solubility, would you expect cholesterol to be soluble in the bloodstream? Where does cholesterol tend to accumulate in the body? Why?

   Cholesterol is insoluble in the bloodstream. It tends to accumulate in fatty tissues and, of course, in the walls (nonpolar membranes) of blood vessels. Note: How then does cholesterol get carried through the bloodstream? That’s the role of the lipoproteins (both the bad LDL and the good HDL) that are so much in the news. (Many adults can quote their LDL/HDL ratios the way sports fans can quote the batting averages of their favorite baseball players.) Lipoproteins act as emulsifying agents, carrying cholesterol through the bloodstream.

8. Vitamins are classified as either water-soluble or fat-soluble. The structures of Vitamin C (water-soluble) and Vitamin A (fat-soluble) are shown below. Identify the features of these molecules that give them their characteristic solubility.
   {13967_Answers_Figure_2}

   Vitamin C contains many polar C—O and O—H bonds. It is a highly polar compound and is thus soluble in water and other polar solvents. Vitamin A consists almost entirely of nonpolar C—H, C—C, and C—C bonds and is a nonpolar compound. It does not dissolve in water, but does dissolve in fatty tissue, which contains nonpolar fats and oils.

9. The simple rule “Like dissolves like” is often used to describe the solubility of a substance in different solvents. Write a short paragraph discussing your evidence for this rule. Include in your discussion where you think this rule works best and where it seems to be less reliable. Give specific examples to back up your statements.

   The rule “like dissolves like” seems to work best at the two extremes. Thus, nonpolar solutes tend to dissolve only in nonpolar solvents (cholesterol in hexane, iodine in hexane or toluene). Similarly, ionic and very highly polar compounds (potassium nitrate and dextrose) dissolve only in water, a highly polar solvent. Compounds of intermediate polarity exhibit both tendencies. Thus, ethyl alcohol is miscible with both water and hexane.

10. (Optional) A drop of motor oil spilled on wet pavement will quickly spread out into a thin film. A drop of water spilled on a greasy plate, however, will bead up into a little sphere. Use these observations, and the nature of solute–solvent interactions, to explain why oil and water do not mix.

    Note: There are many misconceptions about why oil and water do not mix. In biology classes, students may learn about the “hydrophobic” effect, which suggests that oil and water molecules somehow repel one another. The fact that oil will spread out into a thin film on wet pavement belies this notion—clearly oil molecules are attracted to surface water molecules, at the very least. In looking at solubility, there are four factors to be considered: the energy required to disrupt attractive forces among solute molecules, the energy required to disrupt attractive forces among solvent molecules, the energy released due to attractive forces between solute and solvent molecules, and the entropy of mixing. Oil and water do not mix because of the second energy term. Water molecules are much more attracted to other water molecules than to oil molecules. The evidence for this is the way water beads up on a sheet of wax paper.

References

This experiment is from Flinn ChemTopic™ Labs, Volume 12, Solubility and Solutions; Cesa, I., Ed., Flinn Scientific: Batavia, IL.

Introduction

“Oil and water do not mix.” How many times have you heard this old saying? As metaphor, it is often used to explain why relationships between opposites are difficult or even impossible. Let’s trace this metaphor back to its source—the nature of oil and water, solutes and solvents, and why some substances do not dissolve in or mix well with others.

Concepts

• Solute and solvent
• Polar vs. nonpolar
• Intermolecular forces
• Miscibility of liquids

Background

A solution is a uniform, or homogeneous, mixture of two or more substances. The word homogeneous means that a solution must be uniform throughout its contents. The composition or concentration of a solution can be changed by changing the amount of the solute (the minor component) dissolved in a given amount of the solvent (the major component). Although many common solutions contain solids dissolved in liquids, both the solute and the solvent may exist in any phase (solid, liquid or gas). Solubility is a characteristic property of a pure substance and can be used to help identify different substances. Thus, a chemistry handbook will usually report the solubility of a substance in different solvents along with other physical properties (e.g., melting point, density).

When a solute dissolves in a solvent, the attractive forces acting between solute particles and those between solvent molecules must be broken and replaced by new attractive forces between the solute and solvent. The nature and strength of the attractive forces among solute and solvent particles influences whether a solute will dissolve in a solvent. Many ionic compounds, for example, dissolve readily in water. Water is a highly polar molecule, with a great degree of charge separation between the oxygen and hydrogen atoms in its O—H bonds. Upon dissolving in water, an ionic compound breaks apart into its component ions, which are attracted to the partially charged ends of the highly polar water molecules.

Molecular compounds consist of molecules—groups of atoms held together by covalent bonds—rather than ions. The physical properties of a molecular compound, including its solubility, depend on the polarity of the molecules. Molecules are classified as polar or nonpolar based on the nature of the electron sharing among the atoms in a molecule. Polar molecules tend to exert stronger attractive forces than nonpolar molecules. The polarity of a compound determines the types of intermolecular attractive forces between molecules and is an important factor influencing the solubility of solutes and solvents.

Experiment Overview

The purpose of this experiment is to investigate the solubility of ionic, polar, and nonpolar compounds in a variety of solvents. The solubility patterns of different solutes and solvents will be used to classify compounds and to understand the nature of the interactions between solute and solvent particles.

Materials

Prelab Questions

1. Is the iodine molecule polar or nonpolar? Explain.
2. The formula of hexane is CH₃—CH₂—CH₂—CH₂—CH₂—CH₃. Based on its structural formula, is hexane a polar or nonpolar compound? Explain.
3. Draw the structure of a water molecule and explain why it is polar. Show by means of a diagram the types of attractive forces acting between water molecules and also between water molecules and dissolved ions such as Na⁺ and Cl^– ions.
4. Review the Safety Precaution section. Why is it important to work with iodine crystals in a well-ventilated lab? What are the hazards associated with hexane and toluene?

Safety Precautions

Ethyl alcohol, hexane and toluene are flammable organic solvents and dangerous fire risks. Keep away from flames and other sources of ignition. Addition of denaturant makes ethyl alcohol poisonous. Toluene is moderately toxic by ingestion, inhalation and skin absorption. Iodine is toxic by ingestion or inhalation. Iodine is also a skin and eye irritant and will stain skin and clothing. Work with these solvents and chemicals in a well-ventilated lab only and avoid breathing their vapors. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

Part A. Solubility of Iodine

1. Obtain four clean test tubes and place them in a test tube rack. Using Table 1 as a guide, add about 2 mL of the appropriate solvent to each test tube. Note: Use a graduated cylinder to measure and add 2 mL of water to test tube 1. Label a clean Beral-type pipet for each of the remaining solvents, then use the appropriate Beral-type pipet to add about the same volume of other solvents to test tubes 2–4.
   {13967_Procedure_Table_1}
2. Using a spatula, add one crystal of iodine to each test tube.
3. Gently swirl or tap each test tube and observe the mixtures. Beneath the name of each solvent in the data table, record whether iodine is soluble or insoluble in the solvent and the color of the solution (if appropriate).
4. Dispose of the contents of the test tubes in the “Iodine Waste” container provided by your instructor.
5. Rinse each test tube twice with water and dry them with a paper towel for use in Part B. Retain the labeled Beral-type pipets for each solvent for use in Part B.

6. Place six clean test tubes in a test tube rack. Using a clean Beral-type pipet, add about 2 mL (40 drops) of water to test tubes 1, 2 and 3. Note: Use the same test tube level for adding 2 mL of solvent as determined in Part A.
7. Add about 2 mL of hexane to test tubes 4 and 5, using the same Beral-type pipet used in Part A to add hexane.
8. Add about 2 mL of toluene to test tube 6, again using the same Beral-type pipet for toluene from Part A.
9. Using Table 2 as a guide, add 20 drops (about 1 mL) of a second solvent to each test tube using the appropriate Beral-type pipet. Note: Use a graduated cylinder to measure and add 1 mL of ethyl alcohol to test tube 1. Use the appropriate Beral-type pipet for each of the remaining solvents to add 1 mL of the other solvents to test tubes 2–6.
   {13967_Procedure_Table_2}
10. Gently swirl or tap each test tube to mix the contents.
11. Next to the name of each solvent pair in the data table, record whether the two liquids form one or two layers upon standing. If the mixtures separate into two layers, report which solvent is the upper layer.
12. Dispose of the contents of the test tubes in the “Organic Waste” container provided by your instructor. Rinse each test tube twice with water and dry them with a paper towel before using them in Part C. Retain the labeled Beral-type pipets for each solvent for use in Part C.

13. Obtain six clean test tubes, place them in a test tube rack, and label them 1–6.
14. Add about 2 mL (40 drops) of the appropriate solvent to each test tube, as shown in Table 3.
    {13967_Procedure_Table_3}
15. Using a clean spatula, add a small amount (about the size of a grain of rice) of dextrose to test tubes 1, 2 and 3.
16. Wipe the spatula with a clean paper towel, then add a small amount (about the size of a grain of rice) of potassium nitrate to test tubes 4, 5 and 6.
17. Gently swirl or tap each test tube to mix the contents.
18. In the data table, record whether each substance is soluble or insoluble in each solvent.
19. Dispose of the test tube contents as directed by your instructor.
20. Rinse each test tube once with about 1 mL of the appropriate solvent, and then add about 2 mL of fresh solvent to each tube. Use the same solvent arrangement shown in Table 3.
21. Using a clean spatula, add a small amount (about the size of a grain of rice) of cholesterol to test tubes 1, 2 and 3.
22. Wipe the spatula with a clean paper towel, then add a small amount (about the size of a grain of rice) of benzoic acid to test tubes 4, 5 and 6.
23. Gently swirl or tap each test tube to mix the contents.
24. In the data table, record whether each substance is soluble or insoluble in each solvent.
25. Dispose of the test tube contents as directed by your instructor. Wash and rinse the test tubes.

Student Worksheet PDF

13967_Student1.pdf