Intermolecular Attractions

Introduction

Help students understand intermolecular attractions with this collection of dramatic demonstrations.

The set of four demonstrations includes:

1. Floating Oil Droplet—Lamp oil is placed in a beaker of less dense alcohol. Water is gradually added until the oil droplets float in the middle of the beaker.
2. Bubble Shapes—Forms made of straws and twist ties are dipped in a soap solution and withdrawn to create beautiful geometric shapes of bubbles.
3. Capillarity—The ability of water and ethyl alcohol to rise in a glass tube is observed and compared by measuring the height each attains in a capillary tube.
4. Vanishing Volume—50-mL portions of deionized water and anhydrous ethyl alcohol are mixed together in a 100-mL graduated cylinder to produce a volume less than 100 mL.

Also included is a supplemental demonstration called Chemical Fleas. Crystals of camphor are added to a Petri dish filled with water. The crystals begin to dart about, until a little oil is added to the water to calm the motion.

The series of demonstrations may be presented in a variety of ways. Each demonstration may be used to introduce specific intermolecular forces, or all the demonstrations can be performed together as a review of intermolecular attractions. A student worksheet is included as an optional assessment tool for the instructor.

Concepts

• Density
• Surface tension
• Polarity
• Hydrophilic
• Hydrophobic
• Adhesive forces
• Cohesive forces
• Intermolecular forces—hydrogen bonding
• Polar molecules
• Marangoni effect

Materials Included In Kit

Additional Materials Required

Experiment Overview

Floating Oil Droplet: An interesting demonstration that incorporates density, surface tension and intermolecular attractions into one demonstration.

Bubble Shapes: Create beautiful geometric shapes of bubbles with some straws and twist ties.

Capillarity: Why does water do the seemingly impossible and rise upwards in a glass tube? Demonstrate the effect that results from the interaction of adhesive and cohesive forces.

Vanishing Volume: When equal volumes of water and ethyl alcohol are mixed, the total volume is less than that of the two liquids before mixing. What happened to the vanishing volume?

Chemistry Fleas: Watch as particles seem to jump around the surface of water, then are calmed by the mere touch of the surface by a finger.

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid and is slightly toxic by ingestion or inhalation. The blue lamp oil is a flammable liquid. Bubbles break with a fair amount of force; keep them away from your face. The bubble solution will make the floor or pavement slippery; take care to avoid slipping. Glycerin may cause an explosion when contacting strong oxidants. Ethyl alcohol is a dangerous fire risk; it is flammable. The addition of denaturant (methyl alcohol) makes ethyl alcohol poisonous. Do not ingest. Camphor when heated produces flammable and explosive vapors. Keep away from flames and heat sources. It is moderately toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Isopropyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18a. The red lamp oil may be disposed of according to Flinn Suggested Disposal Method #18b. The Floating Oil Droplet demonstration solution may be disposed of according to Flinn Suggested Disposal Method #26b. The soap solutions may be disposed of according to Flinn Suggested Disposal Method #26b. The ethyl alcohol may be disposed of according to Flinn Suggested Disposal Method #26b. The glass tube and ruler can be stored and reused. The resulting Vanishing Volume solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The camphor may be disposed of according to Flinn Suggested Disposal Method #18b.

Prelab Preparation

Bubble Shapes

Preparation of Soap Bubble Solution: To make one liter of bubble solution, mix 100 mL of dishwashing liquid with 50 mL of glycerin in a 1000-mL beaker. Add this to 850 mL of distilled water. The mixture should be stirred, not shaken, otherwise excessive amounts of suds will be produced. Note: For larger structures, up to four liters of solution may be needed in order to completely submerge the structures—it may be best to prepare the solution in a bucket or other large container. The solution can be stored and reused.

Preparation of Geometric Shapes: Use scissors to cut the twist ties in half and the coffee stirrers into 3-inch lengths.

Creating a Three-Way Junction

1. Place two twist ties on top of one another, fold them into a V-shape, and insert the twist ties together into one end of a coffee stirrer (see Figure 1).
2. Bend each twist tie backwards in opposite directions to form a T-shape (see Figure 2).

3. Place a third twist tie across the top of the other two twist ties, and insert the ends into two coffee stirrers (see Figures 3 and 4).

Creating a Four-Way Junction

4. Repeat steps 1 and 2 twice to create two additional T-shaped assemblies.
5. Arrange the two T shapes on top of one another to form a cross shape (see Figure 5).
6. Slide a coffee stirrer over each of the exposed twist ties to create the cross shape (see Figure 6).

Creating a Five-Way Junction

7. Repeat steps 1–3.
8. With a new stirrer, repeat steps 1–2.
9. Insert one twist tie into a new coffee stirrer.
10. Insert one of the twist ties from step 7 into the stirrer from step 9 to obtain the assembly shown in Figure 7.
11. Place the two exposed twist tie ends into the assembly created in step 8, creating a five-way juncture (see Figure 8).

12. Combine three-, four- and five-way junctions as needed to build one of the model structures shown in Figure 9.

13. Wrap an extra twist tie around one edge of the structure and then back up around itself in order to create a handle for dipping the model.

Procedure

Floating Oil Droplet

1. Place 200 mL of isopropyl alcohol in a 500-mL hydrometer cylinder or tall-form beaker.
2. Use a graduated Beral-type pipet to place 2–3 mL of red oil into the isopropyl alcohol. Note: Droplets of red oil will form on the bottom of the cylinder.
3. Use a wash bottle to slowly add distilled or deionized water to the cylinder. Add the distilled water down the side of the beaker. Do not mix the solution. As the water is added, the oil droplets will clump together and start to float.
4. Continue adding water until the droplets float in the middle of the cylinder.
5. (Optional) Have the students complete the worksheet questions and discuss the results.

Bubble Shapes

1. Submerge a model in the bucket containing the soap solution.
2. Pull the model out in one smooth motion. Note: It may take several “dips” in order to achieve the internal geometric soap patterns arranged as desired.

Capillarity

1. Place the Petri dish on an overhead projector. Use a wash bottle to add distilled water to fill the dish about ¼ full.
2. Place the ruler at the top of the overhead screen.
3. Turn the overhead on.
4. Place the capillary tube in the Petri dish below the surface. Observe the water rise in the tube.
5. Hold your index finger over the top of the capillary tube.
6. Raise the tube out of the dish and place it sideways along the ruler. Adjust the tube’s position to reveal the height of the water in the column, in millimeters. Have students note this value.
7. Ask the students what is causing the solution to rise up the tubing.
8. Repeat steps 1 through 6 for anhydrous ethyl alcohol. How does the height compare to that of distilled water?

Vanishing Volume

1. Carefully measure out exactly 50 mL of water in a 100-mL graduated cylinder.
2. Carefully measure out exactly 50 mL of anhydrous ethyl alcohol into a second 100-mL graduated cylinder.
3. Pour the water from the first graduated cylinder into the graduated cylinder containing the ethyl alcohol.
4. Stir the mixture of alcohol and water with a stirring rod and wait about one minute for the bubbles to come out of solution.
5. Observe that the final volume of liquid in the cylinder is less than 100 mL.

Chemistry Fleas

1. Place a Petri dish on the overhead projector.
2. Fill the Petri dish half full with water.
3. Tell the class that you have flea eggs which will begin to hatch and move when you place them in water.
4. Sprinkle camphor crystals around the water surface. Follow their movements on the overhead.
5. After about 1 minute, tell the class you will now put the fleas to sleep.
6. Rub your earlobe with your thumb and index finger. Place your fingers in the water and the “fleas” (camphor crystals) stop moving.

Student Worksheet PDF

13261_Student.pdf

Teacher Tips

• This kit contains enough chemicals to perform the Floating Oil Droplet demonstration at least seven times: 1500 mL of isopropyl alcohol, 25 mL of red oil and 10 Beral-type pipets.

• This kit contains enough chemicals to make four liters of bubble solution.
• For the Bubble Shapes demonstration, hold the model over the bucket or a demonstration tray—it will drip. A wet surface from broken bubbles will be very slippery. Always cover the tabletop and floor with towels when working with bubbles. It may also be a good idea to work with bubble solutions outdoors.
• The bubble solutions commonly available in toy stores are dilute soap or detergent solutions that are good for making small bubbles, but not particularly effective for large bubbles. For large bubbles, use the recipe given in the Preparation section.
• Use distilled or deionized water only when preparing a soap bubble solution to prevent interference from dissolved metal ions present in tap water. If the solution does not seem to work well, let it sit for a few days to a week. Aging seems to improve the characteristics of soap solutions.
• For larger models the twist ties and stirrers may be cut into smaller sizes.
• A common reason for a bubble breaking is the water in the walls of the bubble draining to the bottom of the bubble. This produces a small droplet at the bottom of a bubble. When the top of the bubble becomes too thin to support the total mass of the bubble, it breaks. Adding glycerin to the soap solution tends to make the bubble stronger by preventing the water from readily draining out of the soap film.
• The colors observed on a soap bubble are the result of thin film interference and changing thickness of the film due to the draining liquid. As the thickness of the soap film changes, the distance the light travels changes and the differential interference reflects different colors in the soap film. The swirling colors observed are a result of uneven thickness in the soap film.
• A video of the Bubble Shapes demonstration, Simple Structures, presented by Bob Becker, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Molecular Geometry video package.
• This kit contains enough materials to perform the Capillarity demonstration at least seven times.
• Rest the capillary tube on the side of the Petri dish with the end in the dye solution. Show the students the increase in distance of the solution in the tube. Why the increase?
• The Vanishing Volume demonstration is even more spectacular if done in a 24-inch glass demonstration tube (Flinn Catalog No. GP9146). Fill the tube with equal volumes of deionized water and anhydrous ethyl alcohol, stopper the ends and begin to mix the solvents by turning the tube. An air bubble will soon appear out of nowhere. Adding equal volumes of water and ethyl alcohol to a volumetric flask or a 50-mL buret also works well.
• Anhydrous ethyl alcohol does not contain any water. Do not use 95% alcohol, it contains 5% water.
•  A video of the Vanishing Volume demonstration, Volumes Don’t Always Add Up, presented by George Gross, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Hydrogen Bonding video package.

Answers to Questions

Discussion

Floating Oil Droplet

The density of the lamp oil is greater than that of isopropyl alcohol. Since the oil molecules are non-polar and isopropyl alcohol is polar, the oil will not dissolve and instead forms droplets that sink to the bottom of the cylinder. When water, with a higher density than either isopropyl alcohol or lamp oil, is added, it forms a solution at the bottom of the beaker. The density of this solution is greater than that of isopropyl alcohol alone. As more water is added, the solution density increases. When enough is added to just match the density of the lamp oil, the droplets will float around the middle of the cylinder.

Bubble Shapes

A detergent molecule is a long chain hydrocarbon with two distinct “ends.” At one end is a nonpolar hydrocarbon group and at the other is a polar group with oxygen atoms. The nonpolar end is not attracted to water, or is said to be hydrophobic. Polar end is very attracted to water, or hydrophilic.

When dissolved in water, detergent molecules will form a “sandwich” around the surface layer water molecules (see Figure 10). The hydrophilic ends of the detergent molecules face the water molecules and the hydrophobic ends face away from the water molecules.

When this solution forms a bubble, it forms a three-layer sphere of detergent—water detergent molecules. When the geometric frames are dipped in the solution, minimum surface areas are produced. The different frames will generate a variety of shapes. In all cases, the intersection of surfaces will be an edge, if three planes intersect, or a point, if four surfaces intersect (see Figure 11).

Capillarity

Cohesive forces are the electrostatic forces that hold molecules together. In water, these are primarily hydrogen bonding and dipole–dipole attraction. Adhesive forces are electrostatic forces between molecules of different substances. In the interaction of a glass tube with water, the adhesive forces between the polar water molecules and the polar Si-O bonds at the surface of the glass are greater than the cohesive forces between the water molecules.

The water is pulled up the sides of the tubing until the weight of the water column just balances the total adhesive forces between the glass and water molecules (see Figure 12). The smaller the diameter of the tubing is, the greater the height of the column.

The water molecules at the surface away from the glass wall are attracted inward and form a downwardly curving surface, familiar to all as the meniscus formed in burets, pipets and other glass apparatus.

Vanishing Volume

When 50 mL of water is added to 50 mL of water or when 50 mL of alcohol is added to 50 mL of alcohol, the final volume will always be 100 mL, as expected. In this demonstration, when the water is added to the alcohol, the final volume is about 10% less than the original volume of the two liquids. The “vanishing volume” is due to differences in packing of the solvent molecules in the mixture versus the pure substances. Molecules of ethyl alcohol actually pack together more closely with water molecules than with other alcohol molecules due to hydrogen bonding. The solvent molecules form a highly laced, 3-dimensional network held together by strong hydrogen bonds (see Figure 13). Each alcohol molecule is able to form as many as three hydrogen bonds with neighboring water or alcohol molecules. The result is an intricate lattice or network of molecules strongly attracted to one another. Looking at the molecules:

Hydrogen bonding is an especially strong form of dipole–dipole interaction. A dipole–dipole interaction is the attraction of the positive end of one polar molecule for the negative end of another polar molecule. In hydrogen bonding, a hydrogen atom serves as a bridge between two electronegative atoms (nitrogen, oxygen or fluorine).

Hydrogen bonding plays a major role in the properties of water and alcohols. Hydrogen bonding between water molecules leads to a very high boiling point when compared to other similar liquids. The effect of hydrogen bonding can also be clearly seen when boiling points for alcohols are compared to nonpolar ethers having the same molecular weight. Consider butyl alcohol and ethyl ether. Both have the same formula (C₄H₁₀O), the same molecular weight (74 g/mole) and the same size. Butyl alcohol, however, boils at 118 °C, while diethyl ether boils at 35 °C. The 80 °C difference in boiling points is due to the hydrogen bonding in the butyl alcohol.

Chemistry Fleas

Molecules of water below the surface are symmetrically surrounded by other water molecules. The forces of attraction are the same on one side of the molecule as the other. At the surface, however, the molecules are more attracted to the molecules of water below the surface than to the air molecules above. The result is a domed shaped surface which minimizes the contact of water and air. Since the molecules on the surface are drawn into the liquid, the surface tends to be as small as possible and taut like the rubber of an inflated balloon. This is surface tension.

The phenomenon observed in the demonstration is called the “Marangoli effect.” When there exists a local difference in surface tension on a liquid surface, a surface movement takes place from the lower surface tension region toward the higher surface tension region. The higher surface tension region literally pulls the liquid from the lower surface tension region.

In this demonstration, camphor is a surfactant and dissolves unevenly in water. Water on one side of the crystal contains more dissolved camphor than the opposite side. The side with less camphor dissolved will have a higher surface tension and will pull the camphor in that direction. When the oil from your ear lobe is placed in the water, it forms a thin layer on the surface of the water which prevents the water molecules from dissolving the camphor and thus the crystals stop moving.

References

Special thanks to Mark Langella, Mahopac High School, Mahopac, NY, for providing Flinn with the idea and procedures for these demonstrations.

Special thanks to Penney Sconzo, Westminster Schools, Atlanta, GA, for sharing the Bubble Shapes procedure with Flinn Scientific.

Special thanks to Dr. Frank DeBoer of North Park College, Chicago, IL, for bringing the Vanishing Volume demonstration to our attention.