Pour a Rainbow

Introduction

It’s not just a clever title—a rainbow of colors can actually be poured from beaker to beaker while students spontaneously erupt in a chorus of ooohs and aaahs! This multi-colored, gravity-defying, attention-getting goo will teach your students about polymerization, hydrogen bonding, solubility, acid/base reactions, pH indicators and a series dilution.

Concepts

• Polymers
• Acid–base indicators
• Hydrogen bonding

Background

Polyox water-soluble resins are specialty polymers that have a variety of uses, including paper manufacture and controlled-release drug formulations. As thickeners and lubricants, they have improved personal care products, paints, inks, building materials, textiles and ceramics. As thermoplastics, they add unique qualities, such as water solubility, degradability and lubricity to plastic products. Polyox is commonly used as a bonding agent in strong-holding denture adhesives like Sea Bond^®.

Polyethylene oxide is a non-toxic, long chain, water-soluble polymer. What makes this water-based polymer so special is its high molecular weight of approximately 4,000,000. The molecular structure of Polyox promotes extensive hydrogen bonding, which allows it to be soluble in water despite the high molecular weight. It has been determined that the length of the polymer chains in a single drop of the gel is approximately 1,700,000 miles long (give or take a few). Over the years, Polyox gained popularity by firefighters, who added very small amounts of the polymer to the water supply in fire storage “pumper” trucks. In theory, the addition of the Polyox reduced the friction caused by the water molecules rubbing against the inside wall of the hose. The reduction of friction made the water flow more quickly through the hose.

In order to illustrate the molecular structure of Polyox, it might be helpful to picture a bowl of spaghetti all tangled up. The spaghetti-like structure causes the polymer to thicken water and provides a strong elastic effect. Although extremely elastic, the Polyox remains fluid, like pancake syrup. The straight chain format of the Polyox molecule with no side chains to attach to other molecule strands allows the separate chains to slide past each other and remain fluid.

The magic of Polyox as a chemical demonstration was originally popularized by Alan McCormick in the 1960s and revitalized by Walter Rohr in the later 1990s. When a small amount of Polyox (polyethylene oxide) is mixed with methyl alcohol and water is added, the Polyox dissolves forming a thick, slippery, gooey, mucous-like gel. Oh, the visual imagery! When the gel is poured back and forth between two beakers, it mysteriously siphons from the higher-held beaker to the lower one. It’s a great demonstration of hydrogen bonding!

Materials

Safety Precautions

Methyl alcohol is extremely flammable and toxic by ingestion and inhalation. Always use caution when handling methyl alcohol or any other flammable solvent. Polyox has a very low level of toxicity and is not easily ingested due to its high molecular weight. The resin is neither a skin irritant, a sensitizer, nor does it cause eye irritation as the dry powder or in solution. Yet, as always, wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Since the gel is 99% water, it can be disposed of in the solid waste disposal or down the drain according to Flinn Suggested Disposal Method #26.

Procedure

Part I. Mastering the Self-Siphoning Action

1. Place approximately 3.0–3.5 grams of Polyox into a clean, dry 500-mL beaker.
2. Add about 25 mL of methyl alcohol to the Polyox. Swirl the beaker to wet the polymer with alcohol.
3. Fill a second 500-mL beaker with approximately 300 mL of water. Add 20 mL of universal indicator solution to the water.
4. Pour the water mixture into the beaker with the Polyox in one controlled, swift motion. If the solution is poured too slowly, the polymer and the water will “gel-block” (clump up), and if the solution is poured too quickly, the mixture is likely to spill everywhere. Note: This part takes some practice.
5. Mix the water and Polyox by pouring the liquid from beaker to beaker until the liquid begins to thicken. Don’t attempt the self-siphoning activity described below just yet. Continue to pour the liquid from beaker to beaker to mix it thoroughly. After just a few minutes of mixing, the mucous-like gel flows very quickly from beaker to beaker. Hold the empty beaker below the full one and start to pour a little of the liquid into the lower beaker. What happens? The liquid will siphon itself from the top beaker to the bottom beaker. Notice how the liquid appears to defy gravity as it crawls up and over the sides of the full beaker in a “self-siphoning” kind of action.

Part II. Adding a Twist of Color

6. After the self-siphoning effect of Polyox has been observed, it’s time to add a twist of color. With all of the Polyox from Part I in one beaker, add 1 mL of 1 M HCl to the empty beaker. Pour the Polyox mixture into this beaker. Notice how part of the gel starts to change color.
7. Add 1 mL of 1 M NaOH to the other empty beaker. Pour the Polyox mixture back into the beaker with the NaOH. Note: The more the liquid is poured from beaker to beaker, the more pronounced the color changes will be. The beauty of the demonstration lies in the fact that only a portion of the liquid turns red while the rest stays green–blue. Over time, however, strands of yellow and orange will also appear.
8. Pour the Polyox mixture back and forth between the beakers several times to start the mixing. Begin to slowly pour the mixture back and forth, allowing the liquid to defy gravity and siphon itself from beaker to beaker. Notice that parts of the liquid begin to turn blue and purple. Due to the high viscosity of the liquid, the color change is gradual from red to yellow to green to blue to purple—and a multitude of colors in between. It is possible to see as many as five individual strands of color in a single pour.
9. If equal amounts of acid and base are added, the liquid will eventually return to its original green color and steps 6 and 7 can be repeated over and over.

Student Worksheet PDF

11896_Student1.pdf

Teacher Tips

• Enough chemicals are provided in this kit to perform the demonstration seven times: 25 grams of Polyox, 200 mL of methyl alcohol, 200 mL of universal indicator solution, 30 mL of 1 M hydrochloric acid solution and 30 mL of 1 M sodium hydroxide solution.

• The alcohol acts as a dispersant to separate the polymer particles and to inhibit the formation of large, insoluble lumps.
• Distilled or deionized water can be used, although tap water works just as well. Being nonionic, the polyethylene oxide gel is not affected by the minerals in ordinary tap water.
• Avoid pouring the polyethylene oxide over carpeting or getting the gel on clothing as it is difficult to clean out of fibers.
• Polyox is an unusual polymer that exhibits some strange behaviors. Practice the demonstration. There is an art to pouring Polyox, especially when the mixture changes color at the same time. Consider the first attempt mere practice. It might take two or three times to really get the hang of it. Note that the measurements provided in the procedure serve as recommendations; exact amounts are not crucial to the success of this demonstration.

  Presentation

• If the Polyox mixture stands for 24 hours, the cloudiness of the liquid will disappear and the colors will appear more vivid.
• Position the beakers directly over a strong light source to illuminate the liquid and intensify the visibility of the color changes.
• After the HCl and NaOH have been added, pour the liquid down a piece of white plastic to better highlight the colors. The individual colors are much easier to see against the white background.

Answers to Questions

1. Describe the appearance and behavior of the polyethylene oxide.

The polyethylene oxide is a thick gel. When poured from one beaker to another, it can siphon up the side of the first beaker, working against gravity, and then form a thick strand that falls to the beaker waiting below.

2. The long polymer chains in polyethylene oxide, which are already extensively bonded to each other, will form even more hydrogen bonds with surrounding water molecules. In this state it resembles intertwined spaghetti. How do you think this contributes to its gel-like appearance and self-siphoning behavior?

The large number of hydrogen bonds among the polymer chains and between the chains and water molecules causes it to form a thick gel rather than a free-flowing liquid. And since these long chain molecules are interconnected, they can stretch out and form more hydrogen bonds with each other, allowing them to be pulled together against gravity.

3. What is a polymer?

A polymer is a large molecule, usually in the shape of a chain, composed of many smaller molecules called monomers.

4. Universal indicator solution had been added to the water before the Polyox solution was poured into. Explain the color changes that occurred when hydrochloric acid was added to one beaker and sodium hydroxide was added to the other.

The color changes are due to the different pH levels tin the gel as it is poured from beaker to beaker, as shown by the universal indicator. Because the gel is so thick, neither the hydrochloric acid nor the sodium hydroxide spreads quickly, and the substance is different pH levels in different places. The pH change occurs slowly throughout the Polyox, resulting in the slow color changes from red to orange, yellow, green, blue and purple.

Discussion

The Chemistry Behind the Rainbow (as told by Steve Spangler)

The inspiration to develop a color-changing Polyox demonstration came about after seeing Walter Rohr perform his famous “liquid light” activity using Polyox. The original intention was to simply color the gel green using food coloring, but none was to be found in the lab. Acting out of desperation, Universal Indicator (green in its neutral state) was used in place of food coloring. Voilà! Green Polyox. Then the “idea light” came on. Would the self-siphoning gel change color if the pH changed? Hmmm?

Universal indicator is used as the dye to color the water. When the Universal indicator/water solution is added to the mixture of Polyox and wetting agent (dry alcohol), the resulting viscous gel will be green with a pH around 7. The addition of the 1 M HCl causes the liquid to gradually turn acidic and the indicator eventually turns from green to yellow to orange to red. Because of the thickness of the gel, however, the HCl concentration might range from 1 x 10^–1 M at the bottom of the beaker to a concentration of 1 x 10^–3 M at the top of the beaker. The color change is much slower than expected and is reminiscent of the effects of a series dilution with HCl all within a single beaker of liquid. Similarly, the addition of an equal amount of 1M NaOH will make the solution basic causing the gel to slowly change from red to orange, yellow, green, blue, and then purple as it is poured from beaker to beaker.

References

Special thanks to Steve Spangler from Regis University in Denver, CO, for sharing this original idea with Flinn.