Opening Day

Introduction

Welcome students to the “wonderful world of chemistry!” The colorful and engaging demonstrations in this “Opening Day” demonstration kit have been specially selected to help students develop observational and critical-thinking skills—and to get them excited about the amazing science that lies ahead.

The set of five demonstrations includes:

1. Let Me Think About That—What was it like to imagine the nature of the atom? Build a “think tube” to help students appreciate the challenges associated with understanding things that cannot be directly observed, such as atoms. Apparently simple manipulations of the tube will have students scratching their heads at the observations—and then scrambling to build a model.
2. Dry Ice Color Show—Add chunks of dry ice to various colored solutions, then watch as the solutions immediately begin to “boil” and change colors. What’s going on here?
3. Mismatched Colors—Dip a red sponge into a red solution and the sponge turns blue! Dip the blue sponge into a blue solution and the sponge turns red! Have students solve this apparent mystery.
4. Colorfully Charged Solution—When the positive and negative leads from a 9-volt battery are placed at opposite ends in a green solution, gas bubbles are observed at each lead and a series of color changes spreads out to produce a rainbow of colors.
5. Rotating Reaction—Most reactions proceed in one direction at a time, forward or backward, but this one “rotates” back and forth. Mixing three colorless solutions produces a yellow solution, which changes to blue, then rotates back to yellow, then to blue, then to back to yellow, and so on. Hard to explain, but fascinating to watch!

Optional worksheets are provided for each demonstration. All five demonstrations can be performed during one class period, or one demonstration can be performed each day of “opening week” in chemistry class!

Concepts

• Models
• Hypothesis
• Sublimation
• Acid–base indicators
• Acid and bases
• pH indicators
• Electrolysis
• Electrodes
• Decomposition of a compound
• Oscillating reactions
• Reaction mechanisms

Background

Let Me Think About That
Start the year off with this exercise in observation and deductive reasoning. The demonstration device consists of a hollow tube with strings and beads coming out of four holes. “Pulling the strings” should show how they are connected inside the tube—but it’s not as easy as it looks! Have students record their observations and then brainstorm to create a working model of the inside of the tube.

Dry Ice Color Show
This eye-catching dry ice demo is sure to get a “cool” reaction from your students! It makes a great demonstration to try at Halloween or any time you want to put on a colorful acid–base show during the year. Just add chunks of dry ice to various acid–base indicator solutions and observe the brilliant color changes. 

Mismatched Colors
Things are not always as they seem. The discrepant event of placing a red sponge in a red solution and having it turn blue is sure to capture your students’ attention. Students will couple their observations from this demonstration with those from the two previous demonstrations to generate possible hypotheses for this bit of chemical “sleight of hand.”

Colorfully Charged Solutions
Demonstrate simple electrolysis in a colorful and dramatic way on an overhead projector. Using a 9-volt battery and a Petri dish filled with a salt solution containing universal indicator, create a spectrum of colors across the entire Petri dish.

Rotating Reaction
Your students will be spellbound when you cap off the “opening day” show with this amazing demonstration. Three colorless solutions are mixed to produce a yellow solution that suddenly turns blue and then yellow again. The solution will oscillate between yellow and blue for several minutes. Forget the worksheet on this one; use the demonstration simply for its entertainment value.

Materials

Safety Precautions

Although the Let Me Think About That demonstration is considered nonhazardous, please follow all normal classroom safety guidelines. Dry ice (solid carbon dioxide) is an extremely cold solid (–78.5 °C) and will cause frostbite. Do not touch dry ice to bare skin; always handle with proper gloves. Household ammonia is slightly toxic by ingestion and inhalation; the vapor is irritating, especially to the eyes. Universal indicator solution contains alcohol and is therefore flammable. Hydrochloric acid is corrosive to skin and eyes and toxic by ingestion and inhalation. Sodium hydroxide solution is corrosive to skin and eyes. Universal indicator is an alcohol-based solution and is flammable; do not use near an open flame. Hydrogen peroxide solution is an oxidizer and a skin and eye irritant. Potassium iodate is an oxidizer; the solution is acidified and contains sulfuric acid. Sulfuric acid is severely corrosive to eyes, skin and other tissue. Starch–malonic acid–manganous sulfate solution is a strong irritant, moderately toxic and corrosive to eyes, skin and respiratory tract. The reaction produces iodine in solution, in suspension and as a vapor above the reaction mixture. The solid iodine is toxic by inhalation. Iodine in solution is irritating to eyes, skin, and respiratory tract. Perform demonstration in well-ventilated room. Avoid all body tissue contact. 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. The Let Me Think About That demonstration tube may be reused from class to class and year to year. All materials in the Dry Ice Color Show may be disposed of according to Flinn Suggested Disposal Method #26b. Extra dry ice may be placed in a well-ventilated area and allowed to sublime. The acid and base solutions in Mismatched Colors can be used several times before they become neutralized or the dyes start to decompose. When ready to dispose, simply mix the two solutions together to neutralize them. Pour the neutral solution down the drain with excess water according to Flinn Suggested Disposal Method #26b. All materials for Colorfully Charged Solutions may be disposed of according to Flinn Suggested Disposal Method #26b. Dispose of the Rotating Reactions reaction mixture according to Flinn Suggested Disposal Method #12a.

Prelab Preparation

Let Me Think About That

1. Cut the nylon string into two 36" pieces.
2. Loop the two nylon cords through the washer as shown in Figure 1. Place the washer in the middle of the tube.
   {12586_Preparation_Figure_1}
3. Pull the ends of the cords through the holes in the tube (see Figure 1).
4. Pull the ends of the cords through the holes of the wooden beads and tie a knot at the end of each cord to hold the beads in place. Place the red bead in the upper left position, the yellow bead in the lower left position, the blue bead in the upper right position and the green bead in the lower right position (see Figure 1).
5. Cover the ends of the tube with the tube caps that have been provided.

Dry Ice Color Show

1. Set five 1-L beakers (or other large transparent containers) in clear view on a demonstration table.
2. Fill each beaker with approximately 750 mL of distilled water (about ¾ full).
3. Using graduated pipets, add 2 mL of indicator to the water in the beakers, in the following order*:
   {12586_Preparation_Table_1}

   *The suggested order shown in the table produces a colorful arrangement of solutions, but any order is okay.

   Each indicator should begin in the basic pH range and change to the acidic range upon addition of CO₂ (dry ice). The basic and acidic colors for each indicator, along with the pH range in which the color transition occurs, are shown in the table.

4. To the beakers containing universal indicator and bromthymol blue, add 1 mL of household ammonia.
5. The indicator solutions should now all be in their basic color range. If they are not, add ammonia dropwise to obtain the basic color as indicated in the table above. Avoid adding excess ammonia or the colors will take too long to change when dry ice is added.
6. Set up reference solutions in the five 100-mL beakers by pouring approximately 70 mL from each large beaker into its corresponding small beaker. Set the reference beakers next to their corresponding large beakers.

Mismatched Colors

Note: This demo works fine with any acid or base concentration above 0.05 M. The procedure works best if the concentrations of the acid and base are similar to one another.

1. Add 100 mL of 1 M hydrochloric acid to a 1000-mL (or larger) beaker. Fill the beaker about ¾ full with tap water.
2. Add enough red food coloring (about 1 mL per 1000 mL solution) to the acid solution in the beaker until it is a deep red color.
3. Add 100 mL of 1 M sodium hydroxide solution to a 1000-mL beaker. Fill the beaker about ¾ full with tap water.
4. Add enough blue food coloring to the basic solution in the beaker until it is a deep blue color.
5. If the sponge is red, wet the sponge with tap water and rinse it out.
6. If the sponge is blue, place the sponge in the blue base solution to convert it to a red color before beginning the demonstration.

Colorfully Charged Solutions

1. Prepare the demonstration solution using the following recipe:

   50 mL universal indicator solution
   250 mL distilled water
   25 g sodium sulfate

2. Place chemicals in a beaker and stir until the sodium sulfate dissolves. Place the mixed solution in a capped container until ready to use. Make the solution the day of the demonstration. Note: This volume of solution (300 mL) is enough to carry out the demonstration as written seven times. If less than 300 mL of solution is needed, decrease the recipe proportionately.

Procedure

Let Me Think About That

1. Pass out Worksheet 1.
2. Have students draw the original positions of the beads for each experiment in the left column of the worksheet.
3. The basic sketch for each manipulation should look similar to Figure 2. (R = Red bead; B = Blue bead; Y = Yellow bead; and G = Green bead)
   {12586_Procedure_Figure_2}
4. In the middle column of the worksheet, students should draw or write what happens in each part of the demonstration.
5. In the right column of the worksheet, students should draw or write a possible hypothesis or explanation for each part of the demonstration.
6. Perform Parts A–C. Be sure to pull each cord until it stops.

Part A

Red and yellow beads on left end appear to be connected by a single string (see Figure 3).

1. Hold the cord attached to the green bead with your right thumb.
2. Pull the red bead.
3. Pull the yellow bead.

Blue and green beads on right end appear to be connected by a single string.

1. Hold the cord attached to the yellow bead with your left thumb.
2. Pull the blue bead.
3. Pull the green bead.

Part B

Red and green beads appear to be connected by a single string (see Figure 4).

1. Hold the cord attached to the yellow bead with your left thumb.
2. Pull the red bead.
3. Pull the green bead.

Part C

Yellow and green beads appear to be connected (see Figure 5).

1. Hold no cords.
2. Pull the yellow bead.
3. Pull the green bead.

Review
Repeat any of the manipulations in Parts A–C and review what was observed.

Review should show that:

Part A. The red and yellow beads appear connected and the blue and green beads also appear connected.
Part B. The red and green beads appear connected.
Part C. The yellow and green beads appear connected.

It should be pretty clear by now that the top and bottom strings on each end are not one single string!

Dry Ice Color Show

1. Use insulated gloves to place a nugget of dry ice on the bottom of an inverted Styrofoam cup.
2. (Optional) Take a burning or glowing splint and place it into the vapor. The CO₂ will extinguish the flame.
3. Have the students record their observations in Worksheet 2.
4. Use insulated gloves to add a nugget of dry ice (about the size of a walnut) to each large beaker of prepared (basic) indicator solution. The dry ice immediately begins to sublime. Vigorous bubbling occurs and a heavy white vapor appears. Shortly afterwards, each indicator solution changes color to its acidic color (see Table 1 in the Prelab Preparation section).
5. Have students make observations about the temperature of the solutions and of the vapor. Have students feel the sides of the beakers. Notice that the vapor is cool (rather than hot) to the touch, as are the water solutions. Explain to the students that “boiling” does not always occur at high temperature—a common misconception—and that the solution is not actually boiling. The solution appears to be boiling because there is such a large temperature difference between the water and the dry ice (see Discussion section).
6. Have the students record their observations in the worksheet.
7. Fill two 100-mL beakers with approximately 75 mL of water.
8. Add 1 mL of phenol red indicator to each beaker. Tell students that this is the indicator in beaker 3.
9. Explain the role of an acid–base indicator. Demonstrate the color change by adding 1 mL of 1 M hydrochloric acid to one beaker and 1 mL of ammonia to the other. The acidic solution color will remain yellow and the basic solution color will change to red. The species responsible for the acidic state is the hydrogen ion, H⁺, and that responsible for the basic state is the hydroxide ion, OH^–.
10. Have students answer the questions on the worksheet.

Mismatched Colors

1. Using tongs or a gloved hand, slowly place the red sponge halfway into the beaker containing the red acid solution. Raise the sponge back out of the solution and notice that the sponge has turned blue. Completely submerge the sponge.
2. Remove the sponge and squeeze out as much liquid as possible back into the acid beaker. Observe that the “rinse water” coming out of the now-blue sponge is red!
3. (Optional) Rinse the sponge in tap water to show that the sponge is actually blue and is not just saturated with a blue solution. This step also minimizes the amount of acid and base that will be transferred between solutions. If most of the liquid has been squeezed out of the sponge (see step 2), this step may not be necessary.
4. Using tongs or a gloved hand, slowly place the blue sponge into the beaker containing the blue base solution. The sponge will immediately turn red!
5. Remove the sponge and squeeze out as much blue base solution as possible back into the blue beaker.
6. Rinse the sponge in tap water, if necessary, to show that the sponge is actually red and it is not just saturated with a red solution.
7. Repeat the demonstration as desired (or requested).
8. Have students record their observations and complete Worksheet 3.

Colorfully Charged Solutions

1. Place the two halves of a Petri dish on the projection stage of an overhead projector.
2. Pour enough demonstration solution into each half of the Petri dish to just cover the bottom of each half dish. Adjust the overhead so that the dishes are in clear focus. Each half dish should appear to be a rich, transparent green color.
3. Break a pencil lead in half. Attach the leads to opposite sides of the Petri dish with the alligator clips. Make sure the tip of each lead is submerged in the green solution and the alligator clips remain out of the solution.
4. To start the demonstration, clip the 9-volt battery into the snaps on the battery clip (see Figure 6).
   {12586_Procedure_Figure_6_Demonstration setup}
5. Let the demonstration run for 5–10 minutes and note the changing colors over time. (A purple color will appear at the cathode very quickly. An orange color at the anode will appear more slowly. Over time, the entire spectrum of universal indicator colors will appear.)
6. Have students record their observations and discuss the results as the demonstration continues. Discuss the various colors as well as why the “extra” dish was included in the demonstration.

Rotating Reaction

1. Using a 50-mL graduated cylinder, measure out 40 mL of 8.6% hydrogen peroxide solution and transfer it to a 250-mL beaker.
2. Using a clean, 50-mL graduated cylinder, measure out 40 mL of the 0.2 M potassium iodate acidified solution and add it to the beaker. Stir, using a stirring rod or magnetic stirrer.
3. Using the third 50-mL graduated cylinder, measure out 40 mL of the starch–malonic acid–manganous sulfate solution. Add this solution to the beaker and stir.
4. Bubbles will begin to appear. In a short period of time, the solution will turn yellow, then blue and finally colorless. The entire process repeats itself over and over again. The yellow to blue to colorless oscillations will continue for about 10 minutes.

Lab Hints

• In the Dry Ice Color Show, the 1-L beakers may be substituted with 400- or 60-mL beakers or 2-L plastic soda bottles with the top third cut off.

Teacher Tips

• The Let Me Think About That demonstration may be done as many times as desired.
• Practice each step in Parts A–C before presenting the entire activity to the students. Some practice will be required to master all of the steps, especially holding the string at the holes without being obvious to students.
• The ends of the cords may be melted with the flame from a match to prevent fraying.
• This kit contains enough chemicals to perform the Dry Ice Color Show demonstration at least seven times: 50 mL of household ammonia needed, 20 mL of 1 M hydrochloric acid, and 20 mL of each of the following indicator solutions—bromcresol green, bromthymol blue, methyl red, phenol red, and universal indicator.
• The indicator solutions in the beakers can be reused from class to class by adding a small amount of household ammonia, dropwise, after the demonstration is complete. Care must be taken not to make the solutions too basic or else the color changes will not occur.
• Plastic soft drink bottles that are cut off at the narrowing point may be used in place of the large beakers.
• Slabs of dry ice can be broken or cracked using a hammer. Wrap the dry ice slab in a towel or place in a zipper-lock bag before striking it with a hammer. Dry ice may be obtained from a local ice cream store or ice company. Look in your local yellow pages under ice or dry ice. Dry ice may also be made using the Dry Ice Maker, Flinn Catalog No. AP4416.
• If the prepared indicator solutions sit in the open air for too long (especially the phenol red), they will begin to change color as CO₂ from the air dissolves in the solution, making it acidic. Adding slightly more ammonia will change the solutions back to their basic colors.
• If distilled or deionized water is not available, use tap water. Be sure to adjust the pH appropriately as some tap water does not have a neutral pH.
• Try other indicators that change color at a pH of near neutral, such as neutral red (yellow to red, 8.0 to 6.8) and bromcresol purple (purple to yellow, 6.8 to 5.2).
• Use the universal indicator overhead color chart (AP5367) to follow pH changes in the universal indicator solution.
• This kit contains enough chemicals to perform the Mismatched Colors demonstration at least seven times. The solutions and the sponge are reusable.
• Food coloring is an excellent dye and will stain fingers and clothing—wear gloves and an apron.
• The concentration of the two solutions are not critical as long as they are above 0.05 M. If the sponge is rinsed out between the acid and the base, then it is not necessary that the two solutions have similar concentrations.
• At a HCl concentration of 0.01 M (pH 2), the sponge will turn blue but will not have the intensity or completeness that the lower pH values give.
• Rinsing the sponge out between each color change will keep the acid and base solutions fresher. It minimizes the amount of acid and base and also the amount of food coloring that is transferred between beakers. Note: The liquid coming out of the sponge is the color of the solution and not the color of the sponge.
• Squeezing out as much solution from the sponge will also keep each solution fresher.
• Rinse the indicator sponge with water prior to use the first time to remove any excess congo red solution.
• This kit contains enough chemicals to perform the Colorfully Charged Solutions demonstration at least seven times: 9-V battery, battery clip with alligator clips, 100 mL universal indicator, 30 g of sodium sulfate, 12 pencil leads and a reusable Petri dish.
• Concepts of pH and electrolysis should be discussed prior to this demonstration. Universal indicator colors, as they relate to pH values, should also be discussed. The Flinn Overhead Color Chart (Catalog No. AP5367) would be a nice visual supplement during this discussion.
• The reactions at the anode and cathode seem to proceed at the same pace. There is bubbling at both the anode and the cathode. A deep purple forms immediately at the cathode and an obvious red/orange at the anode. Both of these color changes occur with similar intensity. Purple, blue, green, yellow and red/orange are all visible after five minutes.
• The demonstration can be repeated using a pH 7 buffer to show the effects of buffering a solution. (When the solution is buffered, no color changes result.)
• The Rotating Reactions demonstration can also be done using 3% hydrogen peroxide, although the color changes will not be as sharp. Therefore an 8–9% solution is recommended for this demonstration.
• A magnetic stirrer can be used to stir the solution throughout the entire demonstration or used to simply mix the solutions well at the beginning and then simply enjoy the oscillations.
• The demonstration can be done in a Petri dish on an overhead projector. Pour equal amounts (about 5 mL) of each solution into a Petri dish. Swirl the solution to mix. The solution will oscillate between yellow and blue for numerous cycles.
• Use only distilled or deionized water. Chloride ions from tap water can contaminate the reaction and stop the oscillations.
• A detailed description of the reaction mechanism for this oscillating reaction my be obtained by calling or writing us at Flinn Scientific. Ask for Publication No. 14127.

Answers to Questions

Let Me Think About That

Dry Ice Color Show

1. Based on your observations of the dry ice on the Styrofoam cup, what are the states of carbon dioxide?

   Carbon dioxide exists in two states, solid and gaseous.

2. What happened in the second demonstration? Include color changes for each beaker.

   There were five beakers set up, each of which contained solutions of different colors. Dry ice was added to the each beaker, and the solutions bubbled vigorously. A thick, white vapor appeared above the beakers, and it looked like the solutions were boiling. The solutions underwent the following color changes:
   Beaker 1 – blue to yellow-green
   Beaker 2 – purple to orange
   Beaker 1 – red to yellow
   Beaker 1 – yellow to red
   Beaker 1 – blue to yellow
   When a burning splint was held above the vapor, the flame was extinguished.

3. Carbon dioxide, like any gas, can dissolve in water. Based on your observations and knowledge of indicators, craft an explanation for the color changes in the beakers.

   Students observed carbon dioxide does not exist as a liquid, only a solid or gas. Indicators have one color in an acidic solution and a different color in a basic solution. From the demo, students know the acid color for phenol red indicator is yellow and the basic color is red. Beaker 3 has this indicator.
   In beaker 3, the solution was initially basic (red). The addition of dry ice caused the solution to gradually turn yellow in color.
   When carbon dioxide gas is dissolved in water, the solution becomes acidic, resulting in the color changes of the solutions.

Mismatched Colors

1. Describe what happened in this demonstration.

   A red sponge was dipped into a red solution. The sponge turned blue, and remained blue even after rinsing with water. The sponge was then dipped in a blue solution and the sponge turned back to red.

2. The sponge surface coating can change color when exposed to different solutions. What have we seen before that acts similarly?

   Indicators can change colors when exposed to basic, then acidic solutions.

3. Based on your observations of this and the previous demonstrations, give an explanation for the observed color changes of the sponge? Propose a method to test your hypothesis.

   The surface of the sponge contains an acid–base indicator that changes from red to blue.
   This can be tested by dipping the sponge in clear acidic and basic solutions. Colors should switch from red to blue.

Colorfully Charged Solution

1. Describe what happened in this demonstration.

   The solution at one side quickly turned purple. Slowly, the solution on the other side turned orange. A variety of other colors appeared at both ends.

2. Indicate the direction of current flow.
   {12586_Answers_Figure_7}
3. Passing an electric current through a water solution breaks down water molecules into hydrogen, H₂, and oxygen, O₂.
   {12586_Answers_Equation_3}
   Hydrogen gas is produced at one electrode, oxygen at the other. Their production occurs simultaneously. As hydrogen is produced by this path:

   4e^– + 4H₂O → 2H₂ + OH^– (where e^– represents an electron)

   Oxygen is produced by this path:

   2H₂O → O₂ + 4H⁺ + 4e^–

   Based on your observations and the use of the indicator chart, at which electrode is hydrogen produced and at which is oxygen being produced?

   Hydrogen is produced at the pencil lead attached to the negative pole of the battery, oxygen at the lead attached to the positive pole.

Discussion

Let Me Think About That
The manipulations presented in Parts A–C of this activity are designed to create discrepancies in the minds of the viewers. In Part A, it appears that the red and the yellow beads are directly connected together and that the blue and green beads are also connected. In Part B, however, the red and green beads seem to be attached to each other. Finally, Part C shows that the yellow and green beads are also attached. Repeat the three parts of this demonstration as many times as necessary for students to develop a working model of the inside of the tube. Some students may be better able to describe in words rather than draw what is occurring. Encourage both written descriptions and drawings to record observations and develop a model.

The goal of this demonstration is to have students hypothesize and develop a possible model of exactly what is happening. The demonstration may be presented as many times as you would like until students fully understand the mechanics behind the demonstration tube. You may choose to reveal the design at the end of the demonstration, or you may decide to keep the secret to yourself—use your discretion!

Dry Ice Color Show
Dry ice is solid carbon dioxide (CO₂). The temperature of dry ice is –78.5 °C (or –109.3 °F), making it extremely cold to the touch. Carbon dioxide is normally found in the gaseous state, making up approximately 0.04% of our atmosphere. It is a colorless, odorless, noncombustible gas with a faint acid taste. Dry ice is made by cooling atmospheric air and compressing the solid into desired forms (e.g., blocks, nuggets, pucks). The different gases that make up atmospheric air (e.g., nitrogen, oxygen) condense at different temperatures, and therefore may be easily separated. Carbon dioxide forms a frosty, white solid at –78.5 °C. As a solid, carbon dioxide can cause frostbite on contact with skin and will stick to moist tissue (such as wet skin or your tongue). Solid carbon dioxide undergoes sublimation upon exposure to air. This means it transforms directly from the solid phase to the gaseous phase without melting to a liquid.

When dry ice is placed in water (as in this demonstration), it sublimes rapidly since the water is so much warmer than the dry ice. The solution appears to boil. As the dry ice sublimes to gaseous CO₂, some of the gas bubbles away quickly and some dissolves in the water. A heavy white cloud of condensed water vapor forms above the liquid (due to the coldness of the escaping CO₂ gas). As the CO₂ gas dissolves in the water, the solution becomes more acidic due to the production of carbonic acid (H₂CO₃), a weak acid, according to the following equation:

The indicators change to their acidic forms as the pH levels of the solutions drop, producing a color change. The time required for the change to occur depends on the initial pH of the solution, the transition point of the indicator, and how much dry ice was added to the solution.

Mismatched Colors
The indicator sponge is saturated with congo red solution. Congo red is a dye, a biological stain, and an acid–base indicator. It has been used as a direct fabric dye for cotton to produce a bright red fabric. Biologists use Congo red as a general contrast stain for cellulose. Congo red is also used as a pH indicator. The color transition is between pH 3.0 and 5.0. Below a pH of 3.0 (very acidic solutions), the indicator is blue. Above pH 5.0, the indicator is red.

When a cellulose sponge is soaked in a Congo red solution, the dye becomes permanently bonded to the cellulose fibers. The active functional groups responsible for the acid–base properties of Congo red are still available and the sponge becomes an “indicator sponge” for acids. It can also be used to check for acid spills on counters after students have used acids. Simply wipe down the work area using the indicator sponge. If it turns blue, the students did not use safe laboratory procedures.

Colorfully Charged Solutions
When an electric current is passed through an aqueous solution containing an electrolyte (NaSO₄), the water molecules break apart or decompose into their constituent elements, hydrogen and oxygen. The overall reaction occurs as two separate, independent half-reactions. Reduction of the hydrogen atoms in water to elemental hydrogen (H₂) occurs at the negative electrode. (–), while oxidation of the oxygen atoms in water to elemental oxygen (O₂) occurs at the anode (+). Each half-reaction is accompanied by the production of OH^– or H⁺ ions as shown:

Negative electrode: 4e^– + 4H₂O → 2H₂(g) + 4OH^–
Positive electrode: 2H₂O → O₂(g) + 4H⁺ + 4e^–

Excess OH^– ions produced at the negative electrode will cause the pH to increase, resulting in a color change of the universal indicator solution from green (neutral, pH 7) to purple (basic, pH ≥ 10).

Excess H⁺ ions produce at the positive electrode will cause the pH to decrease, resulting in a color change of the universal indicator solution from green to an orange/red color (acidic, pH ≤ 4). The electrolysis half-reactions can also be followed by observing the production of gas bubbles at the negative electrode (H₂) and positive electrode (O₂).

Universal indicator is an acid–base indicator that is different colors at different pH values. All colors will be visible in the Petri dish as electrolysis progresses and as the pH conditions continually change due to diffusion and neutralization. Rotating Reactions
This oscillating reaction is known as the Briggs-Rauscher (BR) Reaction and was developed by Thomas S. Briggs and Warren C. Rauscher of Galileo High School in San Francisco. The reaction mechanism is very complex. During the reaction, oscillations occur in the concentration of iodine and iodide ions. The yellowish color is attributed to the rise in I₂ concentration; the blue-black color of the starch–iodine complex results from the rise in both I^– and I₂ concentrations. The colorless solution is caused by the decline in I₂ concentration and the continued rise in I^– concentration.

The blue-black starch–iodine complex is amylose-iodine. Amylose is the linear starch fraction which is composed of chains of 1,4 linked α-glucose units as shown in in Figure 8. The color of the complex, blue-black, comes from the pentaiodide anion, I₅^– formed when I₂ and I^– concentration are elevated. Though normally an unstable anion, it becomes stable as a part of the starch complex. The overall BR reaction is:

IO₃^–(aq) + 2H₂O₂(aq) + CH₂(CO₂H)₂(aq) + H⁺(aq) → ICH(CO₂H)₂(aq) + 2O₂(aq) + 3H₂O(l)

This reaction consists of two component reactions that create an intermediate molecule HOI. The two reactions are themselves very complex, consisting of ten steps. Iodine (I₂) and iodide ions (I^–) are produced as intermediates in various steps of these reactions.

In the proposed reaction mechanism, the concentration of HOI rises and falls, triggering oscillations in the I^– and I₂ concentrations in solution. When I₂ and I^– concentrations are high, the solution is blue; when I₂ is high and I^– is low, the solution is yellow; and when I₂ is low and I^– is high, the solution is clear. The oscillations continue until either malonic acid or iodate ions are consumed.

References

Special thanks to Robert Lewis, Downers Grove North High School, Downers Grove, IL, and Jeff Hepburn, Dowling High School, West Des Moines, IA, for providing the idea and instructions for the Let Me Think About That activity.

Flinn Scientific would like to thank Lee Marek, Chemistry teacher (retired), Naperville North HS, Naperville, IL, for bringing the Dry Ice Color Show demonstration to our attention to share with other teachers.

Special thanks to Mike Shaw, West Stokes High School, King, NC, for the Colorfully Charged Solutions demonstration idea.