A Christmas of Chemistry

Introduction

Make the holiday season joyous with seven demonstrations guaranteed to delight and enlighten the entire class.

This set of seven demonstrations includes:

1. Winter Wonderland—An aqueous saturated solution is mixed with a less polar solvent, isopropyl alcohol. This causes the precipitation in the mixing layer, creating “snowflakes” that drift through the bottom aqueous layer. A short demonstration, five minutes, with preparation and set up done an hour beforehand.
2. Christmas Tree Forest—Cut out paper trees are placed in a solution of various dissolved ionic compounds. The trees are set aside and overnight all the solvent has evaporated and a beautiful “snow”-covered tree is produced. A class participation demonstration that should take no more than twenty minutes.
3. Cozy Holiday Fire—Mix alcohol with a solution to produce a flammable gel. Sprinkle two compounds on the gel, ignite, and red and green flames result. The gel can be made ahead of time and stored until the demonstration.
4. Santa in a Snowstorm—A solution is heated and poured into a jar with a holiday figurine. When the solution cools, “snow-like” precipitation forms and a holiday figurine is engulfed in a “blizzard.” This demonstration can be prepared once ahead of time and demonstrated over and over again.
5. The Littlest Christmas Tree—Cut out a small triangle of copper and place it on a slide under a microscope. When a drop of silver chloride is added, fine needle-like crystals radiate from all three sides of the copper. The class can view this demonstration one at a time or, using a microscope video camera and monitor, all together.
6. Blinking Holiday Colors—Add different indicators to two solutions and turn them holiday green and red. In less than a minute, they suddenly switch colors, red goes green and green goes red. Preparation of solutions for this demonstration should be done ahead of time.
7. Invisible Christmas Message—Take two indicator solutions and write a greeting with them on paper. Once dried, the message is invisible. Spray the paper with a dilute basic solution and the greeting is revealed in red and green.

This demonstration kit can be presented in a variety of ways. An entire class period can be used to present all seven, one demonstration can be done for each day of Christmas or the demonstrations can be done over several class sessions.

Concepts

• Solubility
• Precipitation
• Solvation
• Crystal formation
• Transpiration
• Gels
• Atomic emission
• Saturation
• Oxidation–reduction reaction
• Crystallization
• Clock reaction
• pH indicators
• Buffers
• pH
• Acid and bases
• Indicators

Experiment Overview

Demonstration I. Winter Wonderland: What would the holidays be without a beautiful blanket of snowflakes? Add two clear liquids together and create a snowfall of flaky, white precipitate.

Demonstration II. Christmas Tree Forest: Now that the snow has fallen, have the students create a forest of delicate, snow-covered pines.

Demonstration III. Cozy Holiday Fire: As the temperature drops, a bright cheery fire is a must. In place of logs, use a colorful display of green and red flames. Create a gel, that when lit and sprinkled with crystals, burns the bright colors of the Christmas holiday!

Demonstration IV. Santa in a Snowstorm: Create a giant “Santa in a Snowstorm” scene while demonstrating solubility principles.

Demonstration V. The Littlest Christmas Tree: It is now time to put up the tree. Add a drop of clear solution to a microscope slide containing a tiny piece of copper and create a beautiful, silver-branched Christmas tree that grows before your very eyes!

Demonstration VI. Blinking Holiday Colors: A green solution and red solution are prepared and then suddenly blink and switch colors after 30 seconds.

Demonstration VII. Invisible Christmas Message: Write a holiday message on blotting paper with two liquids. Once dried the message is invisible. As a fitting end to this series of demonstrations, spray the blotting paper with a dilute base solution and reveal your special holiday greeting.

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid and a fire hazard. It is slightly toxic by ingestion and inhalation. Household ammonia is slightly toxic by ingestion and inhalation; both liquid and vapor are extremely irritating especially to the eyes. Use caution when handling both laundry bluing and the crystal growing solution to prevent spilling on clothes. Ethyl alcohol is a flammable liquid and a dangerous fire risk—an ABC class fire extinguisher should be close by while conducting the demonstration. Addition of denaturant makes ethyl alcohol poisonous—it cannot be made non-poisonous. Lithium chloride is moderately toxic by ingestion. Boric acid is slightly toxic by ingestion and an irritant to skin in dry form. Benzoic acid is moderately toxic by ingestion. Avoid contact with skin, eyes, clothing and respiratory tract as it is a severe irritant. Use exhaust ventilation to keep airborne concentrations low. Keep the bottle capped to avoid breathing the vapors. The silver nitrate solution is moderately toxic by ingestion, irritating to body tissues. Avoid all body tissue contact. Silver nitrate solution will stain skin and clothing. Formaldehyde is an alleged carcinogen; however, recent studies indicate no significant risk of cancer from low level exposure to formaldehyde. Formaldehyde is a strong irritant; avoid breathing vapor and avoid skin contact. Formaldehyde is highly toxic by ingestion, inhalation and skin absorption. The use of formaldehyde in this demonstration does not present an unnecessary risk. Use a fume hood to prepare solution. Sodium sulfite is moderately toxic; possible skin irritant. Sodium bisulfite is slightly toxic; a severe irritant to skin and tissue as an aqueous solution. The indicator solutions contain ethyl alcohol; moderately toxic by ingestion. Avoid body tissue contact with all these chemicals. Sodium hydroxide solution (0.1 M) is a body tissue irritant. Avoid exposure to eyes and skin. 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. Isopropyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18a. Potassium sulfate, the trees, crystals, sodium chloride, lithium chloride, calcium acetate, the slide and the reaction products for The Littlest Christmas Tree, sodium bisulfite and the blotting paper with message may be disposed of according to Flinn Suggested Disposal Method #26a. The final snow mixture, laundry bluing solution, ammonia solution, ethyl alcohol, leftover gel, silver nitrate and the reaction products of Blinking Holiday Colors may be disposed of according to Flinn Suggested Disposal Method #26b. Boric acid may be disposed of according to Flinn Suggested Disposal Method #24a. The benzoic acid solution should be neutralized, then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24a. Formaldehyde may be disposed of according to Flinn Suggested Disposal Method #2. Sodium sulfite may be disposed of according to Flinn Suggested Disposal Method #12b. The indicator solutions may be disposed of according to Flinn Suggested Disposal Method #18b. The sodium hydroxide solution may be disposed of according to Flinn Suggested Disposal Method #10.

Prelab Preparation

Demonstration I. Winter Wonderland

Prepare a saturated solution of potassium sulfate. Weigh out 30 g of potassium sulfate (K₂SO₄) and transfer to a 400-mL beaker. Add 200 mL of distilled or deionized water and stir for 30 minutes. All the potassium sulfate may not dissolve. Decant off the solution.

Demonstration II. Christmas Tree Forest

1. Distribute two sheets of blotting paper with printed trees and have students cut out the 20 tree silhouettes.
2. Make 10 pairs of trees by placing together one tree silhouette with a marked 1½" line at top and one with 1½" line at bottom (see Figure 6).

3. Cut the 1½" line on each tree silhouette. Slide the tree silhouette with bottom cut over the one with a top cut to form a three-dimensional tree with four sides at 90° angles (see Figure 7).

4. (Optional) Add a drop of food coloring to the tips of each branch of the trees. This will add color to the crystals that form.

Demonstration III. Cozy Holiday Fire

Clean and rinse all glassware with deionized or distilled water. Sodium ions in tap water will give the flame a yellow tint and mask the intensities of the red and green colors.

Demonstration VI. Blinking Holiday Colors

Formaldehyde solution: Measure out 10 mL of 37% formaldehyde in a 10-mL graduated cylinder and transfer to a 500-mL Erlenmeyer flask. Fill to 500-mL mark with deionized or distilled water. Stir to mix.

Sulfite/bisulfite solution: Prepare this solution within 24 hours of use. Dissolve 3.2 g of sodium sulfite and 10.5 g of sodium bisulfite in another 500-mL Erlenmeyer flask containing about 300 mL of distilled or deionized water. Fill to 500-mL mark with deionized water. Stir to mix.

Procedure

Demonstration I. Winter Wonderland

1. Fill the 500-mL tall-form beaker or hydrometer cylinder with the 200 mL of saturated potassium sulfate solution.
2. Carefully add 100 mL of isopropyl alcohol to the saturated potassium sulfate solution.
3. A “cloud” of potassium sulfate (K₂SO₄) forms immediately and a snowfall begins as the potassium sulfate precipitates out and drifts to the bottom of the container.

Demonstration II. Christmas Tree Forest

1. Measure out 180 g (9 tablespoons) of sodium chloride and add to a 600-mL beaker.
2.  Add 140 mL (9 tablespoons) of hot deionized or distilled water to the beaker. Stir to dissolve the salt.
3. Carefully add 140 mL (9 tablespoons) of laundry bluing and 25 mL (1.5 tablespoons) of household ammonia. Stir.
4. Pour 30 mL of the solution into each Petri dish. The cover as well as the Petri dish can be used in this demonstration.
5. Place the Petri dishes in a place where they will not be disturbed. Place a tree in each Petri dish. The crystals will start to form in 1–12 hours, depending on the rate of evaporation. Do not move or touch the trees once the crystals start to form. The crystals are very fragile.

Demonstration III. Cozy Holiday Fire

1. Weigh out 3 g of calcium acetate in a 250-mL beaker.
2. Measure out 10 mL of deionized or distilled water and add it to the calcium acetate. Stir to dissolve most of the solid (not all of the solid will dissolve).
3. Measure 75 mL of ethyl alcohol using a graduated cylinder and transfer it to the beaker containing the calcium acetate mixture. A gel will form immediately as the alcohol is added. Do not stir the mixture.
4. Using a spatula, transfer the gel equally to two separate Pyrex evaporating dishes or Pyrex Petri dishes.
5. Weigh out 10 g of boric acid and sprinkle it evenly over the gel in one dish.
6. Weigh out 10 g of lithium chloride and sprinkle this evenly over the gel in the second dish.
7. Place each dish on a nonflammable ceramic fiber square or tile and away from combustible materials.
8. Use a match or butane safety lighter to carefully light the gels. Turn off the lights to make the flames more noticeable. The lithium chloride dish will burn a brilliant red and the boric acid dish a bright green. Notice the formation of calcium carbonate on the inside of each dish.
9. The dishes will get extremely hot! Use caution and do not handle the dishes until they have completely cooled.
10. Wash hands, work area and equipment thoroughly when finished.

Demonstration IV. Santa in a Snowstorm

1. Slowly heat about 1.8 L of tap water in a 2-L beaker using a hot plate. Do not allow the water to boil.
2. As the water is heating, add the 20 g of benzoic acid supplied with the kit. Continue to heat and stir the mixture until the benzoic acid completely dissolves. Do not boil the solution.
3. Remove the beaker from the hot plate and allow the solution to cool.
4. Thoroughly clean and dry the 3-L plastic bottle.
5. Glue the holiday figurine on the inside of the lid of the 3-L plastic bottle using a hot glue gun. Put some hot glue on the bottom of the figurine and immediately place the figurine in the center of the lid. Hold it in place for a few seconds. Allow the glue to cool for several minutes. Apply an extra line of glue around each foot for more support.
6. Observe the benzoic acid solution as it begins to cool. Snowy-looking crystals will appear.
7. After the solution has completely cooled to room temperature and the figurine is secure to the lid, stir the snow mixture, and then quickly pour it into the 3-L bottle.
8. Slowly fill the bottle with additional tap water. Leave a 1- to 2-inch gap of air in the bottle.
9. Cap the bottle tightly. The snow scene should now be able to be turned upside down and the holiday figurine will stand in the snowstorm.
10. If desired, place some electrical tape around the bottle lid to seal it completely.
11. If the holiday figurine comes loose, it can be repaired by opening the bottle, thoroughly drying the lid and the holiday figurine and regluing. Replace the lid, adding a bit more tap water, if necessary.

Demonstration V. The Littlest Christmas Tree

1. To make the small tree, cut out a small triangle, about 2-mm tall and 0.5-mm wide, from the copper strip (see Figure 1).

2. Center the copper tree on a microscope slide and place the cover slip over it (see Figure 2).

3. Bring the tree into focus. Set the magnification so that the tree is centered and does not fill the entire field (see Figure 3).

4. Have each student view the tree or optionally, set up the video camera and display the tree on the monitor.
5. Now add 1 drop of 0.3 M silver nitrate (AgNO3) along the side of the cover slip. (see Figure 4). The solution is wicked under the cover slip and when it reaches the copper tree, silver dendritic crystals will start to grow from the edges of the copper.

6. As they grow outward, the silver crystals resemble pine tree branches and needles (see Figure 5).

7. Move the slide slightly and change magnification to follow the growth of these beautiful crystals.

Demonstration VI. Blinking Holiday Colors

1. Place the two hydrometer cylinders or tall-form beakers on display. Put a 400-mL beaker, with a stirring rod, behind each.
2. Use a 250-mL graduated cylinder to transfer 250 mL of the dilute formaldehyde solution into each 600-mL cylinder or beaker.
3. Rinse the 250-mL graduated cylinder with distilled or deionized water and use it to transfer 250 mL of the sulfite/bisulfite solution to each of the 400-mL beakers.
4. Use a 10-mL graduated cylinder to transfer 5 mL of the “green-to-red” indicator solution to the first 400-mL beaker. Stir to mix. The solution will be green.
5. Rinse the 10-mL graduated cylinder with distilled or deionized water and use it to transfer 5-mL of the “red-to-green” indicator solution to the second 400-mL beaker. Stir to mix. This solution will be bright red.
6. Quickly pour the contents of each 400-mL beaker into the 600-mL cylinder or beaker in front of it. Stir each solution.
7. In 20 to 30 seconds, the first cylinder or beaker will suddenly turn bright red, with the second one switching to green.

Demonstration VII. Invisible Christmas Message

1. Pour approximately 10 mL of each indicator into separate 50-mL beakers.
2. Use a cotton-tipped applicator to write a message on the blotting paper, using a separate applicator for each color.
3. Allow the message to dry and fade completely.
4. Affix the blotting paper over the demonstration tray. Place 10 mL of 0.2 M NaOH solution in a spray bottle. Add 40 mL of deionized water to the spray bottle and mix.
5. When ready to display the message, spray the blotting paper with the 0.04 M NaOH solution and reveal your red and green wish of holiday good cheer!

Teacher Tips

• This kit contains enough chemicals to perform the Winter Wonderland demonstration as written seven times: 210 g of K₂SO₄ and 700 mL of 70% isopropyl alcohol.

• Gently stirring the middle and lower layers will mix more isopropyl alcohol and saturated K₂SO₄ solution and create a “lower altitude” cloud for another snowstorm.
• This kit contains enough chemicals to perform the Christmas Tree Forest demonstration as written seven times: 1 L of laundry bluing, 200 mL of ammonia solution, 1300 g of sodium chloride and 14 sheets of blotting paper.
• Using hot water helps the salt dissolve faster and also allows for more rapid crystal formation.
• Use proper decanting techniques to prevent spilling and possible staining by the bluing solution.
• All the sodium chloride may not dissolve. The excess can be left in the 600-mL beaker.
• This kit contains enough chemicals to perform the Cozy Holiday Fire demonstration as written seven times: 70 g of H₃BO₃, 70 g of LiCl, 25 g of Ca(C₂H₃O₂)₂ and 500 mL of 95% ethyl alcohol.
• Be sure to use only borosilicate (e.g., Pyrex^®) glassware to prevent the glass from shattering or melting.
• Ethyl alcohol burns with a slight yellow-orange flame. This color may flicker as the red and green flames are produced.
• Do not add lithium chloride or boric acid to the calcium acetate solution. This will prevent the gel from forming.
• For Santa in a Snowstorm, a snowman or other holiday figurine may be substituted for Santa depending on availability.
• Encourage students to bring in their own figurines (whatever they find interesting) to add alongside (or in place of) the included figurine. Solid plastic figurines work the best. You may consider purchasing inexpensive figurines at a local discount or craft store.
• Student-size models may be made following the same procedure. “Microscale” the demonstration down to 1 gram of benzoic acid and 75 mL of water. Use a 4-oz ointment bottle (Flinn Catalog No. AP8445) or a baby food jar. Fill the bottle to the brim with tap water.
• Do not try to speed up the cooling process of the supersaturated solution by using ice; beautiful crystals form when the benzoic acid is allowed to crystallize slowly.
• If the solution is still hot when added to the jar, the figurine will have a good chance of coming unglued and falling off. Be patient.
• If your tap water is very hard, deionized water may be used.
• Only one figurine and bottle is included. Prepare ahead of time.
• This kit contains enough chemicals to perform The Littlest Christmas Tree demonstration more than seven times: 20 mL 0.3 M AgNO₃; 1.2 cm x 15 cm copper strip.
• Use caution when cutting the copper strip to avoid any sharp edges or metal slivers.
• Switching from a back-lit image to a top-lit image will highlight the shiny silver needles.
• This kit contains enough chemicals to perform the Blinking Holiday Colors demonstration more than seven times: 70 mL of 37% formaldehyde, 25‑g of sodium sulfite, 80 g of sodium bisulfite, 40 mL of red-to-green indicator solution and 40 mL of green-to-red indicator solution.
• The sulfite/bisulfite solution must be fresh to avoid having both ions oxidized by oxygen in the air. This reduces the amount of each ion in solution and also produces sulfuric acid, which changes the pH of the initial solution and interferes with the clock time and color changes.
• The green colors are not Kelly green; the reds may have an orange tint.
• This kit contains enough chemicals to perform the Invisible Christmas Message demonstration as written seven times: 70 mL of each indicator solution, 80 mL of 0.2 M NaOH and 10 sheets of blotting paper.
• Set up a special developing area for spraying the sodium hydroxide solution. This can be done over the demonstration tray to contain the sprayed mist. Always spray in a direction away from others in the lab. For good housekeeping it is important to clean up any residual basic solution from the work area after the demonstration is over.
• After 3–5 minutes, the green color will fade to yellow. This is due to the reaction of CO₂ with moisture in the air to form carbonic acid (H₂CO₃)—the absorption of this weak acid reduces the pH enough to change one of the indicators in the green color, thymolphthalein, from blue to clear. Subtracting blue from green leaves the words with a yellow tint. Spraying the paper again with the 0.1 M NaOH solution restores the green color.

Discussion

Demonstration I. Winter Wonderland

Isopropyl alcohol and water are miscible—mutually soluble in all proportions. This miscibility is due to the similar polarity and hydrogen bonding characteristics of water and isopropyl alcohol. Isopropyl alcohol is polar but, because of its organic portion, less polar than water and therefore less able to dissolve ions. When isopropyl alcohol is added to an aqueous salt solution, the polarity of the resulting mixture decreases and reduces, in turn, the solubility of the salt ions in solution.

A saturated potassium sulfate solution already contains the maximum concentration of potassium and sulfate ions. In this demonstration, the polarity of the solution changes when isopropyl alcohol is added and the concentration of these ions that can be solvated and kept in solution dramatically decreases. The result is a loss of solvation and the formation of potassium sulfate precipitate.

If the isopropyl alcohol is added slowly, three layers are formed in the mixture: an upper isopropyl alcohol layer, a middle layer of isopropyl alcohol and K₂SO₄ solution and a saturated K₂SO₄ lower layer. A cloud of unsolvated K₂SO₄ immediately forms in the middle layer due to the decrease in polarity and loss of solvation. K₂SO₄ particles formed in this layer grow larger and heavier, before they begin to slowly fall through the isopropyl alcohol–water interface into the saturated K₂SO₄ layer. After all the precipitated K₂SO₄ has fallen from the upper layers, the mixture can be stirred to mix more saturated K₂SO₄ solution with the isopropyl alcohol and precipitate more K₂SO₄.

Demonstration II. Christmas Tree Forest

The porous blotting paper allows for both capillary action and evaporation. The solution rises through the blotting paper due to capillary action and the solvent slowly evaporates, beginning at the top edges and tips of the tree branches. As the solvent evaporates, crystals begin to form. Like tree and plant transpiration, the evaporation causes more solution to be pulled up through the tree to the edges, where further evaporation leads to a build-up of more crystal deposits.

The bluing solution is composed of a colloidal dispersion of the blue pigment prussian blue (Fe₄[Fe(CN₆)]₃•xH₂O). This is combined with an ammonia solution and a concentrated solution of sodium chloride. The fluffy white crystals that form are believed to be ammonium chloride and potassium sodium ferrous ferrocyanide [KNaFe₂(CN)₆]. The KNaFe₂(CN)₆ results from the breakdown of the prussian blue crystals by ammonia in solution.

Demonstration III. Cozy Holiday Fire

Mixing a concentrated solution of calcium acetate with ethyl alcohol produces a flammable gel, also known as canned heat or Sterno^®. As the ethyl alcohol and calcium acetate solution are mixed, the calcium acetate immediately precipitates out of solution and forms a gel with the ethyl alcohol. A gel is a solid dispersed in a liquid that develops a structure resistant to flow. The formation of the gel is a physical, not chemical change.

As the gel is ignited, the ethyl alcohol evaporates and begins to burn, producing carbon dioxide and water.

The flame heats up the boric acid and lithium chloride, with some gaseous lithium (Li) and boron (B) atoms being produced as a result. As these metal atoms M(g) are heated in the flame, their electrons absorb a specific amount of energy, putting the atom in an excited state M(g)* (see Figure 8). These electrons naturally move back to their lowest or ground state energy levels, releasing the energy difference in the form of radiant energy (hυ) (see Figure 9).

The wavelength of the energy emitted is unique to each atom. For lithium, this corresponds to red light; for boron, green.

Demonstration IV. Santa in a Snowstorm

A solution is formed by dissolving a solute, such as benzoic acid, in a solvent, such as water. The process of a solid solute dissolving in a solvent is a surface phenomenon. Dissolving is a surface phenomenon because it is those molecules or ions at the surface of the solid, not those in the interior, or bulk, of the solid, that interact with and dissolve in the surrounding solvent. As the surface ions dissolve, the next layer of ions now becomes the surface layer. This new surface layer interacts with the ions already in solution. This interaction at the surface of a crystal continues until the crystal is completely dissolved or until the solution can accept no more solute.

A solution is said to be unsaturated when it contains a lower concentration of solute than it can at a given temperature and pressure. A saturated solution contains as much solute as it can at a given temperature and pressure. When the temperature is increased, the solubility usually increases and more solute will dissolve. If this solution is then cooled, the “extra” solute that dissolved with heat will once again precipitate. This is the physical phenomenon occurring when the benzoic acid precipitates out as “snow” in this demonstration.

Demonstration V. The Littlest Christmas Tree

The reaction is a single replacement, oxidation–reduction reaction, where copper is oxidized to copper(II) ions and silver ions are reduced to silver metal.

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

The slide and the cover strip restrict the growth of silver crystals, creating the dendrite, or thread-like spikes of pure crystalline silver.

Demonstration VI. Blinking Holiday Colors

The same clock reaction is occurring in both beakers. The only difference is the color of the pH indicators used. The pH of the solutions, and the indicator colors, remain constant through the initial period, about 30 seconds. At this time the pH of the solutions quickly increases, causing a sudden change in the indicator colors.

The clock reaction is a formaldehyde–sulfite/bisulfite reaction. The sulfite (SO₃^2–) and bisulfite (HSO₃^–) act as a buffer system, where the bisulfite is the weak acid and the sulfite its conjugate base.
The initial solution of sulfite/bisulfite buffer is slightly acidic with a pH of approximately 6.4.

Formaldehyde reacts with sulfite to form hydroxymethyl sulfonate ions and hydroxide ions, according to Equation 3.
As the reaction proceeds, the sulfite ions are consumed and hydroxide ions produced. This causes a shift to the right of Equation 1 and a shift to the left of Equation 2. This buffering keeps the pH of the solution essentially constant until all the bisulfite ions (HSO₃^–) have been consumed. Without the bisulfite ion, no buffering occurs. Hydroxide ions produced in Eqation 3 are therefore not consumed in Equation 2 and the pH rapidly rises to approximately 10.5.

Both indicator solutions are mixtures of two or more indicators. For the green-to-red indicator solution, the acid color is green, the basic color is red, and the transition occurs at approximately pH 10. For red-to-green indicator, the acid color is red, the basic color is green, and again, the transition occurs at approximately pH 10.

Demonstration VII. Invisible Christmas Message

Acid–base indicators are usually organic dyes that are weak acids. If the indicator is represented by HInd, and its conjugate base by Ind^–, then its equilibrium can be represented by the equation The strength of each weak acid can be represented by K_a, its equilibrium constant in water. The acid form (HInd) of each of these indicators has one color and its conjugate base (Ind^–) has another. The ratio of these two determines the color of the solution. When the color of HInd predominates and when
the color of Ind^– predominates.

This means that the color change for indicators occurs over a hydronium ion concentration range of about 100, corresponding to a pH range of 2.

When this ratio of equals 1, [H₃O⁺(aq)] = K_a, or pH = pK_a.

This represents the midpoint pH of the color transition for each indicator. The weaker the acid, the higher the pH of the transition. Therefore, each indicator has a pH range for its color change equal to:

pH range of HInd ≈ pK_a ±1

In this demonstration, the red color indicator contains a combination of phenolphthalein and meta-nitrophenol. The green color indicator contains a combination of thymolphthalein and meta-nitrophenol. The colors and pH range for the color change of each indicator are listed in Table 1. The indicator solutions are initially colorless. When sprayed with 0.1 M NaOH, the indicators are converted to their Ind^– forms. The red-purple and yellow produce the red color and the blue and yellow produce the green.

References

Special thanks to Bob Becker, Kirkwood HS, Kirkwood, MO, for providing Flinn Scientific with instructions for The Littlest Christmas Tree demonstration.

DeKorte, John M., Pocket Guide to Chemistry & Chemical Reactivity, 4th Edition, Saunders College Publishing; 1999; pp. 295–297.

Katz, David A., Chemistry in the Toy Store, 5th Edition. Community College of Philadelphia, Philadelphia, PA, 1990.

McDuffie, Jr., Thomas E., and Jacqueline Anderson. Chemical Experiments from Daily Life. Portland, ME: J. Weston Walch Publisher, 1980.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison; 1985; Vol. 4, pp. 70–74.

Summerlin, Lee R. and James L. Ealy, Jr., Chemical Demonstrations: A Sourcebook for Teachers, American Chemical Society, 1985.